3 things to consider:
What is the material the positive is made from? Is it porous, non- porous, and will the heating of the plaster possibly cause and expansion?
What does the object look like- does it have undercuts or details that are important to maintain?
What is your plan with the final objects? Do you need the objects to be soft for further modification? If so, you may need additional parts of the mold for easy release.
1. Decide if you want to use a clay bed to lay your object on or if you want to lay it directly on the surface (the clay bed is easier to seal.
2. Soap your object if porous. Polish it to a silky finish.
3. Determine where your walls need to be for each section before you start. This will ensure that you are working smart not hard. Use a clay maquette or a sketch to help you. If you don’t care about the object you can also use a sharpie directly on the object.
4. Using slabs, create walls for each section. Be careful to seal each section (rubber finger tools work great for this.) Mix and pour plaster one section at a time. Let each section set up. Make keys, then pour the next section.
5. Use the amount of clay it takes to properly caulk any cracks. Wipe away any extra clay. A small amount of clay can seal without creating a large seem.
6. After the mold has been poured and has set, gently vibrate the mold apart. Remove the positive. Put the mold back together for cleaning so that you don’t inadvertently clean the fettle lines (the place where the mold pieces meet up on the interior of the mold. )
Sanding the mold compresses the mold, gets rid of plaster that might chip off, and helps it to last in the future.
Even store bought slip should be sieved before use. This eliminates lumps that will have to be sanded out later and allows even water distribution.
Deflocculate: To literally break up the “flock” of clay particles by creating a negative charge and causing particles to repel each other. This allows us to add less water with more fluidity and allows particles to bond with better compression and with out air bubbles. We add this deflocculant to slip (Darvan or Sodium Silicate)
Coddle: The objects which contain our plaster while it sets around the object we are casting. Sometimes Aluminum Flashing, or boards we clamp together.
Fettle: The seam of the mold.
Fettle Line: The line of an object where the undercut occurs (often still visible on plastic cast objects, such as pens)
Undercut: The point where if an immoveable object (such as set plaster) is wrapped around it will become locked and cannot be removed.
Key: the small indentation that keeps a mold from sliding apart. Two primary types: Standard Keys and Slide Keys.
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