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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

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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

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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).

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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

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Gypsum productsGypsum products

Plaster of Paris Dental stone

Die stone

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Physical propertiesPhysical properties

TypeType W/PW/P PorosityPorosity Compressive Compressive strengthstrength

Abrasion Abrasion resistanceresistance

Setting Setting expansionexpansion

plasterplaster 0.450.45 HighHigh 8.8 MPa8.8 MPa LowLow HighHigh

StoneStone 0.300.30 moderatemoderate 20.6 MPa20.6 MPa ModerateModerate ModerateModerate

High strength High strength stonestone

0.230.23 LowLow 34.3 MPa34.3 MPa HighHigh LowLow

High strengthHigh strength

High-expansion High-expansion stonestone

0.200.20 LowLow 48.0 MPa48.0 MPa High High High High

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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.

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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.

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Reproduction of detail:Reproduction of detail: Greater porosity decreases surface detail Greater porosity decreases surface detail

productionproduction

The impression material should be compatible The impression material should be compatible with the gypsum to improve detail reproduction. with the gypsum to improve detail reproduction. The best compatibility is between silicon and The best compatibility is between silicon and polyethers and gypsumpolyethers and gypsum

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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

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Plaster

Plaster

Flask

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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 plasterDental stone casts

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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

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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

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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

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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.

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Manipulation (cont.)Manipulation (cont.)

Mixing:Mixing:

Manual: Rubber bowl Manual: Rubber bowl

and spatula. Powder and spatula. Powder

is sifted into water to is sifted into water to

ensure good wetting ensure good wetting

and avoid clumps, and avoid clumps,

and avoid air bubblesand avoid air bubbles

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Mechanical (Mechanical (also used with vacuum)also used with vacuum)

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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.

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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

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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.

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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

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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.

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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.

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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

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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.

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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.

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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:

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1.1. Double-pour methodDouble-pour method

1 2

3

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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?

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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.

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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.

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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

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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?

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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

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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

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Investment materialsInvestment materials

Used to form metal Used to form metal casting through the lost casting through the lost wax techniquewax technique

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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