2. INTRODUCTION
Gypsum is a mineral that is mined in various parts of
the world.
The word gypsum derived from Greek word meaning
¶to cook· referred to burnt or calcined mineral.
Chemically it is CaSo4;2H2O i.e. calcium sulphate
dihydrate.
Three types of gypsum products are available in
dentistry ²
1. Model plaster
2. Dental stone
3. High strength dental stone or die stone
3. 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 trade
1} Selenite 2} Satin spar
3} Alabaster 4}Plaster of Paris
4. CLASSIFICATION
ACCORDING TO ADA SPECIFICATION NO 25-
TYPE I -IMPRESSION PLASTER
TYPE II -DENTAL PLASTER
TYPE 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)
5. Types of gypsum products
Gypsum Manufacture Use
Products
Impression Earlier plaster was Preliminary impression
Plaster modified for use as of edentulous ridge in
(Type ² I) impression material by fabrication of dentures.
Now rarely used. addition of chemicals and [MUCOSTAIC
flavoring agents IMPRESSION]
Dental Plaster Heating the gypsum Weakest and least
(Type ² II) powder in open container. expensive of all gypsum
Plaster of paris This direct and rapid products.
heating results in powder Making preliminary cast
F-hemihydrate that consist of porous for CD
Model plaster and irregular particles. To secure cast to
articulator ,
to fill a flask in denture
construction
6. Dental Stone Made for gypsum by
(Type ² III) carefully controlled
Hydrocal calcinations under steam Stone is stronger
pressure in a closed
Class I stone container. The method and more expensive
E-hemihydrate releases the water of than plaster.
crystallization slowly and Use for making cast
resultant powder particles for diagnostic purpose.
are more uniform in shape Cast for CD and RPD
and less porous. construction
Dental Stone, Made by calcining in calcium It is strongest and most
high strength chloride solution. This expensive. Densite is
(Type IV) calcination result in a the densest of all the
Class II Stone, powder particles that is material.
Densite, very dense and cuboidal in Used mainly for cast and
Improved shape, with a reduced dies for inlay and crown
stones, Die surface area. fabrication.
Stone
7. Dental Stone, Same as Type IV, For making very
high strength, Added with strong dies (has
high expansion accelerators and highest
(Type V) retarders and compressive
surface tension strength) this is
reducing agent like achieved by
LIGNON SULFATE lowering the w/p
ratio and
increasing the
setting expansion
to compensate
for alloy
solidification
shrinkage.
8. 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
9. MANUFACTURE
CaSO4.2H2O CaSO4.1/2 H2O CaSO4
Gypsum 110-130oC Plaster or Stone 130-200oC
CaSO4
Hexagonal
CaSO4 anhydrite
Orthorhombic
anhydrite
200-1000oC
Commercially, the gypsum is ground and
subjected to temperature 110-120 0C to drive
off the water of crystallization and produce
calcium sulphate hemihydrate .
10. CALCINATION
CaSO4;2H2O
boil,
CaCl2
open kettle high
pressure Type IV Type V
steam
Type II
Type III
11. CaSO4.1/2H2O
-hemihydrate -hemihydrate
The difference between and
hemihydrate are the result of differences
in crystal size, surface area and degree of
lattice imperfection
12. -hemihydrate -hemihydrate
y When dihydrate is y When dihydrate is
heated under steam heated in an open
pressure kettle or kiln.
y Dense prismatic y Spongy irregularly
crystals shaped crystals.
y Stronger harder y Less strong products
products on mixing on mixing with water
with water (Type I,II)
(type III,IV,V)
y Smaller w/p ratio y Larger w/ p ratio
15. SETTING OF GYPSUM
PRODUCTS
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.
16. SETTING PROCESS
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)
17. T s tti g r cti c rst s
f ll s:
i y r t is ix it t r,
s s si is f r t t is fl i
r l .
W t r c ss t r t it i y r t .
T is s t r t s l ti is s rs t r t
it i y r t ,s t l tt r r ci it t s t.
l ti is l g rs t r t it
i y r t .
it c ti s t iss lv , till f rt r
i y r t r ci it t s t f s l ti .
18. 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.
19. THEORIES OF SETTING
Theories of Setting
of Gypsum products
Colloidal Theory
Hydration Theory
Dissolution precipitation Theory or Crystalline
Theory
20. 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 is
consumed, the mass converts into a solid gel.
21. 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
22. NUCLEI
OF
CRYSTALLIZATION
SPHERULITIC
GROWTH
OUTWARD THRUST AS
SPHERULITES MAKE
CONTACT
23. PROPERTIES
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:P
The W:P ratio is an important factor in determining the
physical and chemical properties of the final gypsum
product
24. Chief factors that determine the amount of
gauging water required are:-
1. Particle size
2. Total surface area
3. Particle size distribution
25. ´For t l s , xc ss ount of sur
ter is l ys necess ry ove the
theoretic lly correct ount required for
hydr tion hich is . 6%
´The excess is needed to e or le ix
that can e oured and shaped
´The excess ater is distri uted as free ater in
the set ass ithout taking part in chemical
reaction and it contri utes to the subsequent
porosities or voids in the set product
26. ´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.
27. The excess water is distributed as free
water in the set mass without taking
part in the chemical reaction .When
the set mass is dried the excess water
evaporates leaves porosity in the
structure weakening it.
28. Recommended ranges of w/p ratio:
´Type I ³ 0.50-0.75
´Type II ³ 0.45-0.50
´Type III³ 0.28-0.30
´Type IV ³0.22-0.24
´Type V ³0.18-0.22
30. SETTING TIME
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
31. 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
32. 1. Loss of gloss test for initial set.
occurs hen excess ater in the mix is
taken up in forming the dihydrate, so the
mix loses its gloss.
This occurs approx. at mins.and the
mass has no measurable compressive
strength.
. Initial ilmore test for initial set
This occurs approx at 13mins this time
is marked by definite increase in strength.
33. Initial gillmore test for initial set:-
the mixture is spread out , the needle of ¼ lb
weight and dia of 1/12µ 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, 1/24µ). The
time elapsed when it leaves only a barely perceptible
mark on the surface is called the final setting time. It
is 45- 0 min.
It is rarely used as an indication for the ready-for-use
stage
36. 3. icat 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. nder a
load of 300gm.setting time is
achieved hen needle can no
longer penetrate to a depth of
mm into the material.
Vicat
penetrometer
37. ´ It consists of a rod eighing 300 g ith a
needle of 1-mm diameter. A ring container is
filled ith the mix, the setting time of hich
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 icat or the initial
setting time
39. 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.
40. CONTROL OF SETTING TIME(S.T.)
The .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.
. o. of nuclei of crystallization can be increased
or decreased.
3. ate of crystal growth is increased or decreased.
41. The operator can vary the S.T.within reason
By altering W:P ratio mixing time.
Decrease in setting time (fast setting) -
o mixing
q W:P ratio
Addition of certain chemicals called accelerators.
Increase in setting time (slow setting) ²
q mixing
o W:P ratio
Addition of certain chemicals called retarders.
42. 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
44. IMPURITIES
´ 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.
45. 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
47. CONTROL OF SETTING EXPANSION
(S.E)
´Less the W:P ratio o S.E.
o mixing time o 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.
48. ACCELERATORS AND
RETARDERS
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
49. 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
50. HYGROSCOPIC S.E.
´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.
51. 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
52. 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.
53. 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.
54. 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.
55. ´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) .
57. STRENGTH
´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 35MPa
1 hour tensile strength of model plaster is 2.3
MPa
59. ´The strength depends on porosity of set
material, which is related to W:P ratio
necessary to make a workable mix.
´Eg ² Plaster (which require the most
gauging water to make a fluid mix) is the
weakest, while improved stone is
strongest.
60. ´The presence or absence of excess of
free water affects strength and two types
of strength are recognized.
Wet strength
Dry strength
61. Wet trength : It is the strength
measured when the sample contains
some or all of the water is excess of the
theoretical amount required for
hydration.
ry trength : It is the strength
measured when the excess of water is not
present.
ry strength is two or more times more
than the wet strength.
63. SURFACE HARDNESS AND ABRASIVE
RESISTANCE
´ DEFINITION - IN MINEROLOGY SURFACE HARDNESS OF
SUBSTANCE IS ITS ABILITY TO RESISTS SCRATCHING
SURFACE HARDNESS ~ COMPRESSIVE STRENGTH
65. PROPORTIONING, MIXING AND CARING
FOR PRODUCTS
The technical use of gypsum products is relatively
simple requiring only
´Mixing bowl
´Mixing spatula
´Room-temperature water
´Appropriate gypsum product
66. PROPOTIONING
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.
67. a. Measuring the Water
The 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.
69. WATER FOR REACTION
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
70. MANUAL MIXING
´ 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.
71. MIXING
´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.
73. VACCUM MIXING
´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
75. CARING FOR CAST
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.
76. 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
77. SYNTHETIC GYPSUM
´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.
78. SPECIAL GYPSUM PRODUCTS
In addition to the standardized gypsum
materials there are some that have been
characterized for special purposes
For 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.
79. ´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.
80. ´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.
82. INFECTION CONTROL
´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
83. 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.
84. REFERENCES
´ 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