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7 common properties of Gypsum Plaster?

properties gypsum plasterBefore indicating some of the differences in gypsum plaster, we shall point out some more very basic characteristics common to all types of plaster; but it is often a very basic and simple assumption that goes unexplained which causes trouble.


1. Gypsum plaster is mixed with water - not with oil, not with turpentine, but with good, clean, room temperature water.


2. Gypsum Plaster is added to the water - never add the water to the dry plaster.


3. Plaster adheres to plaster. There are times when one wishes this were not true but, generally speaking, we make this property work for us when necessary, as in patching and repairing. When we do not wish plaster to adhere, we must protect one plaster surface from another by using a separator or "parting agent". This is a coating that is applied to a plaster surface which has already set before wet plaster is poured over that surface. Lacquer, shellac, thin oil, mould grease, and soap solutions are all variations of parting agents.


4. Plaster is primarily an indoor material. Attempts have been made to weather-proof plaster for outdoor use by such means as immersing it in boiling linseed oil, but these methods have not proved very successful.


5. The setting time of plaster is affected by several factors. Plaster sets up (becomes solid) not only according to the plaster-to-water ratio but also according to the temperature of the water and the speed and duration of agitation. 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.


6. Gypsum Plaster should always be carefully stored. Since this material is so hygroscopic (water-seeking), it should always be stored in as dry an atmosphere as possible, and its age should be noted. Older plaster runs the risk of having already absorbed moisture, making for lumping and unevenness. In the gypsum cements Hydrostone, Hydrocal and Ultracal, the age of the stored material determines the speed of setting, which takes longer, by as much as 20 minutes, the older the material gets.


7. Gypsum Plaster can be used for both models and moulds. As a model material, it is ideal because of its workability; as a mould material, its faithfulness to detail cannot be surpassed.