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:

• low price,
• ease of preparation,
• very good reproduction of surface details,
• Excellent dimensional stability.

Disadvantages:

• rigidity
• difficult to remove from undercut spots,
• inaccuracy
• drying of the mucous membrane of the patient's mouth.

Types

• 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.

Types I and II are so-called β-positionswires and types III and IV are so-called α-positionswires

Links

Related articles

• Modeling materials

Bibliography

• DOSTÁLOVÁ, Taťjana. Fixní a snímatelná protetika. 1. edition. Praha : Grada Publishing, a.s, 2004. 220 pp. ISBN 80-247-0655-5.

• HUBÁLKOVÁ, Hana – KRŇOULOVÁ, Jana. Materiály a technologie v protetickém zubním lékařství. 1. edition. Praha : Galén, 2009. 301 pp. ISBN 978-80-7262-581-9.