The calcium sulfate hemihydrate dissolves in the mixing water. Heat is liberated, as shown by the following reaction: When any of the various types of calcium sulfate hemihydrate are mixed with water, the hemihydrate is changed back to dihydrate by the process of hydration. Gypsum-based investments are presented in Chapter 10, Materials for Fixed Indirect Restorations and Prostheses. Other types of gypsum products are produced for special uses, such as fast set, mounting of casts on articulators, and impressions. It displays higher setting expansion and is referred to as Type V stone. A newly developed high-strength stone with a higher compressive strength than that of Type IV stone is also available. High-strength stone is often referred to as Type IV stone, die stone, densite, or modified alpha-hemihydrate. This material is used because high strength and surface hardness are required during the fabrication process the fabrication of crowns is described in the next chapter. Figure 9.3 shows an example of an improved stone cast and several dies for the fabrication of crowns. High-strength stone is the strongest and most expensive of the three gypsum products, and it is used mainly for making casts or dies for crown, bridge, and inlay fabrication. This method of calcination results in a powder particle that is very dense, is cuboidal in shape, and has a reduced surface area. High-strength stone, or improved stone, is also made from gypsum by calcining the gypsum but in a calcium chloride solution. It is often referred to as alpha-hemihydrate, Type III stone, or Hydrocal. The stone is usually light tan in color, but it can be obtained in other colors. It is used mainly in making casts for diagnostic purposes and casts for complete and partial denture construction, which require greater strength and surface hardness than that of plaster. Stone is stronger and more expensive than plaster. 9.4 B) is more regular, more uniform in shape, and less porous compared to that of plaster. This method of calcination slowly releases the water of crystallization from the crystal so that the resultant powder particle ( Fig. Stone is made from gypsum by carefully controlled calcination under steam pressure in a closed container. In the past, plaster was modified for use as an impression material by the addition of chemicals and was called impression plaster (see Chapter 8, Impression Materials). Plaster is usually white in color and sometimes is referred to as beta-hemihydrate or Type II. (Courtesy of Diane Schwegler-Berry, NIOSH, Morgantown, WV.) This device simulates the patient’s occlusion and mastication process and is shown in Figures 1.8C and 11.6F these photographs illustrate the use of plaster to secure the cast to the articulator.įIGURE 9.4. It is used mainly when strength is not a critical requirement, such as preliminary casts for complete dentures and attaching casts to a mechanical device called an articulator. Plaster is the weakest and least expensive of the three gypsum products. The resulting powder consists of porous, irregular particles ( Fig. This direct and rapid heating in open air drives part of the water of crystallization from the crystal and shatters the crystal. It is manufactured by grinding the gypsum rock into a fine powder and then heating that powder in an open container. Plaster was the first gypsum product available for dentistry. The manufacturers add other chemicals to improve handling and properties. These differences in powder particles are responsible for their different properties, which make them suitable for various uses. Plaster, stone, and improved stone differ in the physical characteristics of their powder particles as a result of differing calcination methods. This process is called calcination and is shown in the following equation: They are produced as a result of heating gypsum and driving off part of the water of crystallization. Chemically, all three are calcium sulfate hemihydrate. In this chapter, three types of gypsum products are discussed: plaster, stone, and high-strength or improved stone. In homes, gypsum plaster is used to make walls in industry, it is used to make molds. Gypsum products are used in dentistry, medicine, homes, and industry. Pure gypsum is white, but in most deposits, it is discolored by impurities. Chemically, gypsum rock is calcium sulfate dihydrate (CaSO 4♲H 2O). Gypsum rock is mined, ground into a fine powder, and then processed by heating to form a variety of products. Gypsum products are made from gypsum rock, which is a mineral found in various parts of the world.
Not all gypsum products display all of these desirable properties equally. Compatibility with the impression material
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