Sunday, June 1, 2008

How To Make Plaster Slabs

In pottery work you will always find a use for any number of flat rectangular slabs in your studio or craft room. Here are detailed instructions on how to obtain and prepare your plaster, as well as construct and pour your plaster mold.

Things You’ll Need:
4 lengths of wood 3/4" X 1" X 12"


Wood Glue

Drywall Nails

Sheet of glass or plywood

Plaster Mix

Mixing Bucket


Ruler or Straightedge

Oil or Vaseline

Constructing The Mold
Step 1:
Start by making the mold, as the plaster will rapidly begin to set once its been mixed. Form the four pieces of wood into a square frame.

Step 2:
Apply wood glue to the points where the pieces meet and stick them together, hold together with firm pressure for a count of twenty.

Step 3:
Reinforce and connect the joints by driving nails into the sides. When you finish you should have a wooden square with each side approximately 12 and 3/4 inches long, one inch high, and 3/4 inches deep.

Step 4:
Place the square in the center of a clean sheet of glass. Plywood will work as well, but plaster doesn't stick to glass while it will hold onto plywood. If you're using plywood coat both the frame and plywood backer thoroughly with oil or Vaseline. If using glass then only coat the wooden framework with oil or Vaseline. Set it aside for future use.

Preparing The Plaster
Step 1:
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 ever 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 mold work is 2 3/4 pounds of plaster to one quart of water.
Step 2:
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.

Step 3:
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 mold.

Step 4:
Once the slaking is complete, pour out any water remaining on the top of the mixture.

Step 5:
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.

Step 6:
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.

Making The Slab
Step 1:
When the plaster is ready, pour it evenly into the frame. The sheet of glass or plywood should be on a table with an absolutely level surface, so that when the plaster flows it will make a layer of even thickness.

Step 2:
Bump the table vigorously immediately after pouring to get the plaster to flow into all the corners of the mold. If air bubbles rise to the surface, blow on them to break them.

Step 3:
In a few minutes the plaster will begin to set and the surface will lose its shine. At this point it takes on the consistency of cream cheese. it is passing through its period of elasticity, which potters call the cream cheese state.

Step 4:
In a few more minutes it will begin to harden, and then begin to get warm. This is the period of crystallization. When it is cold to the touch, crystallization is completed and it is safe to remove the wooden frame and lift up the slab.

Tips & Warnings
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 molds. Too little plaster in your mix results in plaster that flakes and crumbles easily Make sure to grease your mold thoroughly or your plaster will never come loose from it.

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