Society of North American • Premier organization for jewelers, designers, and metalsmiths • October 2008 27

TECH NEWS

This issue’s article comes from the creators of the new Clay. Innovations such as this keep our medium fresh and exciting. Bill gives a good overview of the material and offers a link to a great downloadable PDF at the end of this article. If you are using Bronze Clay, or any other metal clay, and have discovered a new technique, firing schedule or process, please share it with us. As always, send your articles or topics to Jim at . Enjoy. Working with Bronze Clay

Bronze clay is a new medium for jewelers and artists. Similar to clay (PMC and Art Clay Silver), it is a clay-like material that turns into metal when fired in a kiln. Unlike commercial bronze, which is an of zinc and , the bronze made from bronze clay is a true bronze alloy of 10% tin and 90% copper. The color of the fired bronze is the same as high-karat when it is polished to a high luster.

Prior to the commercial introduction of bronze clay, my wife, Hollace Ann (LaceyAnn) Struve, and I made 273 different formula- tions of the clay. We had lots of encouragement, feedback and help from Chris Darway, Celie Fago, Hadar Jacobson, Jennifer Kahn, Jeanette Landenwitch, Tim McCreight, Yvonne Padilla, Kevin Whitmore, CeCe Wire, and Gordon Uyehara during and after the development of bronze clay.

The techniques for working with bronze clay can be broken down into working with the wet clay, drying the clay, working with the dry clay, firing the dry clay, finishing the bronze, and getting more information. Society of North American Goldsmiths • Premier organization for jewelers, designers, and metalsmiths • October 2008 28

Working with the Wet Clay

Bronze clay is similar to silver clays, in that it is somewhat less stiff than modeling clay. Most of the techniques used for silver clays can be used with bronze clay. One exception is forming joints. More attention to detail is required to join wet bronze clay pieces than wet silver clay pieces.

Tools for shaping modeling clay are widely available and can be used to shape bronze clay. When working with the wet clay, olive oil can be used to prevent it from sticking to surfaces, tools and hands. No special safety precautions are needed while handling the clay since it contains only copper and tin powders and other materials that are generally recognized as safe. The wet clay can be pressed into molds or onto patterned surfaces. Its fine texture allows very fine details to be reproduced. The clay can also be shaped by hand or rolled into cylinders or snakes be- tween two sheets of glass or Plexiglas. To form the clay into a sheet, place it inside a plastic page protector so that it will not dry out. A light coat of olive oil on the inner surfaces of the page protector will prevent the clay from sticking. Thickness spacers, e.g. playing cards, are placed on each side of the clay and a cylinder, e.g. PVC pipe, is rolled over the clay to form a sheet of the desired thickness. Sheets of wet clay can be easily cut into any shape. Sheets can also be draped over objects, e.g. a sphere, to make three-dimensional objects. You can also use metal or rubber stamps to make patterns on the surface of the sheet. Small cracks that may develop when working with the wet clay can be smoothed out with a wet paintbrush. Drying the clay

The clay can be air dried or for faster results, dried in a vegetable dehydrator, on a coffee warmer, or with a hair dryer. Sheets that are draped over objects should be dried on the object. Fast drying can lead to distortion, so it is better not to use too high a temperature. The drying time depends on the thickness of the clay – thin pieces will dry in less than an hour in a dehydrator. The clay shrinks about 4% when drying, so it may develop cracks if the shrinkage is prevented, e.g. drying a on a cylinder.

Working with the Dry Clay

Dry bronze clay is not as brittle at dry silver clay. It can be bent a fair amount without cracking, but it will crack if bent too far. Slight distortions from drying can be corrected by bending the dry clay and holding it in place for awhile. The dry clay can be filed, sanded, drilled, and carved. Dockside micro carving tools work very well for carving the surface of the dry clay. Small imperfections or cracks may be removed with a wet paintbrush.

Wet clay can be applied to embellish the dry clay piece. The dry clay should be dampened with a paintbrush where the wet clay is to be applied, and the wet clay should be pressed into place and held for 15 seconds or longer to establish a bond. Dried pieces can be joined by wetting the surfaces, then pressing them together, and holding them in place for 15 seconds or longer. When the surfaces to be joined do not fit precisely, a small amount of slip can be applied to one of the wetted surfaces. The slip can be made from the wet clay by adding water (about half the volume of the clay). The slip should be thick, not thin. The dry clay should be finished to perfection since it is much easier than finishing the bronze after firing. Society of North American Goldsmiths • Premier organization for jewelers, designers, and metalsmiths • October 2008 29

Firing the Dry Clay

Unlike fine silver, both copper and tin will react with the oxygen in the air at the temperatures required to fire the clay into bronze. Bronze clay must therefore be fired in an oxygen-free atmosphere. To accomplish this, the clay is surrounded by activated carbon inside a container. The carbon reacts with the oxygen in the air before the copper or tin does. The reaction of oxygen with carbon produces carbon dioxide (CO2 ). Since carbon dioxide is denser than air, the container fills with carbon dioxide and provides the required oxygen-free atmosphere.

Granulated activated carbon is readily available since it is used in air filters, water filters and aquarium filters. The size of the granules is between 12 and 30 or 40 mesh, usually stated as 12x30, 12x40, 1230, or 1240. Common sources of the carbon in acti- vated carbon are coal and coconut shells. The coal-derived carbon will produce a range of colored patinas on the bronze whereas the carbon derived from coconut shell produces little or no patina on the bronze. The coal-derived carbon is softer and dustier than that from coconut shell. Do not use charcoal if your kiln is inside since charcoal gives off noxious fumes.

The container can be stainless , foil, high-fire ceramics, or alumina. Low fire ceramics will take the heat but they break when heated with activated carbon inside. Other container materials such as , nickel and copper should work but have not been tried.

To fire your dry clay, first place a ½ to 1-inch layer of the carbon inside the container and place your pieces on top of that layer. Fill the container with carbon and put the lid on. The lid is not really necessary but it serves to reduce the loss of carbon. The container is then placed in a kiln and fired to make the clay into bronze. Since most kilns do not have heating elements in the floor, the con- tainer should be supported on posts so that the bottom of the container is heated. Kilns that open from the front leak cold air which cools the front of the container. You should place your pieces along the sides and back of the container if you use this type of kiln.

At least three things happen during firing: removal of the non-metallic components, formation of the bronze alloy, and fusing of the metal particles (called ). At some point in this process the rate of temperature rise is important. What is happening during the slow rise in temperature is not fully understood at present, so the firing schedule is not fully worked out.

One firing schedule that has been successful is ramp at 250F/hour to 1550F and hold at 1550 for 3 hours. This schedule has been shown to work for pieces up to ½ inch thick. Thin pieces, 0.06 inches or less are well formed with a faster ramp (500F/hour to 750F/hour) to 1550F and hold at 1550F for 2 hours. Other people have used a full ramp to 1000F-1200F and then a slow ramp (200F/hour to 250F/hour) to 1550F and hold at 1550F for 3 to 3.5 hours. Lower final temperatures (down to 1450F) have also been used successfully.

Finishing the Bronze

After firing, the clay becomes bronze metal and any technique that is used for finishing bronze will work. The bronze can be sanded, filed, drilled, hammered, bent, polished, and patina applied. The resulting bronze is very strong, so thin, durable lightweight jewelry is possible.

Getting More Information

A PDF brochure is available at: and the author can be contacted at: .

SNAG News will pay up to $125 per page (up to 4 pages) for informa­tive technical articles aimed at any level—from intermediate to ad- vanced, from student to the highly skilled professional. If you teach a workshop, have a clever bench trick or technique of interest to other jewelers and metalsmiths, or have more academically based technical research, please share it with our SNAG community. Send your submission to Jim Bové, 1497 Route 136, Washington, PA 15301, [email protected]