Classification of gypsum

1. 11

2. 22
Introduction Introduction

Gypsum is a mineral naturally found in nature.Gypsum is a mineral naturally found in nature.

Dental applications:Dental applications:

Impression materialsImpression materials

Constructing casts and diesConstructing casts and dies

Making replicas Making replicas

3. 33
Ideal properties of die materialsIdeal properties of die materials

Dimensional accuracy. Dimensional accuracy.

Ease of use. Ease of use.

Accurate reproduction of fine detail. Accurate reproduction of fine detail.

Inert. Inert.

Good strength. Good strength.

Inexpensive. Inexpensive.

Abrasion resistant. Abrasion resistant.

Good color contrast Good color contrast

4. 44
Properties and behaviourProperties and behaviour

Chemical propertiesChemical properties: : (exothermic reaction)(exothermic reaction)
Ca sulfate dihydrate heatCa sulfate dihydrate heat water loss groundwater loss ground Ca sulfate Ca sulfate
hemihydratehemihydrate

The production of various types of gypsum is basically the same:The production of various types of gypsum is basically the same:

If the heating processIf the heating process occurs in atmospheric pressure at 115°C, occurs in atmospheric pressure at 115°C,
the powder particles are porous and irregular (b hemihydrate, the powder particles are porous and irregular (b hemihydrate,
plaster).plaster).

if heating process is under pressure, powder particles are if heating process is under pressure, powder particles are
regular and less porous (a hemihydrate, or dental stone).regular and less porous (a hemihydrate, or dental stone).

5. 55

A further increase in pressure and refining of the A further increase in pressure and refining of the
powder by grinding results in denser stone, high-powder by grinding results in denser stone, high-
strength or die stonestrength or die stone

When stone is mixed with silica: dental When stone is mixed with silica: dental
investmentinvestment, , a material that can withstand high heat a material that can withstand high heat
and stress produced when molten metal is forced and stress produced when molten metal is forced
into molds to form indirect restorationsinto molds to form indirect restorations

6. 66
Gypsum productsGypsum products
Plaster of Paris Dental stone
Die stone

7. 77
Physical propertiesPhysical properties
TypeType W/PW/PPorosityPorosityCompressive Compressive
strengthstrength
Abrasion Abrasion
resistanceresistance
Setting Setting
expansionexpansion
plasterplaster 0.450.45HighHigh 8.8 MPa8.8 MPa LowLow HighHigh
StoneStone 0.300.30moderatemoderate20.6 MPa20.6 MPa ModerateModerate ModerateModerate
High strength High strength
stonestone
0.230.23LowLow 34.3 MPa34.3 MPa HighHigh LowLow
High strengthHigh strength
High-expansion High-expansion
stonestone
0.200.20LowLow 48.0 MPa48.0 MPa High High High High

8. 88
Physical properties (cont.)Physical properties (cont.)

Strength and hardnessStrength and hardness: : Affected byAffected by

PorosityPorosity

Shape of particles (large irregular particles don’t Shape of particles (large irregular particles don’t
condense well leading to decreased density).condense well leading to decreased density).

Amount of water mixed with the powder. Higher Amount of water mixed with the powder. Higher
porosity requires more water to be mixed to porosity requires more water to be mixed to
produce a cast which will be weaker.produce a cast which will be weaker.

9. 99

Dimensional accuracy:Dimensional accuracy:

The higher the setting expansion, the lower The higher the setting expansion, the lower
the accuracy. the accuracy.

Setting expansion results from the growth of Setting expansion results from the growth of
crystals as they join.crystals as they join.

Solubility: Solubility:

Set gypsum is not highly solubleSet gypsum is not highly soluble

The greater the porosity the greater the The greater the porosity the greater the
solubility.solubility.

10. 1010

Reproduction of detail:Reproduction of detail:

Greater porosity decreases surface detail Greater porosity decreases surface detail
productionproduction

The impression material should be compatible with The impression material should be compatible with
the gypsum to improve detail reproduction. The the gypsum to improve detail reproduction. The
best compatibility is between silicon and best compatibility is between silicon and
polyethers and gypsumpolyethers and gypsum

11. 1111
Classification of gypsumClassification of gypsum

Impression plaster (Type I) Impression plaster (Type I)
*seldom used**seldom used*

Model plaster (Type II): used Model plaster (Type II): used
forfor

Diagnostic castDiagnostic cast

Articulation of stone castArticulation of stone cast

Art portion of working castArt portion of working cast

Flasking procedure for Flasking procedure for
acrylic dentures (cast end)acrylic dentures (cast end)
The mix produces a weak cast The mix produces a weak cast
compared to dental stone. It’s compared to dental stone. It’s
available is fast and regular available is fast and regular
sets.sets.
Impression (negative replica), poured
into gypsum to make a cast (positive
replica

12. 1212
Plaster
Plaster
Flask

13. 1313

Dental stone (Type III): Dental stone (Type III):
used for makingused for making

Full or partial denture Full or partial denture
modelsmodels

Orthodontic modelsOrthodontic models

Flasking procedure for Flasking procedure for
acrylic dentures (teeth acrylic dentures (teeth
end)end)
It requires less water,It requires less water,
stronger than plasterstronger than plaster
Dental stone casts

14. 1414

Dental stone, high strength Dental stone, high strength
(Type IV):(Type IV):

AKA die stone used in AKA die stone used in
fabricating wax patterns of fabricating wax patterns of
cast restorations (crown cast restorations (crown
&bridge)&bridge)

ImplantsImplants

Precision attachments workPrecision attachments work

Often colored pink or greenOften colored pink or green

Almost 2 times stronger than Almost 2 times stronger than
type III stonetype III stone
Die stone used in the fabrication
of cast crown restoration

15. 1515

High strength, high expansion dental stone (Type V)High strength, high expansion dental stone (Type V)

Colored blue or greenColored blue or green

Most costly of all gypsum materialsMost costly of all gypsum materials

Lowest W:P ratio, higher compressive strengthLowest W:P ratio, higher compressive strength

The need for higher expansion (0.1%) was to The need for higher expansion (0.1%) was to
compensate for the solidification shrinkage of compensate for the solidification shrinkage of
some alloys used as base metals used for dental some alloys used as base metals used for dental
castingcasting

16. 1616

Selection: based on the desired properties and Selection: based on the desired properties and
dental application. e.g.:dental application. e.g.:

For a diagnostic cast, plaster can be used. For a diagnostic cast, plaster can be used.

For a working cast, strength is required and For a working cast, strength is required and
accuracy, dental stone is the gypsum of choiceaccuracy, dental stone is the gypsum of choice

Working models for cast restorations require die Working models for cast restorations require die
stonestone
Manipulation Manipulation

17. 1717
Manipulation (cont.)Manipulation (cont.)

Proportioning (W:P ratio)Proportioning (W:P ratio)

Golden ruleGolden rule: Manufacturer instructions should : Manufacturer instructions should
always be followed.always be followed.

Variations in W:P ratio affect the set materials’ Variations in W:P ratio affect the set materials’
properties such as strength and accuracy. properties such as strength and accuracy.

Too much water the mix will be runny, the model will Too much water the mix will be runny, the model will
be weaker.be weaker.

Too little water difficult to mix, chance to get bubbles Too little water difficult to mix, chance to get bubbles
inside the mixture.inside the mixture.

18. 1818
Manipulation (cont.)Manipulation (cont.)

Mixing:Mixing:

Manual: Rubber Manual: Rubber
bowl and spatula. bowl and spatula.
Powder is sifted into Powder is sifted into
water to ensure good water to ensure good
wetting and avoid wetting and avoid
clumps, and avoid air clumps, and avoid air
bubblesbubbles

19. 1919
Mechanical (Mechanical (also used with vacuum)also used with vacuum)

20. 2020
Manipulation (cont.)Manipulation (cont.)

Initial setting time-working timeInitial setting time-working time

Working time start after mixing for 1 minuteWorking time start after mixing for 1 minute

Initial setting time: time elapsed from the start of Initial setting time: time elapsed from the start of
mix until loss of gloss (8-10 minutes)mix until loss of gloss (8-10 minutes)

6-10 minutes of working time are available to pour 6-10 minutes of working time are available to pour
the gypsum. the gypsum.

21. 2121

Final setting time: is reached when the materials can be Final setting time: is reached when the materials can be
safely handled, the gypsum is cool (exothermic reaction safely handled, the gypsum is cool (exothermic reaction
is over).is over).

Setting expansion: Results from crystal growth during Setting expansion: Results from crystal growth during
setting. Can be decreased by the addition of potassium setting. Can be decreased by the addition of potassium
sulfate, sodium chloride, borax.sulfate, sodium chloride, borax.

Hygroscopic expansion. If gypsum soaked during Hygroscopic expansion. If gypsum soaked during
setting, water fills pores and increases volumesetting, water fills pores and increases volume

It’s recommended to separate the cast from impression It’s recommended to separate the cast from impression
after 1 hour.after 1 hour.

Strength increases 2-3 times after 24 hoursStrength increases 2-3 times after 24 hours

22. 2222
Tests for initial setting, and final Tests for initial setting, and final
setting timessetting times

Loss of gloss test for initial setting time: loss of gloss Loss of gloss test for initial setting time: loss of gloss
occurs as water is taken up by gypsum to form the occurs as water is taken up by gypsum to form the
dihydrate. The materials does not have measurable dihydrate. The materials does not have measurable
compressive strengthcompressive strength..

Initial Gillmore test for initial set: needles are used to Initial Gillmore test for initial set: needles are used to
indent the material until no indentation can be seen = indent the material until no indentation can be seen =
initial setting time. initial setting time.

23. 2323

Vicat penetrometer (A) for setting Vicat penetrometer (A) for setting
time: used for the next stage of time: used for the next stage of
setting (refer to figure)setting (refer to figure) after loss after loss
of gloss, the plunger rod is of gloss, the plunger rod is
released onto the mix. Time released onto the mix. Time
elapsed until the rod no longer elapsed until the rod no longer
penetrates is the setting time.penetrates is the setting time.

Gillmore test (B) for final setting Gillmore test (B) for final setting
time: a heavier Gillmore needle time: a heavier Gillmore needle
to determine final setting time. to determine final setting time.
B
A
B

24. 2424

25. 2525
Ready for use criteriaReady for use criteria

The ability to judge readiness of gypsum to be The ability to judge readiness of gypsum to be
handled improves with experience.handled improves with experience.

Technically, the material is considered ready Technically, the material is considered ready
when compressive strength reaches 80% of the when compressive strength reaches 80% of the
strength attained after 1 hour.strength attained after 1 hour.

Most products are ready to use in 30 minutes.Most products are ready to use in 30 minutes.

26. 2626

Clinical tip: before separating cast from Clinical tip: before separating cast from
impression, ensure that no part of the tray is impression, ensure that no part of the tray is
connected to the gypsumconnected to the gypsum

Clinical tip: if alginate impression dried before Clinical tip: if alginate impression dried before
cast separation, soak in water for 15 minutes. cast separation, soak in water for 15 minutes.

27. 2727
How to control setting timeHow to control setting time
1.1.Changing water : powder ratioChanging water : powder ratio
Increasing water Increasing water Decreasing waterDecreasing water
Retarded settingRetarded setting Shorter setting timeShorter setting time
Weaker model or castWeaker model or cast Mix difficult to manipulateMix difficult to manipulate
Inaccurate modelInaccurate model Bubbles inclusion in mixBubbles inclusion in mix
---------------- Inaccurate modelInaccurate model

28. 2828
2.2.Spatulation: rapid and prolonged spatulation Spatulation: rapid and prolonged spatulation
accelerates setting and also increases setting accelerates setting and also increases setting
expansion.expansion.
3.3.Temperature: increasing water temperature Temperature: increasing water temperature
to a certain level will accelerate setting. to a certain level will accelerate setting.

29. 2929
4.4.Accelerators and retarders:Accelerators and retarders:

Manufacturers add accelerators or retarders to Manufacturers add accelerators or retarders to
gypsum. These chemicals increase or decrease gypsum. These chemicals increase or decrease
gypsum solubility respectively which will alter gypsum solubility respectively which will alter
setting time accordingly.setting time accordingly.

Clinicians can add accelerators such as Clinicians can add accelerators such as
potassium sulfate or set gypsum (slurry water), potassium sulfate or set gypsum (slurry water),
they act as sites for crystallization. they act as sites for crystallization.

Setting reaction retarders: blood, saliva, alginate. Setting reaction retarders: blood, saliva, alginate.
If left on impression, can affect surface details of If left on impression, can affect surface details of
impression. Impression surface need to be impression. Impression surface need to be
properly rinsed before being poured. properly rinsed before being poured.

30. 3030

Pouring of the cast: the cast is composed of two Pouring of the cast: the cast is composed of two
parts which are prepared separatelyparts which are prepared separately

The anatomical part (hard and soft tissue), impression The anatomical part (hard and soft tissue), impression
poured using a vibratorpoured using a vibrator

Art portion or base, which is important to aid in handling Art portion or base, which is important to aid in handling
and articulating the casts. Can be poured in different and articulating the casts. Can be poured in different
ways:ways:

31. 3131
1.1.Double-pour methodDouble-pour method
1
2
3

32. 3232
2.2.Single stepSingle step

Both anatomical and art portions of the cast Both anatomical and art portions of the cast
are prepared at the same time. This method are prepared at the same time. This method
requires skill and accurate timing. Difficulty requires skill and accurate timing. Difficulty
encountered:encountered:

If mix is too runny?If mix is too runny?

If mix started to initially set?If mix started to initially set?

33. 3333
3.3.Boxing methodBoxing method

A strip of wax is fitted around the impression then A strip of wax is fitted around the impression then
gypsum is poured. The wax border should extend at gypsum is poured. The wax border should extend at
least 0.5 inch above the highest point of the least 0.5 inch above the highest point of the
impression.impression.

34. 3434
Storage and clean upStorage and clean up

Gypsum should be stored in airtight dry area. Gypsum should be stored in airtight dry area.
Prolonged exposure to moisture can retard Prolonged exposure to moisture can retard
setting due to decreased solubility of powder.setting due to decreased solubility of powder.

Relevant equipment should be kept clean to Relevant equipment should be kept clean to
avoid unwanted acceleration of setting by set avoid unwanted acceleration of setting by set
gypsum. gypsum.

35. 3535
Infection controlInfection control

Casts should have set for 24 hours before Casts should have set for 24 hours before
being disinfected if necessary. being disinfected if necessary.

Spray rather than immerseSpray rather than immerse

Disinfectants commonly used:Disinfectants commonly used:

Sodium hypochloriteSodium hypochlorite

iodophors iodophors

Chlorine dioxideChlorine dioxide

36. 3636
Trimming Trimming

Plaster bases are recommended since trimming Plaster bases are recommended since trimming
them is easier than dental stone.them is easier than dental stone.

If base is made from stone, it should be soaked If base is made from stone, it should be soaked
in water for 5-10 minutes to soften it before in water for 5-10 minutes to soften it before
trimming. Important considerations when trimming. Important considerations when
trimming?trimming?

37. 3737
Trimming considerationsTrimming considerations

Proportion of base to Proportion of base to
anatomical partanatomical part

ParallelismParallelism

Use of wax bite registrationUse of wax bite registration

Outer border of castOuter border of cast

Shaping of anterior part of Shaping of anterior part of
upper and lower archesupper and lower arches

38. 3838

Metal plated die stone: silver or copper plated Metal plated die stone: silver or copper plated
to increase abrasion resistanceto increase abrasion resistance

Epoxy die stone: resin and a hardener are used Epoxy die stone: resin and a hardener are used
to increase abrasion resistance.to increase abrasion resistance.

Resin reinforced die stone: resin is Resin reinforced die stone: resin is
incorporated into the gypsum material to incorporated into the gypsum material to
increase abrasion resistance.increase abrasion resistance.
Other types of Die stoneOther types of Die stone

39. 3939
Investment materialsInvestment materials

Used to form metal Used to form metal
casting through the lost casting through the lost
wax techniquewax technique

40. 4040
References References

Dental materials, clinical applications for Dental materials, clinical applications for
dental assistants and dental hygienists. dental assistants and dental hygienists.
Chapter 12 Chapter 12

Phillips’ science of dental materials. Phillips’ science of dental materials.
Chapter10 Chapter10