1. Gypsum products
and investment

2. Most gypsum products are optained from natural gypsum rock.
It’s the dihydrate form of Calsium sulfate ( CaSo4.2H2O)
On heating , it loses 1.5 g mol of its 2 g mol of water and is converted to calsium sulfate hemihydrate ( CaSo4. ½ H2O).
When hemihydrate form mixed with water , the reverse reaction takes place.
CaSo4. ½ H2O+ 11/2 H2O CaSo4.2H2O cal/g mol
Hemihydrate form called β-calsium sulfate hemihydrate

3. Hydrocal: gypsum 125 °c + pressure
Plasters : gypsum ° c
β-calsium sulphate hemihydrate
İrreguler shape and porous
Modelling and lab. Plasters
Hydrocal: gypsum °c + pressure
Uniform in shape and denser than plasters
α-calsium sulphate hemihydrate
Densite: gypsum boiling in a %30 calsium chloride
High strength dental stone

4. Types of gypsum Plasters Low strength dental stone
High –strength / low expansion dental stone
High –strength / high expansion
Alternatively as type 2,3,4 and 5 in ADA spesification no,25

5. Ideal properties of die materials
Dimensional accuracy.
Ease of use.
Accurate reproduction of fine detail.
Inert.
Good strength.
Inexpensive.
Abrasion resistant.
Good color contrast

6. Chemical Reaction
100 g plaster g water = 118 g calsium sulfate dihydrate
100 g plaster + 45g water = = 26.4

7. Recommended Ratios of Water to Powder for Gypsum Products

8. Methods to Accelerate and Inhibit the set
ACCELERATE_ speed
Potassium sulfate
Sodium chloride(salt)
Slurry- from unclear bowl or spatula
HEAT
INHIBIT slow
Borax
Hydrocolloid gels
Sodium citrate
BLOOD
COLD

9. Effect of Spatulation
. When the chemical reaction starts, and some calcium sulfate dihydrate is formed. During spatulation the newly formed c.s.d. Breaks down to to smaller crystals and starts new centers of nucleation
An increase in the amount of spatulation (either speed or time or both) causes more nuclei centers to be formed and that shortence the setting time.

10. Effect of Temperature
Increasing the temperature from room temperature of 20° c to 37° c increases the rate of the reaction slightly and shortence the setting time .
However , as the temperature is raised over 37°c the rate of reaction decreases.and the setting time is lengthened.
At the 100°c the solubilityies of dihydrate and hemihydrate are equal . In which case tno reactiıon occures.and plaster does not set.

11. Effect of colloidal systems
Colloidal systems such as agar and alginate retard the setting of gypsum products.
If these materials are in contact with calsium sulfate hemihydrate during setting, a soft , easily abraded surface is optained.
Acceleratores such as potassium sulfate are added to improve the surface quality of the set against agar or alginate.

12. Effect of Humidity
When the relative hummidity increases to %70 and above , moisture in the air can cause some coversion of hemihydrate to dihydrate
Because dihydrate crystals can accelerate the reaction by providing more nuclei for crystalization, the initial result is acceleration of setting.

13. Properties and behaviour
Chemical properties: (exothermic reaction)
Ca sulfate dihydrate heat water loss ground Ca sulfate hemihydrate
The production of various types of gypsum is basically the same:
If the heating process occurs in atmospheric pressure at 115°C, the powder particles are porous and irregular (b hemihydrate, plaster).
if heating process is under pressure, powder particles are regular and less porous (a hemihydrate, or dental stone).

14. A further increase in pressure and refining of the powder by grinding results in denser stone, high-strength or die stone
When stone is mixed with silica: dental investment, a material that can withstand high heat and stress produced when molten metal is forced into molds to form indirect restorations

15. Gypsum products
Plaster of Paris
Dental stone
Die stone

16. Physical properties Type W/P Porosity Compressive strength
Abrasion resistance
Setting expansion
plaster
0.45
High
8.8 MPa
Low
Stone
0.30
moderate
20.6 MPa
Moderate
High strength stone
0.23
34.3 MPa
High strength
High-expansion stone
0.20
48.0 MPa

17. Physical properties (cont.)
Strength and hardness: Affected by
Porosity
Shape of particles (large irregular particles don’t condense well leading to decreased density).
Amount of water mixed with the powder. Higher porosity requires more water to be mixed to produce a cast which will be weaker.

18. Dimensional accuracy:
The higher the setting expansion, the lower the accuracy.
Setting expansion results from the growth of crystals as they join.
Solubility:
Set gypsum is not highly soluble
The greater the porosity the greater the solubility.

19. Reproduction of detail:
Greater porosity decreases surface detail production
The impression material should be compatible with the gypsum to improve detail reproduction. The best compatibility is between silicon and polyethers and gypsum

20. Classification of gypsum
Impression plaster (Type I) *seldom used*
Model plaster (Type II): used for
Diagnostic cast
Articulation of stone cast
Art portion of working cast
Flasking procedure for acrylic dentures (cast end)
The mix produces a weak cast compared to dental stone. It’s available is fast and regular sets.
Impression (negative replica), poured into gypsum to make a cast (positive replica

21. Plaster
Plaster
Flask

22. Dental stone (Type III): used for making
Full or partial denture models
Orthodontic models
Flasking procedure for acrylic dentures (teeth end)
It requires less water,
stronger than plaster
Dental stone casts

23. Dental stone, high strength (Type IV):
AKA die stone used in fabricating wax patterns of cast restorations (crown &bridge)
Implants
Precision attachments work
Often colored pink or green
Almost 2 times stronger than type III stone
Die stone used in the fabrication of cast crown restoration

24. High strength, high expansion dental stone (Type V)
Colored blue or green
Most costly of all gypsum materials
Lowest W:P ratio, higher compressive strength
The need for higher expansion (0.1%) was to compensate for the solidification shrinkage of some alloys used as base metals used for dental casting

25. Manipulation
Selection: based on the desired properties and dental application. e.g.:
For a diagnostic cast, plaster can be used.
For a working cast, strength is required and accuracy, dental stone is the gypsum of choice
Working models for cast restorations require die stone

26. Manipulation (cont.) Proportioning (W:P ratio)
Golden rule: Manufacturer instructions should always be followed.
Variations in W:P ratio affect the set materials’ properties such as strength and accuracy.
Too much water?
Too little water?
Too much water, runny mix, weaker cast
Too little water, mix is difficult to manipulate, does not produce a flowable mix.

27. Manipulation (cont.) Mixing:
Manual: Rubber bowl and spatula. Powder is sifted into water to ensure good wetting and avoid clumps, and avoid air bubbles
Strong vigorous mixing motion against sides of the bowl.
Mix for 1-2 minutes until mix is smooth, homogenous and glossy.

28. Mechanical (also used with vacuum)

29. Manipulation (cont.) Initial setting time-working time
Working time start after mixing for 1 minute
Initial setting time: time elapsed from the start of mix until loss of gloss (8-10 minutes)
6-10 minutes of working time are available to pour the gypsum.

30. Final setting time: is reached when the materials can be safely handled, the gypsum is cool (exothermic reaction is over).
Setting expansion: Results from crystal growth during setting. Can be decreased by the addition of potassium sulfate, sodium chloride, borax.
Hygroscopic expansion. If gypsum soaked during setting, water fills pores and increases volume
It’s recommended to separate the cast from impression after 1 hour.
Strength increases 2-3 times after 24 hours

31. Tests for initial setting, and final setting times
Loss of gloss test for initial setting time: loss of gloss occurs as water is taken up by gypsum to form the dihydrate. The materials does not have measurable compressive strength.
Initial Gillmore test for initial set: needles are used to indent the material until no indentation can be seen = initial setting time.

32. A
Vicat penetrometer (A) for setting time: used for the next stage of setting (refer to figure) after loss of gloss, the plunger rod is released onto the mix. Time elapsed until the rod no longer penetrates is the setting time.
Gillmore test (B) for final setting time: a heavier Gillmore needle to determine final setting time. B B

34. Ready for use criteria
The ability to judge readiness of gypsum to be handled improves with experience.
Technically, the material is considered ready when compressive strength reaches 80% of the strength attained after 1 hour.
Most products are ready to use in 30 minutes.

35. before separating cast from impression, ensure that no part of the tray is connected to the gypsum
if alginate impression dried before cast separation, soak in water for 15 minutes.

36. How to control setting time
Changing water : powder ratio
Increasing water
Decreasing water
Retarded setting
Shorter setting time
Weaker model or cast
Mix difficult to manipulate
Inaccurate model
Bubbles inclusion in mix

37. Spatulation: rapid and prolonged spatulation accelerates setting and also increases setting expansion.
Temperature: increasing water temperature to a certain level will accelerate setting.
Water temperature above 100°F will retard setting, above 212°F gypsum will not set.

38. Accelerators and retarders:
Manufacturers add accelerators or retarders to gypsum. These chemicals increase or decrease gypsum solubility respectively which will alter setting time accordingly.
Clinicians can add accelerators such as potassium sulfate or set gypsum (slurry water), they act as sites for crystallization.
Setting reaction retarders: blood, saliva, alginate. If left on impression, can affect surface details of impression. Impression surface need to be properly rinsed before being poured.

39. Pouring of the cast: the cast is composed of two parts which are prepared separately
The anatomical part (hard and soft tissue), impression poured using a vibrator
Art portion or base, which is important to aid in handling and articulating the casts. Can be poured in different ways:

40. Double-pour method 1 2 3

41. Single step
Both anatomical and art portions of the cast are prepared at the same time. This method requires skill and accurate timing. Difficulty encountered:
If mix is too runny?
If mix started to initially set?

42. Boxing method
A strip of wax is fitted around the impression then gypsum is poured. The wax border should extend at least 0.5 inch above the highest point of the impression.

43. Storage and clean up
Gypsum should be stored in airtight dry area. Prolonged exposure to moisture can retard setting due to decreased solubility of powder.
Relevant equipment should be kept clean to avoid unwanted acceleration of setting by set gypsum.

44. Infection control
Casts should have set for 24 hours before being disinfected if necessary.
Spray rather than immerse
Disinfectants commonly used:
Sodium hypochlorite
iodophors
Chlorine dioxide

45. Trimming
Plaster bases are recommended since trimming them is easier than dental stone.
If base is made from stone, it should be soaked in water for 5-10 minutes to soften it before trimming. Important considerations when trimming?

46. Trimming considerations
Proportion of base to anatomical part
Parallelism
Use of wax bite registration
Outer border of cast
Shaping of anterior part of upper and lower arches

47. Other types of Die stone
Metal plated die stone: silver or copper plated to increase abrasion resistance
Epoxy die stone: resin and a hardener are used to increase abrasion resistance.
Resin reinforced die stone: resin is incorporated into the gypsum material to increase abrasion resistance.

48. Investment materials
Used to form metal casting through the lost wax technique

49. References
Dental materials, clinical applications for dental assistants and dental hygienists. Chapter 12
Phillips’ science of dental materials. Chapter10