DENTAL PLASTER

1. DENTAL
PLASTER

2. ACKNOWLEDGMEN
T
We would like to express our special thanks of gratitude to
our teacher Dr. Sanjeev Joshi and Dr. Parveen who gave us
the golden opportunity to do this wonderful project on the
topic dental plaster, which also helped us in doing a lot of
research and we came to know about so many new things
we am really thankful to them.
Secondly we would also like to thank our parents and friends
who helped us a lot in finalizing this project within the limited
time frame.

3. INDEX
1. INTRODUCTION: GYPSUM PRODUCTS
2. ITS USES IN DENTISTRY
3. TYPES OF GYPSUM PRODUCT
4. INTRODUCTION: DENTAL PLASTER
5. PRODUCTION
6. SETTING REACTION
7. GRAPH
8. MANIPULATION
9. WORKING TIME
10.SETTING TIME AND MIXING TIME
11. TESTS FOR SETTING TIME
12.CONTROL OF SETTING TIME
13.MODIFIERS FOR SETTING TIME
14.STRENGTH
15.COMMERCIALY AVAILABE DENTAL PLASTER
16.REFRENCES

4. INTRODUCTION: GYPSUM
PRODUCTS
• Gypsum (CaSO4.2H2O); (Calcium Sulfate Dihydrate) is a mineral mined in
various parts of the world.
• Gypsum products are supplied as fine hemihydrate powders that are
produced by heating ground gypsum particles.
• After mixing with water, the mixture reverts back to gypsum, this unique
property of gypsum products has led to numerous applications through
history.
• Gypsum and gypsum products are used today for many applications
including building construction, soil conditioning, food additives,
pharmaceuticals, medical devices and dental applications.

5. USES OF GYPSUM PRODUCTS IN
DENTISTRY
•Gypsum products are used in dentistry for the preparation of study models for oral and
maxillofacial structures.

6. •Various types of dental plaster are produced, modified for specific property
requirements and used to form molds and casts on which dental prostheses and
restorations are constructed.
• Plaster mixed with fillers is known as gypsum dental investment. Such dental
investment are used to form molds for the casting of dental restorations with molten
metal.
•It is also used as an important auxiliary materials for dental laboratory operations that
are involved in the production of dental prostheses.

7. • To mount cast on articulator.

8. • For bite registration.
• For dental investment

9. TYPES OF GYPSUM
PRODUCTS
1. Impression plaster (type I)
2.Dental plaster (type II)
3.Dental stone (type III)
4.Dental stone, high strength (type IV)
5.Dental stone, high strength, high expansion (type V)

10. IMPRESSION
PLASTER
•These impression materials are composed of plaster of Paris, to
which modifiers have been added to regulate the setting time and
the setting expansion.
• Impression plaster is rarely used anymore for dental impression.
• Its has been replaced by less rigid materials, such as the
hydrocolloids and elastomers.
• Plaster is primarily restricted to use as a final impression in the
construction of full dentures.

11. DENTAL PLASTER
• Dental Plaster or laboratory Type II plaster is the beta form of calcium sulfate
hemihydrate (CaSO4 .1/2H2O)
• A mixture of Plaster of Paris and water is placed in an impression tray and
pressed against the tissues of, for example, a patient’s edentulous jaw. The
plaster is allowed to harden or set and the impression is taken
• It is now used to fill as flask used in denture construction when setting
expansion is not critical and the strength is adequate according to limits cited in
the ADA specification or ISO standard.
• It is usually marketed in the natural white color, thus, contrasting with stones,
which are generally colored.

12. DENTAL

13. POWDER PARTICLES OF BETA- HEMI
HYDRATE

14. DENTAL STONE
• Dental Stone or laboratory Type III plaster is the alpha form of
calcium sulfate hemi hydrate (CaSO4. ½ H2O).
• It is also called high-strength stone. Here, the gypsum is
calcined by boiling it in 30% calcium chloride solution.
• It is intended for the construction of casts in the fabrication of full
dentures to fit soft tissues.
• Type III stones are preferred for casts used to process dentures
because the stone has enough strength for this purpose

15. DENTAL

16. DENTAL

17. POWDER PARTICLES OF ALPHA-HEMIHYDRATE

18. DIFFERENCE BETWEEN
DENTAL PLASTER AND DENTAL
STONE

19. DENTAL STONE, HIGH
STRENGTH
• The principal requisites for a die material are strength, hardness and
minimal setting expansion. To obtain these properties, modified alpha hemi
hydrate.
• The cube-shaped particles and the reduced surface area produce such
properties without undue thickening of the mix.
• This material is also called die stone.
• A hard surface is necessary for a die stone because the tooth
preparation is covered with wax and carved flush with the margins of the
die.
• The average dry surface hardness of the die stone is approximately 92.

20. DIE
STONE

21. DENTAL STONE, HIGH STRENGTH,
HIGH EXPANSION
• The gypsum product exhibits an even higher compressive
strength than the type IV dental stone.
• Higher expansion is required in the stone die to aid in
compensating for the alloy solidification shrinkage.
• One should avoid the use of type V stones for producing dies for
inlays and on lays since the expansion may lead to an
unacceptably tight fit.

22. POWDER PARTICLES OF TYPE IV AND TYPE V DENTAL
STONE

23. PROPERTIES OF GYPSUM
PRODUCTS

24. PRODUCTIO
N• These materials are produced by calcining calcium sulfate dihydrate (gypsum).
• The gypsum is ground and subjected to temperatures of 110 ͦ C to 130 ͦ C ( 230 ͦ F to 266 ͦ F)
in open containers to drive off part of the water of crystallization. This corresponds to the first
step the reaction.
• The principal constituent of gypsum- based products is calcium sulfate hemihydrate
(CaSO4.1/2 H2O)
• The resulting particle is a fibrous aggregate of fine crystals with capillary pores known as
plaster of paris or dental plaster in dentistry.
• As the temperature is further raised, it becomes an anhydrite. This process is known as
calcination.
• Calcination Process:
CaSO4.2H2O
110 – 130 ͦC
CaSO4. ½ H2O
Gypsum (calcium sulfate
dihydrate)
Dental Plaster (calcium sulfate
hemihydrate)

25. SETTING
REACTION
• There are three theories of gypsum setting.
• The colloidal theory proposes that, when mixed with water, hemihydrate enters into the
colloidal state through a sol-gel mechanism. In the sol state, hemihydrate particles are
hydrated to form dihydrate, thereby entering into an active state. As the measured
amount of water is consumed, the mass converts to a solid gel.
• The hydration theory suggests that rehydrated plaster particles unite through hydrogen
bonding with sulfate groups to form the set material.
• The most widely accepted mechanism is the dissolution-precipitation theory, which is
based on dissolution of the hemihydrate particles in water followed by instant
recrystallization to the dihydrate. This reaction has become possible because the
solubility of hemihydrate in water is four times greater than that of the dihydrate near
room temperature.

26. CaSO4. ½H2O + 3H2O
2 CaSO4.2H2O + unreacted (CaSO4)2.2
H2O + HEAT
Calcium sulfate
hemihydrate
Calcium sulfate dihydrate
REACTION :

27. MANIPULATION OF DENTAL
PLASTER:
PROPORTIONING
• The strength of a plaster is inversely proportional to the W/P ratio, it is important to
keep the amount of water as low as possible.
• However, it should not be so low that the mix will not flow into every detail of the
impression.
• Once optimum W/P is determined the same proportions should be used subsequently.
• The water and powder should be measured by using an accurate graduated cylinder
for the water volume and a weighing balance for the powder.
• The powder should not be measured by volume, because powder varies from product
to product and does not pack uniformly. Powder will pack harder as the container
remains unused.
• If the container is shaken, the volume increases as a result of entrapment.
• Pre-weighed envelopes, have become popular, because they promote accuracy waste,
and save time..

28. W/P RATIO
• The ratio of the water to the hemihydrate powder is usually expressed as W/P Ratio.
The ratio is usually abbreviated as W/P .
• If 100 g of plaster is mixed with 60 ml of water, the W/P ratio is 0.6.
• The W/P ratio is important factor in determining the physical and chemical properties of
the final gypsum product.
• As the W/P ratio increases, the setting time increases, the strength of the gypsum
product decreases, and the setting expansion decreases.
• Although the W/P ratio varies for the particular brand of plaster the following are some
typical recommended ranges. Type II plaster, 0.45 to 0.50.
• Using plaster or stone that is supplied in preweighed bags sometimes results in mixes
that are too thick or too thin.

29. VARIOUS EQUIPMENT REQUIRED FOR DENTAL
PLASTER PROPORTIONING

30. MIXING AND POURING
• If mixing is performed by hand, the bowl should be parabolic in shape, smooth and
resistant to abrasion.
• The spatula should have a stiff blade and a handle that is convenient to hold.
• Entrapment of air in the mix must be avoided to prevent porosity, leading to weak spots
and surface inaccuracies.
• Place a measured amount of water in the bowl, and shift the weighed powder into the
water as initial hand mixing is performed.
• Then stir the mixture vigorously, periodically wiping the inside of bowl with the spatula
to ensure the wetting of all of the powder and the breaking up of any agglomerates, or
lumps.
• Continue mixing until a smooth mix is obtained, usually within a minute. A
• We must add the measured water first, followed by gradual addition of preweighed
powder.
• The powder is incorporated during approximately 15 sec of mixing with a hand spatula,
followed by 20 to 30 sec of mechanical mixing under vacuum.

31. STEPS OF
MANIPULATION

35. MIXING OF DENTAL PLASTER

36. CUT SECTION OF A IMPROPERLY
MIXED DENTAL PLASTER CAST

37. CUT SECTION OF PROPERLY MIXED DENTAL
PLASTER CAST

38. CARING OF THE
CAST
• Dimensions of cast will be relatively constant under ordinary conditions
of room and temperature and humidity.
• However, it is sometimes necessary to soak the gypsum cast in
preparation for other procedures.
• When a dry cast is immersed in water, negligible expansion may occur
if the water is saturated with calcium sulfate.
• The safest method for soaking the cast is to place it in a water bath
with gypsum debris remaining on the bottom of the container to provide
a saturated solution of calcium sulfate.
• It is not safe to store or heat or heat a stone cast in air at a
temperature higher than 55 ͦ C
(133 ͦ F)

39. INFECTION
CONTROL
• The possible cross-contamination of dental office personnel by microorganisms,
including Hepatitis B virus, HIV via dental impressions has prompted study of the
effect of spray and impression disinfecting techniques on impression materials.
• Disinfection solution can be used that do not adversely affect the quality of
gypsum cast. Alternatively disinfectants can be incorporated either in the powder or
dissolved water.
• The same microwave used in drying gypsum casts has also been evaluated in
infection control. One study that a 5-min irradiation at 900 W reduced
microorganisms counts.
• The widespread availability of a spectrum of disinfected dental plaster casts with
proven efficacy and unimpaired physical properties will undoubtedly strengthen the
barrier system of infection control in the dental laboratory.

40. MIXING TIME (MT)
Mixing time is defined as the time from the
addition of the powder to the water until the
mixing is completed. Mechanical mixing of plaster
is usually completed in 20 to 30 seconds, hand
spatulation generally requires at least a minute to
obtain a smooth mix.

41. WORKING TIME
(WT)
Working time is the time available to use a workable mix, one
that maintains a uniform consistency to perform one or more
tasks. It is measured from the start of mixing to the point
where the consistency is no longer acceptable for the
product’s intended purpose. For example, sufficient working
time might be needed to pour an impression, pour a spare
impression, and clean the equipment before the gypsum fully
sets. Generally, a 3 min working time is adequate.

42. SETTING TIME
(ST)
The powder is mixed with water and the time that elapses
from the beginning of mixing until the material hardens is
known as setting time. This is usually measured by some
type of penetration test using the instruments. A number of
stages occur in the setting of gypsum product. By use of an
actual strength test on an dental plaster. In this figure 1 min
is indicated for the mixing time (MT), with an additional 3 min
for the working time (WT), that is, pouring into an impression

43. TESTS FOR SETTING TIME
(ST)
LOSS OF GLOSS TEST FOR INITIAL
SET:
As the reaction proceeds, some of the excess water is
taken up in forming the dihydrate so that the mix loses
its gloss. This loss of gloss at approximately 9 min, at
which the mass still had no measurable compressive
strength. Therefore it could not be safely removed from
the mold.

44. GILLMORE NEEDLE

45. GILLMORE TEST FOR INITIAL
SET
•There are two types of Gillmore needles are shown.
•The smaller one is more frequently used for the setting
time of dental cements, but it is some times used on
gypsum products.
• The mixture is spread out, and the needle is lowered
onto the surface. The time at which it no longer leaves
an impression is called initial set.
• This event is marked by a definite increase in strength.

46. VICAT TEST FOR SETTING
TIME
• The next stage in the reaction is determined by use of
another instrument, the Vicat penetrometer.
• The needle with a weighted plunger rod is supported
and held just in contact with the mix.
• Soon after the gloss is lost the plunger is released. The
time elapsed until the needle no longer penetrates to
the bottom of the mix is known as the setting time.

47. VICAT NEEDLE

48. GILLMORE TEST FOR FINAL SETTING
TIME
The next stage in the setting process is measured by the
use of the heavier Gillmore needle. The elapsed time at
which this needle leaves only a barely perceptible mark
on the surface is called the final setting time.

49. COMPRESSIVE STRENGTH OF A TYPE II MODEL PLASTER DURING
SETTING

50. CONTROL OF THE SETTING
TIME
Setting time must be controlled for different applications.
Theoretically at least three methods can achieve such control.
1. The solubility of the hemihydrate can be increased or decreased. For example
if the solubility of the hemihydrate increased, super saturation of the calcium
sulfate increases, and the rate of crystalline deposition also increased.
2. The number of nuclei of crystallization can be increased or decreased. The
greater the number nuclei of crystallization, the faster the gypsum crystals form
and the sooner the hardening of the mass occurs because of crystalline
intermeshing.
3. The setting time can be accelerated or retarded by increasing or decreasing
the rate of crystal growth, respectively
Thus the operator can vary the setting time within reason by changing the W/P ratio
and mixing time

51. IMPURITIE
SIf the calcination is not complete and gypsum particles remain, or if the
manufacturer adds gypsum, the setting time is shortened because of the
increase in potential nuclei of crystallization.
FINENES
SThe finer the particle size of the hemihydrate, the faster the mix hardens. Not
only is the rate of the hemihydrate dissolution increased, but the gypsum
nuclei are also more numerous. Therefore a more rapid of crystallization
occurs
W/P RATIO
The more water used for mixing, the fewer nuclei there are per unit volume.
Consequently, the setting time is prolonged.

52. MIXING
Within practical's limit, the longer and more rapidly the plaster is mixed the
shorter is the setting time
TEMPERATUR
ELittle temperature change occurs between 0 ͦC (32 ͦ F) and 50 ͦC (120 ͦ F). If
the temperature of the plaster-water mixture exceeds 50 ͦC (120 ͦ F)
a gradual retardation occurs. As the temperature approaches 100 ͦ C
(212 ͦ F), no reaction takes place.

53. EFFECT OF W/P RATIO AND MIXING TIME ON SETTING
TIME

54. MODIFIERS FOR SETTING
TIME
RETARDER
S• Retarders generally act by forming an adsorbed layer on the hemihydrate to reduce its
solubility and on the gypsum crystals present to inhibit growth.
• Organic materials, such as glue, gelatin, and some gums, borax, potassium citrate and
sodium chloride behave in this manner.
•If the chemical added modifiers increases the setting time, its is known as retarder.
ACCELERATOR
S• The most effective and practical method for controlling the setting time is the addition of
certain chemical modifiers to the mixture of plaster.
• If the chemical added modifiers decreases the setting time, its is known as accelerator.
• The us of cooler water will not have a significant effect on the setting time. Thus, to ensure
the working time , one should purchase a regular setting product and/or use minimum mixing
times and slower rates of mixing.

55. COMPRESSIVE STRENGTH ON ADDITION
OF
ACCELERATORS AND RETARDERS

56. STRENGT
H
• The strength of gypsum products is generally expressed in terms
of compressive strength, although tensile strength should also be
considered in order to secure a satisfactory guide to the total
strength characteristics.
• Wet strength: It is the strength obtained when the water in
excess of that required for hydration of the hemihydrate is left in
the specimen.
• Dry strength: When the excess water in the specimen has been
driven off by drying, the strength obtained is dry strength.

57. COMPRESSIVE STRENGTH AS A FUNCTION OF W/P RATIO
FOR THE FIVE TYPES OF GYPSUM PRODUCT

58. SETTING
EXPANSION
• Setting expansion is due to the outward thrust of growing crystals. It is
observed based on the mechanism of crystallization i.e., The process of
outgrowth of crystals from the nuclei of crystallization .
• Setting expansion may range from 0.06%-0.5%
• It is of two types,
• Normal setting expansion
• Hygroscopic setting expansion
• Normal setting expansion refers to the setting expansion when a gypsum
product is allowed to expand in air.
• Hygroscopic setting expansion refers to the expansion of a gypsum product
when it is allowed to set, immersed in water.

59. DIFFERENCE BETWEEN NORMAL &
HYGROSCOPIC SETTING EXPANSION

60. DIFFERENCE BETWEEN NORMAL &
HYGROSCOPIC SETTING EXPANSION

61. LATEST ADVANCEMENT IN DENTAL
PLASTER:
• Orthodontist prefer white stone for study model and may even
treat surface with soap for an added sheen.
• Nowadays mounting plaster have been developed which are fast
setting and have low setting expansion.
• A type of plaster changes color to denote when it is ready for
use.
• Another trend is the addition of a small amount of plastic or resin,
which reduces brittleness and improves resistance

62. SOME COMMERCIALY AVAILABLE DENTAL
PLASTER:
NEELKANTH
DENTAL PLASTER
VICTOR DENTAL
PLASTER

63. ANSWORTH
DENTAL
PLASTER
DENTEX DENTAL
PLASTER

64. REFERENC
ES
• Phillips Science of dental materials 11th edition.
• Phillips science of dental materials 12th edition.
• Google images.

65. THANK YOU