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Gypsum and Crystals in Dentistry
45 Questions
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Gypsum and Crystals in Dentistry

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@LuxuriantRubellite

Questions and Answers

Which type of gypsum is classified as dental stone with high strength and high expansion?

  • Type I Gypsum
  • Type IV Gypsum
  • Type III Gypsum
  • Type V Gypsum (correct)

    • Type II gypsum is used primarily for taking impressions.

    False

    What is produced during closed calcination?

    α-hemihydrate

    Type ___ Gypsum is classified as the Plaster Model or Plaster of Paris.

    <p>II</p> Signup and view all the answers

    What measurement tool is used to assess the initial setting time of gypsum?

    <p>Vicat needle</p> Signup and view all the answers

    Match the following types of gypsum with their primary uses:

    <p>Type I Gypsum = Impression taking Type II Gypsum = Study cast/model records Type III Gypsum = Working cast Type IV Gypsum = Die stone</p> Signup and view all the answers

    Type IV gypsum has loosely packed crystals.

    <p>False</p> Signup and view all the answers

    What shape are the crystals in α-hemihydrate gypsum?

    <p>Prismatic</p> Signup and view all the answers

    The correct pressure used during closed calcination is ___ PSI.

    <p>16</p> Signup and view all the answers

    What is the maximum temperature that can be reached during the final setting of gypsum?

    <p>30°C</p> Signup and view all the answers

    The initial setting time (I.S.T) is measured from the start of mixing until the second increase of temperature.

    <p>False</p> Signup and view all the answers

    What is the recommended soaking time for gypsum in water before mixing?

    <p>20 seconds</p> Signup and view all the answers

    The time available to use a workable mix is known as ________ time.

    <p>working</p> Signup and view all the answers

    Match the terminologies with their definitions:

    <p>Mixing Time (MT) = Time taken to mix powder and water Initial Setting Time (I.S.T) = Time until the mix reaches a semi-hard stage Working Time (WT) = Time available to use a workable mix Final Setting Time (F.S.T) = Time until the cast is fully set and can be separated</p> Signup and view all the answers

    What is the main purpose of setting time in dental materials?

    <p>To prevent premature separation</p> Signup and view all the answers

    How long does mechanical mixing generally take to complete?

    <p>30 seconds</p> Signup and view all the answers

    Final setting time is typically 20 minutes for gypsum products.

    <p>True</p> Signup and view all the answers

    A lower water-powder ratio tends to result in shorter mixing times and greater setting expansion.

    <p>False</p> Signup and view all the answers

    What is the significance of knowing the final setting time (F.S.T)?

    <p>Cast separation</p> Signup and view all the answers

    What effect does increasing the amount of spatulation have on setting time?

    <p>It shortens the setting time.</p> Signup and view all the answers

    Hand spatulation generally requires at least ________ to obtain a smooth mix.

    <p>one minute</p> Signup and view all the answers

    The ideal W/P ratio for high-strength dental stone is typically between __________.

    <p>0.18 - 0.22</p> Signup and view all the answers

    What is the significance of knowing the initial setting time (I.S.T)?

    <p>Time to trim off excess material</p> Signup and view all the answers

    Which of the following factors affects setting expansion?

    <p>W/P ratio</p> Signup and view all the answers

    Chemical modifiers tend to increase setting expansion in dental materials.

    <p>False</p> Signup and view all the answers

    List one chemical modifier that can be used in dental materials.

    <p>Potassium sulfate or borax.</p> Signup and view all the answers

    Setting expansion is usually measured through a __________ test.

    <p>penetration</p> Signup and view all the answers

    Match the type of material with its water-powder ratio:

    <p>Plaster Impression = 0.50 - 0.75 Dental Stone = 0.28 - 0.30 Die Stone = 0.22 - 0.24 High Strength Dental Stone = 0.18 - 0.22</p> Signup and view all the answers

    The setting expansion for die stone is 0% to 0.10%.

    <p>True</p> Signup and view all the answers

    What is the effect of accelerators on the setting time of gypsum products?

    <p>They speed up the setting time</p> Signup and view all the answers

    Retarders create a surface coating of crystals that makes gypsum less soluble to water.

    <p>True</p> Signup and view all the answers

    What is the primary result of the interaction and entanglement of gypsum crystals?

    <p>Strength and hardness</p> Signup and view all the answers

    The best accelerator for gypsum products is __________ at concentrations greater than 2%.

    <p>K2 SO4</p> Signup and view all the answers

    Match the following materials with their corresponding compressive strength (MPA):

    <p>Laboratory plaster = 27.5 MPA Dental Stone = 62 MPA Die Stone = 79 MPA</p> Signup and view all the answers

    What happens to the setting expansion when the concentration of NaCl increases?

    <p>Setting expansion decreases</p> Signup and view all the answers

    The dry strength of gypsum is twice the wet strength after 24 hours.

    <p>True</p> Signup and view all the answers

    What should be done to prolong the shelf-life of gypsum products?

    <p>Store in an airtight container</p> Signup and view all the answers

    A higher water-to-powder (W/P) ratio generally results in __________ setting expansion.

    <p>decreased</p> Signup and view all the answers

    Which of the following is a commonly used retarder for gypsum products?

    <p>Borax</p> Signup and view all the answers

    What happens to the strength of a cast when the water/powder (w/p) ratio increases?

    <p>It decreases</p> Signup and view all the answers

    Dry strength of a cast is less than wet strength.

    <p>False</p> Signup and view all the answers

    What should be used for disinfecting casts according to the guidelines?

    <p>1:10 dilution of sodium hypochlorite solution</p> Signup and view all the answers

    The __________ strength of a cast is twice that of the wet strength.

    <p>dry</p> Signup and view all the answers

    Match the following properties of a good cast with their descriptions:

    <p>A = Must have high strength B = Should be hard C = Accurate and stable D = Compatible with impression material E = Good color contrast F = Should be smooth and free of bubbles</p> Signup and view all the answers

    What is the effect of adding both accelerators and retarders to gypsum products?

    <p>It lowers both wet and dry strength</p> Signup and view all the answers

    Study Notes

    Types of Gypsum Products

    • Large crystals viewed microscopically appear spongy and irregular in shape.
    • Type IV Gypsum: High-strength dental stone suitable for detailed dental work.
    • Type V Gypsum: High-strength dental stone with high expansion, ideal for investment models.

    Calcination Process

    • Closed calcination occurs in an autoclave at 16 PSI and 120° to 130°C.
    • This process produces α-hemihydrate, characterized by smaller, regularly shaped crystalline particles.

    ADA Classification of Gypsum Products

    • Type 1: Impression plaster, used for taking impressions.
    • Type 2: Plaster of Paris, used for making study casts and records.
    • Type 3: Dental stone, stronger than plaster, used as a working cast that can withstand applied forces.
    • Type 4: Die stone, known for high strength and lower expansion.
    • Type 5: Dental stone, high strength and high expansion, used as an investment model.

    Instruments for Measuring Setting Times

    • Vicat needle measures initial setting time (1/4 lb).
    • Gillmore needle measures final setting time (1/4 lb for initial and 1 lb for final setting).

    Exothermic Setting Reaction

    • Maximum temperature reached during final hardening can hit 30°C at the center.
    • Temperature increase duration depends on material bulk and thermal properties.

    Mixing and Setting Times

    • Gypsum powder should initially soak in water for 20 seconds.
    • Mechanical mixing is ideal for 20-30 seconds while hand spatulation should require about one minute.
    • Initial setting time is tracked from mixing until semi-hard stage (7-13 minutes).
    • Final setting time is observed until the temperature decreases a second time (approximately 20 minutes).

    Importance of Initial and Final Setting Times

    • Initial Setting Time (I.S.T) allows for trimming excess plaster or stone.
    • Final Setting Time (F.S.T) prevents premature separation of the cast, avoiding breakage.

    Water-Powder Ratio (W/P Ratio)

    • Affects compressive strength and setting expansion; less ratio means longer mixing time and greater setting expansion.
    • Varying W/P ratios lead to different compressive strengths across types, with dental stone having the highest strength.

    Practical Factors Influencing Setting

    • High hygroscopic setting expansion corresponds to gypsum immersion in water.
    • Accelerators (e.g., potassium sulfate) speed up setting, while retarders (e.g., borax) slow it down.

    Strength Measurement

    • Strength and hardness are determined by the crystal interaction and entanglement.
    • Measured in terms of compressive strength, laboratory plaster stands at 27.5 MPA and dental stone at 62 MPA.

    Care of Gypsum Products

    • Store in airtight containers to prevent deterioration.
    • Shake the container before use and utilize dry spoons to avoid moisture contamination.

    Properties of an Ideal Cast

    • Should exhibit high strength to reduce the chance of fracture.
    • Must have a hard surface to prevent scratches during carving.
    • Accurate measurement and stability are imperative to control setting expansion.
    • Should be compatible with impression materials to avoid toxic reactions.

    Disinfecting Solutions for Casts

    • Follow manufacturer's guidelines or immerse in a 1:10 dilution of sodium hypochlorite for 30 minutes.
    • Non-disinfected models can be aseptically wrapped and sterilized using ethylene oxide in clinical settings.

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    Description

    Explore the fascinating world of gypsum and its various forms in dental applications. This quiz covers large crystals, pores, and the specific types of gypsum used for high strength and expansion in dental stones. Test your knowledge on the closed calcination process and its significance in creating dental materials.

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