Gypsum (2)

DEPARTMENT OF
ORTHODONTICS
Government dental college
Trivandrum
www.indiandentalacademy.com
INDIAN DENTAL ACADEMY
Leader in continuing dental education

GOOD
AFTERNOON

Gypsum products
and its
orthodontic
applications

Gypsum products
History of gypsum.
Chemistry.
Manufacturing.
Setting process.
Properties of gypsum products.

Gypsum products
Classification of gypsum products.
Recent developments.
Orthodontic applications.
Construction of a model.
Disposal of gypsum.

History of Gypsum
Gypsos – Greek word – chalk.
Oldest traces of plaster – Anatolia and Syria –
9000 years old.
Egypt's – CHEOPS PYRAMID – 5000 years
old.
Windows made up of transparent gypsum – the
SELENITE gypsum.

Alabaster – king Solomon's temple.
Phillips Pfaff introduced it to dentistry.
Plaster of PARIS – MONTEMATRE – near
to Paris - large manufacture of gypsum.
King of France – all walls covered with
plaster to prevent fire – after the big
London fire destroyed the city, in 1966.

Gypsum
Gypsum – naturally occurring
mineral (sedimentary rock)
coloured from white, grey to
brown according to the nature of
impurities.
Chemically – calcium sulfate
usually the dihydrate form
(caso4.2 H2o).
Structurally – monoclinic space
lattice. www.indiandentalacademy.com

Gypsum products
Refers to various forms of calcium sulfate,
hydrous and anhydrous manufactured by
calcinations of gypsum, the dihydrate.
Also obtained from the byproducts of
phosphoric acid manufacture.
All these products are generally called
gypsum plasters.

Chemistry
In the temperature ranges 20˚-700˚ C,
three phase transformation occurs in the
CaSO4.2H2O system.
First two stages represent stages in
dehydration of gypsum, & these are
followed by a further transformation into
anhydrous CaSO4.

1. CaSo4.2H2o  CaSo4.1/2H2o + H2o.
(dihydrate) 40-45oc (hemihydrate)
Gypsum Plaster or Stone
2. CaSo4.1/2H2o  γ CaSo4
90-100oc (hexagonal anhydrite)
[soluble form]
3. γ CaSo4  β CaSo4
300-400oc (orthorhombic anhydrite)
[insoluble form]

Hexagonal anhydrate is unstable below
80˚c & if cooled to a lower temperature,
it rapidly rehydrates in the presence of
water vapor, to hemi hydrate.
β CaSo4 is the stable anhydrous form in
this system & exists as the mineral
insoluble anhydrite .

Theoretically hemi hydrate is the stable
form of CaSo4only in the temperature
range 45-90˚ C .It exists as the metastable
phase under dry condition at lower temp.
It has been shown that hydration can
occur if particles are exposed to
atmosphere under conditions where the
water vapor pressure is high.

Manufacturing
Commercial manufacturing.
Methods of calcining.
1.Dry calcination.
2.Wet calcination.
Difference between POP and STONE.

Commercial
manufacturing
At 45o-90o – more than 12 hrs.— to
complete – not feasible.
Higher temperatures used commercially.
Single step conversion to hexagonal
anhydrite – cooled down to temp below
80o – hemihydrates obtained.

Methods of calcining
OPEN AIR, DRY CALCINATION – no
water for crystal reorganization – rough
porous hemihydrate – α form – POP.
Loss of water under dry conditions leaves
parallel channels. so hemihydrates
produced by dry calcinations are powders
with low apparent densities, high relative
surface area & poor packing abilities.

WET CALCINATION -- β hemihydrate.
(STONE)
Sufficient water -- to allow reorganization,
so prismatic crystals of hexagonal CaSO4.

They rehydrate to hemihydrate on
cooling to the air , but this secondary
conversion cannot be accompanied by
recrystallization .
The final particles are pseudomorphic,
monoclinic crystal structure of
hemihydrate but retaining the hexagonal
crystal habit of anhydrous CaSO4
precursor.

Wet calcination
Autoclaving
117Kpa
123oc
5-7 hrs
Hydrocal
Stone
Investment
material
Boiling
30%Cacl2
Densite
Improved
stone
Die stone
Autoclaving
1%sodium
succinate
130-140oc
Crystacal

Plaster Stone Die stone

Difference between
POP and STONE
plaster
(β-hemi hydrate)
stone
(α-hemi hydrate)
Porous, irregular Prismatic, regular
Very low apparent
density
High apparent density
Needs more water to
float particles (mix)
Needs less water
Low strength and
hardness
High strength and
hardnesswww.indiandentalacademy.com

Setting process
Setting mechanism.
Theories of setting.
Stages in setting.
Water requirements (w:p ratio).
Working time/setting times.
Setting expansion.

Setting mechanism
Reversal of calcining.
CaSO41/2H2O + 3/2H2O  CaSO42H20+∆
When 1 g mole of CaSO41/2H2O is made
to react with 1.5 g mole of water 1g mole
of CaSO42H2O is produced with evolution
of 3900 cal of heat.

Theories of setting
1.Crystalline theory -- Henry louis le
chatelier.
2.Colloidal gel theory – Mahaelis.

CRYSTALLINE THEORY (1887) :
-- due to difference in solubility of
dihydrate and hemihydrate.
--hemihydrate --- 6.5g/l
--dihydrate --- 2.4g/l at 20oc.
Hemihydrate + water  two phase
suspension is formed hemihydrate
dissolves to form a super saturated
solution  dihydrate crystallises out of it.

Since dihydrate is less soluble and
hemihydrate is used up, hemihydrate
continues to dissolve.
Two centers are there—
precipitation centre - site of dihydrate
formation
dissolution centre - site of hemihydrate
dissolution

COLLOIDAL GEL THEORY (1893) :
1. The CaSo4 exists as the dispersed
phase in the colloidal system.
2. There is a small delay before the
dihydrate crystals precipitate out of the
colloid – induction period.
3. As crystals grow the mass thickens
and the consistency increases.

Stages in setting
FLUID STAGE : continuous aqueous
phase present -- viscous liquid,
exhibiting pseudo plasticity -- flows under
vibration -- glossy surface.
PLASTIC STAGE : gypsum crystals
continue to grow at the expense of
aqueous & viscosity of mix increases -
glossy surface disappears.

FRIABLE STAGE : Continuous
crystal growth converts the plastic
mass into a rigid solid, initially weak &
friable.
CARVABLE STAGE : Slowly
continuous growth converts mix into
solid mass which can be carved.

Water requirements &
W/P ratio
Reaction of 1g mole of plaster with 1.5g
mole of water produces 1g mole of
gypsum material.
145g of plaster +27g of water 
172 g of gypsum
100 g of plaster requires 18.6 g of
water (ideal w:p ratio = 0.18).

In practice -- Usual to mix 100 g of plaster
45 g of water to make it of usable
consistency. Only 18.6 g of water takes
part in the reaction & rest is distributed as
free water in the set mass .
Differing water requirements of plaster,
stone, high strength stone are the result
mainly of diff. in apparent densities of
powder.

Working & setting
times
Mixing time : time from the addition of
powder to the water until mixing is
completed.
Mechanical mixing -- 20-30 sec.
Hand spatulation -- 1 min.

WORKING TIME : Time available to use a
workable mix -- even consistency that may
be manipulated. 3 min working time is
generally adequate.
SETTING TIME : The time elapsed from
the beginning of mixing until the material
hardens is known as the setting time.
Usually measured by some kind of
penetration tests.

Tests to find setting
times
TESTS FOR INITIAL SETTING :
(a) Loss of gloss:
(b) Gillmore test:
(c) Vicat test:
TESTS TO FIND OUT FINAL SETTING
TIME:
(a) Gillmore test:
(c) Vicat test:

TESTS FOR INITIAL SETTING :
(a) Loss of gloss: As the reaction
proceeds -- excess water is taken up --
so that the mix loses its gloss.
It occurs at approximately 9 min.

(b) Gillmore test: The mix is spread
out in a shallow container. The needle
is lowered on to the surface and the
time at which it no longer leaves an
impression is called initial set. The
needle used is ¼ pound or 113.4g
weight with a 2.13 mm point.

(c) Vicat test: A rod carrying a needle
of 300g weight, 5cm length, tip cross
section area 1mm is used. The time
elapsed until the needle no longer
penetrates to the bottom is the initial
setting time.

TESTS TO FIND OUT FINAL SETTING
TIME:
(a) Gillmore test: Here one pound or
453.6g needle with 1.06 mm diameter
point is used. The time elapsed until the
needle leaves only a barely perceptible
mark on the surface is called the final
setting time.
(b) Vicat test is also used to find out the
final setting time.

Ready for use criterion
Subjective measures of the time at which
the set material may be safely used.
Technically it may be considered the time
when compressive strength is at least
80% of that attained at 1 hr.
Most modern products reach the ready for
use stage in 30 minutes.

Control of setting time.
Theoretically there are at least
3 methods by which setting time can be
controlled.
Increasing or decreasing the
solubility of hemihydrate.
Increasing or decreasing the nuclei of
crystallization.
Increasing or decreasing the rate of
crystal growth.

Variables affecting
setting time
Impurities -- ↑nuclei -- ↓ setting time.
Fineness -- ↑ nuclei -- ↓ setting time.
W : P -- > water -- ↓ nuclei -- ↑ setting time
Mixing – increased mixing within limits
-- ↑ nuclei -- ↓ setting time.
Temperature – 2 effects.
1. as temp increases solubility ↓.
2. mobility increased.

After 30oc – solubility is reduced – setting
time increased.
At 100oc -- solubility of hemihydrate and
dihydrate are equal, so there is no
reaction and the material does not set.
Between temp range of 20o – 30oc –
setting reaction is fastened– increase in
mobility of Ca++ -- so setting time is
reduced.

Humidity : ↓ solubility of hemihydrate.
--↑ setting time.
Colloidal systems : agar, alginate,
biological fluids (blood, saliva) decrease
the setting time by adsorbing on the
hemihydrate and dihydrate particles.
Adsorption on the nucleation sites reduce
the setting time more than on the
hemihydrate.
Accelerators and retarders : most
effective way. www.indiandentalacademy.com

Accelerators and
retarders
Accelerator : influences the rate of
dissolution of the hemihydrate.
without increasing the solubility of the
dihydrate

Accelerator :
- 2% NaCl (above 2% act as a retarder).
- 3.4% Na2SO4 (above 3.4% act as a
retarder).
- 2% K2SO4- Commonly used.
Syngenite [K2 (CaSO4) 2H2O] formed which
crystallizes rapidly.
-Tera alba (ground gypsum) 1%.
- Lime (CaO) .1% & Gum Arabic 1%.
- Potassium sodium tartrate (Rochelle salt).

Retarder : forms an adsorbed layer
on the hemihydrates to reduce the
solubility, and on the gypsum crystals
to inhibit its growth.
Citrates
Acetates
Borates
Ortho rhombic anhydrite (very stable.
Reacts less with water. So act as a
retarder).

Setting expansion
Mechanism.
Types of setting expansion
1.Normal setting expansion,
2.Hygroscopic setting expansion.
Factors affecting setting expansion.

Setting expansion
Volumetrically if the equivalent volumes of
hemihydrate water and the reaction
product are compared the volume of
dihydrate formed is less than the
equivalent volume of hemihydrate and
water.

CaSO41/2H2O + 3/2H2O  CaSO42H20+∆
vol → (159.767) → (148.405)
Change in volume = -7.11%
(contraction ???)
However a setting expansion is actually
observed. This can be rationalized on
the basis of crystallization mechanism.

Crystallization process is pictured as an out
growth of crystals from nuclei of crystallization.
These crystals intermesh with and obstruct
the growth of the adjacent crystals. An out ward
thrust develops that produces an expansion of
the entire mass.
During the induction period a contraction of
the mix occurs. Visible setting expansion is
evident only after initial set.

Hygroscopic expansion
If the setting process is allowed to occur
under water the setting expansion may be
double in magnitude. (Mahler D.B, and
Ady A.B ).
This is because the setting takes place
under water, water of hydration is
replaced and the distance between the
particles remains the same.

Normal linear setting
expansion = 0.15%.
Hygroscopic setting
expansion = 0.30%
(twice).

Factors affecting setting
expansion
Composition : finer the particles more the
expansion (α hemihydrate > βhemihydrate).
w/p ratio : the higher the w:p ratio lesser is
the expansion. (less entanglement of
crsytals).
Spatulation : expansion is more with more
spatulation. (more crystals/ unit volume).
Accelerators and retarders : they decrease
the setting expansion.

Properties of gypsum
products
strength (compressive and tensile)
1.Wet strength,
2.Dry strength,
3.Factors affecting strength.
Surface hardness and abrasion
resistance.
Dimensional stability.
Surface detail reproduction.

Strength
usually expressed as compressive
strength. The water content of the set
product affects its strength. The
strength is inversely related to the w/p
ratio.
1 hr compressive strength values are
Model plaster – 12 MPa,
Dental stone – 31 MPa,
High strength stone – 45 MPa.www.indiandentalacademy.com

There are two types of strength for the
set plaster.
Wet strength (Green strength),
Dry strength.
Wet strength : This is also known as
green strength. This is obtained when the
water in excess of that required for
hydration of the hemihydrate is left in the
test specimen.

Dry strength : When the specimen has
been dried of the excess water the
strength obtained is the dry strength. Dry
strength is 2 or more times than the wet
strength. As the last traces of water leaves
fine crystals of gypsum precipitate.
If water is added the small crystals are
the first one to dissolve and the reinforcing
anchors are lost.

Tensile strength
Important because bending tends to occur
because of lateral force applications,
such as removal of cast from the
impressions.
1hr wet tensile strength = ½ dry strength.

Model plaster = ½ high strength stone.
Model plaster = 1/5th of its
compressive strength
The high strength stone has a tensile
strength of 8 MPa and compressive
strength of 80 MPa under dry
conditions ( 1/10 th).

Factors affecting strength
W : P ratio = indirectly proportional,
(tensile strength is less affected by
variations in w:p than compressive
strength)
Mixing time : within limits it is directly
proportional, but over mixing leads to
breakage of already formed crystals
and less interlocking – less strength.

effect of loss of water on compressive
strength of dental stone :The hardened mass
of stone contains 8.8% excess water. On 7.5%
water loss there is a sharp increase in the
compressive strength. And when all 8.8% water
is lost the compressive strength is 55 MPa.
Incorporation of accelerator & retarders
decrease the strength.
-- adulteration + decrease in
intercrystalline cohesion.

Material
w/p
ratio
Comp
strength
Model
plaster
0.45 12.5
0.50 11.0
0.55 9.0
Dental
stone
0.27 31.0
0.30 20.5
0.50 10.5
High
strength
stone
0.24 38.0
0.30 21.5
0.50 10.5
w/p
ratio
Mixing
time
Compressive
strength
MPa psi
0.45 0.5 23.4 3400
0.45 1 26.2 3800
0.60 1 17.9 2600
0.60 2 13.8 2000
0.80 1 11.0 1600
W:P on strength
Mixing time & strength

Surface hardness and
abrasion resistance
related to the compressive strength
(directly prop).
Surface hardness increases at a faster
rate than the comp strength and is
maximum when dry strength is obtained.

can be increased by impregnating
gypsum with epoxy resins, and light
cured dimethacrylate resins.
Another method is to mix the dental
stone with 30% colloidal silica solution .
Measured using Rockwell hardness
number, (RHN).
(92 for type IV and 82 for Type III).

Dimensional stability
All gypsum products have good
dimensional stability. Following setting
the changes are immeasurable and this
material is rigid to resist deformation
when work is being carried out on them.

Surface detail
reproduction
The surface detail reproduction is not as
great as epoxy dies or electro plated dies
because the surface of the set gypsum is
porous at the microscopic level.
Type I & II can reproduce a groove of
width 75µ, where as Type III, IV & V can
reproduce a groove of width 50 µ.

The presence of air bubbles interfere
with the fine reproduction of the
detail.
Contamination of the impression with
saliva can also affect the surface
detail reproduction. Rinsing the
impression and blowing away the
excess water can overcome this
problem.

Classification of gypsum
products
A. International Standards
Organizations – Classified gypsum in
1983 into 4 types
ISO Type 1 -Impression plaster
ISO Type 2 - Plaster
ISO Type 3 - Stone
ISO Type 4 -High strength stone

ADA classification
Impression Plaster Type I :
Composed of plaster of Paris
modifiers added are :
1.K2SO4 is added to ↓ the setting
expansion.
2. Borax is added to counteract the
acceleration in setting time caused by
K2SO4.

3. Alizarin red -- to impart a pink color.
( 1+2+3 supplied as an aqueous
solution which is called anti expansion or
AE solution).
4. Gum tragacanth -- improve the
cohesiveness.
5. Starch -- disintegrate the plaster
when kept in boiling water
facilitating the removal of cast.
6. Rarely used to make final or wash
impression in complete dentures.www.indiandentalacademy.com

Model plaster Type II :
It is now used preferably to fill a
flask in denture construction where setting
expansion is not critical and the strength is
adequate.

Dental stone Type III [Balanced
stone] [Hydrocal] [Class I stone] :
Available in colored form, usually in
green color used for fabricating casts.
This is called balanced stone because it
contains both accelerators.(K2SO4 /
rock salt) and retarders (Borax/Sodium
tetra borate).

High strength stone Type IV [Densite]
[Class II stone] :
It is called die stone. Principal
requisition for a die stone are strength
hardness and minimum setting expansion.
The particles are cubical in shape.

High strength high expansion Type V :
Has high compressive strength
than type 4 stone. Improved strength is
attained by making it possible to mix at a
even lower W/P ratio. In addition setting
exp has been increased from a max. of
0.10% to 0.30% to compensate for
greater solidification shrinkage of newer
base metal alloys.

Type
Set time
(min)
Set expansion
At 2hrs
Comp strength at 1 hr
w/p ratio
Min Max Kg/cm2 Psi
Type
I 4 1 00.00 00.15 40 20 580 290 0.5-0.25
Type
II 12 4 0.00 0.30 90 1300 0.45-0.50
Type
III 12 4 0.00 0.20 210 3000 0.28-0.30
Type
IV 12 4 0.00 0.10 350 5000 0.22-0.24
Type
V 12 4 0.00 0.30 490 2000 0.18-0.22www.indiandentalacademy.com

Recent developments
Impregnation of gypsum with a polymer
like polystyrene, polyester, acrylics and
epoxy resins -- occupy the porosities –
increased strength and toughness.

Incorporation of wetting agents such
as lignosulphonates -- reduces water
requirement of stone -- stronger and
denser stone.
These additives -- retard the setting
time and increase the setting
expansion.
Overcome by the addition of K2SO4.

Some fast setting stones are developed
and are ready to use in 5 min. It has a
little working time.
Another type of stone which changes
color when it is ready for use.
Plastics and resins are added to
decrease the brittleness and improve
the resistance to scratching during
carving of wax pattern in newer
products.

synthetic gypsum
It is possible to make α hemihydrate and
β hemihydrate from the byproducts or
waste products of the manufacture of
phosphoric acid -- more expensive -- but
the properties exceed those of the latter.
Power plants with lime or lime stone
scrubbers convert sulfur dioxide to
synthetic gypsum.

Orthodontic applications
model plaster :
1. used to make the base of orthodontic
casts & study models,
2. mounting the cast on the articulator,
and
3. for flasking for acrylization.

dental stone , (hydrocal or
orth.stone, quick stone):
for preparing casts, study models.
gypsum bonded investment: Used
as soldering investment

SOLDERING INVESTMENT:
The investment for soldering contains
quartz and hemihydrate binder --
designed to have lower setting and
thermal expansions than casting
investments, so that the assembled parts
do not shift in position during setting and
heating of the investment.

Special gypsum products
(A) Orthodontic stone [ADA Type III] :
This is hard accurate stone of super white
color for optimal aesthetics. Has ample
working time -- multiple models with one
mix. Has a glossy surface when the
finished models are treated with model
glow or soap.

(B) Quick stone [ADA Type III] :
It is an economic dental stone with
consistent handling properties and
fast setting time. Its very smooth mix
makes it ideal for a variety of lab
procedures.

Properties Ortho. Stone Quick stone
w/p ratio 28:100 28:100
Working time 5-7 min 3-5 min
Setting time 11 min 10 min
Comp. strength
(wet)
31MPa 27MPa
Comp. strength
(dry)
59MPa 55MPa

Construction of a model
Proportioning.
Mixing and pouring.
1. manual.
2. manual + mechanical.
3. power driven mechanical
spatulator under vacuum.
Finishing and caring of the cast.

Proportioning.
1.Strict adherence to the
stated W/p ratios is
recommended.
2. Pre weighed envelops
have become popular
because they promote
accuracy, reduce waste
and save time.
Powder should always be
measured by a weighing
balance and not by vol.
(use of scoop), since the
powder does not pack
uniformly.www.indiandentalacademy.com

Mixing and pouring.
Hand mixing :
Flexible rubber bowl,
Water is placed into the bowl first and then
powder is sifted through,
A stiff-bladed spatula is used to strop (push
hard against the side-wall) against the
mixing bowl while rotating the bowl in one’s
hand.
After about 1 minute, a smooth glossy
mixture is produced

Hand & Mechanical spatulation :
Powder is incorporated during approx. 15 sec.
of mixing by the hand, followed by 20-30
seconds of mechanical mixing under vacuum
by a mixer.
PROPERTIES HAND
MIX
POWER DRIVEN MIXED
UNDER VACUUM
SETTING TIME 8.0 7.3
COMPRESSIVE
STRENGTH AT 24
HRS
43.1 45.5
SETTING EXP. AT 2
HR (%)
.045 .037
VISCOSITY
(CENTIPOISE)
54000 43000

Disinfection of the
impression
The disinfectants can be mixed with
water when the casts are poured. Mc Gill
in 1988 used 5% phenol and Ivanovski et
al 1995 used 2% gluteraldehyde.
The casts and dies can be immersed in
sodium hypochlorite solution (1:10
dilution) for 30 min after each clinical
stage.

iodophor can also be sprayed over
the impression.
Autoclave sterilization of the casts
has been suggested by White and
Brochhurt in 1996. Some loss of
strength and surface hardness and an
increase in dimension may occur.

Pouring of the casts
After disinfecting the impression -- dried with
a paper towel or a gentle spray of
compressed air.
SURFACTANT is also sprayed for better
wetting.
Using the wax spatula a little bit of material is
added while it is on the vibrator. It should be
made sure that each tooth is full before we
proceed to the next tooth.

Once the impression is poured, it should be
either placed in a humidor or 2%potassium
sulfate while the stone hardens.
A rough stone surface results if excess water
remains in the cast before pouring.
However the impression should not be
completely dried – gel adheres to the cast on
removal – also syneresis and distortion of the
impression takes place.

Separating the models
Setting process goes through 3 stages:
1.Loss of gloss: When the mix
becomes less shiny. Do not manipulate plaster
beyond this point.
2.Gypsum feels warm to touch
3.Gypsum reaches room temperature
Alginate can be removed from the stone after
the stone has reached the third stage. This
usually takes an hour.

Compatibility with
hydrocolloids
With hydrocolloid impressions, the surface
of the model may remain soft or peel off
while removing the set cast from the
impression -- due to the retarding effect
which hydrocolloids have on setting of
gypsum products.

This can be overcome in two ways.
By immersing the impression in a
solution containing an accelerator for
the setting of gypsum product before
pouring the impression with the
gypsum.
By incorporating a plaster hardener
in the impression material.

The hardening solution act in 2 ways:
(a) act as an accelerator overcoming the
retarding action of the gel
(b) reacts with the gel to produce a
surface layer that reduces or prevents
syneresis and retarding action of gel.
Various hardening chemicals employed are
potassium sulfate, zinc sulfate, manganese
sulfate, and potash alum. The most effective
is 2% potassium sulfate solution.

Finishing the cast
The models are soap polished. The soap
solution is boiled for about 30 min and the
models are kept in the solution for about
10 min. The casts are then dried and
wiped with soft silk or nylon cloth or
chamois cloth.

When soap reacts with gypsum calcium
sulfonate is formed which gives the
shining surface. Nowadays commercial
model glows are available which can be
applied on the model surface.

Caring the cast
[a] Solubility
If the cast immersed in running water the
linear dimension may decrease approx
0.1% for every 20 minute. The safest
method is to place it in a water bath in
which plaster debris is allowed to remain
constantly at the bottom to provide a
solution of calcium sulfate.

[b] Temperature
If the storage temp is raised between 90-
110˚ C shrinkage occurs as the water of
crystallization is removed and the
dehydrate returns to the hemihydrate. The
contraction of the plaster is greater than
that of stones. So it is not safe to keep
stone cast or heat a stone cast in air at
temp higher than 55˚C.

Caring for the gypsum
products
Gypsum products are sensitive to the
changes in the relative humidity of the
environment.
It has been shown by Torrance and
Darwell in 1990 that if the particles are
exposed to the atmosphere where the
water vapor pressure is high, hydration of
the hemihydrate crystals can occur.

Gypsum products made with thinner mix
appear to be affected more than those
with low w/p ratio.
Skinner and farmer in 1942 proposed
that it is safe to store gypsum products
at a relative humidity of 70%.
First manifestation of deterioration of
plaster is increase in setting time. Best
method is to store it in closed metal
containers.

Disposal of gypsum
products
The finer particles of gypsum when
inhaled can cause respiratory problems.
Gypsum products are considered as
hazardous materials by the national
board of gypsum.
These products are kept in special
polythene packs and containers which
are taken away by the authorities.

These products can be recycled and the
dihydrate is converted to hemihydrate --
can be used again.
The recycled gypsum can be used for
agricultural purposes, in power plants,
can be used in the manufacturing of
cements.

Scientists from Cairo university Ibrahim R
M, Seniour S H, and Sheehab G T, have
tried to prepare the dental stone from
previously fabricated dental casts.
Reheating at diff. conditions were tried by
them in an autoclave.
Results indicated that calcium sulfate
dihydrate can be reproduced using
previously fabricated casts.

Advantages
Inexpensive.
Easy to use .
Good accuracy and dimensional
stability.
Has the ability to reproduce details.

Disadvantages
Poor mechanical properties.
Brittle, tensile strength.
Poor abrasion resistance.
Incompatibility with hydrocolloids.

CONCLUSION
Alternatives to gypsum do exist but are rarely
used. They are generally less stable, difficult to
use and are more expensive.
Certainly the advantages and uses of gypsum
out weigh its disadvantages.
Gypsum products are inexpensive mold-
making materials that have been used in
dentistry since the 1850s because of their low
cost and ease of mixing and are still used
widely.

Thank you

Reference
Philips science of dental materials –
Anusavice
Dental materials a programmed review of
selected topics – William. J. O’Brien
Restorative dental materials – Craig
Applied dental materials – J. F. Mc Cabe
Notes on dental materials – Combe
Applied dental materials – John. A.
Anderson

Thank you
For more details please visit

Gypsum (2)

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