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"

Hammer

Wood Glue

Drywall Nails

Sheet of glass or plywood

Plaster Mix

Mixing Bucket

Water

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.

How To Pick Tools Needed For Pottery

When initially working with clay and learning the art of pottery, many amateur potters make the mistake of going out and buying all kinds of shaping tools. Such tools are only really needed by the time your finished works are of a smooth and professional look. Until then, you can make do with pens, pencils, and plastic forks, knifes, and spoons. For those who believe themselves ready to advance to the next level of this ageless art, here's a brief explanation of the most used potter's tools and what to look for when buying them.


Loop Tools
The loop tool is a simple metal loop set into a handle. Depending on the shape of the loop they can be used for cutting swaths from a finished clay pot for large designs or uniformly thinning the pot's walls. They are also available with thin, malleable wire but the stainless steel loops tend to last longer. Also people often have the option of choosing between double ended loop tools with stainless steel handles or single ended loop tools with finished wooden grips. Go for the wooden handles as they are easier to clean and their larger size grants a greater degree of control over the tool.

Ribbon Tools:
Ribbon tools are used by potters for medium-duty cutting, shaping and slicing. They use the same stainless steel ends as the loop tool, simply on a smaller scale. Also because of their smaller size, where the end of a loop tool may when being used to deal with the contours of the item being sculpted, a ribbon tool will stay rigid and cuts more deeply. It's less common to find a single ended ribbon tool, so don't try as it will simply lead to a great deal of frustration. Often they are sold in kits with many ribbon tools having different shaped heads for different tasks. They can all prove to be useful and are cheaper when in kits, so it's probably best to buy them as such. Stainless steel handles can be cleaned by scrubbing them in hot water more easily than wooden ones, having no grain to collect dirt and clay. But because of that inherent grain, wooden handles make the tool easier to hold and manipulate. The choice is up to you.

Potter's Rib:
A potter's rib, often simply called a rib, is a simply shaped tool, usually wood, that is used to manipulate or alter the shape of any vessel being thrown on a potter's wheel. They work in a manner similar to a woodworker's lathe, shaping or stripping the clay dependent on the angle of the tool and how much pressure is applied. Usually they are found in two shapes, kidney and rectangular. The kidney shape helps smooth edges, pushing round indentations into the vessel. The rectangular shape is used to scrape off excess clay or push sharply angled indentations into the vessel. Traditionalists still use wooden ribs, but they can be found made out of various metals, and plastics. Some have even been seen made from stone. In my opinion the easiest to hold rib is made from wood, but the rib that's easiest to clean and can round edges the smoothest is made from Teflon.


Needle Tool:
The needle tool is a long, thin heat-treated steel needle used to make a clean smooth cut with a minimum of drag when removing the uneven top edge of thrown items. The width of the needle is crucial, too thick and the needle pushes clay aside rather than cuts through it. Too thin and the needle fails to pull through the clay, potentially bending or breaking the needle or chipping pieces out of the top of your thrown item. One that is slightly smaller than the average nail you might use to hang a picture frame is acceptable for most situations. The needle itself should have a pointed sharp tip. If such a needle dulls over time (though this is unlikely) simply stroke it lightly across a whetsone, giving the handle a quarter turn after each stroke. Handles come in plastic, wood and stainless steel in different widths. Any of the materials works just as well as this tool need not be handled with the same skill as the others. What you would want to take into consideration is the width of the handle. They range from slightly thinner than a pencil to as thick as a piece of rebar. The larger a person's hands, the larger the handle would need to be to feel comfortable.

Modeling Tool:
The purpose of a modeling tool is less to cut or remove clay from the work or sculpture as to push it into place, create fine detail, and smooth over the rough edges. This tool is not used in pottery a great deal, but it's the clay sculptor's best friend. A smooth-edged butter knife will work just as well when you're starting out. A dull number two pencil will work as a stylus for finer detail. They are comparatively cheap and easy to find, coming in plastic, wood, and metal of lots of different shapes. The metal modeling tools have a tendency to cut into the clay rather than push it into place and I would not recommend them. Plastic tools are more resilient and can withstand more pressure that wooden ones without splintering. They're also heat resistant so they can be thrown into the dishwasher for cleaning, whereas a wooden modeling tool would likely split.

Tips & Warnings
Before you spend the money to purchase all these tools, look around the house for items that would work just as well. You'd be surprised how many normal household implements have unforeseen uses.

Using potter's ribs requires a still and the steady application of light pressure, misuse of this tool on a potter's wheel can result in flying clay in all directions and light injuries to the work hand.