SLIP-CASTING Learning the basics To find out more, visit: http://virginiadecolombani.wordpress.com/ INDEX 02. What is slip-casting? 03. Slip-casting manifesto 04. Making a ptototype 05. Planning mould sections 06. Making a casting mould 09. Casting 11. Glossary 12. References WHAT IS SLIP-CASTING? 2 “Slip-casting is a technique for the mass- production of pottery, especially for shapes not easily made on a wheel. A liquid clay body slip is poured into plaster moulds and allowed to form a layer, the cast, on the inside cavity of the mould. For a hollow cast mould, once the plaster has absorbed most of the liquid from the outside layer of clay the remaining slip is poured off for later use. The cast piece is removed from the mould, “fettled” (trimmed neatly) and allowed to dry. This produces a greenware piece which is then dried before firing, with or without decoration and glaze.” Advantages Disadvantages This process was introduced into many European porcelain factories in the eighteenth century, and • Complex forms can easily be • Slow production rate was commonly employed for the casting of terra achieved • Limited control over tolerances cotta sculpture in the nineteenth century. • Ideal for producing hollow ware • Labour intensive • Does not require much finishing • Moulds may be heavy to handle (usually only fettling) • Large scale production requires • Efficient use of material many moulds, which themselves • Slip is fairly inexpensive require storage • Lets itself well to small-scale production runs or limited editions SLIP-CASTING MANIFESTO 3 You can’t learn “ceramics” in a Come prepared You are going to get dirty Don’t be scared of making week mistakes Have a design in mind before Don’t be scared of getting Don’t come expecting to learn starting and preferably some dirty. Use your hands, get You are probably not going everything there is to learn tools familiar with the material to get it right the first time. straight away. Take baby steps Experiment as much as pos- and focus on one aspect at the sible as the best results often time come from experimenting and making mistakes. Experience comes with time MAKING A PROTOTYPE 4 MDF Clay Blue Foam Other Materials Easy to work and tool with, A model can easily be made This is a material that is A model may also consist more durable. Prototype is with clay. It is preferable to relatively easy to handle and of very soft materials safe and intact, great for allow the model to dry to very light. It is quite expensive such as mattress foam or achieving sharp, transitional the leatherhard stage before so use it reasonably. While textiles. However, during the lines and edges. Laminating making the mould. You should moulding it is very delicate and construction of the mould, together with wood glue might preferably use a clay body that can get scratches easily so deformation may occur due take a long to dry etc. but once is similar in color to the work you have to handle it with care. to the weight exerted by it’s glued, it’s firm and stable. after firing it to avoid traces of Otherwise it is ideal for more the mould. The material can clay of a different color in the organic shapes that can be become compressed, resulting final piece. hard to obtain with wood. in staggered mould seams. PLANNING MOULD SECTIONS 5 Factors to take into consideration • Each undercut makes another mould section necessary • Manageability of the mould is another factor determining the placement of mould sections. • The weight of each mould section and of the entire mould • The way the mould is filled and emptied • How to hold together the different sections One Piece Two or Multi-piece together (straps, tape or wires) Ideal for hollow shapes The purpose of making the • Where to position your pouring hole such as bowls or glasses. mould in two or more pieces This option is a time saving is to permit certain shaped • How the mould is going to stand; its stability technique that results in an castings, when leather hard, and strength (especially when doing multi- accurate mould which also to be removed from the mould sectioned moulds) minimizes seam lines. without breaking. MAKING A CASTING MOULD 6 Prepare your cottles. Fill the bottom half of Make sure your model is well Carve a couple of keys into Rectangular cottles are made the mould box with clay placed and that your parting the clay surface by using any from wooden planks, often (preferably recycled) and line is somewhat parallel to rounded end, such as a marker. held together by clamps. The place your model on it. your work cottles. When it is These round depressions will wood should be soaped to Create a pouring hole by in place, smoothen the clay help hold the mould together keep the plaster from sticking adding some extra clay where surface. when casting. Then, with a soft and the cottles should be you are going to want an brush, cover the model with a sealed with clay to prevent opening. thin layer of vaseline. plaster from leaking out. Be careful, as the pressure of the liquid can be surprising 7 Put some water into a clean Fill the mould box with plaster. Turn the mould over and Gently clean the surfaces and bowl. Avoid lumps and make The plaster should cover at proceed to lift off the clay from remove any crums or lumps. sure to have enough water. least 1 inch over the highest model and plaster half of the Add some vaseline to the Slowly add plaster to the point of the model. Let it mould. Make sure to keep the model. water until you start seeing set for around 20 minutes. It clay in the pouring hole and all small “islands” that don’t sink. should start feeling warm when the cottles in place. Place your hand in bottom of it is dry enough to handle. bowl and shake it from left to right (to remove lumps and air bubbles). 8 Once the second half of the Remove the clay from the Pour some water in the mould Carefully open the mould. With mould has been filled, let it set pouring hole with the help of a and let it rest for a minute or a soft brush, wash both halves for around one hour. Once it is knife, making sure not to ruin so. This procedure should with water, making sure to dry, remove the cottles. the mould facilitate the opening of the remove any remainders of clay. mould. Set it to dry in a warm place. The time it takes for the mould to dry depends on a lot of things but, to be sure, let it rest for at least a couple of days. When it starts to feel warm, you will know it is almost if not fully dry CASTING 9 If you buy an already-made Seal the mould parts with Pour the slip into the mould. While the slip is drying, remove slip, make sure to stir it well either straps, wires or tape. You will notice that after one the excess slip that spilled before using it. It is always It doesn’t really matter what minute or so, the level of the outside of the mould with a good to pour some (better too you use, as long as you make slip will lower. This is because knife. The longer you leave much than not enough) in a sure they don’t move. Once it starts to set inside the the slip inside the mould, the smaller pitcher. Make sure to the mould is sealed, set it in mould. When this happens, thicker it will become. sieve it. This is necessary to a vertical position with the pour more slip into the hole. remove any dry bits that might pouring hole facing upwards. have falled into the bucket. 10 Once you believe that your After 10 minutes, whe the Lay the mould on a side. After the removed casting has product is thick enough, gently slip stops dripping, turn the Remove the straps and gently dried 30 mins, fettle it until the tilt the mould and slowly drain mould straight up again. Let tap the upper mould piece surfaces are even. If there are out all the excess slip into a the mould rest in this upright before lifting it all the way. Use any air holes, they can be filled container. Position it upside- position for 30 or more the help of a knife if necessary. by hand with soft clay. Set the down, vertically, and place minutes. Cut off the excess clay from casting away until it is bone- it on two parallel pieces of the pouring hole. See if the dry. You may then sandpaper wood. Leave it there until the casting will lift out by hand. If the surfaces very gently, where slip stops dripping. Make sure it is too fragile, let it rest for necessary. It is now ready to that the outflowing slip does 10-15 mins before carefully be fired in the kiln. not block the gate. removing it and positioning it vertically GLOSSARY 11 Bisque: Pottery which has been fired once, without glaze, to a Leather hard: Stage of the clay between plastic and bone dry. temperature just before vitrification Clay is still damp enough to join it to other pieces using slip. For example, this is the stage handles are applied to mugs. Bone Dry: Completely air dried Maturing Point: The temperature at which the clay becomes Cottle: The word used for the device that holds the liquid hard and durable plaster around the mould Pouring hole: Opening in mould to pour slip into Earthenware: A low-fired clay, often red, white and buff colored.