Gypsum Products Dental Materials Question And Answers

Gypsum Products

 

Question 1. Write a short note on the impression plaster.
Answer:

Impression plaster is a gypsum product. It is also known as type I or dental plaster or impression plaster. Impression plaster was one of the earliest impression materials in dentistry

Composition of impression plaster:

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Manufacture and Setting Reaction:

The powder form of plaster is manufactured by calcination in which gypsum is subjected to high temperatures to remove water content from it. This leads to conversion of calcium sulphate dehydrate to calcium sulphate hemihydrates in which other modifiers are added.

• CaSO₄. 1/2 H₂O + 3H₂O CaSO₄.2H₂O + 3900 Cal/gmol

Manipulation of impression plaster:

The premeasured powder is sifted into a flxible bowl which consists of a premeasured quantity of water and the material is mixed with a straight stainless steel spatula till a smooth mix is obtained. As the water/powder ratio is slightly high, it is not possible to load a thin consistency mix onto a stock tray so a closely adapting custom tray made of either acrylic resin or shellac base plate is used.

Once the impression is made to pour a cast, a separating medium such as varnish, lacquer, or detergent soap are needed since both the cast material and the impression material used are gypsum products. After about 30 to 60 minutes, the cast should be removed from the impression material by immersing the assembly into hot or warm water.

Advantages of impression plaster:

• Its hydrophilic nature and thin consistency allows it to flow and record fine details with least pressure.
• Dimensional changes are very minimum as compared to hydrocolloids.
• It is economical as the manipulation of material does not require any costly equipment.

Disadvantages of impression plaster:

• The taste is unpleasant.
• Due to the water sorption nature, it produces an increased dry sensation in the mouth of the patient.
• Gag reflux can cause increase fluidity during impression-making.
• It cannot be used in undercuts because of brittleness.
• Material can irritate the underlying soft tissues due to heat evolved during the settling process of dental plaster.
• At times separation from the cast is difficult if separating medium is not applied.

Uses of impression plaster:

• To record completely edentulous arches and regions.
• Used in dental soldering procedures for assembling and recording the relationship of crowns and politics.
• As bite registration material.