Sunday, September 2, 2012

05 Molding Design Nº 3

A figure that has been modeled in oil-clay is very soft and does not possess a great deal of permanence, so the fragile figure must, therefore, be transferred into another more durable material for designing ball and socket joints, and refining to the desired surface finish. The method of transferring is to make a mold around the oil-clay figure; and when the mold has set, removing the oil-clay figure from the mold, then casting the more durable material into the mold. For the purpose of making a ball-jointed doll, the more durable material is carving wax, and the mold material may be any of several different mold materials, each with its own properties, advantages, and disadvantages.

I am going to discuss three mold materials that may be used to transfer oil-clay into carving wax; namely, plaster, hot-pour moulage, and silicone rubber. Plaster may be considered an inflexible, or hard rigid mold material. Hot-pour moulage is a soft, semi-flexible mold material. Silicone rubber is a very flexible mold material. I will cover plaster first, then moulage, and finally, silicone rubber.


The word gypsum is derived from the Greek γύψος, gypsos. Plaster of Paris is one of the most used sculptor's materials. The material derives its name from the earth of Paris and its surrounding regions, which contains an abundance of the mineral gypsum, from which Plaster of Paris is made. It is also known as sulphate of lime, hemihydrate of calcium sulphate, and hydrate sulphate of lime. It has a hardness of 2 in the Mohs scale. Commercially, gypsum is secured from underground deposits by mining or quarrying. It must be crushed, screened, pulverized, and then heated before it is usable for mold making or casting.

Plaster of Paris is prepared by the partial calcination or dehydration of gypsum by heat. Gypsum, when calcined at about 350 degrees F., loses about 75 per cent of its water of crystallization, and is transformed into Plaster of Paris. When the roasted powder is mixed with sufficient water to render the mass creamy in consistency, it recovers the water it possessed before calcination, and the mass sets to a uniform, inert, and solid mass of substantially the same composition as the original gypsum.

Plaster has certain important properties. One of these is that when it is properly mixed with water, and allowed to set, it is homogeneneous. That means that it becomes a solid that has no grain, no soft or hard spots, no lumps or knots. Because it is homogeneous, its predictability is dependable; for a given type and age of plaster, many plaster molds can be madethat are consistent in their water-absorbing properties. This property assures consistent quality in objects slip-cast in the molds.

Another imortant property of plaster is that it has a very low coeffiecent of expansion. This means it does not expand very much when it sets in a solid form. Since it is a homogeneous material, the percentage of expansion remains constant, so it can be taken into consideration once its extent has been determined.

Compared to some other mold materials, plaster is relatively inexpensive, making it a financially reasonable material to use when learning how to make molds. Since plaster cannot be reclaimed, being readily available and inexpensive is a good thing.

Plaster also has other properties, including setting time, hardness, strength, water required, workability, fineness, and surface characteristics. Of all the different types of plaster available, including No.1 Industrial Molding Plaster, No.1 Casting Plaster, Art Plaster, and the Gypsum Cements, I will be referring to No.1 Industrial Molding Plaster from now on. No.1 Molding Plaster, often referred to as soft plaster or plaster of Paris, is the softest, the most porous and has no surface hardening additives. It is easily carved and best fitted for models and waste molds. It is best used at a consistency of from 67 to 80, which sets in 20 to 35 minutes.

The setting time generally means the elapsed time from the moment the plaster is added to the water until the mix becomes solid enough to handle. During the first part of the setting time, the plaster is very fluid and can be poured. A little later, it is in a plastic state during which it can be worked with templates and tools. The later stage of thickening is called the period of plasticity. This period can best be learned by mixing test batches. This period is a valuable property to know. However, for most mold making, the period when the plaster is still liquid and easy to pour is the most important to know. Setting time and the period of plasticity are both influenced by the amount of water used to mix the given amount of plaster, or, the water to plaster ratio.

Normal consistency means the amount of water needed to mix a given amount of plaster to a standard fluidity. Both the plaster and the water are measured by weight, and the required amounts are expressed as a numerical ratio. A mixture of one pound of water to one and one-half pounds of plaster, can be expressed as two parts water to three parts plaster, or a 2:3 ratio. Usually the equation is expressed in terms of 100 parts of plaster. The 2:3 ration would then become 67:100. When 100 parts of plaster are added to 67 parts of water, the mixture has a particular consistency which is expressed as the number 67 alone. The number 70 would express the consistency of the mix when 100 parts of plaster are added to 70 parts of water, by weight.

Since the consistency number refers to the amount of water used per 100 parts of plaster, the higher the consistency number, the more fluid the mix will be. When less water is used and the mix is less fluid, the setting time and the period of plasticity for the mix are comparatively short. When more water is used and the density of the mix is reduced, the setting time is longer.

The consistency not only affects the setting time but the hardness and the compressive strength of the set plaster as well, which in turn is closely related with resistance to breakage and useful life. The higher the consistency number, that is, the more water used, the softer and weaker the final plaster will be. U.S. Gypsum rates the consistency values as follows:
94 to 77, soft to medium;
76 to 59, medium to hard;
anything less than 58, hard to extra-hard.
So, as the amount of water is increased, hardness and strength are lost in the set plaster. For long mold life, strength is more important than workability (carving).

The best all-around mix for making plaster molds for slip casting doll parts is a water to plaster ratio of 1 lb. of water for every 1.5 lbs. of plaster (consistency number 67). This is easy to calculate and the mix provides a good, firm plaster mold.

Here are some useful, common properties of plaster which are very basic and simple, the knowing of which can help avoid trouble:

Plaster is always mixed with water. Do not mix it with oil, nor with turpentine, but with good, clean, room temperature water.

Plaster is added to water. Never add the water to the dry plaster.

Plaster adheres to plaster. This is useful when patching and repairing. Otherwise, to protect one plaster surface from another, a separator or parting agent is used. This is a coating that is applied to a plaster surface that has already set before liquid plaster is poured over that surface. Lacquer, shellac, thin oil, mold-grease, and soap solutions are all variations of parting agents.

Dry, wet, or set, plaster should never be put in the drains. Always have a mold to pour mixed plaster into, a disposable trash container to dump excess plaster, and a bucket of water to rinse plaster mixing containers, tools, and hands. The water in the bucket can be dipped out after the plaster settles, and the plaster residue in the bottom of the bucket can then be scooped out and placed in the trash bin. Putting any plaster down a drain will most certainly clog up the drain. Be prepared ahead of time when using plaster.

Plaster is primarily an indoor material. Attempts to weatherproof plaster have generally been unsuccessful.

The setting time of plaster is affected by several factors. Plaster sets up (becomes solid) not only according to the water-to-plaster ratio (consistency), but also according to the temperature of the water and the speed and duration of agitation when mixing. The warmer the water, the faster the set; also, the longer and more violent the agitation, the faster the set. Extraneous material can also be added to the water to affect the setting speed: alum or salt increases it, alcohol or glue-size retards it.

Plaster should always be carefully stored. Plaster seeks water, so it should always be stored in a dry area, and its age should be noted. Older plaster may have already absorbed water making for lumping and unevenness.

Plaster can be used for both models and molds. As a model material, it is ideal because of its workability; as a mold material, its faithfulness to deatil cannot be surpassed.

Plaster of Paris is a rigid mold material. It is the material of choice when the object has no undercuts or very few undercuts; and when those undercuts are simple. It is also employed as a reinforcing material for various kinds of flexible mold materials.

This pretty much covers plaster, the material. Next I will be going over ways to design a plaster piece mold.

The Materials and Methods of Sculptors. Jack C. Rich.
Plaster Mold and Model Making. Chaney & Skee.

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  1. Thank you, Cerchio Fatato. I am happy to know that you find it useful.


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