Podcast
Questions and Answers
What is the chemical formula for the gypsum used in dental applications?
What is the chemical formula for the gypsum used in dental applications?
- CaSO4⋅2H2O (correct)
- CaSO4⋅1/2 H2O
- CaSO4
- CaSO4⋅3H2O
What is the result of open calcination of gypsum?
What is the result of open calcination of gypsum?
- Calcium Carbonate
- Calcium Sulfide
- Calcium Sulphate Dihydrate
- Calcium Sulphate Hemihydrate (correct)
What temperature range is used in open calcination?
What temperature range is used in open calcination?
- 100-110°C
- 130-140°C
- 120-130°C
- 110-120°C (correct)
Which characteristic is not associated with Plaster of Paris?
Which characteristic is not associated with Plaster of Paris?
What is the impact of adding 30% calcium chloride to gypsum?
What is the impact of adding 30% calcium chloride to gypsum?
What is the primary process used to produce α-hemihydrate/Dental Stone?
What is the primary process used to produce α-hemihydrate/Dental Stone?
Which type of gypsum is referred to as Dental Stone?
Which type of gypsum is referred to as Dental Stone?
What defines the crystal shape of β-hemihydrate compared to α-hemihydrate?
What defines the crystal shape of β-hemihydrate compared to α-hemihydrate?
Which property is not associated with Type IV Gypsum?
Which property is not associated with Type IV Gypsum?
What is the purpose of the Vicat needle in relation to gypsum products?
What is the purpose of the Vicat needle in relation to gypsum products?
What is the purpose of the Gillmore needle?
What is the purpose of the Gillmore needle?
What occurs during the exothermic setting reaction of gypsum?
What occurs during the exothermic setting reaction of gypsum?
How long should gypsum be soaked in water before mixing?
How long should gypsum be soaked in water before mixing?
What is the approximate temperature rise during the setting process?
What is the approximate temperature rise during the setting process?
What is the working time (initial setting time) for a workable mix?
What is the working time (initial setting time) for a workable mix?
How long does the final setting time typically take?
How long does the final setting time typically take?
What is the main purpose of the initial setting time (I.S.T)?
What is the main purpose of the initial setting time (I.S.T)?
What should occur after the final setting time (F.S.T)?
What should occur after the final setting time (F.S.T)?
What defines the setting time of gypsum material?
What defines the setting time of gypsum material?
What is the standard time for mechanical mixing of gypsum?
What is the standard time for mechanical mixing of gypsum?
What happens to gypsum products when they are allowed to set while immersed in water?
What happens to gypsum products when they are allowed to set while immersed in water?
Which substance is considered the best accelerator for gypsum products when used at concentrations greater than 2%?
Which substance is considered the best accelerator for gypsum products when used at concentrations greater than 2%?
What effect does fineness of the gypsum crystals have on setting time?
What effect does fineness of the gypsum crystals have on setting time?
How does increasing the water-to-powder ratio affect setting time?
How does increasing the water-to-powder ratio affect setting time?
Which of the following are classified as retarders that create a surface coating of crystals?
Which of the following are classified as retarders that create a surface coating of crystals?
What happens when gypsum is subjected to temperatures above 50°C?
What happens when gypsum is subjected to temperatures above 50°C?
How does particle size affect the setting time of gypsum products?
How does particle size affect the setting time of gypsum products?
What is the relationship between the water/powder ratio and setting expansion?
What is the relationship between the water/powder ratio and setting expansion?
What is the setting time effect of using Type I Impression Plaster?
What is the setting time effect of using Type I Impression Plaster?
Which of the following is true about the wet strength of gypsum products compared to dry strength?
Which of the following is true about the wet strength of gypsum products compared to dry strength?
What is the consequence of having too thick a mixture in gypsum setting?
What is the consequence of having too thick a mixture in gypsum setting?
How should gypsum products be stored to prolong their shelf life?
How should gypsum products be stored to prolong their shelf life?
What is the typical mixing time for gypsum products?
What is the typical mixing time for gypsum products?
What should be done before using powder from a bottle of gypsum product?
What should be done before using powder from a bottle of gypsum product?
Which of the following is true about setting expansion?
Which of the following is true about setting expansion?
What is the primary characteristic of Type V High Strength, High Expansion Dental Stone?
What is the primary characteristic of Type V High Strength, High Expansion Dental Stone?
Study Notes
Gypsum Overview
- Gypsum is composed of calcium sulfate dihydrate (CaSO4⋅2H2O).
- Commonly used in dental applications includes dental plaster and dental stone.
- Ground gypsum can exist as solid particles or powder.
Calcination Processes
-
Open Calcination:
- Conducted at 110-120°C (230-250°F).
- Produces β-hemihydrate, known as Plaster of Paris, characterized by irregular orthorhombic crystals.
-
Closed Calcination:
- Performed in an autoclave under 16 PSI at 120-130°C.
- Yields α-hemihydrate, known as Dental Stone, with smaller, prismatic crystals.
ADA Classification of Gypsum Products
- Type 1: Impression/Soluble Plaster - Used for impressions.
- Type 2: Plaster Model/Plaster of Paris - Used for models and records.
- Type 3: Dental Stone (Class 1) - Stronger and used as working casts.
- Type 4: Die Stone - High strength, low expansion, used for precise applications.
- Type 5: High Strength High Expansion Dental Stone - Used in investment models.
Gypsum Characteristics
- Crystals are differentiated by size, shape, and packing:
- β-hemihydrate has larger, irregular crystals with loose packing.
- α-hemihydrate features smaller, more regularly shaped crystals, tightly packed.
Measuring Setting Times
- Initial Setting Time (IST): Time from mixing until reaching a semi-hard state typically within 7-13 minutes.
- Final Setting Time (FST): Time until the material achieves its final hardness, usually around 20 minutes.
Exothermic Setting Reaction
- Temperature during final hardening can rise up to 30°C, depending on the mass of material used.
Mixing and Working Time
- Mixing Time: Approximately 20-30 seconds for mechanical mixing; hand spatulation requires at least one minute.
- Working Time: Generally allows around 3 minutes of usable time after mixing.
Chemical Reaction of Gypsum Setting
- Setting reaction involves the conversion of calcium sulfate hemihydrate to calcium sulfate dihydrate with heat release:
- H2O + CaSO4⋅1⁄2 H2O → CaSO4⋅2H2O + Heat
Factors Affecting Setting Time
- Purity and Fineness: Smaller particles dissolve quicker, leading to faster setting times.
- Water-Powder Ratio: More water leads to longer setting times; less water may cause incomplete mixing.
- Additives:
- Accelerators (e.g., K2 SO4) can shorten setting time.
- Retarders (e.g., Borax) can prolong it.
Strength and Hardness
- Measured compressive strength varies:
- Laboratory plaster: 27.5 MPa (4000 PSI)
- Dental Stone: 62 MPa (9000 PSI)
- Die Stone: 79 MPa (11500 PSI)
Care of Gypsum Products
- Store products in airtight containers to maintain integrity.
- Shake containers before use and utilize clean, dry utensils to avoid contamination.
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Description
This quiz explores the properties and applications of gypsum, particularly in dental practices. It discusses the differences in types such as dental plaster and dental stone, highlighting their strength and surface characteristics. Understand how gypsum is utilized in impression materials and dental solutions.
