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United States Gypsum Castings

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United StatesGypsum
Castings

Solid

These pieces are usually rather flat (wall plaques) or small pieces with relatively thin sections of plaster. After proper weighing, soaking and mixing, the plaster is poured slowly into the mold. Be careful to avoid trapping air at the face of the piece as they will have to be patched later. To break up such air bubbles, slurry is poured into the mold and poured out again into the mixing container. After a short time, the mold is again filled with slurry. Vibrate the filled mold by pounding it on the work table. This action causes air bubbles to raise to the back of the cast. The plaster is then screeded off the back of the casting to give a flat and smooth back.

 If the casting is a wall plaque, a hanger should be inserted into the leveled piece before the plaster sets hard.

Splash

Place mold down on the table. Mix plaster to plasticity. Splash the plaster on or over the surface of the mold until it is ¼ to ½ in. thick. Next, take sisal or hemp, and dip or soak it in the plaster. Press this plaster-wetted sisal against the built-up plaster coating, smoothing the surface with your hands. This technique is used to increase strength and reduce weight of large flat pieces.

 

Hollow or slush

This method is used for large pieces to reduce the weight of the piece and to speed drying. When making hollow casts, use more than one mold at a time. This is an efficient procedure that gives the plaster time to thicken before successive pours are made.

 

  

 

First, the mold is filled 1/3 to 1/2 full. Then the plaster is slushed by rolling and tilting the mold until its entire surface is coated. Excess plaster is poured back into the mixing container. The second mold is prepared similarly and the process repeated.

When making the second pour, make sure the first coat has thickened enough to stay in place when slushing the mold wall. The process is repeated three or four times, until proper wall thickness is built up. Thickness varies from piece to piece depending on size and shape of casting. Timing is important. If too much time elapses between coats, delamination may occur, causing a weak casting. Hollow casting is an art; it takes practice to learn successful techniques and timing.

Rotational Casting Machines for hollow casting are available from Plaster Master Industries. 

Trimming and Patching

After removing the casting from the mold, there may be some fins, rough edges, small bubbles or pin holes that must be trimmed off and filled.

Trim and patch the cast when it is wet or as soon as possible after it is removed from the mold. If the casting has dried, the area to be patched should be moistened with water.

Trim fins and rough edges with the edge of a knife or an open wood rasp with removable bottom plate. After using the rasp on wet plaster, clean it with a wire brush, before the plaster in the opening of the rasp sets hard.

Trim and Patch the cast

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