gypsum products and its uses in dentistry

Gypsum is a naturally occurring mineral, chemically known as calcium sulfate dihydrate (CaSO₄·2H₂O).

In dentistry, gypsum products are among the most widely used materials, primarily for making casts, models, and molds.

They are obtained by controlled heating (calcination) of gypsum to form calcium sulfate hemihydrate (CaSO₄·½H₂O), which, when mixed with water, rehydrates back to the dihydrate, hardening into a rigid solid.
Setting time: Controlled by temperature, additives, and water–powder ratio.

Setting expansion: Slight expansion occurs due to crystal growth. (Plaster > Stone > Die stone).

Strength: Directly related to density → lower water/powder ratio → higher strength.

Dimensional stability: Susceptible to moisture; requires proper storage.

Compatibility: Compatible with most impression materials, but can be inhibited by alginate if not properly treated.
According to the American Dental Association (ADA), gypsum products are classified into five main types:

Type I – Impression Plaster

Rarely used now.

Has low setting expansion.

Historically used for impression making (especially edentulous ridges).

Brittle, lacks accuracy, replaced by elastomers.

Type II – Model Plaster (Plaster of Paris)

Produced by open kettle heating (irregular, porous crystals).

Used for study casts, mounting casts on articulators, and flasking dentures.

Weakest, least abrasion resistant.

White in color, relatively fast setting.

Type III – Dental Stone

Made by calcination under steam pressure → denser, more regular crystals than plaster.

Stronger and harder than Type II.

Used for diagnostic casts, opposing casts, and removable prosthesis work.

Yellow commonly.

Type IV – Dental Stone, High Strength/Low Expansion (Die Stone)

Produced by calcination in presence of CaCl₂ (modified process).

Very dense, small, regular crystals.

High strength and abrasion resistance.

Low setting expansion → ideal for fabricating dies for crown and bridge work.

Type V – Dental Stone, High Strength/High Expansion

Similar to Type IV, but designed to compensate for casting shrinkage of newer high-fusing alloys.

High strength and higher setting expansion (0.1–0.3%).

Commonly reddish or green.

3. Chemical Setting Reaction

When hemihydrate is mixed with water:

𝐶
𝑎
𝑆
𝑂
4
⋅
1
2
𝐻
2
𝑂
+
1.5
𝐻
2
𝑂
→
𝐶
𝑎
𝑆
𝑂
4
⋅
2
𝐻
2
𝑂
+
Heat (exothermic)
CaSO
4
​

⋅
2
1
​

H
2
​

O+1.5H
2
​

O→CaSO
4
​

⋅2H
2
​

O+Heat (exothermic)

Reaction is exothermic (releases heat).

The set gypsum is calcium sulfate dihydrate again.
Applications in Dentistry

Impression plaster: Rare, historical.

Model plaster (Type II): Study models, orthodontic casts, flasking procedures.

Dental stone (Type III): Working casts for dentures, opposing casts.

Die stone (Type IV & V): Fabrication of dies for inlays, crowns, bridges, implant prosthesis.

Investment material: Modified gypsum (with silica) used in casting gold alloys.
Advantages

Easy to manipulate.

Accurate in reproductions

1. GUIDED BY : PRESENTED BY : Dr. VIVEK CHOUKSE Dr. KASHIVISHWESHWAR KASTURE Dr. DURGARAJU MACHA PG PROSTHODONTICS Dr. ASHWIN AIDASANI Dr. ABHAY NARAYANE Dr. RANJEET GANDAGULE Dr. ABHILASHA MASIH Dr. KRISHNA NANDA 1 GYPSUM PRODUCTS

2. CONTENTS INTRODUCTION CLASSIFICATION AND USES TYPES CALCINATION AND MANUFACTURING CHEMICAL NATURE AND MANIPULATION THEORIES OF SETTING PROPERTIES WATER POWDER RATIO USES IN DENTISTRY AND PROSTHODONTICS INVESTMENT MATERIAL DIE MATERIAL STORAGE AND CARE 2

3. INTRODUCTION The mineral gypsum has commercial importance as a source of plaster of Paris. The term plaster of Paris was given this product because it was obtained by burning the gypsum from deposits near Paris, France. The dihydrate form of calcium sulfate, called gypsum, usually appears white to milky yellowish and is found in a compact mass in nature. 3

4. 4 The primary applications of gypsum products in dentistry include the production of study models for oral and maxillofacial structures and their use as auxiliary materials for dental laboratory operations involved in the production of dental prostheses. They are also used to form models and casts on which wax dental prostheses and restorations are constructed

5. CLASSIFICATIONS According to ANSI/ADA specification No. 25 (ISO 6873 :2013) Type 1 - Impression plaster / dental plaster for impressions Type 2 - Model plaster/ laboratory plaster class I – for mounting class II – for models Type 3 - L ow- to moderate-strength dental stones/ Hydrocal /US Gypsum Company/class I stone Type 4 - High-strength/low-expansion dental stones/Die stone/ Densite /Improved stone/ class II stone Type 5 - High-strength/high-expansion dental stones/ crystacal 5

6. Differences between α and ß- hemihydrate α - Hemihydrate (Stone) ß- Hemihydrate (Plaster ) C onsist of cleavage fragments Fibrous aggregate of fine and crystals in the form of rods or prisms crystals with capillary porous. Manufacturing process : Wet Manufacturing process : Dry calcination calcination Crystals are prismatic and more Crystals are spongy, irregular regular in shape and compact. in shape and porous. W/P Ratio: 0.3 W/P Ratio: 0.5 Set gypsum is stronger and Set gypsum is of lower harder. Strength. High abrasion resistance Low abrasion resistance. 6

7. 7 A, Plaster of Paris (β-hemihydrate particles); crystals are spongy and irregular in shape. B, Dental stone (α-hemihydrate); crystals are prismatic and more regular in shape than those of plaster C ,Improved stone (modified α-hemihydrate); the cube-shaped particles have a reduced surface area that improves the flow of the mixture.

8. CALCINATION OF GYPSUM The process of dehydration of gypsum is called as calcination where removal of water is by heating. The gypsum is ground and subjected to temperatures of 110 to 130 C (230 to 250 F) to drive off part of water of crystallization. As the temperature is further increased, the remaining water of crystallization is removed. 8

9. 9 Open kettle / dry calcination High steam under pressure / wet calcination Beta type of gypsum / plaster Type I and type II gypsum Temp – 120-180 degree celcius Crystals don’t get reorganized Low density Increase surface area Poor packing ability Need more amount of water (50ml for 100gm) Alpha type of gypsum / stone Type III ( dental stone) Allow complete solution conversion and recrystallisation Increased density Decrease surface area Good packing ability (regular particles) Require less water for packing (30ml for 100gm) Pressure + heat + 30%cacl2 Die stone , improved die stone Product is more denser Caf2 and mgf2 act as deflocculant and don’t allow particles to stick together

10. 10

11. 11 CaSO 4 . 2H 2 O Gypsum (calcium sulfate dihydrate ) CaSO 4 . ½ H 2 O Plaster or stone (calcium sulfate hemihydrate CaSO 4 Hexagonal anhydrite CaSO 4 Orthorhombic anhydrite 110 -130 C 130 -200 C 200 -1000 C Manufacturing of gypsum

12. CHEMICAL NATURE OF GYPSUM - gypsum is calcium sulphate dihydrate and occurs naturally at many sites around the world. - dental plaster is provided in the hemihydrate form. once water is added to this, the hemihydrates reverts to the dihydrate with the liberation of heat. 12

13. 13

14. MANIPULATION OF GYPSUM ARMAMENTARIUM Rubber bowl Spatula 14 2 Techniques MANUAL MANIPULATION VACUUM MIXER

15. Manual manipulation of gypsum 15

16. WATER POWDER RATIO 16

17. THEORIES OF SETTING OF GYPSUM Setting reaction of gypsum 1g mol of calcium sulphate hemihydrate + 1.5g mol of water Hemihydrate reverts to dihydrate 1 g mol of calcium sulphate dihydrate by hydration + heat liberation Various theories put forth based on this concept as follows 17

18. THEORIES OF SETTING COLLOIDAL THEORY HYDRATION THEORY DISSOLUTION–PRECIPITATION THEORY 18

19. COLLOIDAL THEORY Put forward by MAHAELIS in 1893 SOL GEL MECHANISM 19

20. HYDRATION THEORY SOLID SET MASS 20

21. DISSOLUTION – PRECIPITATION THEORY Put forth by HENRY LOUIS LE CHATELIER in 1887 Most widely accepted theory Based on the principle of gradual dissolution of hemihydrate and instant crystallization of dihydrate forming set mass with interlocking crystals 21

22. The precipitation of dihydrate occurs because of difference in the solubility of dihydrate and hemihydrate forming crystallization of dihydrate hence called CRYSTALLINE THEORY Crystals of dihydrate called SPHERULITES (needle like appearance) 22

23. PROPERTIES OF GYPSUM 23 SETTING TIME COMPRESSIVE STRENGTH TENSILE STRENGTH SETTING EXPANSION HYGROSCOPIC EXPANSION

24. COMPRESSIVE STRENGTH AND TENSILE STRENGTH 24 COMPRESSIVE STRENGTH When a body is placed under a load that tends to compress or shorten it, the internal resistance to it is called compressive strength. TENSILE STRENGTH A load that tends to stretch or elongate the body, the internal resistance to this is called tensile strength.

25. 25

26. 26

27. 27

28. MIXING TIME The time from addition of powder to the water until mixing is completed is called the mixing time. Mechanical mixing is usually completed in 20 to 30 seconds. Hand spatulation generally requires at least a minute to obtain a smooth mixture. WORKING TIME The time from the start of mixing to the point where the consistency is no longer acceptable for the product’s intended purpose is the working time. Generally, a 3-minute working time should allow sufficient time for mixing, pouring an impression before the gypsum becomes unworkable. 28

29. MEASUREMENT OF SETTING TIME LOSS OF GLOSS METHOD As reaction proceeds, the gloss disappears form the surface of plaster mix (Sometimes used to indicate initial set). EXOTHERMIC REACTION The temperature rise of the mass may also be used for measurement of setting time, as the setting reaction is exothermic. . 29

30. TYPES OF PENETROMETERS VICAT NEEDLE GILLMORE NEEDLE INTITAL GILLMORE TEST FOR INTIAL SET The smaller 113.4 gms (1/4 th pound) needle has a 2.13mm point(1/12 th inch in diameter). The time at which it no longer leaves an impression is called the initial set. Noted as ‘initial gillmore ’. It indicates partial progress of a setting reaction. 30 PENETRATION TESTS By using penetrometers

31. INITIAL SETTING TIME When the mix can resist penetration by a Gillmore needle, which has a tip 2.12 mm in diameter and weighs 113.4 g, the time elapsed is called the initial setting time. FINAL SETTING TIME The elapsed time at which a heavier Gillmore needle, weighing 453.6 g and with a tip 1.06 mm in diameter, leaves only a barely perceptible mark on the surface is called the final setting time. 31

32. GILLMORE TEST FOR FINAL SETTING TIME It is measured by the use of the heavier gillmore needle - 453.6 gms (1pound) in weight and 1.06mm in diameter(1/24 th inch) The elapse time at which this needle leaves only a barely perceptable mark on the surface is called the Final Setting Time The rod holding the needle weighs 300 gms , the needle is 1 mm in diameter and 5cm long VICAT TEST FOR SETTING TIME 32

33. 33 VICAT NEEDLE APPARATUS GILLMORE NEEDLES

34. SETTING EXPANSION The setting expansion without water immersion is usually called normal setting expansion HYGROSCOPIC EXPANSION The expansion that occurs under water is known as hygroscopic setting expansion The hygroscopic setting expansion is a physical phenomenon and is not caused by a chemical reaction any more than is the normal setting expansion. 34

35. APPLICATION IN DENTISTRY AND PROSTHODONTICS Impression materials Study models Casts and die materials Investment materials – plaster with silica to form refractory moulds into which molten metal is cast Mounting onto articulators Bite registration – to record centric jr 35

36. INVESTMENT MATERIAL Dental casting investment A material consisting principally of an allotrope of silica and a bonding agent; the bonding substance may be gypsum (for use in lower casting temperatures) or phosphates and silica (for use in higher casting temperatures) GPT9 Investing The process of covering or enveloping, wholly or in part, an object such as a denture, tooth, wax form, crown, etc., with a suitable investment material before processing, soldering, or casting GPT9 36

37. Basic composition 37

38. Types Of Investment Materials 38

39. GYPSUM BONDED INVESTMENT TYPE 1 Used for casting inlays & crowns. casting shrinkage is compensated by thermal expansion of investment TYPE 2 used for casting inlays & crowns casting shrinkage is compensated by hygroscopic expansion of investment TYPE 3 Used for construction of partial denture with gold alloys 39

40. Composition Of Gypsum Bonded Investment Refractory material (silica) 55-65% Binder ( -hemihydrate ) 25-35% Modifying agents 2-3% 40

41. REFRACRTORY MATERIALS Silica exists in 4 allotrophic forms: QUARTZ TRYDYMITE CRISTOBALLITE FUSED QUARTZ 41

42. α QUARTZ ß QUARTZ α CRISTOBALLITE ß CRISTOBALLITE α TRIDYMITE MIDDLE TRIDYMITE ß TRIDYMITE Effect of temperature on refractory material 42 575 C 200-270 C 117 C 163 C

43. BINDER MATERIAL  - hemihydrate form of gypsum Effect of temperature on binder : 200  C to 400C – Shrinkage 400 C to 700C - Slight expansion Above 700C – large contraction 43

44. MODIFIERS REDUCING AGENT carbon or copper powder. -Provides non- oxidising atmosphere in mold -Reduces any oxide formed on metal ADDED MODIFIERS eg. boric acid & sodium chloride -Regulate setting expansion and setting time COLOURING AGENT 44

45. MANIPULATION OF GYPSUM BONDED INVESTMENT Hand Spatulation Mechanical mixing or vacuum mixing 45

46. SETTING TIME According to ADA Specification No. 2 Setting time should not be less than 5 minutes and not more than 25 minutes. Modern investments – 9 to 18 minutes SETTING EXPANSION NORMAL EXPANSION HYGROSCOPIC EXPANSION THERMAL EXPANSION 46

47. NORMAL SETTING EXPANSION Aids in enlarging the mold to compensate partially for the casting shrinkage of the gold. ADA Specification No. 2 Type I Maximum setting expansion 0.6%. Modern investment material is 0.4%. 47

48. HYGROSCOPIC SETTING EXPANSION - occurs when gypsum product is allowed to set under or in contact with water. - 6 or more times the normal setting expansion According to ADA specification no.2 for Type II investment material Minimal setting expansion in water – 1.2% Maximal setting expansion in water – 2.2% 48

49. THERMAL EXPANSION Directly related to the amount and type of silica present . ADA specification no. 2 Must be not less than 1% nor greater than 1.6% at 700 C Must be between 0% & 0.6% at 500 C. 49

50. PHOSPHATE BONDED INVESTMENT Classification According to ADA specification No.42(old)/ 126, ISO 9694-1998 Type I : for inlays, crowns & other fixed restorations. Type II : for partial dentures & other cast, removable restorations. 50

51. Composition Filler/ Refractory - Cristoballite - Quartz Aids in: - high temperature thermal shock resistance - high thermal expansion Binder Magnesium oxide (basic) Phosphate ion (acid) Carbon – Modifier - clean castings - facilitate divesting 51

52. SETTING REACTION Ammonium phosphate reacts with magnesium oxide to give the investment green strength or room temperature strength. NH 4 H 2 PO 4 +MgO+5H 2 O NH 4 MgPO 4 + 6H 2 O ADVANTAGES H igh strengths. High thermal expansion than gypsum bonded investments Ability to withstand temperatures that reach 900 C. Altered with use of colloidal silica quantity and ratio Slight alteration in liquid to powder ratio increased expansion. 52

53. DISADVANTAGES When used at high temperatures of greater than 1375°C results in mold breakdown & rougher surfaces on castings. Make divesting a difficult & tedious task. 53

54. Ethyl Silicate-Bonded Investment ADA SPECIFICATION NO.126/91(OLD) ISO11246:1996 Also called silicate bonded investments or silica bonded investments. USED FOR Cobalt-chromium alloys Nickel-based alloys COMPOSITION Binder – silica gel Magnesium oxide- strengthener Refractory – Quartz or Cristoballite 54

55. ADVANTAGES Ability to cast high temperature cobalt- chromium & nickel chromium alloys. Attain good surface finish. Low distortion & high thermal expansion 1.6% at 600 C DISADVANTAGES Ethyl alcohol vapour liberated High temperatures may cause ignition. Complicated & time consuming procedures. More prone to cracking during the burnout & firing process. 55

56. DIE MATERIALS Die - The positive reproduction of the form of a prepared tooth in any suitable substance. GPT 9 56

57. IDEAL REQUIREMENTS OF A DIE MATERIAL Dimensional stability. Dimensional accuracy Compatibility with impression materials Abrasion and fracture resistance. (compressive strength) Color contrast with wax pattern. Ease of use. Economical. 57

58. MATERIALS USED FOR MAKING DIES Gypsum products- Type IV Dental Stone Type V Dental stone Die stone - Investment combination Electroformed dies- Silver plated Copper plated Epoxy Resins Polyurethane 58

59. ALTERNATIVE DIE MATERIALS Silicophosphate cement Amalgam Metal sprayed dies Ceramic dies 59

60. TYPE IV DENTAL STONE Used for making dies It has increased strength and hardness Setting expansion is less it is 0.1% W:P ratio is 22 to 24 ml water per 100 gms of powder Cuboidal particles and reduced surface area- increased compressive strength increased hardness Adequate Surface detail reproduction 60

61. TYPE V DENTAL STONE HIGH STRENGTH,HIGH EXPANSION Setting expansion is increased from 0.10% to 0.30%. Make excellent dies or casts. Indication: Base metal alloys Are ready for use after approx. 1 hr. Higher compressive strength than type IV dental stone. 61

62. PROPERTIES OF GYPSUM DIES 62

63. ADVANTAGES It has good strength , hardness . Compatible with all impression material Dimensionally accurate. Dimensional stability Inexpensive Good colour contrast with casting waxes. Easy to use 63 DISADVANTAGES Poor resistance to abrasion.

64. DIVESTMENT Gypsum bonded investment material that contains silica It is highly accurate for gold alloys Setting expansion : 0.9%, thermal expansion : 0.6% at 677 C Divestment Phosphate(DVP): Metal ceramic alloys. 64

65. COMPOSITION Powder : Gypsum bonded investment Liquid : Colloidal silica ADVANTAGES Greater accuracy Lesser distortion DISADVANTAGE Loss of master die during procedure 65

66. ELECTRFORMED DIES Obtained by Electroplating the impression. Electroplating - the process of covering the surface of an object with a thin coating of metal by means of electrolysis. GPT 9 Process of making a die is known as Electroforming Jacobe (1934) used it first Wanja (1937) Applied it in dentistry 66

67. The types of dies obtained by electroforming are- Copper plated dies Silver plated dies 67