VOCABULARY TERM: PLASTER CASTING

Plaster Casting. Preparing a three-dimensional cast for a more permanent, transferable, reproducible form in plaster of Paris. Plaster casting requires a craftsman skilled in preparing the model, mixing the plaster with water, pouring the plaster and removing air bubbles. Plaster casting is easy to learn and proficiency comes early, but there are many variables – kinds of plaster, time, temperature, release agents – in addition to the caster's skill. While some of these variables are the operator's choice, as hard or soft plaster, the use dictates others. For example, a super fine hard plaster is preferred in the coin and medal field but the caster may prefer one brand over another. Hydrocal and Densite are brands of commercial plaster used in the modeling of coins and medals.

Preparing the model. The original model can be any medium the modeler worked in – clay, plasteline, wax, plaster, wood, metal, whatever. This needs to be transformed into a more permanent, three-dimensional form and plaster is the best medium for accomplishing this. The original model, in most instances, may be porous or rough. So the first step is to smooth the surface of the model with steel wool, emery, fine sandpaper or light rubbing.

Second step is a sealer. Brush the model with a very light coating of shellac. If the brush strokes are visible, then too much has been applied. If the model is white plaster, the caster may wish to use a tinted shellac (to make it different from the casting). An orange shellac, diluted 3 or 4 to 1 and will give a reddish tint to the coating applied to the model. Allow ample time for the shellac to dry, and no longer be tacky.

Third step is a release agent. Clay or plasteline models generally do not need a release agent when cast in plaster, every other medium does. Even so, a light dusting of talc on clay will prevent it from sticking in the mold. For a model made of plaster use a very light coating of vegetable oil, or Mazola oil. Talc, liquid green soap or silicon spray can also be used as a release agent on plaster.

Fourth step is to build a fence around the model. This can be of any thin sturdy material, as galvanized tin, lead, or similar. Clay cannot be used for this step because of the weight of the plaster could break through the clay. For plaster casts intended to be pantographically reduced a flange of one to two inches needs to be added to the cast.

Mixing plaster with water. Choose a container for the proper amount of plaster necessary for one entire cast – no more, no less. The container is best if it is flexible (for later cleaning) and have a round bottom. Pour in the water, about 85% of the volume needed; the water may be hot, cold or room temperature. Hot water slightly increases setting time, cold water slightly retards it.

If a harder plaster is required it can be accomplished by substituting lime water for a portion of the tap water.

Pour the dry plaster into the water, letting it sink to the bottom. Keep pouring a little at a time, keeping some plaster on the surface of the water at all time. It will reach a saturation point. Stop adding plaster. The consistency will be fairly fluid – this is ideal.

Insert a large flat spoon for stirring. This is called spatulation. Do not remove the spoon from the mixture. Do not whip, do not do anything to introduce air into the mixture.

Stir the spoon on the bottom of the container with a flat swishing motion. There are mechanical spatulators – these have blades, both at bottom and near the top but must be covered by the liquid. Do not allow any air into the mixture.

The longer time for spatulation, the less time required for setting (one minute stirring requires 10 minutes setting time, two minutes stirring requires 7 minutes setting, and three minutes stirring requires 5 minutes setting time.)

Ideally the caster wants sufficient time to work with the pouring then have the plaster set quickly. The slower the setting time the more skillful must be the caster. However the caster can lessen the setting time by using hotter water and longer spatulation, as mentioned, but he can also do this chemically by adding a hastener (or a hardener to quicken the drying) – or a retarder if the caster wants a slower setting time). Liquid gypsum hardeners and retarders are available commercially.

Pouring the liquid plaster. After the model is smoothed, sealed, dried and coated with a release agent, and the plaster is mixed, the model is given a very fine spray of water from an atomizer bottle. The liquid plaster is poured – only a small amount! – first in the high points (lowest cavities in the model) with this small amount.

With a 100% bristle brush and by blowing almost as hard as a human can, move the liquid plaster around to make certain there are no bubbles and all cavities are filled. Then the caster moves on to other areas like the legend or lettering; blowing and brushing the liquid plaster back and forth until a film of plaster covers every surface.

After all surfaces have been checked for no voids and no air bubbles, then the remainder of the plaster is slowly poured over the model to fill the mold made by the fences. The plaster should be poured to a level about one inch above the highest point of the model.

Setting and drying. All this procedure, of course, has to be accomplished within the three to ten minutes of the setting time. Plaster begins setting as soon as it comes in contact with the water.

Heat will be given off as the plaster dries. It is warm to the hand but the heat diminishes as the drying continues, for up to two hours. When the caster can place his hand on the plaster and it is cool to the touch the casting can be separated from the mold.

Separating the cast from the mold. After the fence is removed a knife is inserted between the cast and the mold at several places around the casting and pried apart. Had the model been properly prepared (with no undercuts) and an adequate release agent used the cast separates easily from the mold.

Lift straight up. Do not tip the casting to either side, but lift straight up. And there is the plaster cast. However dry the mold appears it is not ready for further use; it must be baked.

It must be further dried in an oven at 125° for from 12 to 14 hours. This removes all moisture and gives the plaster its hardness and permanency.

The sculptor may cast his own model, or the caster may be a separate craftsman (specializing in plaster casting). Either way the thoroughly dried plaster cast is then viewed by the artist. The sculptor – despite the amount of time in preparing the original pattern – will undoubtedly want to touchup the cast, sharpening up detail and deepening some areas for greater contrast.

Coloring plasters. The plaster cast will be chalk white. The surface can be tinted (as for photographing), or the plaster can be colored entirely throughout by adding the coloring (as blueing) when it is mixed with water.

To read the complete entry on the Newman Numismatic Portal, see:
Plaster Casting (https://nnp.wustl.edu/library/dictionarydetail/516504)

To read the earlier E-Sylum article, see:
VOCABULARY TERM: PLASTER CAST (https://www.coinbooks.org/v27/esylum_v27n20a13.html)

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