Using the plaster molds for preparation of the design model is customary between designers and model makers. Plaster is a material with great molding and forming capabilities and at the same time it is cheap and accessible. But using these capabilities is only possible if the designer has the right awareness about plaster properties and features. Plaster is the gentlest material for molding, but if you know its requirements!
Since most of the designers have sufficient knowledge about basic and general principles of plaster using, from plaster grout preparation to its stiffening as well as finishing, therefore we don’t mention it again, and here only the practical and key points that have been gathered from various experiences and the study of different references are cited, that getting information about them enhances your abilities about your plaster works.
After pouring plaster into water, please wait about two minutes (it differs according to the plaster type). If you agitate it sooner, the plaster will convert to a clump and its softening will be difficult.
The plaster mortar stiffening time depends on gypsum type and its cooking temperature.
If you use warm water, the plaster dough will stiffen faster.
The more the water of plaster mortar, the later the plaster stiffens and the lower will be the resistance of its toughened mortar and vice versa.
In general the exotic materials in plaster or water delay its stiffening. But salt addition in very low amount (lower than two percent of plaster weight), results in quick plaster stiffening. If the added salt is more than two percent, it will stiffen pretty later.
If we put some alum in water, gypsum will stiffen faster.
During plaster drying out, it is possible to put it in front of a heater for faster drying, but if the plaster heats a lot, so that you cannot touch it, it will soften and fall down.
After plaster stiffening and toughening, it appears unbelievable water absorption tendency. If in this moment a drop of clay grout pour over a plaster, it immediately (maximally in a few minutes) will changes into a clay dough. So, because of this properties of plaster it is used as the mold of ceramic dishes mold.
If plaster is poured on a wood or metal scrap or plaster itself, it will instantly adhere, unless a separator stuff is applied in between. Soap starch (liquid soap) is perfect for this purpose. But notice that, once this soap is used, the plaster has to be separated from the model prior to its full drying, since this soap can’t act perfectly after plaster full drying. The types of films, Vaseline, and other oils can be used as a separator. The quantity of a separator significantly matters as well. If the amount of separator stuff is low, the initial model will adhere to the mold, and if the separator is too much, the inner surface of the model will be uneven.
Starch must exactly be rubbed on the model before plaster pouring. If the starch is rubbed on the model and then left for about half or one hour, so the starch will lose its effect.
Plaster does not adhere to glass.
If a plaster mortar is placed next to metals, after a while it will combined with metals and produce sulfate.
Plaster is the only mortar that increases in volume by 1 percent after toughening.
For a final finishing and furbishing of plaster surfaces, we can use the dead plaster. Continuously agitate the dissolved and stiffening plaster, and gently add little water. Every time agitate it as far as no excess water to be absorbed by the plaster. The acquired mortar is a dead plaster, which can be easily put on the plaster surfaces with a trowel and have a smooth surface.
A dead plaster never has any mechanical resistance and it falls down with minute finger pressure. Thus only use it when your work is over and you just want to have a smoother surface. You ought not to fill the large pores with it, since it will break down very quickly.
In order to dyeing a sculpture, it is better, while you pour plaster into water, also add 5 percent quicklime to the plaster, and after plaster stiffening, submerge the sculpture in one of the sulfates solution.