Plaster of Paris, 3.24

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. 

Qualitative examples abound. Perfect crystals of sodium carbonate, sulfate, or phosphate may be kept for years without efflorescing, although if scratched, they begin to do so immediately. Too strongly heated or burned lime or plaster of Paris takes up the first traces of water only with difficulty. Reactions of this type tend to be autocat-alytic. The initial rate is slow, due to the absence of the necessary linear interface, but the rate accelerates as more and more product is formed. See Refs. 147-153 for other examples. Ruckenstein [154] has discussed a kinetic model based on nucleation theory. There is certainly evidence that patches of product may be present, as in the oxidation of Mo(lOO) surfaces [155], and that surface defects are important [156]. There may be catalysis thus reaction VII-27 is catalyzed by water vapor [157]. A topotactic reaction is one where the product or products retain the external crystalline shape of the reactant crystal [158]. More often, however, there is a complicated morphology with pitting, cracking, and pore formation, as with calcium carbonate [159]. 

Slip casting of metal powders closely follows ceramic slip casting techniques (see Ceramics). SHp, which is a viscous Hquid containing finely divided metal particles in a stable suspension, is poured into a plaster-of-Paris mold of the shape desired. As the Hquid is absorbed by the mold, the metal particles are carried to the wall and deposited there. This occurs equally in all directions and equally for metal particles of all sizes which gives a uniformly thick layer of powder deposited at the mold wall. 

Forming. Most refractory shapes are formed by mechanical equipment, but some very large or intricate shapes requke hand mol ding ki wooden, steel-lined molds with loose liners to permit easy removal of plaster of Paris molds. 

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. 

The reaction reverses when water is added to plaster of Paris and the mixture sets back to the dihydrate accompanied by a slight increase in volume and the evolution of heat. 

The hermhydrate (stucco) is normally produced by heat conversion of the dihydrate from which 3/2 H2O is removed as vapor. The resulting powder is also known as plaster of Paris [26499-65-0]. Stucco has the greatest commercial significance of these materials. It is the primary constituent used to fabricate products and ia formulated plasters used ia job- or shop-site appHcations. 

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. 

Mixtures containing sulfated castor oil were used to increase the lubricity of water base drilling fluids (123). Sulfated castor oil is also used in dishwashing compounds as a hand softener. A typical cleaning composition contains sodium dodecylben2ene sulfonate, sulfated castor oil, ethanol, and water. A sulfated derivative of castor oil is used as a dispersant for plaster of Paris, reducing the water needed to form a plastic slurry (124). Pesticide emulsions can be stabilized using ethoxylated castor oil (125). 

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. 

Pivalyl-l, 3-indandione, see Pindone Plaster of Paris, see Calcium sulphate Platinum Metal 1... 

Plaster of Paris Platinum, soluble salts (as Pt) Polychlorobiphenyls, see Chlorodiphenyls Polytetrafluoroethylene decomposition products Propane... 

Calcium carbonate Glycerin Mist Limestone Plaster of Paris... 

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. 

Alabastergips, m. plaster of Paris (best grade). Alakreatin, n. alacreatine. 

Formgips, m. plaster of Paris (good grade), formhaltend, a. retaining form or shape. Form-heizung, /. (Rubber) mold cure, -holz, n. plastic wood. 

Gipfelwert, m. top (or peak) value, maximum. GIpa, m. gypsum calcium sulfate plaster of Paris. >abdruck, abguss, m. plaster cast, -arbeiter, m. plasterer, gipsartig, a. gypseous. 

The reaction of lead with concrete differs from that of aluminium and of zinc in that it is not normally rapid during the early wet stage. It is, however, progressive in damp conditions, and this is said to be due to the fact that the concrete prevents the formation of a protective basic lead carbonate film on the surface of the lead. The packing of lead cables in plaster of Paris is reported to be of doubtful value in preventing corrosion from surrounding concrete. 

Metalline Nitroleum (Nitroglycerin-metal). Dynamite in which powdered red lead (Pb304) is used as an absorbent. Plaster of Paris may be added to the mixt Ref Daniel (1902), 437... 

The thin-layer technique (CA 60, 6691) utilizes aliquots of proplnt ether extract (I) and the ether soln (II) of a known mixt. II consists of nitrates of glycerol and glycol, di-Bu or di-Et phthalates, Et or Me centralites, DNT, and diphenylamine. The chromatoplates are made of 85 15 silica gel and plaster of Paris. These plates, containing spots of I and 11, are developed with 1 1 C6H6-petroleum ether, then sprayed with specific detectors by color. The method is much quicker and easier than chemical analysis and simpler than infrared spectroscopy and column chromatography... 

Hydraulic cements. These cements are formed from two constituents one of which is water. Setting comprises a hydration and precipitation process. Into this category fall Portland cement and plaster of Paris. 

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