Tuesday, April 1, 2014

09 Plaster Production Molds Nº 13

I am going to discuss parting agents for plaster mold making. It is important to understand that a plaster mold works by absorbing moisture from the slip. As moisture is absorbed into the walls of the plaster mold, the slip next to the walls thickens. When the slip reaches the desired thickness, the excess slip is poured back into the slip jug. The result is a hollow casting. In order for the plaster mold to absorb moisture from the slip, the plaster mold must be dry.

Plaster sets through a chemical reaction with water. It can react with the water in air (hunidity), so plaster should be stored for a minimal amount of time in closed bags. Fresh plaster is essential for making quality molds.


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Plaster sticks to plaster. Because of that, a parting agent must be used when making multiple piece plaster molds. The parting agent should act as a mold release, so that the pieces of the mold will not stick to each other when the mold is being made. It should also be so thin that it does not fill-in any detail in the model. The recommended parting agent for making plaster molds for slip casting is a natural soap. Do not use a detergent.

Many of the plaster mold making books I have looked at, recommend English Crown Soap. I have used Tincture of Green Soap with great success. Murphys Oil Soap and Simple Green, also available from the grocery can be used. I am currently using KIRKLAND environmentally friendly liquid dish soap, purchased at COSTCO.

The important thing to know is that mold soap works by chemical reaction with the plaster form a thin layer of soapscum on the surface of the plaster. It is essential that your mold soap container does not become contaminated with bits of plaster. The calcium in plaster chemically unites with stearate in soap to produce a surface of Calcium stearate.

I mix the liquid soap 50/50 with good clean water.
  • 1/2 Cup of Liquid Soap plus
  • 1/2 Cup of Water
will make 1 Cup of Mold Soap, which can be stored in a glass jar with a tight fitting lid. I always pour some of the mold soap into a small container to use it for soaping the mold.


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For soaping models or molds, you may use natural sponges, such as Elephant Ear sponges, 3 to 5 or 6 inches in diameter. The softer, the better. Synthetic sponges are to be avoided. The best tool I ever used was a 1 inch wide Japanese Hake Brush (pictured above).

To properly "soap" a piece of plaster, one must simply insure that the pores of the plaster near the surface have absorbed enough mold release (soap) to prevent the plaster from sticking; the visual signal that this condition has been attained is the appearance of a gloss or semi-gloss on the surface. ~ pg. 20. Plaster Mold and Model Making. Chaney & Skee. (1973).

When I first learned how to make multiple piece plaster slip casting molds, I applied the soap to the mold surface, and worked up a lather with a brush. Then I removed the lather with the cleaned brush. I would repeat this three times, or until the surface had a satin sheen to it.

Chaney & Skee recommend doing it this way:

...apply the liquid soap liberally over the plaster surface of the model or mold. The plaster takes only 15 or 20 seconds, to absorb the soap, which can be seen to soak in. As soon as the plaster has absorbed the this first coating, apply another coating of liquid soap. Continue to repeat these procedures, alternately waiting for absorption and then resoaping the plaster, until a gloss appears on the plaster surface. It may take as many as eight or ten repetitions, all within a few minutes, to acquire this gloss. When the gloss appears over the entire mold or model, enough soap has been applied.

When this glossy condition has been attained, use the finishing sponge to pick up any surplus soap that has accumulated, especially in dips and hollows of a piece. No suds should remain, nor should any part be touched by the hands, once the plaster has been soaped. If it is necessary to handle a piece after having soaped it, the piece should be resoaped in that area.

Pay careful attention to small crevices and intersections, as they tend to gather suds; use a small bristle brush to remove froth from these trouble spots. Remember that, whenever a bubble of soap remains, the detail of the piece will be missing in the plaster cast over it.

Often in the rush of things, doubt may arise as to whether you have or have not soaped a certain piece; in these cases, always soap again &em; play it safe.

This completes the soaping operation and the plaster surface is then ready to receive fresh plaster. Remember, avoid touching soaping areas.
~ pg. 20-21. Plaster Mold and Model Making. Chaney & Skee. (1973).

Multiple piece plaster mold making is a Craft Skill, and as such, requires much practice to get good at it.

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