Gypsum materials
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Gypsum Materials. DA 122 Dental Materials. Applications for Gypsum. Used to make a model from an impression Use of model determines type of gypsum used Gypsums vary in Strength Dimensional accuracy Resistance Reproduction detail Water/powder ratio Setting times. Calcination.

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Gypsum materials

Gypsum Materials

DA 122 Dental Materials


Applications for gypsum

Applications for Gypsum

  • Used to make a model from an impression

  • Use of model determines type of gypsum used

  • Gypsums vary in

    • Strength

    • Dimensional accuracy

    • Resistance

    • Reproduction detail

    • Water/powder ratio

    • Setting times


Calcination

Calcination

  • Manufacturing process used for

    gypsum production

  • Gypsum is mined rock; then

    ground into a fine powder

  • Powder is heated until specific

    amount of water is driven out

    of the gypsum

    = CALCINATION


Types of gypsum materials

Types of Gypsum Materials

See pages 859-860 in Dental Assisting text


Type i impression plaster

Type I: Impression Plaster

original impression material

rigid, fractures and breaks easily

used for edentulous models

Ratio: 60 ml water : 100 gm powder


Type ii lab plaster model plaster

Type II: Lab Plaster/Model Plaster

  • Slightly stronger than Type I

  • Used for: diagnostic casts or study models

  • Used to fabricate bleach trays, night guards, sports mouth guards

  • Used in treatment planning, measuring dentition, legal record of, comparison of before and after treatment

  • Ratio: 50 ml water : 100 gm powder


Type iii lab stone

Type III: Lab Stone

  • Stronger than plaster

  • Normally yellow (“buff”)

  • Used for: study models, working casts, models for partial and full dentures

  • Ratio: 30 ml water : 100 gm powder


Orthodontic stone

Orthodontic Stone

  • White

  • Used for: diagnosis and treatment of orthodontic cases

  • Ratio: 30 ml of water : 100 gm powder


Type iv die stone

Type IV: Die Stone

  • Positive replica of prepared tooth

  • Strong and resistant to abrasion

  • Most accuracy and detail

  • Used for: dies and where strong model is needed in casting metals or making crowns

  • Ratio:

    • 24 ml water :

      100 grams powder


Type v high strength die stone

Type V: High-strength Die Stone

  • High strength and high expansion

  • Recently added by the ADA

  • Strongest accepted gypsum product

  • Ratio: 18-22 ml water : 100 gms powder


Water powder ratio

Water : Powder Ratio

  • Less water

    • Shortens setting time; dry and crumbly

  • More water

    • Thin mixture; weaker product with air bubbles

  • After mix starts: adding water to thin or powder to thicken =weakens final product

  • Increasing water temperature = faster set with no affect on quality of final product


Armamentarium

Armamentarium

  • Flexible rubber bowl

  • Stiff narrow spatula

  • Water measuring device

  • Scale

  • Model vibrator, with protective covering

  • Room temperature water


Mixing process hydration

Mixing Process (hydration)

  • Add powder to water; let it dissolve into water

  • Slowly mix particles with spatula (20 seconds)

  • Add powder to desired creamy, putty-like thickness (peanut butter) (1 minute)

  • Material will stick to spatula when lifted from bowl, when it is proper thickness

  • Vibrate to remove air bubbles (1 minute)


Mixing techniques

Mixing Techniques

Using vibrator to remove air bubbles

Checking for proper thickness


Caution re mixing

Caution re: mixing

  • DO NOT WHIP: will add air and bubbles

  • DO NOT OVERMIX: will add weak spots to model


Setting process dihydration

Setting Process (dihydration)

  • Exothermic reaction (release of heat) drives off water to harden gypsum material

  • Material will heat up and then cool down


Setting time 40 60 minutes

Setting Time (40 – 60 minutes)

  • Initial Set:

    • Time between start of spatulation until material loses gloss

    • Firm and solid enough to handle

    • Still moist and pliable (cold and wet)

  • Final Set:

    • After all heat is driven away

    • Cool, hard and dry


Factors affecting setting time

Factors Affecting Setting Time

  • Type of gypsum powder

  • Water and air temperature

  • Water : powder ratio

  • Addition of

    • Retardants (slow down = cold water)

    • Accelerators (speed up) = warm water OR

      • SLURRY (mix of watery trimmed plaster residue)


Microbes

Microbes

  • Microbes can live within gypsum for 7 days

    • PPE very important!


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