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Why is ‘Consistency’ and ‘Water-to-Plaster Ratio’ Important?

gypsum plaster ratioA basic property of any gypsum plaster is its "normal consistency", or the amount of water required to mix a given amount of plaster to a state of fluidity. Both the gypsum plaster and the water are measured by weight, and the required amounts are expressed as a numerical ratio. A mixture of 1kg of water to 1.5kg of gypsum plaster, for example, can be expressed as 2 parts water to 3 parts plaster, or a 2:3 ratio. Usually, the equation is expressed in terms of 100 parts of plaster. The 2:3 ratio would then become 67:100. When 100 parts of plaster are added to 67 parts of water, the mixture has a particular consistency and this is expressed by the number 67 alone. The number 80 would express the consistency of the mix when 100 parts of plaster are added to 80 parts of water.



As the 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 therefore 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 lengthened.

The consistency affects not only the setting time but the hardness and the compressive strength of the set plaster as well, which in turn correlates closely with resistance to breakage and useful life. The higher the consistency number, that is, the more water required, the softer and weaker the final plaster that results. This is because plaster becomes solid by the formation of tightly interlaced gypsum crystals; as more water is added, these crystals are pushed further apart, thus making for a weaker structure.

A consistency number in the 65 to 85 range indicates that the set plaster will be of medium hardness. 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. It is extremely important, therefore, to consider the ultimate use of the plaster mix not only in choosing the type of plaster to use but also in adjusting the water-to-plaster ratio for the chosen plaster. Remembering that as you increase the amount of water you lose hardness and strength in the set plaster, you must decide whether workability (such as carving) or strength (such as longer mould life) is more essential for your purpose.


Water-to-plaster ratio

The water-to-plaster ratio is important in another respect as well. When proper proportions are being mixed, the gypsum plaster gradually reaches a creamy state, becoming opaque when seen on the hand or stirrer. It is not thin and watery. When it has reached the opaque stage, it is practically ready to be poured. One can be misled, however, if the water-to-plaster ratio strays too far. If too much plaster is present, the mix will appear to be creamy and opaque in a much shorter time; but when poured it will sit with water on the surface and take an extra-long time to set. This results in a non-homogenous piece - the plaster will be porous on top and hard on the bottom. If too much water is present, the mix will take an extra long time to reach the creamy stage and then, all of a sudden, it will set overly fast. The piece will still have good homogeneity, but the set plaster will be softer than it would have been had the desired ratio been used. In either case, after you have gained experience and have become acquainted with the material, you will achieve a "feel" for the plaster; you will be aware of any imbalance in the mix early enough in the mixing process to remedy it by adding a dash of one or the other ingredient to bring about the balance.

It is advisable, then, to mix several small batches (approximately 1/2kg) of the plaster you have chosen for your particular work, until you have acquired a little of the feel of a proper ratio. Learning to recognize the various stages in setting as the changes occur, the precise consistency to use, and the proper time to pour or shape any particular plaster can best be determined by your own experience with it, since many variables in shop and studio techniques influence the results.