All these types differ in their physical form , but have the same chemical structure.

The term Plaster of Paris was given to this product as this was obtained by burning the gypsum from deposits near Paris , France .

OTHER FORMS OF GYPSUM Gypsum has variety of names

that are widely used in the mineral trade1} Selenite 2} Satin spar 3} Alabaster 4}Plaster of

Paris

ACCORDING TO ADA SPECIFICATION NO 25-

TYPE I -IMPRESSION PLASTERTYPE II -DENTAL PLASTERTYPE III - DENTAL STONE (Hydrocal, Alpha hemihydrate)TYPE IV - DENTAL STONE (HIGH STRENGTH) (Die stone, Densite,Modified alpha

hemihydrate)TYPE V - DENTAL STONE (HIGH STRENGTH, HIGH EXPANSION)

Gypsum Products

Manufacture Use

Impression Plaster(Type – I)Now rarely used.

Earlier plaster was modified for use as impression material by addition of chemicals and flavoring agents

Preliminary impression of edentulous ridge in fabrication of dentures. [MUCOSTAICIMPRESSION]

Dental Plaster(Type – II) Plaster of paris -hemihydrate Model plaster

Heating the gypsum powder in open container. This direct and rapid heating results in powder that consist of porous and irregular particles.

Weakest and least expensive of all gypsum products. Making preliminary cast for CDTo secure cast to articulator ,to fill a flask in denture construction

Types of gypsum products

Dental Stone(Type – III)HydrocalClass I stone -hemihydrate

Made for gypsum by carefully controlled calcinations under steam pressure in a closed container. The method releases the water of crystallization slowly and resultant powder particles are more uniform in shape and less porous.

Stone is stronger and more expensive than plaster.Use for making cast for diagnostic purpose.Cast for CD and RPD construction

Dental Stone, high strength (Type IV) Class II Stone, Densite,Improved stones, Die Stone

Made by calcining in calcium chloride solution. This calcination result in a powder particles that is very dense and cuboidal in shape, with a reduced surface area.

It is strongest and most expensive. Densite is the densest of all the material.Used mainly for cast and dies for inlay and crown fabrication.

Dental Stone, high strength, high expansion (Type V)

Same as Type IV, Added with accelerators and retarders and surface tension reducing agent like LIGNON SULFATE

For making very strong dies (has highest compressive strength) this is achieved by lowering the w/p ratio and increasing the setting expansion to compensate for alloy solidification shrinkage.

Based on crystal structure:1. Beta hemihydrate- Type I , II fibrous aggregate of fine crystals with

capillary pores. Fluffy porous and least dense

2. Alpha hemihydrate- Type III, IV, V cleavage fragments and crystals in

form of rods or prisms Higher density and more crystalline 

CaSO4.2H2O CaSO4.1/2 H2O CaSO4

Gypsum Plaster or Stone CaSO4

Hexagonal CaSO4 anhydrite

Orthorhombic

anhydrite

Commercially, the gypsum is ground and subjected to temperature 110-120 0C to drive off the water of crystallization and produce calcium sulphate hemihydrate .

200-1000oC

110-130oC 130-200oC

CaSO4;2H2O boil, CaCl2

open kettle high

pressure Type IV Type V

steam

Type II

Type III

CaSO4.1/2H2O

α -hemihydrate β -hemihydrate

The difference between α and β hemihydrate are the result of differences in crystal size, surface area and degree of lattice imperfection

α -hemihydrate β -hemihydrate

When dihydrate is heated under steam pressure

Dense & prismatic crystals

Stronger & harder products on mixing with water

(type III,IV,V)Smaller w/p ratio

When dihydrate is heated in an open kettle or kiln.

Spongy & irregularly shaped crystals.

Less strong products on mixing with water

(Type I,II)

Larger w/ p ratio

The following reaction takes place in the hardening process CaSO4.2 H2O+ 3H2O 2(CaSO4 .2H2O) + heat

3900Cal/g mol)

The product of the reaction is gypsum.The heat evolved in the exothermic reaction

is equivalent to the heat used originally in calcination.

This chemical reaction takes place regardless of whether the gypsum material is used as impression material , Die material, or a binder in casting investment.

There is distinct difference in the solubility of hemihydrate and dihydrate.

Hemihydrate is 4 times more soluble in water than dihydrate near room temp.(200c)

The setting reaction can be understood as follows:

Hemihydrate is mixed with water,a suspension is formed that is fluid and workable.

Water becomes saturated with hemihydrate.

This saturated solution is super saturated with dihydrate,so the latter precipitates out.

Solution is no longer saturated with hemihydrate.

So it continues to dissolve, untill no further dihydrate precipitates out of solution.

The process continues until most of the hemihydrate is converted to dihydrate.

The crystals of dihydrate are spherulitic in nature and grow from specific growth sites called nuclei of crystallization.

Theories of Setting of Gypsum products

Colloidal Theory Hydration Theory Dissolution precipitation Theory or

Crystalline Theory

Colloidal Theory: When mixed with water , plaster enters into the colloidal state

through sol-gel mechanism . In the sol state ,hemihydrate particles

are hydrated to form the dihydrate, thereby entering into an active state .

As the measured amount of water isconsumed, the mass converts into a solid

gel.

Hydration Theory This theory suggests that rehydrated

plaster particles join together through hydrogen bonding to the sulfate groups

to form the set material.Dissolution precipitation Theory: by Louis

Chatelier in 1885 This theory is based on dissolution of

plaster and instant recrystilization of gypsum, followed by interlocking of the crystals to form the set product

Water/Powder ratio-The proportion of water to powder used to

make a workable mix of a particular gypsum product is called water/powder ratio

It is the quotient obtained when the weight or volume of the water is divided by the weight of powder.

It is usually abbreviated as W:PThe W:P ratio is an important factor in

determining the physical and chemical properties of the final gypsum product

Chief factors that determine the amount of gauging water required are:-

1.Particle size2.Total surface area3.Particle size distribution

For dental use , an excess amount of measured water is always necessary above the theoretically correct amount required for hydration which is 0.186%

The excess is needed to make a workable mix that can be poured and shaped

The excess water is distributed as free water in the set mass without taking part in chemical reaction and it contributes to the subsequent porosities or voids in the set product

The proper W:P ratio for each product depends on physical characteristics of powder particles

Therefore plaster requires more measured water (gauging water) to float the irregular porous particles than the dense particles of stone

A mixture of Calcium oxide (0.1%) & Gum arabic (1%) reduces the amount of water necessary to mix gypsum products: resulting in Improved properties.

Recommended ranges of w/p ratio: Type I — 0.50-0.75 Type II — 0.45-0.50 Type III— 0.28-0.30 Type IV —0.22-0.24 Type V —0.18-0.22

Mixing time- Time from addition of powder to water until mixing is completed. Mechanical mixing:20-30 secs Hand spatulation:1 min.Working Time: The time available to use a

workable mix, one that maintains an even consistency that may be manipulated to perform one or more tasks. General 3 min working time is adequate

Setting time When the powder is mixed with water, the time that elapses from the beginning of mixing until the materials hardens is known as setting time.

This is usually measured by some type of penetration test, using the following

instruments- Vicat penetrometer Gilmore needle

1. Loss of gloss test for initial set. occurs when excess water in the mix

is taken up in forming the dihydrate, so the mix loses its gloss.

This occurs approx. at 9mins.and the mass has no measurable compressive strength.

2. Initial Gilmore test for initial set This occurs approx at 13mins & this

time is marked by definite increase in strength.

Initial gillmore test for initial set:- the mixture is spread out , the needle of

¼ lb weight and dia of 1/12” is lowered onto the surface and the time at which it no longer leaves an impression is called the initial set. This time is marked by a definite increase in strength.

Gillmore test for final setting time:- this uses a heavier gillmore needle(1lb,

1/24”). The time elapsed when it leaves only a barely perceptible mark on the surface is called the final setting time. It is 45-60 min.

It is rarely used as an indication for the ready-for-use stage

GILMORE NEEDLES

a) MATERIAL IS UNSET

b) MATERIAL IS SET

3.Vicat test for setting time:it has an inbuilt gauge for judging the setting characteristics. The setting material is indented by a needle of 1mm dia. Under a load of 300gm.setting time is achieved when needle can no longer penetrate to a depth of 2mm into the material.

Vicat penetrometer

It consists of a rod weighing 300 g with a needle of 1-mm diameter. A ring container is filled with the mix, the setting time of which is to be measured. The rod is lowered until it contacts the surface of the material, then the needle is released and allowed to penetrate the mix. When the needle fails to penetrate to the bottom of the container, the material has reached the Vicat or the initial setting time

Ready for use criterion

It is a subjective measure of the time at which material may be safely handled

Technically it is the time when compressive strength is atleast 80% of that attained at 1 hour.

Most modern products reach the ready to use state in 30mins.

The S.T. of gypsum products can be controlled by manufacturers particular formulation & accordingly either a fast setting or a slow setting product can be purchased. Theoretically there are 3 methods –1. The solubility of hemihydrate can be increased or

decreased.2.No. of nuclei of crystallization can be increased or decreased.3.Rate of crystal growth is increased or decreased.

The operator can vary the S.T.within reason By altering W:P ratio & mixing time.Decrease in setting time (fast setting) - mixing W:P ratioAddition of certain chemicals called

accelerators. Increase in setting time (slow setting) – mixing W:P ratio Addition of certain chemicals called

retarders.

The operator can accelerate the setting time by adding gypsum (<.20%),Potassium sulfate or sodium chloride(<.28%)

Retarder act by forming an adsorbed layer on the hemihydrate to reduce its solubility and on the gypsum crystals to present to inhibit growth. Organic materials like glue, gelatin and some gums behave in this manner.

Nacl when added in small quantities will act as accelerators , BUT in large quantities will act as retarder

Other factors- Impurities Fineness Temperature

Presence of set gypsum particles, shortens the setting time because of the increase in the potential nuclei of crystallization.

Orthorhombic anhydrite: Induction period is increased. Hence setting time increased.

Hexagonal anhydrite: Induction period is decreased.

hence setting time is decreased.

Fineness: Finer the particle size of the hemihydrate… …faster is the set.Temperature: Little change occurs between 0*C to 50*C. If temperature of the plaster exceeds 50*C then gradual retardation occurs. As the temperature approaches 100*C, no reaction takes place. At higher temperature there is a tendency for the gypsum crystals formed to be converted back to hemihydrate form

Setting Expansion

This thrust results in an external expansion with resulting internal porosity in set mass.

Less the W:P ratio S.E. mixing time S.E. However the most effective method for

control of S.E. is the addition of chemicals. (accelerators or retarders)

Accelerators or retarders have an effect of reducing setting expansion and are referred as Anti expansion agents.

Accelerators : It increases the solubility of hemihydrates without increasing the solubility of dihydrate. Thus accelerates the process. Eg. NaCl upto about 2%

NaSO4 upto 3.4%

K2SO4 > 2% Terra Alba/Set calcium sulfate dihydrate. Liquids of High pH accelerate the setting reaction while the liquids of low

pH like saliva retard the setting reaction

Retarders : Certain chemicals form a coating on the hemihydrates particles and thus prevent the hemihydrates from going into solution in the normal manner.

Eg. Citrates, Acetates, Borates Colloidal system such as agar &

alginate retard the setting reaction by being adsorbed on the hemihydrate or the dehydrate nucleation sites thus interfering in hydration reaction

So far we have assumed that the plaster or stone is allowed to set in air.

If the setting process is allowed to occur under water, the setting expansion is more than double in magnitude.

Stage I : Initial mix is represented by three round particles of

hemihydrate surrounded by water

As the crystals of dihydrate grow, they contact each other, and the S.E. begins

Stage II : Reaction started and crystal of dihydrate start forming In left, water around particles is reduced

by hydration and particles are drawn more closely together by surface tension action of water.

In right, because the setting is taking place under water, the water of hydration is replaced and distance between the particles remains the same.

Stage III : The water around particles is again

decreased in the eg. on left. The particles with their attached crystals

tend to be drawn together as before but the contraction is opposed by the outward thrust of growing crystals.

On the other hand the crystals in right diagram are not so inhibited, as water is again replenished.

Stage IV & Stage V: The effect become more marked. The crystals being inhibited on the left become intermeshed and entangled much sooner than those on right, which grow much more freely during early stages before the intermeshing finally prevents the further expansion.

The basic mechanism of crystal growth is same in both instances, and both phenomenon are true setting expansion.

To distinguish between them, S.E. without water immersion is often termed as Normal setting expansion (NSE) where as the expansion that occurs under water is known as Hygroscopic setting expansion (HSE) .

W:P ratio HSE mixing time HSE

The strength of gypsum products is usually measured in terms of compressive strength. As seen in setting reaction the strength develops rapidly during the first 30 to 45 min as the hydration is completed. The wet strength of dental plaster is

9MPa and Improved stone is 35MPa1 hour tensile strength of model plaster

is 2.3 MPa

The strength depends on porosity of set material, which is related to W:P ratio necessary to make a workable mix.

Eg – Plaster (which require the most gauging water to make a fluid mix) is the weakest, while improved stone is strongest.

The presence or absence of excess of free water affects strength and two types of strength are recognized.

Wet strength Dry strength

Wet Strength : It is the strength measured when the sample contains some or all of the water is excess of the theoretical amount required for hydration.Dry Strength : It is the strength measured when the excess of water is

not present.Dry strength is two or more times more than the wet strength.

W:P ratio strength. mixing time strength. accelerator or retarder strength.

DEFINITION - IN MINEROLOGY SURFACE HARDNESS OF SUBSTANCE IS ITS ABILITY TO RESISTS SCRATCHING

SURFACE HARDNESS ~ COMPRESSIVE STRENGTH

Detail Reproduction:ANSI/ADA Specification No.25 requires Type I & Type II to reproduce a groove of 75 µ m Type III , IV & V to reproduce a groove of 50µm.

The technical use of gypsum products is relatively simple requiring only

Mixing bowl Mixing spatula Room-temperature water Appropriate gypsum product

Because the strength of a stone is indirectly proportional to the W:P ratio, it is most important to keep the amount of water as low as possible. However it should not be so low that the mix will not flow in every detail of the impression.

Thus, water and powder must be proportioned accurately for optimum properties.

a. Measuring the WaterThe water is usually dispensed by volume in

a graduated cylinder, as 1 gm of water has a volume of very close to 1 ml. b. Measuring the Powder The power can be weighed in grams with a simple balance or scale. Volume dispensers may be used, but volume dispensing of the power is not accurate because of varying packing effect on the powder. Weighing with a scale is a simple and

convenient method to ensure accurate proportions.

MIXING WATER:-TOTAL AMOUNT OF WATER REQUIRED FOR CHEMICAL REACTION TO OCCUR

REQUIRED WATER:- THAT AMOUNT OF WATER THAT REACTS WITH HEMIHYDRATE IS KNOWN AS

REQUIRED WATER.

STANDARD VALUE 18.6 ML/100GM OF POWDER

The preferred method of mixing is to add the measured water first, followed by the gradual sifting of the pre weighed powder into it.

Guess work of repeatedly adding water and powder to achieve proper consistency is to be avoided, as it may cause an uneven set in the cast, resulting in low strength and distortion.

Hand mixing involves using spatula at a rate of 2 revolutions /sec.

Done in a flexible plastic or rubber bowl with a stiff bladed spatula to combine the powder and water. The mix should be smooth, homogenous, workable and free of air bubbles. A minimum of air inclusion in the mixed product is desirable to prevent surface bubbles and internal defects.

Often, mixing is done mechanically with a vaccum mixing and investing machine.

This provides a gypsum mix that is free of air bubbles and homogenous in consistency.

Many other devices are available which will mix gypsum products mechanically and they are used where the application of product is critical

Once the setting reaction in the cast have been completed, its dimension will be relatively constant thereafter under ordinary conditions of room temperature and humidity.

However it is sometimes necessary to soak gypsum cast in water. If the stone cast is immersed in running water, its linear dimension may decrease approximately 0.1% for every 20 min of such immersion.

The safest method is to soak the cast in saturated solution of calcium sulfate.

Storage of set plaster or stone at room temperature produces no significant dimensional change.

However if the storage temperature is raised to between 90o and 100oc, a shrinkage occurs as the water of crystallization is removed and the dihydrate reverts to hemihydrate.

Therefore, it is not safe to store or heat a

stone cast in air at temperature higher than 55oC

This is done with waste products or by-products of phosphoric acid production

The synthetic product is usually much more expensive than that made from Natural gypsum, but when properly made its properties are equal to or exceed those of the latter.

In addition to the standardized gypsummaterials there are some that have beencharacterized for special purposesFor Eg.Orthodontist prefer a white stone or

plaster for study models and may even treat the surface with soap solution for added sheen.

These products have a longer working time for ease of trimming.

The mounting stones or plasters are used to mount the cast on articulator, are fast setting and have low S.E.

The mounting plaster has low strength to permit easy trimming and to separate the cast readily from articulator

Since 1991, a plethora of new dental stones have appeared mostly as time savers.

One type is ready to use in 5 min. but it has little working time.

Another product changes color to help denote when it is ready for use.

Most recently, the trend is to add a small amount of plastic or resin that reduces brittleness and improves the resistance to scratching during the carving of wax pattern.

There is always a risk of cross contamination to a dental office personnel by micro-organism, including

hepatitis B and HIV , via dental impressions.

Thus the impressions should be disinfected using spray and immersion disinfecting techniques

If the impression is not disinfected it is prudent to disinfect stone cast

Usual disinfectants for stone cast include spray disinfectants, hypochlorites and iodophores

Disinfecting solutions can be used that do not adversely affect the quality of gypsum cast.

Alternatively, dental stone containing disinfectant may be employed.

When patients with known cases of infection are being treated, overnight ethylene oxide gas sterilization is an option.

Phillips Science of DENTAL MATERIALS- 10th &11th Edition CRAIG’S Dental Materials- Properties and

manipulation. 8th edition Clinical aspects of dental

materials .Theory, practice and cases. Marcia Gladwin.

Applied dental Materials. McCabe