Gypsum

Dental plasters are used for many purposes. These mainly include imprinting and preparation of models. We rank them among model materials, impression plaster belongs to imprinting materials.

Composition
$$CaSO_4 \cdot \frac{1}{2}H_2 O$$ It is a calcium sulfate hemihydrate. But other substances are added to it.
 * Potassium sulfate – reduction of expansion, acceleration of solidification,
 * borax – retarder to prolong the setting time,
 * diatomaceous earth, quartz, limestone – improves brittleness.

Advantages and disadvantages of impression plaster
Advantages: Disadvantages:
 * low price,
 * ease of preparation,
 * very good reproduction of surface details,
 * Excellent dimensional stability.
 * rigidity
 * difficult to remove from undercut spots,
 * inaccuracy
 * drying of the mucous membrane of the patient's mouth.

==== Types ==== Types I and II are so-called β-positionswires and types III and IV are so-called α-positionswires
 * Type I – impression plaster.
 * Mixing ratio: powder 100 g, water 40–70 ml.
 * Setting time: 4 minutes.
 * Strength: 4 MPa.
 * Expansion two hours after solidification: maximum 0,15 %.
 * Application: preprints for laboratory production of total removable dentures, fastening models to articulation apparatus (they can be easily removed than when using another type of plaster).
 * Type II – alabaster gypsums.
 * Mixing ratio: powder 100 g, water 60 ml.
 * Processing time: 6-7 minutes.
 * Setting time: 12–14 minutes (16–20 minutes).
 * Strength: 9 MPa.
 * Solidification expansion: 0,1 %.
 * Application: making preliminary, situational models, connecting the working model to the articulator (for example, when processing a full-shell cast crown).
 * Type III – hydrocals.
 * Mixing ratio: powder 100 g, water 28–32 ml.
 * Processing time: 4-7 minutes.
 * Setting time: 8–14 minutes (12–16 minutes).
 * Strength: 20.7 MPa.
 * Solidification expansion: 0.14–0.20%.
 * Application: making of working models without the need for maximum strength and low abrasion.
 * Type IV – density/stone.
 * Mixing ratio: powder 100 g, water 22–23 ml (21–25 ml of water).
 * Processing time: 5-6 minutes.
 * Setting time: 10 minutes (12–16 minutes).
 * Strength: 34.5 MPa.
 * Solidification expansion: 0,1 %.
 * Application: precise and strenuous work in the laboratory.
 * Type V – stone .
 * Mixing ratio: powder 100 g, water 18–20 ml.
 * Setting time: 12–16 minutes.
 * Strength: 48.3 MPa.
 * Solidification expansion: 0.1–0.3%.
 * Field of application: models for the formation of insertions and replacements from base metal alloys (used to compensate for their contraction).
 * Special plasters:
 * orthodontic gypsums,
 * plaster for assembling models,
 * fast-setting gypsum,
 * gypsums that change color when solidifying.

Related articles

 * Modelové a modelovací hmoty

Reference
