Uses of Plaster

Plaster is used to create complex detailing for use in room interiors. These may be geometric (simulating wood or stone) or naturalistic (simulating leaves, vines and flowers). These are also often used to simulate wood or stone detailing found in more substantial buildings. 

In the past the famous painters used thin layer of wet plaster to paint on it, known as intonaco. The pigments sink into this layer so that the plaster itself becomes the medium holding them, which accounts for the excellent durability. 

Plasters have been in use in passive fire protection, as fireproofing products, for many decades. The finished plaster releases water vapour when exposed to flame, acting to slow the spread of the fire, for as much as an hour or two depending on thiekness. It also provides some insulation to retard heat flow into structural steel elements, which would otherwise lose their strength and collapse in a fire. Early versions of these plasters have used asbestos fibres, which have by now been outlawed in industrialized nations and have eaused significant removal and re-coating work. 

More modem plasters fall into the following eategories  

One differentiates between interior and exterior fireproofing. Interior produets are typieally less substantial, with lower densities and lower eost. Exterior produets have to withstand more extreme fire and other environmental conditions. Exterior products are also more likely to be attraetively tooled, whereas their interior eousins are usually merely sprayed in place. 

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An important consideration for a three-dimensional macrosculpture is selecting the material from which the sculpture will be made. For example, the Statue of Liberty would not be effective as a wire sculpture. The function of the Statue of Liberty is to welcome travelers to the New York harbor as a gateway to the United States. In wire, the statue would be missed entirely by travelers. The materials used in a sculpture can determine its success or failure as a work of art. In Activity 5.4, students will be instructed about the use of plaster, sometimes called plaster of Paris, as a medium for three-dimensional macrosculpture. The preparation of plaster of Paris and the chemical changes that occur during preparation are discussed. 

It is interesting to consider that the old traditional warm-region winemakers, in their use of plaster, were adding acid and SO2 to low acid musts just as we do today. 

Peltier, L.F. (1961) The use of plaster of Paris to fill defects in bone. Clin. Orthop., 21, 1-31. 

Setting of Plaster Accelerators and Retarders Uses of Plaster... 

Lamhe CM (1958) Preparation and Use of Plaster Molds. In Kingery WD (ed) Ceramic Fabrication Processes. John Wiley Son, New York... 

CaSO4-0-5HiO, prepared by heating gypsum at 130 "C, is used as plaster of Paris. Gypsum and anhydrite are used for H2SO4 production and gypsum is used as a soil additive and as an inert additive to pharmaceuticals and insecticides. 

Although the use of simple diluents and adulterants almost certainly predates recorded history, the use of fillers to modify the properties of a composition can be traced as far back as eady Roman times, when artisans used ground marble in lime plaster, frescoes, and po22olanic mortar. The use of fillers in paper and paper coatings made its appearance in the mid-nineteenth century. Functional fillers, which introduce new properties into a composition rather than modify pre-existing properties, were commercially developed eady in the twentieth century when Goodrich added carbon black to mbber and Baekeland formulated phenol— formaldehyde plastics with wood dour. 

In the eadiest known paintings, the primitive cave paintings, paint was appHed directly onto the cave wall, with tittle or no preparation. As early as the Old Kingdom in ancient Egypt, however, wall surfaces were specially prepared using a coating of plaster. In time, the refinement and complexity of the preparation layers increased until in the Renaissance several layers of different composition and fineness were superimposed. Other preparations used, especially in the Far East, consisted of a clay layer. 

Residential Construction. Owing to rising energy costs, the cost and low thermal conductivity are of prime importance in wall and ceiling insulation of residential buildings. The combination of insulation efficiency, desirable stmctural properties, ease of appHcation, abiHty to reduce air infiltration, and moisture resistance has led to use of extmded polymeric foam in residential constmction as sheathing, as perimeter and floor insulation under concrete, and as a combined plaster base and insulation for walls. 

The use of porous formers ia the dippiag process, or porous molds prepared from plaster of Paris or uaglazed porcelaia with a surface pore size smaller than the majority of mbber particles, has been widely adopted ia the latex iadustry. With the porous porcelaia formers, the mbber particles are filtered oa the surface of the formers. The mbber latex coagulates because of its high coaceatratioa to form a film of increa sing thickness as more water is absorbed iato the ceramic. Its rate of iacrease diminishes sharply beyoad an optimum period of time, however, depending on the various characteristics of the ceramic. 

Those made of plaster of Paris have a finite life for that reason, but residues can be removed from ceramic molds, by use of an inorganic acid to clean the surface, and a chelating agent such as EDTA for deeper cleansing. 

Forming-Die Alloys. The tonnage of slab zinc used in this appHcation is small. The use of zinc alloy dies started in the aircraft industry during World War II (119). Zinc-based alloys cast in sand and plaster molds continue to be used for short-mn dies for steel and aluminum stampings in the automotive and aircraft industries (120). Considerable cost savings are realized with these low melting zinc-based alloys which are easy to poHsh, machine, weld, and remelt. 

Hemihydrate. The abiUty of plaster of Paris to readily revert to the dihydrate form and harden when mixed with water is the basis for its many uses. Of equal significance is the abiUty to control the time of rehydration in the range of two minutes to over eight hours through additions of retarders, accelerators, and/or stabilizers. Other favorable properties include its fire resistance, excellent thermal and hydrometric dimensional stabiUty, good compressive strength, and neutral pH. 

Impression Plasters. Impression plasters are prepared by mixing with water. Types I and II plasters are weaker than dental stone (types III and IV) because of particle morphology and void content. There are two factors that contribute to the weakness of plaster compared to that of dental stone. First, the porosity of the particles makes it necessary to use more water for a mix, and second, the irregular shapes of the particles prevent them from fitting together tightly. Thus, for equally pourable consistencies, less gypsum per unit volume is present in plaster than in dental stone, and the plaster is considerably weaker. 

The use of monomer-polymer doughs has been largely confined to the production of dentures. A plaster of Paris mould is first prepared from a supplied impression of the mouth. Polymer powder containing a suitable polymerisation initiator is then mixed with some monomer to form a dough. A portion of the dough is then placed in the mould, which is closed, clamped and heated in boiling water. After polymerisation, which usually takes less than half an hour, the mould is cooled and opened. This technique could also be usefully employed for other applications where only a few numbers-off are required but does not seem to have been exploited. 

Absorption - Processes water can be removed from a material by the capillary action of porous bodies. An example is the cream of clay and water used for casting pottery, which is deprived of the greater part of its water by placing it in molds of plaster of Paris. The capillary character of this mold withdraws the water from the liquid clay mixture and deposits upon itself a layer of solid clay, the thickness of which is controlled by the time of standing. Certain types of candies, such as gumdrops, are dried mainly by contact with the starch molds in which they are cast. The drying effect of sponges, towels and materials of this kind is due to this same action. 

Drying Rooms For drydng materials that cannot be agitated, the drying room, and tray drier (which there are many varieties) can be used. In the former, the material may be spread on pans and these stacked in a rack, which leaves a space between the trays for air circulation. The room has loosely fitted side walls of plaster board, or similar material, or even canvas, and contains a steam radiator. The natural circulation of the warm air removes the moisture the drying period is generally long, for example, one or two days. 

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