Toward a Standard Working in clay without any standards is not only diffi­ cult but unproductive, and eventually discourages any cre­ ative drive. When cracks occur, casseroles break or a kiln fires inefficiently, all the time and effort of producing the work are negated. Creating pottery is a combination of many skills—from preparation of the clay to design and forming through final glazing and firing. Within each ac­ tivity there is a standard, and failure to meet any one of them may detract from the final result. There are standards in pottery, which, though not spe­ cifically stated anywhere, do exist. While these standards may have a degree to them (a good, better, best quality), they are defined by how recipients perceive the work.These recipients may be the creator of the work, a person receiv­ ing a gift, or a customer purchasing a piece. Discovering standards There are standards for design, clay preparation, throw­ ing, handbuilding, glaze formulation, safety, kiln firing, deco­ ration, etc. Knowing what the standards are for each pro­ cess helps us to become better at making the pottery we want to make. These standards are made apparent through instructors, literature, research and hands-on experience. Striving to meet those standards on a good-better-best scale helps us keep a balance so we won’t become stuck at one skill before moving on to another. Becoming the best de­ signer, handbuilder, or kiln technician won’t make one the best potter. But by starting with being good at each skill level then working toward becoming better, anyone can achieve a standard that’s readily recognizable. Applying standards Standards are based on both knowledge and ability. For the beginner, the standards are few and may be as simple as completing a whistle that survives the firing and makes a sound when blown. But even at that, those are two stan­ dards that must be met to measure the success of the piece. For the more accomplished amateur or professional, the standards are higher, as in the case of creating a casserole that is both aesthetically pleasing and can survive hundreds of heating and cooling cycles in an oven without cracking, or having a glaze that is safe and doesn’t chip or crack.The success of this piece means meeting many standards that are commonly accepted in industry and the marketplace. Finding help In this issue, you’ll find help in producing and working to a higher standard. Slabs that crack at the seams when attached were substandard for Anna Holcombe, so she de­ scribes how she eliminated the problem; a couple of cus­ tomers returned cracked ovenware pieces to Dick Lehman, so he created a methodology for testing a replacement clay body; and concern over potters not using the right respira­ tors (or none at all) in the studio led Jeff Zamek to research the issue and he reports his findings. Improving your work, your methods and your studio to achieve a higher standard is worth pursuing—-just get started, stay at it and enjoy your improvements along the way.

Pottery Making illustrated 1 2 Spring 1999 ClayLinks by Barbara Coultry ...... 6 Check out this new feature for finding valuable information on the web Building a Better Box by Anna Calluori Holcombe with Patrick Taddy...... 9 Eliminate cracking at the seams using tar-paper support Drawing the Line by Hanna Lore Hombordy...... 14 Discover a new use for an obsolete tool How to Throw Large Pieces by Joining Cylinders by George Juliano with Elizabeth Hudgins...... 17 Stack your cylinders for taller forms All Polished Up by Sumi von Dassow...... 21 Put a shine on your pots without glaze Primal Screens by Lili Krakowski...... 26 Make your own sieves using readily available materials Oxygen Probes by Mel Jacobson...... 29 Help your fuel burn better using state-of-the-art tools Globe Whistles by Craig Hinshaw...... 33 Get in tune with the world Creating Glazes by Michael Hamlin ...... 36 Mix basic glazes to build your palette Respirators for Potters by Jeff Zamek...... 38 Learn the basics about face masks, and breathe easier in your studio Testing Ovenware by Dick Lehman...... 43 Establish a testing standard for your functional pots

Cover: “Still Lifebox VI,” 21 inches in height, earthenware clay, terra sigillata, trailed slip, low-fire glaze, electric-kiln fired to Cone 04, by Anna Calluori Holcombe. Photo: Richard Bergen. See story page 9.

Pottery Making illustrated 3 Editor:Bill Jones Production Manager:John Wilson Graphic Design:Debi Hampton Graphic Production:Robin Chukes Advertising Manager: Steve Hecker Advertising Assistant:Renee Fairchild Customer Service:Mary Hopkins Publisher:Mark Mecklenborg Editorial, Advertising & Circulation Offices 735 Ceramic Place PO Box 6136 Westerville, OH 43086-6136 Phone: (614) 794-5890 Fax: (614) 794-5892 E-mail: [email protected] Website: www.potterymaking.org Pottery Making Illustrated (ISSN 1096-830X) is published quarterly by The American Ceramic Society, 735 Ceramic Place, Westerville, OH 43081. Standard mail (A) postage paid at Westerville, OH. Opinions expressed are those of the contributors and do not necessarily represent those of the edi­ tors or The American Ceramic Society. Subscription Rates: One year $15, two years $27, three years $36. Add $8 per year for sub­ scriptions outside North America. In Canada, add GST (R123994618). All payments in US dollars. Allow 6-8 weeks for delivery. Advertising: Ad rates and ad information are available on the website or by contacting Steve Hecker at the address above or by phone at (614) 794-5809. Change of Address:Send your change of ad­ dress via e-mail or to the Circulation Depart­ ment. Allow six weeks advance notice. Contributors: Writing and photographic guide­ lines are available on request and on the website. Mail manuscripts and visual materials to the edi­ torial offices with a self-addressed postcard for no­ tification of receipt. Indexing: Feature articles are indexed in the Art Index and daai (design and applied arts index),avail­ able through public and university libraries. Photocopies: Permission to photocopy for per­ sonal or internal use beyond the limits of Sec­ tions 107 and 108 of the U.S. Copyright Law is granted by The American Ceramic Society, pro­ vided that the base fee of US$5 per copy, plus US$0.50 per page, is paid directly to the Copy­ right Clearance Center, 222 Rosewood Dr., Danvers, MA 01923 USA (508) 750-8400.This consent does not extend to other kinds of copy­ ing, such as copying for general distribution, for advertising or promotional purposes, or for cre­ ating new collective works. Requests for special photocopying permission and reprint requests should be directed to the Director of Publica­ tions, The American Ceramic Society, P.O. Box 6136,Westerville, OH 43086-6136. Back Issues:When available, back issues are $5 each, plus $2 postage. Add $5 for orders outside North America. In Canada, add GST. Postmaster: Send address changes toPottery Making Illustrated, PO Box 6136,Westerville, OH 43086-6136. Form 3579 reauested.

Copyright © 1999 The American Ceramic Society All rights reserved 4 Spring 1999

[email protected] This is my e-mail address. Please send your favorite sites and discoveries to me at this address. Also, put my by Barbara Coultry e-mail address into your address book. When you find a particularly good site that youPottery think Making Enter “ceramics” in a search engine and you’ll be Illustrated’s readers might be interested in, send me the buried in sites built by potters, suppliers, museums, gal­ link. leries, chemical engineers, gift shops, archaeologists and What am I looking for? If it has to do with ceram­ probably even NASA. Pottery sites alone have exploded ics and has good beginner, intermediate or advanced onto the Internet. ClayLinks is here to act as your guide. information, I want to know about it. Also, since we ceramics artists aren’t inspired only by things made of This issue introduces you to those must-see sites where clay, if you find a non-ceramics site that has solid in­ the range of information is extremely wide and helpful. formation, elements of inspiration, or contains ideas Bookmark the four reviewed sites since their content for new things we might do with clay, by all means periodically changes. share it. Examples might be a page devoted to polymer Before we begin, I have a few hints. clay or a gallery of Inuit soapstone carving. 1. Be patient. Ceramic sites tend to be photo-heavy, I can’t personally answer everyone’s e-mail—I’m sure you understand—but, believe me, I’ll be checking which can slow up the works. out each and every link you’re kind enough to send. 2. Though you shouldn’t have to type “http://” in front of a Web address, now and then you will. 3. Remember that most Web sites are created by one or just a few people. If you find a site you truly like, send e-mail to say how much you appreciate what they’ve done.You’ll brighten another person’s day! And now, on with the show

The CeramicsWeb The Mining Company - Ceramics http ://art. sdsu. ed u/ceram icsweb http://ceramics.miningco.com Hosted by San Diego State University, this site should I be love your The Mining Co., and this site is one of the reasons. Here first stop. Click on absolutely everything. Try Educationis a true for potpourri ar­ sure to warm a clay artist’s heart. Hosted by ticles from Ice Age Ceramics through Pyrometry to Majolica.Steven Goldate, a ceramic artist in Australia, this is the place for Then, in GlazeBase, type in the qualities of a glaze you’reglazes, look­ articles, tips and techniques, events, bulletin board, chat, ing for, hit the “Start Search” button, and see what glazessafety, it links, newsletter, and/or history. If you’re new to the site, comes up with.Try a similar search in Clayart Glazes. Materialsdon’t get depressed about having missed everything that’s been Data is where you can search for the chemical analysis andposted use there since 1996.The articles have been archived so you of various ceramic materials. Don’t skip Health/Safety, can IRC positively & wallow in them. Do so. I guarantee satisfaction. #potters, Software, Search Links, and Clayart List (see Clayart below). And, if you’ve been here before, it’s time for a return trip.

6 Spring 1999 ClayNet International http://www.home.vicnet.net.au/~claynet/ ClayNet International looks plain, but you’ll find a treasure trove of information by clicking on the arrow next to “Where would you like to go today?” Choose either ad­ While they don’t have the full range of the other dress to get there. sites described, the following sites provide a lot of Minnesota Clay USA information. http ://www. m m .com/m nclay us Another commercial site—click on “News Archive” and Brad Sondahl’s Pottery Tips “Tech Releases.”Their “Animations” are delightful and so http://www.camasnet.com/~asondahl/potterytips.html is their “Windows Calendar.” A great site for making your own tools, a few hints and tips, and some glaze recipes. Carl Baker - Stoneware & Raku Pottery http://www.acga.net/cbpotter Krueger Pottery, Inc This site has excellent FAQs and educational materials, in­ http:// www.kruegerpottery.com cluding information on raku firing. This commercial site provides tips on glazes, firing and throwing. If you’re a teacher—and even if you’re David not—check Hewitt Pottery out “Weber Ware.” http://www.dhpot.demon.co.uk How-to’s on agateware, slip, glaze and materials. Pottery Guide http://www.potteryguide.com Folk Art & Craft Exchange Pottery Guide contains articles for beginners and http://www.folkart.com/~latitude interme­ diates, a bulletin board, a free newsletter, and a forum. A site loaded with inspiration.

Clayart Digitalfire Corporation http ://www. u ky. ed u/Artso u rce/vce/clay art. htm I http://www.digitalfire.com http://www.egroups.com/list/clayart Digitalfire created Foresight, glaze database software for DOS, http://www.potters.org/categories.htm and Insight, glaze recipe calculation and database software for For the moment, ignore all those addresses while I tell youWindows. about It also has a textbook on glaze formulation,The Magic Clayart. It’s a forum, a special sort of bulletin board. Oneof person Fire, and a huge electronic book, Magic of Fire Reference, enters a question or observation or maybe an which opinion, details ceramic then materials. Click the Education tab for an others respond. The process is just like writing e-mail. To viewarea brimming with articles on clay, glaze, firing, kilns, and ma­ what others have written, you have a choice of two terials.They methods. also have listings of books, magazines, and newslet­ You can use your news server, which is usually a partters, of plus your a glossary e- of ceramic terms. Don’t forget to subscribe mail program, or you can visit the archives to read what’sto the been free newsletter. If you want, download a trial version of going on or to search for particular subjects. the software, a good way to decide if you want to buy the pro­ Directions: Type in the first address if you’d like to join.You’llgram. be sent several pages giving you all the information you need for using and enjoying the forum.The second address will start your news reader. Choose from any of the last three addresses to see what it’s all about.You won’t be disappointed.

Pottery Making illustrated 7 8 Spring 1999 by Anna Calluori Holcombe with Patrick Taddy

began using the box format in my work while in graduate school at Louisiana State University iin the 1970s. I’ve continued to use it where the concept of container is important in the series. The idea of something mundane becoming precious by being placed in a box has always fascinated me. Working with slabs can be frustrating. Stress cracks often develop at the seams if one waits until the slabs are stiff enough to build with, thereby limiting the form possibilities. I was searching for a way to reduce the cracking and give myself more flexibility in my forms. In a conversation with a colleague, the fact that a certain clay artistwas using tar paper for the purpose of building with slabs piqued my interest. In 1985, I was living in a new housing develop­ ment under construction, and construction workers regularly threw away partial rolls of tar paper. I picked up one such roll and brought it back to my studio to experiment with. What I discovered from my experimentation is there are a number of advantages to using this tech­ nique. One is that I can put slabs together before the leather-hard stage, virtually eliminating stress cracks in the seams. Second, it allows me to con­ struct larger pieces as the slabs are supported by the tar paper.Third, I can form the slabs into shapes that will hold until they stiffen. Last but not least, I am able to paddle, roll and pinch the slabs into shape without marring the surface. In a classroom or workshop situation this tech­ nique allows the student a quick and easy way to “Still Lifebox V,” 14 inches in height, earthenware clay, terra construct a box form in a short amount of time. I’ve sigillata, underglaze, low-fire glaze, electric-kiln fired to had numerous opportunities to teach this technique, Cone 04. My intention in this work is to capture the arcane most recently in Scotland.There I had to find a sub­ qualities of everyday mundane objects. This interest in the still life was fired up in 1990 while in Bologna, Italy, where stitute, because roofing materials are considerably dif­ I saw the Georgio Morandi Retrospective Exhibition. I was ferent in Europe. I found that a waxed stencil pa­ amazed at how Morandi could paint such simple everyday per—a type of waterproofed poster board—worked objects repeatedly and yet brilliantly capture their mystical almost as well, although the tar paper is stronger and qualities. more waterproof.

Pottery Making illustrated PHOTOS: PATRICKTADDY

Figure 1 Figure 2 Figure 3 Using a white china marker, trace graph Cut the shapes out with heavy-duty scis­ Spray water onto the slab and the back paper templates of the box sides, top and sors. of the tar paper forms. bottom. Tip: Designs can be transferred using chalk rubbed on the back side of the design and the image redrawn through the paper onto the tar paper.

Figure 6 Turn the pieces over and bevel the side edges by using a ruler placed about 1/4 Figure 5 inch from the edge, holding the knife Cut out all the slabs using a knife with a blade at a 45° angle and then cutting. stiff blade. Allow them to set for a while Figure 4 to slightly stiffen, or cover them with plas­ Press tar paper into place and tic and allow them to set overnight. lightly roll to secure it.

10 Spring 1999 Figure 7 Figure 8 Figure 9 Build the walls on to the bottom slab— Add the top part of the box. Note: Since Leave the tar paper on the constructed scoring and slipping are critical. Paddle the top pieces support each other, have box until it is leather hard and the sides the sides together with a wooden spoon, all parts scored and ready to be joined can support themselves. add a coil on the inside seams, then ahead of time. paddle again.

Figure 10 Peel away the tar paper and dispose of it since it buckles too much to reuse it. Figure 11 The box is ready to be polished off and finished using metal ribs and Surforms Anna Calluori Holcombe is Professor of Art and to conceal the seams and square the cor­ Department Head at Kansas State University. Her ners. work has been exhibited nationally as well as in Italy, Germany, New Zealand and Australia. She is also a Fellow ofNCECA (National Council on Education for the Ceramic Arts). Patrick Taddy, collaborator and photographer for the process shots, is a graduate of Nova Scotia College of Art and Design in Halifax. He is currently en­ rolled in the MFA program in ceramics at Kansas State University.

Pottery Making illustrated 11 “Still Lifebox VI,” 21 inches in height, earthenware clay, terra “Still Lifebox VIII,” 19½ inches in height, earthenware clay, terra sigillata, trailed slip, low-fire glaze, electric-kiln fired to Cone sigillata, underglaze, mishima, low-fire glaze, electric-kiln fired 04. All my work is made of earthenware clay, which has a high to Cone 04. “In 1991, I spent my sabbatical as an artist-in- iron content. “I use nylon fiber in the clay for strength in the residence at the Archie Bray Foundation in Helena, Montana. I slabs, and mullite, instead of grog, for its better thermal quali­ worked alongside potters making functional pots meant to be ties. The surface is low-fire underglazes and glazes, as well as used and enjoyed. This led to my research on modern and con­ terra sigilatta (a refined slip, with commercial stains added for temporary artists depicting the everyday object, such as color). I sometimes incorporate the slip techniques of mishima, Cezanne, Picasso and Elisabeth Murray. I am interested in the an inlaid slip, or sgrafitto, scratching through slip to get to the play of the actual object, which is three dimensional, as op­ clay. The pieces are typically fired once in an electric kiln to posed to the visual image, which is merely two dimensional. I about 18OO°F.” utilize this imagery to challenge the viewer’s perception of space and three dimensionality. The notion of container is an impor­ tant one, as it is also important for the still-life objects, which are precariously balanced on top of the box.”

Richard Zakin ’s Clay Body Terra S igillata Tar P aper

Cedar Heights Redart.... 33 lb Cedar Heights Redart.... 1000 gr Tar paper (also known as “roof felt”) Ball clay...... 26 Calgon water softener...... 5 gr can be purchased at most building Nepheline syenite...... 7 Water...... 2500 gr supply stores. Ask for the heavier 30- Barium carbonate...... 1 This will make a red terra sigillata, which lb. weight that comes in 200-sq.-ft. Mullite or cordierite...... 16 is good for mixing darker colors rolls. The cost is approximately $13 for a roll. Grog (optional)...... 10 93 lb Cedar Heights Goldart... 1000 gr I add about 2 tablespoons of nylon fiber Calgon water softener...... 5 gr to the dry mix and mix thoroughly be­ Water...... 2000 gr fore adding water. This will make a white terra sigillata, which is good for lighter colors. I add about a teaspoon of (Mason) stain to an 8-oz. cup of terra sigillata as a stan­ dard measure and then add more or less for the shade of color desired.

12 Spring 1999

Drawing the Line

by Hanna Lore Hombordy

“Three Pods,” to 10 inches in length. Lines were drawn with black underglaze, then transparent glaze applied.

y work in clay spans the last 20 or so years, and a lot of that time has been spent looking for the solution to assorted problems. One M challenge was to discover a quick and easy way to draw lines on clay both in wax resist and with glazes or underglazes. I wanted a tool with a reservoir, and one that could give me adjustable line width. One of the advantages of being over 55 is that one has such a long period of learning experiences to fall back on. I remember from my earlier life while working in advertising and industrial design that we used a tool called a ruling pen in combina­ tion with a T-square to draw straight lines of even width. I decided to try one with ceramic materials instead of ink.

14 Spring 1999 I knew ruling pens were designed to make perfectly straight lines but I saw no reason why I couldn’t use one for freehand drawing, much like a pencil or brush. Results of my experiments were encouraging. The ruling pen has a screw to adjust the width of the line, which gave me flexibility in adjusting to various mate­ rials.The liquid used should be diluted enough to flow freely, but it shouldn’t be too runny. One fills the reservoir Figure 1 with a dropper or with a loaded brush. With a straightedge, ruling pens are the traditional tool for produc­ The pen should be held at a slant to ing straight lines. Cardboard under the ruler prevents smudging of the surface, not upright. Some pens have the line. sharper points than others, which af­ fects the tendency to pick up dust or bits of clay from the surface. If a crumb should clog the point, a fine brush can be used to clear it out. If the flow is reluctant to start up, a dab at the point with a finger or a damp cloth should encourage flow. And the smoother the surface, of course, the easier it is to coax liquid over it. I tried the ruling pen on greenware, bisque and glaze, and was able to get a satisfactory result on all of them. I now commonly use ruling pens with wax Figure 2 resist to separate glazes, but I really en­ joy them as a freehand drawing tool, Using the ruling pen filled with dark wax to draw decorative lines filled with black underglaze. on a bisque plate. Different colored glazes are then applied. Another handy use for the ruling pen is in applying gold luster lines over al­ ready glazed pieces. Keep in mind that this requires an especially thorough cleaning of the reservoir. With practice, other decorative techniques will come to mind. While they are not a product of modern technology, it is still possible to buy ruling pens. Merely call a sup­ plier of art, architectural or engineer­ ing materials, or consult one of the na­ tional art supply catalogs. The cost of a ruling pen is approximately $6, and well worth it.

Figure 3 Black underglaze in a ruling pen decorates a bisque form, which is then sprayed with transparent glaze.

Pottery Making illustrated 15 In these plates titled “Shades of Waves” (left) and “Burst of Color” (right), wax resist lines were first applied, then glaze applied between these lines.

This 16x12-inch tile panel, titled “Mother Earth,” “Bud,” 9 inches in height, black underglaze on is an example of black underglaze lines covered stoneware, covered with transparent glaze. with a transparent glaze. The ruling pen was de­ signed for straight lines, but there is no reason why it cannot be used for freehand drawing, much like a pencil or brush.

Hanna Lore Hombordy earned a BFA, with honors, from the School of Art and Design, Pratt Institute, New York. Before settling in Ventura, California, in 1973, her main occupation was in the field of creative design and graphic arts. Since 19,73, most of her creative work has been in clay Hombordy’s work has been included in numerous publications and in over 20 national juried shows, as well as the California State Fair and local art exhibits.

16 Spring 1999 How to Throw Large Pieces by Joining Cylinders Vase, 16 inches in height, raku fired. Joining cylinders works very well and is easily incorporated by potters as they learn to throw larger pots. The “acid test” is that by George Juliano with Elizabeth Hudgins these joined pieces are capable of withstanding even raku firing without cracking.

eorge Juliano, a large, strong potter, had no trouble centering 10 to 20 pounds of clay until April of i 998 whenG he was diagnosed with a rare type of cancer. Then, the debilitating effects of both the cancer and the chemotherapy sapped his strength and his energy. No longer able to center a large amount of clay, yet unwilling to give up the size and type of pots he loved and is known for, George be­ gan to experiment with alternative methods for throwing larger pots. He finally developed a method that met his desire for large pots yet did not take as much strength and stamina. Today, even though he’s finished the chemo­ therapy and has regained his health and strength, George says he will continue to use the method he developed since the possibilities Vase, 18 inches in height, raku fired. George Juliano for throwing even larger and stronger pots are philosophizes about his experience with cancer by say­ limited only by the size of the kiln he uses. ing that this just shows that good can come out of even the worst situations. In this case, he discovered an easier way to throw large pots because of the weakness he experienced during chemotherapy.

Pottery Making illustrated 17 1. Create the bottom section by center­ 2. After opening the centered clay, throw 3. Carefully bevel the lip to a 45° angle ing a ball of clay weighing anywhere from a cylinder approximately 9 inches tall so so that the inside wall is higher than the 5 to 10 pounds. that the bottom is ½ inch thick and the outside. walls are ¾ inch thick.

4. Using an old comb, or similar scratch­ 5. Take a careful measurement with cali­ 6. Trim the bottom around the base and ing tool, score the lip so that it is lightly pers so that you can match the diameter set aside for later work. roughened. This is done so that the join­ exactly. Note the measurement on a piece ing of the two pieces will be more solid of paper for later use. since the ridges, when dampened, will work together easily through a combina­ tion of throwing and capillary action.

7. Approximately four hours later, throw 8. Using the notation from the earlier 9. Bevel the lip to a 45° angle in the op­ a similar cylinder about 7 inches tall with work, carefully check the piece to make posite direction from the first piece so that 5 /s-inch-thick walls. This time, however, sure that the width matches the earlier this cylinder is higher on the outside than throw all the way to the bat so that the piece exactly. the inside. cylinder will be a hollow tube.

18 Spring 1999 10. Score the lip with a comb tool. Trim 11. When the bottom piece is fairly solid 12. Take the second cylinder (the one that the base of this cylinder to get rid of ex­ and the top is still damp but capable of is hollow to the bat), dampen the scored cess clay, then set the section aside for being joined, place the first (bottom) piece area on this section, and firmly set it up­ about an hour. on the wheel and lightly dampen the side down onto the first piece. scored lip.

13. Using a piece of flat wood (similar to 14. Cut the top cylinder from the bat. 15. At this point, wet your arm with a a tongue depressor used by doctors) go sponge to eliminate friction. over the seam with little up-and-down marks, so that the two pieces are worked together slightly.

16. Stand up and begin to throw the two pieces together. Make 17. To make certain that the inside is joined well, George spends certain that the two pieces are firmly joined and that the joining time throwing down from the top over the joined area, then he line is no longer visible. checks to make certain that the walls are consistent through­ out. He continues throwing as he normally would, forming the piece and finishing it.

Pottery Making illustrated 19 George Juliano has been teaching high school ceramics for 28 years and is cur­ rently teaching pottery at James W. Robinson Jr. Secondary School in Fairfax, Virginia. He is also teaching two graduate classes in pottery for Virginia Com­ monwealth University. Elizabeth Hudgins is a retired English teacher and is now working as an iconog- rapher, freelance writer, and photographer. She studies ceramics with Mr. Juliano.

Vase, 17 inches in height, raku fired, by George Juliano.

20 Spring 1999 by Sumi von Dassow

In addition to the numerous variations on “primitive” firing processes, burnished pottery can also be decorated with lusters or enamels. Any over-glaze finish that fires at Cone 018 or below is potentially useful with burnished pottery. Gold leaf and other non-fired finishes may also be used. Once you begin burnishing, you are free to experiment!

Though burnishing is a time-consuming process, and impractical for functional ware, the process is simple, and the result fascinating enough to make it worth any potter’s time to experiment with. The only materials needed are a suitable clay, water, and oil; the only tool, a polished stone. The firing is to a low temperature, can be done in any type of kiln or even outdoors, and releases no toxic fumes.

a didn’t know people still burnished pots!” ex­ does not make the surface as watertight as a glaze, bur­ claimed a fellow potter as she looked at a display nishing reduces the porosity of the clay. At the same Iof my work. Indeed, not only are people still bur­ time, in a warm climate, the slight porosity of a bur­ nishing (polishing the surface of a pot by rubbing it with nished pot allows evaporation, which actually keeps the a smooth stone), the technique is gaining adherents with contents cooler than it would be in a non-porous vessel. the increasing popularity of “primitive” firing methods. Before the discoveries of glaze and high-temperature Burnishing can enhance the effects of sawdust and pit firing, burnishing represented a significant technologi­ firing, or be used instead of glaze to impart a finish that cal advance over bare clay pots. Burnishing works by is as beguiling to the touch as to the eye. pressing down the clay platelets on the surface of the The technique of burnishing developed in many parts pot, so they all face the same way and reflect light the of the world long before the use of glaze was developed. same way. As clay is fired, it loses its platelet structure, so In the new world, glaze (along with the use of the wheel) the higher the firing temperature, the more burnish you was an importation of European conquerors.To this day lose.With a good clay, Cone 018 is hot enough to harden the highly valued pottery of the Pueblo Indians of the the clay without sacrificing too much sheen. Since no southwestern United States, such as the famous Maria glaze is to be applied, no second firing is necessary, though Martinez and her successors, is coil built and burnished. the burnished pot offers many possibilities African potters, such as Magdalene Odundo. similarly In most places nowadays, since the seepage of water carry on the tradition of burnishing to create sophisti­ through the clay eventually erodes the finish, burnish­ cated and beautiful pottery. ing is used to finish decorative work only. With prices In parts of the world where “traditional” pottery is for Southwestern pottery ranging into the tens of thou­ still made and used, burnishing serves a practical func­ sands of dollars, surely no collector is going to fill a pre­ tion. Pottery that is unglazed and fired in the open to a cious work of art with water and use it as a vase! relatively low temperature is quite porous, rendering it inconvenient for cooking and storing water. Though it

Pottery Making illustrated 21 Figure 1 Figure 2 Figure 3 To prepare a pot for burnishing, it should Dip your fingers in water then wet the Re-wet the inside of the rim. Work the be sanded perfectly smooth when it is whole pot lightly, inside and out. water in well and wipe your fingers dry. bone dry. In addition to the pot and the stone, you need a towel or rag, a bowl of water and bit of vegetable oil. Use the towel or rag to wipe excess water off your finger and to wipe the stone clean as nec­ TIP essary throughout the process.

For your first burnishing attempts, start with a small rounded form. Sharp angles, S-curves, flaring lips, grooves, etc. are difficult to burnish. NOTE

It is important to start burnishing at the rim then continue in a spiral pattern around the pot working to­ ward the foot. Once a burnished patch has dried, if it is rubbed again with the stone it gets scratched. For this reason, once you begin you can’t stop until you’re finished. Working in a spiral pattern ensures that you are never working on a patch adjacent to a section that has completely dried: by the time you are working on the bottom, the top—which may have Figure 7 Figure 8 been burnished an hour earlier— As you burnish, bits of clay will collect on As you progress down the body of the is completely dry again, but the the surface you’re working on. Use the pot, you can dampen fairly large areas bottom inch or two is still damp and back of your hand to wipe these bits off and run the stone quickly over it horizon­ can still be safely rubbed so they don’t cause scratches. Wipe clay tally and then more carefully vertically. with the stone. off the stone with the towel or your thumb. Some people find it easier to burnish with a circular motion.

22 Spring 1999 Figure 4 Figure 5 Figure 6 Rub with the stone on the dampened area Once the inside rim has been burnished After burnishing all the way around just until the clay becomes smooth; avoid all the way around the pot, wet the top of below the rim, continue in a spiral pat­ leaving unburnished streaks. If you have the lip and burnish. The lip tends to be tern. When you dampen a new patch to to rub hard to get the clay smooth, or if the most difficult area and should be care­ burnish, slightly overlap the previously scratch-marks from sanding remain vis­ fully smoothed and rounded by sanding burnished area above it. As long as an ible, you’re not using enough water. prior to burnishing. already-burnished spot has not changed color (indicating it has dried completely), it is safe to go over it with the stone.

Figure 9 Figure 10 Figure 11 You may find it easier to hold the pot up- The last spot to be burnished is just above The second step is to immediately cover side-down with a hand inside. the foot (which is left unburnished). Note the entire pot with a light coating of veg­ the color difference between the just-bur­ etable (cooking) oil. This soaks in and nished area near the foot and the dry area leaves a whitish scum on the surface. at the rim.

Pottery Making illustrated 23 Figure 12 Holding the pot with a hand inside to avoid touching the surface and marring the bur­ nish, the pot is re-burnished wherever the Figure 13 oil has dried to a whitish scum. Burnish around patches of wet oil until they dry. If Lightly rubbing the pot with a chamois af­ necessary, extra oil can be wiped off with ter oiling and reburnishing will remove any extra oil and slightly improve the sheen. a finger; if the stone scratches the oiled Figure 14 surface, add more oil and wait for it to The burnished pot can now be decorated soak in. Rubbed again with the stone, the by incising or by painting with terra The finished pot, painted with terra clay takes on a high gloss. This step is sigillata, or it can be immediately bisque- sigillata, fired and partially blackened much faster than the first, and it doesn’t fired to Cone 018. by smoking in a second firing. matter what part of the pot is reburnished first. If there are patches of wet oil, you can work around them and reburnish the parts which are ready. During this step the pot will be easily marred by finger­ prints and should be held with a hand in­ side. This will also help you avoid rubbing THE CLAY THE STONE the oil off while you work.

The best type of clay for bur­ A suitable burnishing stone nishing is a smooth red earth­ can be found at any lapidary enware. A Cone 6 (not 06) clay shop, and often in museum gift will be the strongest and most shops or flea markets. Any sort durable after firing to Cone 018 of stone that has been tumbled or so. (A low-fire clay is often still in a rock polisher may be a per­ so underfired at such a low tem­ fect burnishing stone. Ideal is a perature that it can be scratched stone that is large enough to with a fingernail, like chalk.) If a grasp easily, and has at least white clay is desired, choose a one perfectly smooth, slightly smooth Cone 5 or 6 clay—not a rounded surface. Any nick, porcelain or a low-fire clay. White bump, or sharp edge is likely to clays are not so strong when scratch the pot as it is being bur­ fired at such low temperatures nished. but are best for bringing out the range of color in a sawdust- or pit-firing. For the traditional all­ black of Southwestern pottery a red clay is best.

NOTE: I use Navajo Wheel from Indus­ trial Minerals Co. in Sacramento, A potter for 20 years, Sumi von Dassow teaches pot­ tery at the Washington Heights Center for California, for red; or B-mix Cone the Traditional Arts in Lakewood, Colorado, and is a 5 from Laguna Clay Co. frequent contributor Pottery to Making Illustrated. Sumi, and her work, can often be found at the Foothills Art Center in Golden, Colorado. Visit her web site at www.well.com/~sumi for more information.

24 Spring 1999 Untitled, 4 inches in height. This pot was decorated by Untitled, 3½ inches in height, by Danielle Steinfeld. This carving with a small loop tool while leatherhard, and by pot was decorated by carving while leatherhard. Thrown incising with a sharp blade after burnishing. and trimmed, burnished and smoke.

“Ribbon Dance,” 8 inches in height. Decorated “Moonflower,” 24 inches in height. Decorated with gold, platinum and copper lusters after with terra sigillata and metallic lusters with bisque firing, then smoked. inset moonstone.

25 Pottery Making illustrated by Lili Krakowski

ieving is one of the more misunderstood areas of Materials s glaze making. People sieve when they needn’t, and don’t when they Frames: While I use scrap ends of For coarse sieves, fiberglass window- should. Sieving has two principal pur­ white 4-inch plastic pipe scrounged screen cloth works well, although it’s poses—it helps mix the ingredients very from plumbers, or asked for at con­ not as pliable as silk-screen nylon and well, and it eliminates lumps and foreign struction sites, frames are serious needs a bit more adjustment to stretch matter from the mix. improv country. Plain wooden picture properly over pipe. It’s more coopera­ Although sieving (or screening) may frames (if deep enough) work well— tive over a squarish frame cloth with not seem essential after a good stir, some though they may need regluing with relatively large holes. materials tend to clump or lump in waterproof glue. A simple frame—a Nylon silk-screen is excellent. It the mix. While adding bentonite, cal­ topless, bottomless box—can be built comes in different finenesses.The one cium chloride, Epsom salts or com­ out of 1x3s as can a more complex I use is meant for printing inks that mercial additives prevents clumping, one (see figure 5) copied from some I contain glitter or tinsel—a relatively lumping and settling out, sieving is still saw in Montreal. The arms allow it to tough cloth with relatively large holes recommended, especially when be suspended with ease over any size (mesh). colorants need to be dispersed bucket. More and more I am using “paint throughout the glaze. One of the glazes Fabric: While nothing stops one socks,” which are bags used to screen I use has 0.5% cobalt carbonate in it. I from using “classic” copper screening paint that is to go into sprayers. A five- cannot trust the best of stirrings to re­ stapled to a wooden frame, I have found gallon bag costs about $3 and is enough ally disperse it well. it to be both costly and short-lived as for 2 large and 2 small sieves. I consider the screening of already- chemical interactions soon clog the I’ve used my silk-screen cloth sieves used glaze far more important than that screen pores. (Some potters have for years and have had no need to re­ of the just-mixed! Although I wash my melted the copper mesh into the plas­ place the cloth. When replacement bisque very carefully, bisque dust and tic pipe, but I tried this and found the becomes necessary, I’ll scrape or file the tiny chips of slip can remain. Trouble process both difficult and smelly.) present cloth off, and start afresh. spots where stuff collects are the un­ derside of galleries and handles, the in­ side of spouts, etc. (If this happens in my one-person studio, imagine what goes on in classrooms or large studios.) Furthermore, not everyone is careful about scraping all glaze from the bucket sides before it dries into flakes, and many potters return half-dried crumbs off a pot’s lip or rim into the bucket. In other words, lumps and little bits of this and that are much more of a prob­ lem in already-used glazes than in brand-new ones. (Lumps and un­ wanted debris are an even bigger threat with slips.) Pottery-supply houses sell excellent screens, but you can quickly make in­ expensive screens yourself. Because cleaning sieves to prevent contamina­ Figure 1 tion from glaze to glaze is time con­ Clamp a scrap piece of plastic pipe (A) to a board (B) using a C-clamp (C) or vise grips. suming, I have one or two (4-inch and (CAUTION: To safely saw the pipe, the board must be clamped in a bench vise or clamped 8- or 10-inch) for each glaze, as well as to a workbench.) Use a square and pencil (D) to mark a neat line for sawing or filing. A saber saw (E) is the ideal sawing tool, but a hacksaw will do. Most rasps or files work well slip. As each sieve costs me less than $2 in smoothing the edge. The point is to get the edge smooth enough so that fitting and each, it’s a worthwhile and timesaving gluing the fabric is no problem. And, of course, safety goggles and a dust mask must be investment. worn while sawing, rasping, or filing.

26 Spring 1999 Using a Sieve

To use the finer sieves, I find ny­ lon nail brushes or baby’s hairbrushes, with the handles cut off and filed smooth, work extremely well to push the material through. Nail, hair, even scrub brushes (except for very stiff and coarse) work very well on any A frame made from 1 x3s and all sieves. Again, since they are may be somewhat com­ cheap, you can eliminate time-con­ plex, like this one that I saw suming cleanup projects, as each glaze in Montreal. The arms allow and slip can have its own brush. it to be suspended with ease Some books give recipes for cer­ over any size bucket. tain glazes that should be screened through a specific mesh. While I would be likelier to use my own sieves and buy a commercial one only if my results were far off from the expected, I suggest you try the proper-mesh screen first then test the glaze made with your homemade sieves. I haven’t counted the holes in my sieves, but they visually compare well with the generally-recom­ mended 80 mesh. Lili Krakowski is a professional potter residing in Constableville, New York. She teaches pottery at the Gibbes Museum of Art in Charleston, South Carolina, during the winter months.

Figure 4 Mix epoxy cement—quick or slow dry­ ing—according to directions. When mak­ ing several sieves at a time, slow drying Figure 3 epoxy is better. Force the glue through Center the cut-out piece of cloth on the the mesh of the cloth. It’s not always pos­ pipe and tighten an automotive hose sible to avoid a smudge of glue where it’s clamp (I) around it. When the clamp is not wanted, but don’t let that discourage almost totally tight, puli the cloth down all you—the sieve will be perfectly good. around—tight as a drum if possible—then Remove the clamp when the cement is Figure 2 give the clamp’s screw a final turn. dry, and trim away the excess with a single-edged razor blade or similar tool. Set the cut and smoothed piece of pipe Tip: These clamps come in all lengths (A) on the sieve cloth (G). Draw a line (H) and I keep several smaller ones (for 4- Tip: I drill a hole at the opposite end of the around the pipe, leaving an ample mar­ inch pipe) and larger ones (for 8-inch and sieve and thread a cord through it so the gin of cloth. The extra cloth facilitates up) in the studio. It’s easier to make a sieve can be hung up. Where a separate tightening the cloth over the pipe, and will number of sieves at one time, but each sieve is made and used for each glaze, be trimmed later. will need its own clamp. label the sieve with the name of its glaze.

Pottery Making illustrated 27

Oxygen Probes by Mel Jacobson O2 PROBES

Taking some of the guesswork out of reduction.

he oxygen probe is an analyti­ Sensing atmosphere inside of kilns cal tool used for fuel is a rather old technology, used by Tfired kilns. It helps the potter industry for years. The tools were understand the amount of reduction very expensive and cumbersome. that is in the kiln chamber from neu­ Murrays design fit nicely into the tral to heavy. Basically, an oxygen pottery niche, and since he was a probe works by comparing the level potter and teacher, he moved his de­ of oxygen on the inside of the kiln sign to fit that venue. Maxwell with fresh air present inside the Murray had created a very simple, probe. When the kiln temperature hand-held meter that would do ba­ goes above 1300°F, the probe senses sically what the industrial sensors Figure 1 oxygen with a zirconium/yttrium (with their digital readouts and Oxygen probes, like the AIC pellet. As the amount of oxygen in graphing) were doing. model shown here, work by com­ the kiln decreases, the amount of paring the level of oxygen on the electrical voltage increases, and this inside of the kiln with fresh air increase in voltage is relayed to a Recommendations present inside the probe. An air- squeeze bulb is used to introduce multimeter. A digital readout of the I think a production potter with an eye to repeating firings time after room air into the probe on the AIC temperature, although not Fahren­ model. heit or Centigrade, just a tempera­ time would find an oxygen probe an ture scale from 1-140, is also included invaluable tool. A college or school with a probe. that has multiple kilns firing on a Like all tools, the oxygen probe is regular schedule would also find an an aid to the potter, but it does not oxygen probe to be useful. make a poorly designed kiln work The cost of an oxygen probe is better. It does, however, enable the high (approximately $800 for the potter to make adjustments to the AIC Oxygen Probe and $650 for the kiln and firing schedule to save fuel; Axner Oxyprobe), it should prove to and helps the potter repeat and ad­ be cost-effective to buy and use one. just firings by allowing him or her In many cases, either tool will pay to analyze firings. for itself just in fuel savings, not to The first oxygen probes for stu­ mention its ability to allow the pot­ dio pottery kilns were introduced ter to repeat success time after time. into this country by Maxwell Murray After testing the AIC and Axner from Architectural and Industrial Oxyprobe models for several years, I Ceramics (AIC),Ltd. of Melbourne, have found them to be almost iden­ Figure 2 Australia, in the mid-80s. With the tical in reading the chamber atmo­ Several years ago, Howard Axner, help of Nils Lou, Oregon potter and sphere of my kilns. And each has of Axner Ceramics, Oviedo, teacher, Murray sold some of the performed to a very high standard. Florida, and Nils Lou developed early models, then expanded his dis­ The AIC probe is very fragile, and their own version of the oxygen tribution network to include Bailey s must be used with care. Transporta­ probe (Oxyprobe) as shown here. tion is very difficult. I carry it in a Nils Lou added a protection tube Ceramic Supply, Kingston, New to the probe, and used a Radio York; A.R.T. Studio Clay Co., paper tube surrounded by bubble Shack™, heavy-duty multimeter Sturtevant, Wisconsin; and Seattle wrap. The Axner probe also needs with a rubber boot for protection Pottery, Seattle, Washington. gentle handling, but is more solid of the meter. Instead of using an than the AIC. air-squeeze bulb to introduce room air to the inside of the probe, the Oxyprobe relies on ambient air flow, and uses almost all heavy-duty, industrial parts.

Pottery Making illustrated 29 Oz PROBES Using a Probe Firing Principles The hand-held meter of the oxy­ While the best use of the oxygen gen probe adjusts with a dial or but­ probe comes in the potter’s ability tons to provide several types of to use a cost-effective amount of fuel, readings.The “temperature” and “at­ he or she must also be aware of the mosphere” settings are the most ways of changing reduction in a kiln. critical for firing the kiln. Here is a Using one or more of the following typical firing sequence I have used techniques leads to economic, well- with success: reduced firings, and easy manipula­ 1. Fire with medium fuel in a neu­ tion of the kiln’s firing schedule. tral atmospheric condition, mean­ • The most important technique for ing slight reduction. changing the atmosphere in a kiln 2. When the kiln reaches approxi­ is to use the damper in the flue. mately 1750°F, or “90” on the The oxygen probe allows you to probe’s temperature readout, put move the damper fractions of an the kiln in light reduction or “3.5” inch to change the amount of re­ on the reduction scale. duction within the kiln. 3. Fire to approximately 1900°F, or • The second technique is by in­ “100” on the probe’s temperature creasing gas pressure, i.e.,fuel con­ readout, then put the kiln in me­ sumption. Too much pressure, however, increases back pressure dium reduction or “6.0” on reduc­ Figure 3 tion scale. in the kiln and causes uncontrolled An oxygen probe can be installed reduction. 4. Fire to completion. (Cone 10 in anywhere on a kiln where a pyrom­ my kiln is usually “134” on the eter would be placed. This figure • The third technique for reducing temperature scale.) depicts the Axner Oxyprobe (top) the kiln is to control the primary When bad weather conditions and AIC Oxygen Probe (bottom) air source by closing off oxygen. occur, or changes in time alloca­ along with the thermocouple block (middle), entering the kiln. The Sources tions, the potter can shift to higher probe can be inserted and removed Check out the following sources fuel pressure, open the damper, and during firing, but this should be for current prices of oxygen probes: control the primary air to achieve done slowly to prevent thermal an acceptable firing.The probe just shock to the probe. Axner Pottery Supply relays the numbers that represent 800-843-7057 the amount of reduction that is oc­ Bailey Pottery Equipment Corp. curring in the chamber of the kiln. 800-431-6067 The potter can see the changes al­ The AIC probe comes in three most immediately, and take appro­ sizes based on the length of the tube. priate action to change or correct Since the tube is expensive, prices the condition. range from $600 to $760 based on I have used an oxygen probe very the length. This probe is sold in effectively to analyze the kilns of 200mm, 300mm and 400mm potters that are having difficulty in lengths.The Axner Oxyprobe is cur­ firing. Attaching the probe through rently listing for $660 and $760, de­ the spy hole of a kiln and wrapping pending on a 7- or 10-inch length. the terminal block and cables in Kaowool for protection, gives me the opportunity to assess the firing. I then make readings while firing Mel Jacobson is a Minnesota potter and teacher. the offender. It gives me a great deal Figure 4 He has studied in Japan and has shown his work of information to assist in correct­ in many venues around the world. Mel is very ac­ The AIC (left) and Axner (right) tive in the computer-generated Clayart symposium ing the firing technique. In most oxygen probes display critical infor­ for potters around the world on the Internet. Visit cases I have been able to point out mation on hand-held multimeters. MeVs site at http://www.pclink.com/melpots. the flaws in firing, and change the Each is capable of registering settings and gas pressure, which have oxygen in the kiln atmosphere and resulted in a good firing. temperature (on a relative scale, not in Fahrenheit or Celsius).

30 Spring 1999 Pottery Making illustrated 31 32 Spring 1999 by Craig Hinshaw

ast spring, at the National Art Education Association Con ference in Chicago, one of theL most rewarding workshops I at­ tended was on how to make clay whistles. Jeff Young, from the Uni­ versity of Central Arkansas, demon­ strated an effective method of whistle making using two craft sticks as tools. Although the presentation was for teaching elementary students, I had a great time making them myself. That fall, fourth-grade teacher Marlyn Snider asked if I had an art lesson that could assist in teaching latitude and longitude. My first thought was clay whistles. The stu­ dents could shape hollow clay spheres for the body of the whistle, and the equator and prime meridian could then be etched into the soft clay cre­ ating a small globe.Turning the globe into a whistle would add excitement and motivation to the project. Note: Jeff Young advised that you should be able to make a whistle in I don’t know why making a ball of clay sing is so enticing, but less than three minutes before teach­ whistle making is always a big hit with students. We have made ing it to children. Good advice! self-portrait whistles, rain forest whistles and purely decorative Whistles are difficult for students to whistles. The globe whistles we made were on display at the school for parent/teacher conferences. Next to the whistles the make. Toward the end of the class, I students included a statement of what they had learned. Par­ recommend any students who feel ents were impressed with the integrated art lesson. Mrs. Snider frustrated to simply remove the thanked me for helping her to teach an otherwise challenging mouthpiece and turn their whistles subject. It seemed to be a perfect lesson, if only clay came in into globes. This allows all students flavors . . . to feel good about his or her work, whether it’s a whistle or not.

Pottery Making illustrated 33 Tools Two craft sticks (Popsicle sticks) are needed. One is broken in half and sanded smooth with a small piece of sandpaper. The other is left unbroken.

Step 1 Beginning with two pieces of clay (about the size of golf balls), two pinch pots are made. To reinforce the geography part of this lesson, I referred to these pinch pots as “hemispheres.”

Step 2 The two hemispheres are placed rim to rim, creating a sphere, and are carefully connected; smoothing the clay from one pinch pot onto the other. With air trapped inside, the globe may be shaped by carefully rolling it around on the desk.

Step 3 From a thumb-size piece of clay, the mouthpiece is shaped into a cube. Again, reinforcing the geography part of the lesson, stu­ dents are instructed to attach the mouthpiece about where North America or the 45th parallel would be. It is important the top of the mouthpiece be level with the top of the whistle.

Step 4 The unbroken craft stick is pushed through the mouthpiece into the hollow area of the sphere. Leaving it in place, the broken craft stick is used to scratch a square hole, where the mouthpiece attaches to the whistle. The hole is scratched down to the unbro­ ken stick. Still using the broken stick, a 45° angle is cut/shaped into this hole (see diagram). Now the unbroken craft stick is care­ fully pulled out of the whistle.

Step 5 The whistles must make a sound while in the greenware state or they will not produce sound after bisque firing. If blowing into the whistle doesn’t produce any sound, do the following: Look into the square hole from the top of the whistle—you should be able to see three things: (1) A 45° angle. (2) A black area inside the globe. (3) A small ledge. If you don’t see a black area, clay is blocking the hole. Force the craft stick down into the whistle. Any clay lodged beneath the 45° angle or next to the ledge may also be forced into the whistle.

Step 6 Once the whistle works, a small hole can be made into the globe at the “south pole” with a sharpened pencil. This allows for a sec­ ond note to be produced. The prime meridian, equator and conti­ nents are now drawn into the surface of the whistle.

Finishing Touches After the whistles were bisqued, the students painted the conti­ nents and oceans using blue and green tempera paint. A coat of clear acrylic polymer was brushed over the finished globes. This gave the whistles an attractive sheen and kept the tempera from coming off on the students’ lips.

Craig Hinshaw is an elementary art specialist in Lamphere School District in Madison Heights, Michigan. Send com­ ments [email protected]

34 Spring 1999 Pottery Making illustrated 35 Creating Glazes USING A GLAZE BASE

by Michael Hamlin

hile taking a glaze calcula­ Working with Gerstley Borate • Add 0.5 grams of VeegumCer to tion class, I developed a Reviewing the test bowls from the dry glaze before mixing with w glaze-base recipe that I’ve “Creating Glazes,” we notice that the water to help keep the glaze from used as the backbone for all the glazes Gerstley borate is a powerful melter forming cracks and pulling away I now use. Its a simple recipe that while the nepheline syenite melts to from the surface of the pot. uses several ingredients discussed in form an opaque milky globule. • Make sure the glaze is not applied “Creating Glazes” (Pottery Making Il­ Combining these two materials cre­ too thickly. A thick application of lustrated, Winter 1999). ates a nice even gloss when the glaze glaze encourages gravity to pull I’m very interested in producing is fired to Cone 4. the glaze more. glazes with strong visual texture, so Gerstley borate causes a glaze to • Use the glaze while it’s fresh. I’ve used Gerstley borate because it thicken when mixed with water. Glazes that contain Gerstley bo­ is a chemical whose qualities help However, since the glaze batch thick­ rate will flocculate (become gel­ induce high gloss and opalescence in ens, there is a problem. In the early like) while sitting in the bucket, a glaze surface. When coloring ox­ stages of firing, the glaze layer will and this will alter the ability to ides are added, the Gerstley borate loosen before the fusing of chemi­ apply the glaze accurately. reacts to develop mottled surfaces, cals begins, then when the glaze Despite the problems that Gerstley which creates a nice breakup of color melts, its surface tension pulls the borate can cause, the beauty it helps in the glaze. glaze into separate areas, leaving bare create in a glaze surface is worth If you’re more interested in a solid patches on the glazed piece. This ef­ some of the trouble. color surface, or one that doesn’t have fect is called crawling.To help allevi­ color variegation, I recommend add­ ate this problem several things can Developing a Matt Glaze ing stains, up to 10% of the dry glaze be done: Substituting 15 grams of dolomite base. for 15 grams of nepheline syenite creates a matt glaze from the glaze recipe. Remember from the test bowls that the fired dolomite did not melt completely, but solidified into a hard mass. The reaction of the dolo­ mite in the kiln firing is a good in­ dication that it will not create a glossy surface when added to glazes.When added to the glaze base, the dolomite creates a smooth matt surface. Firing Regimen These glazes have been fired to Cone 4 using the standing witness cones produced by Orton as a guide. Once Cone 4 is achieved, the con­ trols are set to medium for one hour and then turned to low for another This bowl is glazed with the Basic Glaze This bowl is glazed with the Basic Matt hour. This soaking period helps Base Variation 1 where I’ve added me­ Glaze Base Variation 1. The dolomite af­ smooth out any pinholes that may tallic oxides. The oversaturation of metal­ fects the way cobalt reacts in a glaze. In­ develop during the firing. lic coloring oxides produces wonderful stead of the usual blues, dolomite oily luster effects. changes cobalt to beautiful lavenders and purples.

36 Spring 1999 Hamlin Basic Glaze Base Cone 4-6 Gerstley borate...... 30 grams Nepheline syenite...... 30 Whiting...... 6 Silica...... 34 100 grams Add: Veegum Cer*...... 0.5 grams Mix with 100 milliliters (100 grams) of water. When fired between Cone 4 and Cone 6, this glaze produces a nice clear gloss that can be used over slips or underglazes with good effect. *Veegum Cer is a product that helps keep glaze from becoming powdery after it dries, and also keeps the glaze from pulling off the pot during firing. Variation 1 Add: Red iron oxide...... 4% Manganese dioxide*.... 10% Copper carbonate...... 4% This glaze is oversaturated with coloring oxides. By oversaturation, I mean that more than usual oxides have been added to the glaze. This oversaturation produces wonderful oily luster effects, and it’s a great glaze for vases or deco­ rative pieces. This sculpture is glazed with the Basic This sculpture is glazed with the Basic Matt Glaze with the addition of cobalt car­ Matt Glaze with black iron oxide (Varia­ CAUTION:Do not use this oversaturated glaze bonate (Variation 2). This is a very large tion 3). for food surfaces. The greater than usual amount amount of cobalt and again this glaze of coloring oxides will cause the glaze to be easily should not come in contact with food sur­ attacked by food acids such as tomato products, vinegar, citrus fruits and coffee. faces. You may want to experiment more with stains to achieve the purple. ^WARNING:Manganese dioxide may irritate skin or eyes. Inhalation of excessive amounts of dust may cause nausea, weakness, chills, and loss of appetite. Prolonged inhalation of man­ ganese dioxide dust or fumes may cause Conclusion manganism (damage to the central nervous sys­ I believe that both of these glaze tem). recipes are wonderful starting points Variation 2 for developing a personal glaze pal­ Add: Mason stain 6336...... 10% ette. Further experiments are en­ couraged. For instance, try increas­ ing the dolomite, add other glaze Hamlin Basic Matt Glaze Base Cone 4-6 chemicals to the base glaze, try elimi­ Gerstley borate...... 30 grams nating the Gerstley borate altogether, Nepheline syenite...... 15 or use only nepheline syenite and do­ Dolomite...... 15 lomite as the flux in the glaze. The Whiting...... 6 possibilities are endless! Silica...... 34 100 grams Variation 1 Add: Cobalt carbonate...... 1% Rutile...... 5% Dolomite changes the typical way cobalt reacts in a glaze. Instead of the usual blues, cobalt in dolomite glazes yields beautiful lavenders and purples. The base for this sculpture was glazed with the Basic Glaze Variation 2, which Variation 2 utilizes the addition of Mason stain 6336 Add: Cobalt carbonate...... 7% (Peacock). CAUTION:This is a very large amount of cobalt Michael Hamlin is a potter living in Columbus, and again this glaze should not come in contact Ohio. He graduated from the Columbus College withof food. You may want to experiment more with Art and Design in 1996 and enjoys researching and developing glazes for his pots. stains to achieve the purple. Variation 3 Add: Black iron oxide...... 5%

Pottery Making illustrated 37 by Jeff Zamek

hen sweeping the studio, Respirator Filters If It Fits... every potter at some point wwonders, “What should I Every respirator has some type of Fit is one of the most important do to protect my self from the clay dust?" filter to trap particles. One of the most considerations when choosing any res­ Imagine what's floating around in your effective filters is a HEPA (High Effi­ pirator, and two factors are essential when placing a respirator on your face. studio when walking or sweeping up at ciency Particulate Air) filter.They were developed more than thirty years ago A complete seal around the face is criti­ the end of the day On days when direct by the Hepa Corporation. The name cal in allowing no secondary air to sunlight enters the studio, it's possible to has since become generic and many bypass the filter. Make sure the mask see raw materials and clay dust in the companies now produce this classifi­ conforms to your face so no outside air; but it's the stuff you can't see that's cation of filters. air is drawn in while breathing. Men the problem. For many years, HEPA filters have with beards must ensure that the mask been the standard for the industry.They forms an unbroken seal around their Clay is a very small hexagonal-plate­ have a 99.97% efficiency rating, which face; otherwise, shaving the area is re­ shaped particle material and can range means they filter 99.97% of solid par­ quired. The second factor is the com­ from 100 microns (\x) to 0.1\x in size ticles down to a 0.3-micron size. Some fort of the mask. If the particular mask depending on the specific type of clay (A particles at 0.3 microns do not have is uncomfortable or heavy, and it isn’t micron is i/1000 of an inch.) Poten­ enough weight to go through the fil­ worn on the required occasions, it of­ tially the most hazardous particle sizes ter, while particles bigger than 0.3 mi­ fers no protection. crons have a larger mass, causing them Two of the respirators I tested—the are below 10 microns, and potters should to travel with greater velocity to the Willson Freedom® 2000 Series and try to cut down the inhalation of par­ filter. HEPA-type filters are recom­ Willson Valuair® Plus—come with ticles to safe levels. Respirators are very mended whenever heavy metals, such diagrams and instructions on how to effective at blocking particles, but no res­ as, chrome, cadmium, vanadium and wear the masks, and informative vid­ pirator is 100% efficient at blocking all cobalt are in the work environment. eos were also available.The instructions particle sizes. Particles in the 0.3-mi- on use and fit for the 3M 8210, 3M 8110S and 3M 8233 N100 are avail­ cron range can zigzag through a filter Respirator Maintenanceable from the dealer or by calling the (even though some get trapped) so that a 3M technical service number. percentage can pass through the respira­ The industry standard (though Respirators with higher efficiency tor,; while larger-size particles travel in a seemingly inexact) for changing filters (99.97% efficiency) filters can cause straight line and get trapped. occurs when any or all of the 3 D’s are greater resistance when breathing due to the use of finer filtering materials. A safe and conservative approach goes encountered—Damage to the respi­ High-efficiency units have an exhala­ a long way in protecting yourself from rator, Dirty respirators or Difficulty breathing through the respirator.While tion valve in front to allow for easier airborne particles—both visible and in­ dirty and damaged respirators are self- breathing. visible. Fortunately this area of studio explanatory, the difficult breathing re­ safety has been thoroughly researched by quires some explanation. As particles industry and potters can take advantage contact the respirator during use and of the latest array of respirators. penetrate below the outer surface of the filter, some filter holes are closed, causing a caHng effect to occur. This results in the filter becoming more ef­ fective in that an increased amount of holes are plugged by the incoming par­ ticles. However, at some point the heightened resistance needed to breathe causes unfiltered air to be drawn through the point of least resis­ tance on the mask seal. Before this state is reached, the filter should be dis­ carded.

38 Spring 1999 Willson™ Freedom® 2000 Series Disposable Respirator The distinctive feature of this respirator is the simple straightforward design and its light weight on the face. It can be purchased in three face-piece sizes for an exact airtight fit on the face, which is a critical factor in any respirator design. The unit has an excellent P1OO filter that is 99.97% effective in trapping particles down to .3 mi­ crons in size. This is the highest rating by NIOSH standards. The mask allows for eye wear and feels comfortable when I swept the studio or mixed dry glazes. However, the inability to replace the filter would be a major drawback if the potter was consid­ ering it for daily use in the studio. Replacing the entire mask and filter each time would be costly. Retail price $18.55. (Source: Willson Division of WGM Safety Corpora­ tion, product information Form No. 9121C.)

Willson™ Valuair® Plus Reusable Respirator The easily replaceable filter extends the service life of this unit, which is a major benefit. The filter cartridges are the same ones used in the Willson Freedom 2OOO series masks (P100). It also has a soft pliable face piece with adjustable straps to fit almost any face contour. From the first time I used it in my studio, it was very comfort­ able and there was low breathing resistance upon inhaling. Wearing eyeglasses does not prevent the mask from fitting your face. I would highly recommend the respirator for ceramics supply companies’ clay mixing operations. It can also function well in situations where there is heavy-duty prolonged dry raw materials mixing in the stu­ dio. Retail price $21.20. (Source: Willson Division of WGM Safety Corporation, product information Form No. 9601.)

3M™ 8210 and 3M 8110S N95 (smaller size) Commonly referred to as a paper dust mask this is a very lightweight filter that can be custom fit by a thin metal band on the upper part of the mask. The electrostatically charged micromedia fiber attracts and holds airborne particles. Two elastic straps on each side of the mask hold it firmly against the face for an effective seal. I found the mask easy to use and lightweight on my face. The respirator was very comfortable when wearing glasses and as an added benefit had no parts to clean. It is low priced which makes for reasonable replacement costs. Studies have proven this mask to be just as effective as rubber face piece respirators. I would buy a box of the masks and replace them regularly (see Respirator Maintenance). It is rated at 95% filtration efficiency, which means the mask stopped 95% of the particles down to a 0.3 micron size. It can filter cobalt, copper, chrome, iron oxide, silica and manganese. I would recommend the 3M 8210 and 3M 811 OS N95 as low-cost effective masks for dry materials encountered in the pottery studio. Retail price @ $.78 cents, box of 20 $15.67. (Source: 3M Product information sheet #3044.)

3M 8233 N100 Particulate Respirator This is a paper dust mask made from advanced electrostatically charged micromedia fibers. It has fully adjustable straps for a secure soft fit on the face. Also incorporated into the mask is a one-way cool flow valve that makes breathing easier through the finer mesh filter. Its efficiency is 99.97%, making it one of NIOSH’s highest-rated filters. OSHA (Occupational Safety and Health Administration) recommends this type of respirator in situations where lead, cadmium, and arsenic are in the workplace. This mask would function well when mixing dry glaze materials that contain cobalt, copper, chrome, iron oxide, silica, and manganese. The higher efficiency rating and high cost per mask is not required for protection against other ceramic raw materials. It could be used in ceramics supply companies’ clay mixing operations where expo­ sure and concentration levels would presumably be higher than in a pottery studio. I would use this respirator only if lead, cadmium, or arsenic were present in the studio. Retail price @ $6.03, box of 20, $120.60. (Source: 3M Product information sheet #3094.)

Pottery Making illustrated 39 New Standards Recommendation Summary

In July 1998, the National Institute For protection against airborne par­ Most pottery studios and commer­ of Occupational Safety and Health ticles found in the pottery studio, the cial clay mixing operations will require (NIOSH) fully implemented new 3M 8210 and 3M 8110 S N95 respira­ an N95 respirator or an N100 respira­ standards for all respirators (Title 42, tors were my first choice. All the respi­ tor. All the respirators evaluated would Code of Federal Regulations, part 84, rators tested would meet the require­ meet and exceed the respirator safety referred to as “42 CFR 84”). These ments of potters, but the 3M respirators requirements of the studio potter. In new respirator specifications were de­ were lighter in weight, easily replace­ addition, all units could be used in in­ signed for higher levels of protection able and comfortable. I do not think dustrial applications where higher levels against particulate hazards in the work­ the 99.97% vs. 95% efficiency respira­ and longer exposure rates would be place. Respirators sold after July 1998 tors would offer a much greater degree expected as compared with a pottery must comply to the new standard and of protection for the materials found in studio or clay-mixing operation. The “42 CFR 84” must be conspicuously pottery studios or commercial clay mix­ higher-efficiency units rated at 99.97% displayed on the respirator package. In ing operations. efficiency (Willson Freedom 2000, addition to the part 84 particulate re­ When purchasing any respirator, WillsonValuair Plus and the 3M 8233 quirements, buyers must be aware that look for the new NIOSH codes. And N100) would be very effective pro­ respirators are rated and designated for always carefully read the instructions tection where lead or cadmium are other factors (i.e., N=no oil in envi­ on maintenance and use of any respi­ present in the studio. However, the 3M ronment, R=oil resistant, and P=oil rator purchased, as there can be differ­ 8210 and 8110S N95 (smaller size) proof). Potters typically need respira­ ences between each model and brand. rated at 95% efficiency would also meet tors labeled N95 (no oil and 95% effi­ studio requirements for safety. cient) or N100 (no oil and 99.97% ef­ Each respirator was comfortable and ficient),but ratings ofR95,R100,P95 adaptable to fit my face, creating an ef­ or PI00 would also be sufficient. fective seal. The 3M respirators, having only paper-like fibers and elastic straps, were lighter in weight as compared with the Willson respirators. Respirator cost is not a consider­ ation when health and safety are in­ volved so deciding to purchase the cheapest respirator was not a factor in the evaluation. NIOSH publishes a pocket guide listing all raw materials and their per­ missible exposure levels.The 3M com­ pany also publishes a similar listing on raw materials, which is updated annu­ ally. Potters can use either guide to de­ cide the safe levels of specific raw ma­ terials in their studios. If needed, the guides can be used to determine the type and efficiency level of respirator needed in the studio. However, if the potter wanted to pursue air quality to a much greater degree, the first step would be to hire an industrial hygien- ist who would monitor the studio (es­ timated cost for the test is $100 to $180) to determine the level of particulate in the air. The information gathered from testing would then indicate the proper respirator type and filter.

JeffZamek, a frequent contributor PMI, to works as a ceramics consultant residing in Southampton, Massa­ chusetts. He is the author What of Every Potter Should Know,published by Krause Publications. Contact Jeff at 6 Glendale Woods Dr., Southampton, MA 01073; or by e-mail [email protected] .

40 Spring 1999 Pottery Making illustrated 41 Correction In the Winter 1999 issue, an image of David Hendley s lotion dispensers failed to print. Here is the photo as it should have appeared on the bottom of page 33 in the article titled “Homemade Dies.”The work illustrates one application of a hollow extrusion created from a die David made. We apologize for the omission.

Extruded lotion-dispenser pots, to 8 inches in height, with slip glazes, stains and glazes. These dispensers illustrate the varia­ tions possible with hollow extrusions.

42 Spring 1999 Production: Testing Ovenware by Dick Lehman Regardless of how we finally come to our

ovenware recipes—or what those recipes— are we need a methodology by which to make thermal shock tests that are thorough, inclusive, severe and comparative.

Lght before Christmas a few years ago, several cus­ dards that exist within industrial ceramics related to “mean tomers returned baking dishes to my studio due of first failure”—the average number of times a piece must R to unexplainable and unexpected cracking (after 2 be shock tested before it will fail.What follows are the meth­ to 3 years of use). The customers assured me that all appro­ ods and approaches I developed after talking with numer­ priate care had been afforded these pieces, and I had no ous other potters, and reflecting on the kinds of pot-failures reason to believe that they had not followed all the normal we had all encountered. safeguards and recommendations for using handmade I decided to test both mugs and baking dishes.The mugs ovenware pieces. Additionally, some mugs were returned to were of a variety of shapes, and utilized all six of the different me that had multiple cracks going about one or two inches clay bodies I was testing. The baking utensils were of two from the rim down the side. These cracks reportedly ap­ kinds—flat-bottomed dishes (two sizes) and bowl-shaped peared through normal or less than normal use. bakers. The tests were made in conventional ovens, and no I had a problem. What to do? The failures were so few, microwave tests were attempted (that is a separate testing considering the number of pots I sell each year, that I could procedure). Furthermore, this test provided no information have just replaced the pieces and chalked it up to the cost of about the effects of “glaze fit” on a particular clay body, or doing business. And it could have been just a coincidence the implications of glaze fit upon thermal shock failure. that these pieces had all come back to me within a few Measuring Results weeks time—perhaps I should just ignore all this. On the other hand, it may have been that I was on the front end of Following each step of the shock test, the pots were indi­ an avalanche of all the ovenware pieces I had made within vidually tested for integrity. The integrity check, though the last three years.That was an awful prospect! somewhat colloquial, tested both the sound of the piece Regardless of not knowing whether there would be more (the noise it makes when it was struck with a metal rod) and failures in my ovenware, I thought it important to try to the look of the piece (could I see any cracks?). improve the clay body if possible. And I hoped to do so without changing the recipe to such an extent that it would alter the colors of all my glazes. So I set out to attempt to create several new ovenware bodies. My knowledge and abilities in ceramic engineering are limited, and my attempts at altering and improving my own ovenware recipe were more intuitive than academic, relying more on anecdotal information and “common knowledge” than on my acquaintance with good science or accepted research. Designing a Test I created and gathered six new ovenware clay bodies, then developed a series of tests to see if any of the new formulations were actually an improvement over what I was already using. I needed a methodology to make ther­ mal shock tests which were thorough, inclusive, severe and comparative; and these testing procedures needed to be ac­ Thrown and altered open-face baker, 13 inches in diameter. This cessible, repeatable and, to some extent, reliable. piece was formed by separating side walls from the “floor” of the I found no published recommendations for uniform ther­ pot (along the two long sides) while it was still on the wheel. The mal shock testing or integrity testing of ovenware bodies— walls were then repositioned in this “Baroque” pattern and reat­ tached to the floor. Pieces that have been so-stressed in this way at least none geared to the low-tech production potter.This during the making process and that have sharp angular direction test was not informed by any of the existing normative stan­ changes, are more prone to thermal shock.

Pottery Making illustrated 43 Testing Mugs The mugs went through two tests, which were designed to provide pro­ gressively more thermal shock. Test 1: The first test was simply to fill each room-temperature mug half full ofboiling water. By filling the mug only half full, I reasoned that there would be more dissonance within the piece it­ self. Not only did the piece suddenly go from ambient temperature to boil­ Once you’ve tested your ovenware, make ing temperature, but only part of the sure you inform users of the care and use piece would make this transition, of the items. thereby causing the piece, within itself, to have both ambient and hot surfaces, which would increase the interior ther­ Most all of us recognize the differ­ mal stress. ence between the beautiful “ring” that a pot of integrity makes when struck, Test 2: The second test was to freeze and the dull “thud/thunk” that occurs the mugs at —10°F for 45 minutes. Upon taking the mugs out of the when pots have been cracked, overly freezer, they were immediately filled stressed or damaged. To perform the sound test, I placed each pot on a metal halfway with boiling water. While the I stand, then struck it on the rim with a mugs tested passed each of these tests, the tests could be continued for per­ metal rod.This striking was done “gen­ haps hundreds of cycles to determine tly” so as not to break the piece with where the clay bodies would eventu­ the rod. Each piece was struck with as ally fail. This would establish my own similar a stroke as possible, and struck “mean of first failure ” at the same position on the pot.While each piece did not sound the same (due Testing Baking Dishes in part to small unavoidable variations For each of the six recipes, I tested in shape and thickness and also differ­ multiple pieces consisting of two basic ences in recipe formulation), I did note shapes—two sizes of flat-bottomed the sound, or “ring,” of each. I com­ baking dishes with right-angled cor­ pared these sounds to “control” ners where the bottoms of the pots met pieces—pots of similar shapes made the side walls; and baking dishes with with the same recipe—which had not rounded bowl-like contours. I devised yet been shock tested. five tests, which would expose pieces The visual test involved putting wa­ to progressively more dramatic shock ter (heavily stained with food color­ in each subsequent step. I determined ing) into each piece for at least 10 min­ that I would continue testing until at utes. I looked for small cracks, which least some (if not all) the ovenware might not be visible to the naked eye, bodies failed. and which might not have revealed Test 1: Baking dishes at room tempera­ themselves in the sound test. I assumed ture were placed into a preheated 500°F that any crack would collect, through oven, and allowed to remain there for capillary action, some of the food col­ 10 minutes.The pieces were then pulled oring, and maintain some color in the out, allowed to cool naturally, then crack when the tinted water was poured tested for integrity. (I included this fairly out. To eliminate as many variables as benign test since I have always cau­ possible, all pots used for this test were tioned my customers never to place the glazed white and I used blue food col­ baking dishes into a preheated oven.) oring to heighten the contrast. While This test was repeated three times for the food color revealed that most cracks each piece, for each of the six clay bod­ went all the way through the bodies, ies. No failures were observed in any there were rare cases where really small of the clay bodies, or any of the shapes. or beginning cracks were visible only on the inside. Test 2: Reasoning that sometimes my customers may put food-filled baking dishes into hot ovens, I tested each piece several times by placing an ambient temperature piece, half-filled (and not

44 Spring 1999 Thrown and altered open-face baker, 12 inches in diameter. The rims have been pinched and thinned on the “ends” to form handles. The re­ sulting variation in thickness of the side wall or rim tests the integrity of an ovenware body.

fully filled as I recommend to my cus­ tomers) with water, into 500°F oven. Having the pieces half-filled created the potential for exacerbating the shock within each individual piece. Repeated tests yielded no failures for any of the six clay bodies. Test 3:1 next froze empty baking dishes in the freezer for 45 minutes, then placed them directly into a preheated 500°F oven where I left them for 10 minutes. Multiple tests yielded no fail­ ures in any of the clay bodies, or in any of the shapes. Test 4: Next, I froze a half inch of wa­ ter in the bottom of the baking dishes before taking them directly to the pre­ heated 500°F oven. The pieces re­ mained in the oven until the ice had melted and the water began to boil.This is the first point at which I experienced failures. Two of the clay bodies failed; however, only the flat-bottomed pieces failed. And both sizes of flat-bottomed bakers failed in these two recipes. The cracks were clear and obvious. Test 5 :1 had predicted that the frozen water test would crack all the pieces. That not being the case, I moved for­ ward to a more dramatic shock test—I froze all the pieces (empty),placed them in the preheated 500°F oven for 15 minutes, then took them directly to the sink where I poured an inch of cold water into them. At this point all 6 bodies failed, but in different ways.Two bodies failed with large audible cracks; two other bodies failed with shorter thinner cracks; one body cracked only on the bottom, but was visible only through the visual dye test; and one body failed only as a flat-bottomed form. This sixth body, when used to make a bowl-shaped baker, survived repeated severe testing with no failure.

Pottery Making illustrated 45 The bowl-like contour of the body of this lidded Small baking dish with altered rim, 6 inches in casserole minimizes and softens direction diameter. The sharp angle change from floor to changes, i.e., the line between the floor and side side wall, altered rim, and asymmetrical handle walls. Such shapes with a smooth transition help placement exacerbate thermal shock problems. to minimize the effects of thermal shock.

Observations composition, which someone other In the process of performing these than ourselves utilizes and enforces. tests, I was able to make several obser­ Use and test your own products before vations. Some may be obvious while marketing them. While this is costly,you others may be merely colloquial. All may avoid the necessity of replacing may be worth considering as you make years worth of product to unhappy your next ovenware forms: customers. • The larger the piece is, the more These tests do not give conclusive likely it is to fail in ovenware appli­ evidence about how a particular cations. ovenware body is going to fare after 3- • The broader and flatter the bottom 10 years of regular use (or, sometimes, of the pot is, the more likely it is to abuse), but the tests do provide imme­ fail. diate comparative evidence regarding • Pots with soft, rounded corners, how different ovenware clay bodies re­ curves, and lines seem to survive bet­ spond to a series of increasingly severe ter than ones with sharp corners and thermal shock tests. direction changes, regardless of the Conclusion size. Even with all the limitations of my • Filling the entire exposed inner sur­ methodology, one can take heart that face of a baking form with food these severe tests indicated that some (thermal mass) lessens the thermal clay bodies (at least in the short run) shock and prolongs the life of the would perform far above the normal piece. An important fact users need expectations and requirements of most to know studio potters, with respect to the en­ • Bowl-shaped forms required addi­ vironments in which they might ex­ tional applications of the most se­ pect their ovenware clay bodies to sat­ vere test in order to cause five of the isfactorily function. six clay bodies to fail (the same five The method I developed and used, clay bodies that failed in the flat-bot- though not foolproof or even nearly tomed-shaped pieces).The soft line fully proven, at least provides a meth­ of the curvilinear form seemed to odology for uniform comparison of distribute and withstand more stress clay bodies in the light of severe and before failing. multidimensional thermal shock. For Recommendations those of us who choose to mix our own Use a proven industrial or pre-mixed clay bodies, it will provide some useful ovenware clay body. This option may not information upon which to make com­ satisfy our individual taste with regard parative, informed decisions. to texture, color, glaze compatibility, or workability. It may or may not be truly a quality ovenware body under the con­ ditions by which any particular indi­ vidual makes and fires. And addition­ ally, to make such a choice determines Dick Lehman is a frequent contributor to ceramics that one will be fully subject to the qual­ periodicals throughout the world. He maintains a ity control methods in mixing and full-time studio and gallery in Goshen, Indiana.

46 Spring 1999 Ovenware Recipes Here are a few ovenware recipes you can test using your own designs, forming procedures and firing meth­ ods. Lehman Ovenware Body Cone 9 Oxidation/Reduction Old Hickory Gold Label ....50.0 parts #6 Tile kaolin...... 16.5 Custer feldspar...... 20.5 Kentucky ball (OM4)...... 20.5 Flint (200 mesh)...... 6.0 Hawthorne (35 mesh).... 12.0 Christy Minerals silica grog (50 mesh)...... 5.5 Molochite (120 mesh)...... 5.5 Kyanite (100 mesh)...... 13.5 150.0 parts While still undergoing testing, this body has proven the most successful so far. Ovenware Body I Cone 9 Oxidation/Reduction Spodumene ...... 40 parts Tennessee #7 ball clay ...... 60 100 parts Peach color, high porosity when ceramic grade spodumene is used; sandy, finely-speckled medium tan, average porosity with the chemi- cal-grade spodumene. Source: Ceramics Monthly, September , 1979

Ovenware Body II Cone 9 Oxidation/Reduction Spodumene...... 30 parts Pyrophyllite ...... 10 Custer feldspar...... 10 Cedar Heights fireclay...... 30 Tennessee #7 ball clay ...... 20 100 parts Bentonite ...... 3 parts Off white to tan, fused body (requires thorough mixing) when ceramic-grade spodumene is used; finely speckled tan, fused body with chemical-grade spodumene. Source: Ceramics Monthly, September, 1979

Ovenware Body III Cone 9 Oxida tion/Reduction Spodumene...... 30 parts Pyrophyllite ...... 10 Custer Feldspar...... 10 Kentucky OM 4 ball clay ...... 20 Bentonite ...... 2 72 parts Pale cream, fused body when ceramic grade spodumene is used; finely speckled tan, fused body with chemical-grade spodumene. Source: Ceramics Monthly, September, 1979

Pottery Making illustrated 47 48 Spring 1999