Plaster or Plaster of Paris is often used to make moulds and other tools for pottery. Learn how to prepare plaster and then use it to make a bat for a potting wheel.
There are many different kinds and brands of plaster, but the best type for a potter is the U.S. Gypsum Company's pottery plaster. It comes in 100 pound bags or 250 pound barrels, but much smaller amounts are sold by ceramic supply dealers. Only buy as much as you will need for a project, plaster does not keep well unless it can be sealed in a completely airtight container. The correct proportion for mould work is 1.25kg. of plaster to 1 qt. of water.
When mixing, measure the water first and put it in a plastic bucket or mixing bowl. Sprinkle the plaster lightly and slowly into the plaster to prevent it from clumping.
When all the plaster has been sprinkled in, allow it to slake for two minutes. Slaking is letting the plaster body absorb the as much of the water as it can. Not doing this results in dry spots and cracks in your mould.
Once the slaking is complete, pour out any water remaining on the top of the mixture.
Stir the mixture by hand. The stirring should be done in such a way as to agitate the mixture and drive out air bubbles. Do not whip or whisk the mixture. The point is to get air out, not in. A good method of stirring is to put the hand, palm upward, on the bottom of the pail and wiggle the fingers vigorously. Continue to mix for two or three minutes.
The plaster should begin to thicken. You can tell its ready when a finger can be drawn over the surface of the mixture and leave a slight trace.
Round plaster bats are useful for wheel work or coil building. Take a pie tin between six and eight inches in diameter.
Use cooking oil or petroleum jelly to lightly coat the interior of the pie tin.
Pour the plaster into the pie tin in a spiral pattern. To ensure that the bat will be of even thickness, let the mould overflow with the plaster and simply scrape the top smooth with a ruler or straightedge.
Freshly made plaster will need to dry for five or six days before it can be subjected to the stresses of use. To speed this process, place the mould near a warm radiator. Don't let it become hot to the touch, as the plaster will be weakened and crumble. Infrared lamps are the best way to quickly solidify plaster, as they can dry the plaster from the inside as well as the outside.
Once the plaster is completely dry, simply overturn the pie tin and your new plaster bat should slide free. Enjoy!
When mixing, sprinkle the plaster into the water slowly. Never pour water into plaster.
Store plaster in an airtight container, otherwise it will absorb moisture from the atmosphere and go bad.
If your plaster contains lumps, sift before using it.
A dirty scum on the surface of the water when you are mixing indicates that the plaster is old and no good.
Plaster can be cast against moist clay or glass without the using of a parting compound.
Wood and metal surfaces should be oiled or greased before plaster is cast against them, and plaster surfaces should be soaped or "sized".
Be thorough when you apply size, you should use a solid bar of soap, not liquid soap.
Take pride in your work. Even though the things you make of plaster are for temporary use, make them with care and accuracy.
Too much plaster in your mix results in plaster too hard and dense. It's not absorbent enough to be used in moulds.
Too little plaster in your mix results in plaster that flakes and crumbles easily.