DENTAL ANATOMY AND MORPHOLOGY: Page 1 Lecture: "Gypsum Products" ___________________________________________________________________________ CHEMISTRY OF GYPSUM PRODUCTS A. Introduction: 1. Calcium Sulfate Dihydrate = CaSO4 • (2) H2O (Gypsum) 2. Calcium Sulfate Hemihydrate = CaSO4 • (½) H2O (Impression Plaster, Model Plaster, Cast Stone, Improved Stone, Die Stone, Binder for Investment) 3. Important Properties: Consistency, Setting Time, Setting Expansion, Crushing Strength (compressive strength), and Porosity. B. Chemistry of Gypsum Product Formation: 1. Setting Reaction (Chemical Reaction):

dental use [CaSO4 • (1/2)H2O] + [(3/2)H2O] ----------> CaSO4 • (2)H2O + HEAT <---------- manufacture

Plaster,Stone,..... + Water ----------> Gypsum + HEAT 2. Setting Mechanism: a. Dissolution of hemihydrate crystals into the water. b. Nucleation and growth of dihydrate cystals from solution. c. Interlocking crystals during growth form hard set mass. 3. Manipulation Stages: a. Mixing Stage b. Working Stage c. Setting Stage C. Manufacture of Gypsum Precursors:

Plaster Powder Stone Powder Diestone Powder

"Plaster of Paris" "Hydrocal" "Densite" 1. Chemical Name: β-cal.sulf.hemi. α-cal.sulf.hemi. α-cal.sulf.hemi. 2. Formula: CaSO4• (½)H2O CaSO4•(½)H2O CaSO4•(½)H2O 3. Powder Shape: Irregular Uniform Uniform 4. Density: Porous Moderately Dense Dense 5. Production Steps: a. Temperature: Heat to 115 °C Heat to 125 °C Heat to 100 °C b. Process: Heat in air. Heat under Heat in 30% steam pressure CaCl2 solution. 6. Dental Products: Model Plaster, Cast Stone, Improved Stone Impress. Plaster Investment or Die Stone

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PROPERTIES A. Important Properties for Gypsum Products: Plaster Cast Stone Improved Stone 1. Consistency: ----- ----- ----- 2. Setting Time: 11 min 7 min 6 min 3. Setting Expansion: 0.20 % 0.10 % 0.05 % 4. Crushing Strength: 1,600 psi 3,000 psi ≥5,500 psi 5. Porosity: 35 % 20 % 10 % VARIABLES INFLUENCING PROPERTIES A. Manufacturing Variables: 1. Hemihydrate Type: 2. Particle Size: 3. Additives: a. Accelerators: Gypsum Particles, Potassium Sulfate (K2SO4) b. Retarders: Borax (Na2B4O7.10H2O), Colloids c. Fillers: Silica d. Pigments: B. User's Variables: 1. Specific Product Selection 2. Proportioning (W/P Ratio) a. Slurry Water 3. Mixing (Spatulation) a. Vigor b. Duration 4. Temperature 5. Additives C. Proportioning of Materials for Gypsum Products: (**Microstone, W/P = 28/100) Plaster Cast Stone Improved Stone 1. Water: a. Reaction Water 18 cc 18 cc 18 cc b. Extra Water 32 cc 12 cc 6 cc c. TOTAL Water 50 cc 30 cc** 24 cc 2. Powder: 100 gm 100 gm 100 gm 3. W/P Ratio: [0.50] [0.30] [0.24] D. Effects of Increases in Variables on Final Properties: (“+” = Increases length of time = postpones; “-“ = Decreases = shortens; “0” = No effect) Setting Setting Time Expansion Strength 1. Accelerators: - - - 2. Retarders: + - - 3. Particle Size: + + + 4. W/P Ratio: + - - 5. Spatulation: - + - 6. Temperature: - 0 0

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Preparation for mixing of plaster, stone, or diestone -- measuring water (cylinder) and powder (balance).

Dispensing of weighed powder into premeasured water within a mixing bowl -- allowing the powder to sift into the water.

Blending the powder and liquid together with a stropping action using a stiff bladed spatula to push the mix along the sides of the bowl.

Smooth blend of stone. Moving the mixed stone into the

alginate impression while vibrating the impression tray to help remove bubbles and encourage flow into impression details.

Microscopic view of growing gypsum crystals that produce setting by pushing against one another and become interlocked as well.

Gilmore needels (small and large) that are used to detect the onset and completion of the setting period.

Example of a stone working cast with a removable die.

Working die that has been waxed to produce an inlay pattern.

INFECTION CONTROL A. Infection Control Approaches: (1. Additives in Impression Materials) 2. Immersion of Impression Materials 3. Additives in Gypsum Products B. Effects of Disinfectants on Impression Materials: 1. Dissolution or plasticization of surfaces of impression materials. 2. Swelling of impression materials. 3. May alter reaction rates of stone. C. Effects on Gypsum Products: 1. Very effective / Some influence on set gypsum properties 2. Less wear resistant surfaces

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MULTIPLE CHOICE STUDY QUESTIONS: 1. The chemical name for GYPSUM is: a. Calcium sulfate anhydrite b. Calcium sulfate hemihydrate c. Calcium sulfate monohydrate d. Calcium sulfate sesquihydrate e. Calcium sulfate dihydrate 2. The chemical formula for GYPSUM is: a. CaSO4 • (1/2) H2O b. CaSO4 • (1) H2O c. CaSO4 • (3/2) H2O d. CaSO4 • (2) H2O e. CaSO4 • (5/2) H2O 3. The setting reaction of plaster with admixed water is: a. Endothermic b. Incapable of being chemically accelerated c. Affected by exposure to light d. Not sensitive to temperature e. Accelerated by the addition of slurry water 4. HYDROCAL is a commercial example of: a. Calcium sulfate anhydrite b. Calcium sulfate hemihydrate c. Calcium sulfate monohydrate d. Calcium sulfate sesquihydrate e. Calcium sulfate dihydrate 5. The production of STONE differs from PLASTER mainly in regard to: a. Gypsum particle size b. Gypsum particle shape c. Processing atmosphere d. Rate of heating e. Time of heating 6. The main ingredient of dental STONE is: a. Alumina b. Silica c. Calcium sulfate hemihydrate d. Starch e. Colorants 7. Compared to plaster, DIE STONE: a. Requires much less water for mixing b. Produces a more porous gypsum product c. Expands much more on setting d. Produces much less heat on setting e. Requires much less spatulation for mixing 8. Crushing strength of a set gypsum product is related primarily to: a. Water temperature during mixing of the materials b. Setting time of the mixture c. Setting expansion of the mixture d. Amount of porosity remaining in the set material e. Mixing technique

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9. The final porosity in set PLASTER is approximately: a. 15% b. 20% c. 20% d. 25% e. 35% 10. The crushing strength of DIE STONE is at least: a. 1,500 psi (10 MPa) b. 2,500 psi (17 MPa) c. 3,500 psi (24 MPa) d. 4,500 psi (31 MPa) e. 5,500 psi (38 MPa) 11. Which one of the following gypsum product mixtures will be the most fluid? a. Plaster, at W/P = 0.50 b. Stone, at W/P = 0.40 c. Stone, at W/P = 0.30 d. Improved Stone at W/P = 0.27 e. Improved Stone at W/P = 0.24 12. Which one of the following should have the highest crushing strength when it is

set and dry? a. Plaster, at W/P = 0.60 b. Plaster, at W/P = 0.50 c. Stone, at W/P = 0.50 d. Stone, at W/P = 0.40 e. Improved Stone, at W/P = 0.50 13. The best way to EXTEND the working time of a STONE mix in the laboratory is to: a. Increase the W/P ratio, and increase the spatulation b. Increase the spatulation, and decrease the temperature c. Increase the temperature, and decrease the W/P ratio d. Decrease the W/P ratio, and add a retarder e. Decrease the temperature, and decrease the spatulation 14. The setting reaction of plaster, stone, and/or diestone: a. Requires the dissolution of the powder particles in the water b. Involves the adsorption of the water in the mixture into the powder particles c. Depends on the presence of chemical modifiers to occur d. Does not involve calcium sulfate hemihydrate e. Produces a major change in volume of the mixture during setting 15. The setting expansion for improved stone (DIE STONE) is: a. 0.05 % b. 0.10 % c. 0.20 % d. 1.00 % e. 2.00 % 16. The setting reaction of plaster is accelerated by all of the following EXCEPT: a. Increased temperature b. Rapid spatulation c. Low relative humidity d. Slurry water e. Potassium sulfate

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17. Which of the following does NOT retard the setting of STONE? a. Borax b. Colloids c. Blood d. Reduced temperature e. Slurry water 18. What is the theoretical amount of water required to react with 100 gms of calcium

sulfate hemihydrate to form calcium sulfate dihydrate? a. 15 cc b. 18 cc c. 24 cc d. 30 cc e. 50 cc 19. Slurry water is: a. Special accelerator solution sold with plaster, stone, and die stone b. Dilute solution of calcium sulfate dihydrate crystals that are nucleating agents c. 5% solution of potassium sulfate in water and glycerin d. Dirty water e. The name of the mixture of powder with liquid before setting occurs 20. Plaster powder exposed to air for long periods of time before use reacts differently

because: a. Humidity is absorbed into the powder and pre-reacts with it b. Oxygen is absorbed onto the particles and retards the reaction c. Oxygen degrades calcium sulfate hemihydrate into a non-hydrated form d. Dirt in the air becomes mixed with the powder and acts as nucleating agents e. Oxygen degrades the accelerators and retarders in the mixture © 2004-2005, Stephen C. Bayne, Chapel Hill, NC 27514