agar and alginate are the most commnly used impression material in dentistry for recording impression of the dental arches and for duplication of teeth and associated structures.

1. 1
Agar and Alginate

2. 2
Introduction
In the field of dentistry, constructing a cast is an important
step in numerous dental procedures.
An impression is a negative replica of a dental structure
which helps in construction of a cast which produce the
accurate oral structure.
To obtain an accurate impression, a suitable impression is
required. There are many materials available now for
making impression of oral tissues like elastomers but in
yester years, before the introduction of this newer
materials, dentist usual take impression with impression
plaster which is rigid.
This procedure follows until in mid 20th century came the
first elastic impression which is organic known as
hydrocolloid impression material.

3. 3

4. 4
Impression
An imprint or negative likeness of the teeth and/
or edentulous areas where the teeth have been
removed, made in a plastic material which
becomes relatively hard or set while in contact
with these tissues. Impressions may be made of
full complements of teeth, areas where some
teeth have been removed or in mouth from
which all teeth have been removed.
A negative likeness or copy in reverse of the
surface of an object; an imprint of the teeth and
adjacent structures for use in dentistry.(GPT8)

5. 5
Impression Materials
Non-elastic
Elastic
– Aqueous hydrocolloids
Agar
Alginate
– Non-aqueous elastomers
Polysulfide
Silicones
– Condensation
– Addition
Polyether

6. 6
Indications
Diagnostic casts
– preliminary
– opposing
Indirect
reconstruction
– fixed
– removable
Bite registration

7. 7
Based upon the type of reaction needed for
heat or chemical reaction:
– Thermoplastic- compound and waxes
– Strictly thermoplastic-reversible hydrocolloids
– Thermoset- impression plaster and ZOE paste
Based upon the reusable of material
– Reversible
– Irreversible
CLASSIFICATION

8. 8
Based upon the setting characteristics
– Rigid- dental impression plaster
– Semi rigid- impression compound, ZOE paste
– Plastic- impression wax and soft resin
– Elastic- hydrocolloids impression material and rubber
base impression
Based on their application
– Edentulous- impression plaster, impression
compound, agar
– Dentulous- plastic and elastic impression material

9. 9
I. According to properties.
Rigid or Inelastic impression materials
– Irreversible – Impression plaster, ZOE.
– Reversible – Impression compound.
Elastic impression materials.
A. hydrocolloid impression materials.
– Irreversible – Alginate.
– Reversible – Agar.
B. Non aqueous elastomeric impression
materials
– polysulfide,
– condensation silicone,
– addition silicone,
– polyether.

10. 10
II. According to setting mechanism.
1. Chemical Reaction (Irreversible).
– Rigid – Impression plaster, ZOE paste.
– Elastic – alginate hydrocolloid,
-non – aqueous elastomers
(polysulfides, polyethers, addition silicone,
condensation silicone.)
2. Physical Change (Reversible).
– Rigid (for edentulous patients) – Impression
compound.
– Elastic (for tooth form, can take impression of
slight undercuts) – Agar hydrocolloid.

11. 11
III. According to use
1. For edentulous ridge – without any
undercut
– Reversible – Impression Compound
– Irreversible – Impression plaster, ZOE.
2. For dentulous ridge.
– Reversible – Agar hydrocolloid.
– Irreversible – Alginate hydrocolloid and
elastomers.

12. 12
Impression
Materials
Non-elastic
Elastic
Aqueous
Hydrocolloids
Non-aqueous
Elastomers
Polysulfide
Silicones
Polyether
Condensation
Addition
Agar (reversible)
Alginate (irreversible)
Plaster
Compound
ZnO - Eugenol
Waxes

13. 13
CLASSIFICATION:
According to the manner in which the
material sets
– Reversible
– Irreversible
According to their use
– Rigid
– Elastic impression materials

14. 14
By Setting
Reaction
By application or Mechanical Properties
Rigid/inelastic (edentulous ridge) Elastic
(tooth form)
Chemical
Reaction
(Irreversible)
Plaster of Paris
ZOE impression paste
Alginate
Hydrocolloids
Non aqueous
elastomers
Temperature
Changes
(reversible)
Impression compound
Wax
Agar Hydrocolloids

15. 15
Ideal requirement of impression material
1. They should be fluid enough to adapt to the oral tissues.
2. They should be viscous enough to be contained in the tray that
is seated in the mouth.
3. While in the mouth they should transform into rubbery or rigid
solid in a reasonable amount of time. Ideally the total setting
time should be less than 7 min.
4. The set impression should not distort or tear when removed
from the mouth.
5. The impression made from these casts should remain
dimensionally stable.
6. The impression should maintain its dimensional stability after
removal of the cast so that a second or third cast can be made
from the same impression.
7. The materials should be biocompatible.
8. The materials associated processing equipment and processing
time should be cost effective.

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Elastic impression materials
Hydrocolloids
Elastomeric impression materials

17. 17
Hydrocolloids
Act as Fourth state of matter (colloids)
It’s a two-phase system
The state is between solution and
suspension particles i.e.1- 200nm
No clear demarcation between solutions,
colloids and suspensions.

18. 18
Sol and Gel
Sol is almost a viscous liquid.
Gel is a jelly like elastic semi solid formed
from sol by formation of fibrils or chains or
micelles become interlocked (gelation).
Gelation occurs by:
– Lowering the temperature
– Chemical reaction

19. 19
If concentration of the dispersed phase in the hydrocolloid is
sufficient
changes to semisolid materials “gel”
Dispersed phase agglomerates forming chains of fibrils
(micelles)
Micelles branches and intermesh to form brush heap
structure
Dispersion medium is held by capillary attraction or
adhesion between the fibrils

20. 20
Gel Strength
The stiffness and strength of the gel are
directly related to the brush heap density
or concentration. More dispersed phase
more fibrils formed on gelation.
When gel is heated Interfibrillar distance
increases due to increase in its kinetic
energy and reduction of their temperature.

21. 21
Dimensional effect
In structure of the hydrocolloid contains
large amount of water volume. If water
content of the gel is reduced, the gel will
shrink and if gel takes up water it will
expand or swell.
Gel loss water by evaporation or exudation
of fluid onto the surface by process of
syneresis. Gel absorbed water by process
of imbibition.

22. 22
Elastic
Aqueous
Hydrocolloids
Non-aqueous
Elastomers
Polysulfide
Silicones
Polyether
Condensation
Addition
Agar (reversible)
Alginate (irreversible)

23. 23
Reversible hydrocolloid
(Agar)
Alphous Pollar (1925) first
found out the use of
impression material.
It was the first successful
elastic impression material
to be used in dentistry
Organic hydrocolloids –
linear polymer of galactose
But recently not used

24. 24
Supplied as
Gel in collapsible
tube (for impression)
Glass jar (syringe
material)
Bulk container (for
duplication)

25. 25
Uses
Widely used at present for cast duplication
For full mouth impression without deep
undercuts
Was used extensively for crown and
bridge impressions before the use of
elastomers

26. 26
Composition
Ingredients % by weight
Agar 13-17%
Borates 0.2-0.5%
Potassium sulphate 1-2%
Wax, hard 0.5-1%
Thixotropic materials 0.3-0.5%
Alkylbenzoates 0.1%
Colouring and flavoring agents Traces
Water Balance
(around 84%)

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Characteristics of composition
Agar- Basic constituents
Borates- Improves strength but retards the
setting of plaster
Potassium sulphate- Counters the retarding
effect of borates
Wax, hard- Acts as filler
Thixotropic materials- Plasticizer
Alkylbenzoates- Preservative
Colouring and flavoring agents- pleasing taste
Water- dispersion medium

28. 28
GELATION PROCESS
Setting of a reversible hydrocolloid, often called
gelation, is a solidification process.
Liquefaction temperature (70°C-100°C)
When cooled, the temperature depends on
several factors, including the molecular weight,
purity of the agar and the ratio of agar to other
ingredients.
More the impurity of the agar more temperature
is needed to liquefy the gel.
The gelation temperature is critical.

29. 29
agar hydrocolloid (hot) agar hydrocolloid (cold)
(sol) (gel)
cool to 43ºC
heat to 100ºC

30. 30
Manipulation
Involves 3 step process
– Preparation of the
material
– Preparation
immediately before
making the impression
– Making of the
impression
Vacuum mixture

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Preparation of the material
Requires special
equipments
Needs an agar
hydrocolloid conditioner
consisting of
– Liquefaction section
– Storage section
– Tempering section

32. 32
Preparation of the material
Reverse the hydrocolloid gel to the sol state
Propylene glycol can be added to the water at higher
attitude
For every reliquification, an additional 3-4 min time
should be allowed because it is difficult to break down the
agar brush-heap structure.
Since liquefaction takes some time and the material can
be stored for several days, it is stored in storage chamber
and can used directly from it.

33. 33
Conditioning or Tempering
Since the storage temperature (65°) is too hot
for oral tissue, tempering of the material is
needed.
A gauze piece is placed on top of tray material
and placed in tempering section
Tempering increases the viscosity of the
hydrocolloid material so does not flow out from
the tray.
Syringe materials is never tempered.

34. 34
Making the Impression
Syringe material is first applied on the base of the
prepared cavities and remaining tooth is covered.
The tempered tray material is placed immediately
in the mouth to form the bulk of the impression
Gelation is accelerated by circulating cool water,
approximately 18° to 21°C through the tray for 3
to 5 minutes.
The impression should be removed suddenly with
a jerk to prevent distortion of the impression.

35. 35
Wet Field technique
The areas are actually flooded with warm water,
then the syringe material is introduced quickly,
liberally and in bulk to cover teeth and occlusion
surface and then tray material is loaded on it.
It is believe that the pressure from the tray
material displaces the syringe hydrocolloids into
the area to be restored along with blood and
debris through out the sulcus.

36. 36
Properties
Syneresis- lost of loose water along with
soluble constituents and form small
droplets of exudates on the surface of the
hydrocolloids
Imbibition- uptake of water by
hydrocolloids when immersed in water.

37. 37
Gelation temperature:
As required by ANSI/ADA-11, after boiling for 8 mins,
the material should be fluid enough to be extruded
from the container. After tempering, the sol should be
homogeneous and should set to a gel between 37°C
and 45°C when cooled.
Permanent Deformation:
Permanent deformation id determined in the same
manner as for alginate and at the time the material is
removed from the mouth. ADA/ANSI specification
requires the deformation be less than 1.5% after the
material is compressed 10% for 30 sec. Tray material
type of agar hydrocolloid shows 1% permanent
deformation. However, a reasonable thickness of
impression material should be present between the
tray and the undercut areas so that compression
higher than 10% does not occur, since higher
compression results in higher permanent deformation.

38. 38
Flexibility:
The ADA specification requirement for flexibility
allows range of 4% to 5% and most agar
impression materials meet this requirement.
Strength:
Agar impression material has a compressive strength of
8000 gm/cm2 which is for above minimum requirement
of 2500 gm/cm2 as ADA specification.
Tear strength of agar is 700 gm/cm2. Since agar
impression are Viscoelastic, the strength properties are
time dependent and higher compression and tear
strength occur at higher rate of loading.
The amount of permanent deformation is clinically
negligible provided that the material has adequately
gelled, the impression has been removed rapidly and the
undercuts present in the cavity preparation is minimal.

39. 39
Compatibility with gypsum:
Not all agar impression materials are
equally compatible with all gypsum
products and the manufacturer’s
suggestion should be considered in the
selection. The impression should be
washed free from saliva and any trace of
blood.
Care should be taken not to dehydrate
the surface of the impression.

40. 40
Dimensional stability:
It should be poured immediately or store in
100% humidity for 1 hr storage.
Reproducibility:
Although reversible hydrocolloids are accurate,
they cannot be poured up more than once.
One can used a pin-indexing technique or a 2
impression double die technique. With the
double die technique, one impression is made
and cut apart into individual dies of the
prepared teeth. The second impression is
made in a triple tray so that both arches and
the occlusal relationship are recorded.

41. 41
Flow- material is sufficient fluid to record
the fine details if it is correctly manipulated
Flexibility- of the gel is between 4-15%
Agar hydrocolloids have elastic recovery of
98.8%
Working time ranges from 7-15 min.
Setting time is about 5 min.
Tear strength~715 gm/cm2
Compressive strength of 8000 gm/cm2

42. 42
Viscosity of the sol
Material should be adequately viscous
Triple-tray technique for bite registration is
usually used by this material as it can be
easily bite through by the patients at
sufficient viscosity.

43. 43
Viscoelastic properties
Its behavior is between elastic liquid and viscous
solid
The elastic recovery of the hydrocolloid is never
complete and it does not return to its original
dimension after deformation.
Amount of permanent deformation is clinically
negligible unless, the material has adequate
gelled, the impression has been removed rapidly
and the undercut present was minimal

44. 44
Distortion during gelation
Contraction occurs during gelation due
physical change
The impression material shrinks towards
the center of the mass, thereby creating a
larger dies.
Since it is a poor thermal conductor, rapid
cooling may cause a concentration of
stress near the tray where the first gelation
occurs.

45. 45
CAST DUPLICATION
Large quantities can be prepared relatively
easily
It is economical because it can be reused.
In the mould, investment is poured to
create a refractory cast which is used in
the fabrication of the cast partial denture
framework.

46. 46
Laminate technique:
(Agar alginate combination impression)
The equipment needed for taking an agar
impression can be minimized by the use of
an agar-alginate, single-tray combination
impression.
The tray hydrocolloid is replaced with a
mixed of chilled alginate that bonds to the
syringe agar.
The agar is injected around the
preparation and the mixed alginate is
promptly seated on top of the agar.

47. 47
Because the hydrocolloid is in contact with the
prepared tooth, maximum detail is required.
The advantages are that the cost of the
equipment is lower because only the syringe
material needs to be heated. Also there is less
preparation time for the same reason. In
addition, the agar is more compatible with the
gypsum than alginates.

48. 48
Advantages of laminate technique
The syringe agar gives better details than
alginate
Less air bubbles
Water cooled trays are not required
It sets faster than the regular agar
technique

49. 49
Advantages
Accurate dies can be prepared if handled
properly
Has good elastic properties
Has good recovery from distortion
Palatable and well tolerated by the patients
Cheap compared to synthetic elastic materials
Can be reused as a duplicating materials

50. 50
Disadvantages
Does not flow well compared to newly
available materials
Tear relatively easily. Greater gingival
retraction for providing adequate thickness
of the materials
Only one model can be poured

51. 51
Elastic
Aqueous
Hydrocolloids
Non-aqueous
Elastomers
Polysulfide
Silicones
Polyether
Condensation
Addition
Agar (reversible)
Alginate (irreversible)

52. 52
Impression tray
A receptacle into which suitable
impression material is placed to make a
negative likeness
A device that is used to carry, confined
and control impression material while
making impression (GPT8)

53. 53
A dental impression tray comprising a frame having limbs
spaced apart,
Each of said limbs having outer and inner edges; a
support spanning the inner edges of said limbs and
forming a support for dental impression material;
An upstanding wall adjacent the outer edge of at least
one of said limbs; and a row of projections adjacent the
inner edge of said one of said limbs,

54. 54

55. 55
Border-Lock impression trays
The Border-Lock impression trays are manufactured by Clan Dental
Products from the Netherlands.
Since September 2003, the following 3 types are available:
disposable trays, autoclavable trays and implant trays for both
dentulous and edentulous cases.
Since 1960 Clan Dental Products produces impression trays that are
developed by J. Schreinemakers.
The Border-Lock trays have excactly the same fit as the original
metal dentulous and edentulous trays.
The name Border-Lock refers to the patented retention system.
Special horizontal slits along the border of the tray provide an
excellent locking mechanism for the impression material.
The Border-Lock tray is the first impression tray with a Semi
Custom fit that offers the dentist an alternative for the conventional
Rim-Lock tray.
It enables dentists to achieve excellent impressions that match in
many cases the results achieved with custom trays.
www.borderlock.com

56. 56
Border-Lock impression trays

57. 57
Is a full arch, closed mouth alginate
impression tray that provides patient comfort
and cuts your chair time in half. It's unique
patent pending design, engages the tongue
to restrict the overflow of impression material
and eliminates gagging. In addition it not only
simultaneously captures both the lower and
upper impressions, it also captures the bite
registration
The closed mouth technique frees the
operator from holding the tray in place while
the material sets. The trays only require half
the amount of alginate that is used in the
single arch technique and there is no clean
up required.
Alfa Triple trays are simple to use and
provide excellent results for all full mouth
alginate impressions including, Fabrication of
Custom trays, whitening trays, study models,
occlusal splints and mouth guards
Alfa Triple trays

58. 58
Plastic impression tray

59. 59
Irreversible Hydrocolloid
(Alginate)
1) An alginate impression of the upper
arch in a special tray
2) An alginate impression of the lower
arch

60. 60
At the end of 20th century, a chemist from
Scotland noticed that certain brown seaweed
(algae) yielded a peculiar mucous extraction. He
named it algin.
S. William Wilding received a basic patent for
use of algin as dental impression material.
It is a linear polymer anhydro-β-d-mannuronic
acid (alginic acid).
The use of irreversible hydrocolloid far exceeds
that of other impression materials available. It is
because, it is easy to manipulate, comfortable
for the patient, relatively inexpensive and not
required elaborate equipments.

61. 61
I Fast setting
Types
II Normal Setting
Supplied as:
Powder in bulk containers
(tins,bins or sachets)
In pre-weight packets for
single impression
Commercial name are
Zelgan (DPI), Jeltrate
(Dentsply) are some of the
available product.

62. 62
Composition
Ingredients % weight Functions
Sodium or Potassium or
Triethanolamine alginate
(soluble alginate)
15% Dissolve in water and reacts
with calcium ions
Calcium Phosphate 16% Reacts with potassium
alginate
Zinc Oxide 4% Acts as filler
Potassium Titanium Flouride 3% Gypsum hardner
Diatomaceous earth 60% Acts as filler
Sodium Phosphate 2% Reacts preferentially with
calcium sulphate
Colouring and flavouring
agents
Traces Eg:-
wintergreen,
peppermint,
anice, orange
etc
For pleasant taste and
acceptance to patients

63. 63
Setting reaction
2 Na3PO4 + 3 CaSO4 Ca3(PO4)2 + 3 Na2SO4
Na alginate + CaSO4 Ca alginate + Na2SO4
(powder) (gel)
H2O

64. 64
Gel structure
Brush heap structure of calcium
alginate fibril network enclosing un-
reacted sodium alginate sol, excess
water, filler particles and reaction by
products.
It is a cross linked structure.

65. 65
Properties of alginate hydrocolloids
Taste and odour:
Alginate impression material has a pleasant taste and
smell. Added a variety of colour, odours to make it as
pleasant to patients. Flavours include strawberry,
orange, mint, vanilla etc.
Flexibility:
It is about 14% at a stress of 1000 gm/cm2. Some also
have lower values (5%-8%). Lower w/p ratio results in
lower flexibility.
Elastic and Elastic recovery:
Alginate hydrocolloids are highly elastic (but less
compared to agar) and about 97.3% elastic recovery
occurs. Thus permanent deformation is more for
alginate (about 1.2%).

66. 66
Reproduction of tissue detail:
Detail reproduction is also lower when compared to
agar hydrocolloid.
Strength:
Compressive strength ranges from 5000-8000 gm/cm2
Tear strength: varies from 350-700 gm/cm2
Factor affecting gel strength: water/power ratio: too
much or too little water reduces gel strength.
Mixing time: Over and under mixing both reduce
strength.
Time of removal of impression: Strength increases if
the time of removal is delay for few minute after
setting.

67. 67
Dimensional Stability:
Set alginates have poor dimensional stability due to evaporation,
syneresis and imbibition
Adhesion:
Alginate impression material does not adhere well to the tray.
Retention to the tray is achieved by mechanical locking feature or
applying adhesive.
Biological properties:
Shelf life:
Materials stored for 1 month at 65°C are unsuitable for
use either failing to set at all or setting too rapidly. Even
at 54°C there is evidence of deterioration, probably
because alginate depolymerizes.
The individual pouches are preferred because of less
chance for contamination during storage.

68. 68
Measuring the powder Measured water
Pouring of water into the powder Mixing of the alginate

69. 69
Manipulation
Preparation of the mix:
Measured powder is put into a pre measured
water that has been placed in a clean rubber
bowl.
A vigorous 8 figure motion is best, with the mix
being swiped or stropped against the sides of the
rubber mixing bowl with intermittent rotation of the
spatula to press out air bubbles.
Mixing time is 45 sec to 1 min.
The resultant mix should be a smooth, creamy
mixture that does not drip off the spatula when it
raised from the bowl.

70. 70
Making the impression:
Before seating the impression, the material
should have developed sufficient viscous body
so that it does not flow out of the tray and choke
the patient.
The thickness of alginate between the tray and
the tissue should be at least 2mm.
Gel strength is double in first 4 min of gelation,
but it does not increase appreciably after the first
4 minute period.
Gelation time:
– Type I 1-2 min.
– Type II 2-4.5 min.

71. 71
Loading the tray:
The surface of the alginate in the tray may
be smoothened out by moistening the
finger with water and running it over the
surface of the alginate.
A small amount of material is taken on the
index finger and applied on the occlusal
surfaces of the teeth and on the rugae
areas. This help to reduce air voids and
improved accuracy.

72. 72
Loading of the tray Alginate impression
Disinfecting the impression

73. 73
Pouring of water into the stone die
Mixing of stone
Pouring of stone to the impression

74. 74
Process of making stone cast

75. 75
Modified alginate
In the form of a sol, containing the water but no
source of calcium ions.
A reactor of Plaster of Paris is supplied
separately. As a two paste system one
containing the alginate sol while the second
contains the calcium reactor.
These materials are said to contain silicone and
have superior resistance to tearing when
compared to unmodified alginates. They may be
supplied in both tray and syringe viscosity.

76. 76
Modified alginate
Dust free alginate impression material,
Two paste system alginate impression
material,
Fast setting alginate impression material
and
Alginate compound as separating
solution.

77. 77
Dust free alginate impression
material
Woody et al. (1977), Brune and Beltesbrekke
(1978), and Knibbs and Piney (1985) have
characterized the airborne particles and assessed
the levels of airborne particles of alginate
impression materials.
The dimensions of these particles are similar to
asbestos fibers, glass, and A12O3, that produce
fibrogenesis and carcinogenesis. Based on this
evidence it was recommended that inhalation of
dust from alginate be avoided.
These alginate powders are made dustless by the
application of a coating of a glycol.

78. 78
Kromafaze Dust-Free Color Changing
Alginate
Empress Dust free alginate

79. 79
Two paste system alginate
impression material
A second approach to provision
of a dustless alginate is the two-
paste system Ultrafine. The base
paste contains all the usual
ingredients alginate impression
material plus water (70%) and a
silicone oil minus the calcium
sulfate. The calcium sulfate is
contained in the catalyst paste
plus water (60%) and a
thickening agent.

80. 80
Two paste system alginate
impression material
A comparison of this material with
conventional alginates has been reported
by Williams and Watkins (1983) and
Eames and Litvak (1984), who observed
that it had higher tear and compressive
strengths than did conventional alginates
and, in spite of the presence of the
humectant, no improvement in
dimensional stability

81. 81
Fast setting alginate impression
material and
Mixing time: 30”
• Total Working time: 1’ 05”
• Time in mouth: 45”
• Setting time: 1’ 50”
• Elastic recovery (ISO 1563): 98 %
• Permanent deformation (ISO 1563):
11%
• Compressive strength (ISO 1563): 1,2
MPA
Characteristics
• Highly elastic alginate
• Extra-rapid processing
• Thixotropic

82. 82
Alginate compound as separating
solution.
The alginate solution is used as a
separating solution in fabrication
of dentures during packing.
It consist of solution of alcohol
with alginate. When it is applied
on to surface, the alcohol
evaporates leaving a thin film
making it a separating media.
The commercially available
product name is cold mould seal.
It is usually used for separating
cast with the acrylic denture.

83. 83
Duplicating impression materials
In the preparation of partial dentures, it is necessary to
make a duplicate of the plaster or stone cast of the
patient’s mouth. This duplicate is required for 2 reasons.
The cast on which the wax pattern of the metal frame
work is formed must be made from a refractory
investment.
The original cast is needed for checking the accuracy of
the metal framework and for processing the resin portion
of the partial denture.
The hydrocolloid type duplicating materials have the
same composition as the impression materials but their
water content is higher.

84. 84
Control of gelation time
The gelation time can be measured from the beginning
of the mixing until the gelation occurs.
Once gelation start, the impression material should not
be disturbed because the growing fibrils will fracture and
the impression would be significantly weakened.
The optimum gelation time is 3-4 min. at room
temperature (20°C). For fast setting, the gelation time is
1-2 min.
It can be best controlled by altering the temperature of
the water for mixing alginate material.
Cooler water- longer the gelation time and vise versa.
It may be necessary to pre cooled the mixing bowl and
spatula especially when small amount of impression
material are to be mixed.

85. 85
Compatible with Gypsum
It has been shown that the surface of a gypsum cast
prepared from a hydrocolloid impression may be too soft
for waxing procedures. This disadvantages can be
overcome in two ways:
– by immersing the impression in a solution containing
an accelerator for setting of the gypsum product,
before pouring the impression with the gypsum; or
– by incorporating a plaster hardener or accelerator in
the material by the manufacturer.
Improved gypsum product compatible to these
hydrocolloid materials have been available but clinician
prefer to immerse impression in a hardening solution i.e.
2% potassium sulfate.

86. 86
Effects of Mishandling
Effect Agar Alginate
Grainy material Inadequate boiling
Storage temperature too low
Storage time too long
Improper mixing
Prolonged mixing
Undue gelation
w/p ratio too low
Separation of tray
and syringe
material
Water-soaked layer of tray
material not removed
Premature gelation of either
syringe or tray material
Not applicable
Tearing Inadequate bulk
Premature removal from mouth
Syringe material partially gelled
when tray seated
Inadequate bulk
Moisture contamination
Premature removal from
mouth
Prolonged mixing

87. 87
External bubbles Gelation of syringe material
preventing flow
Undue gelation, preventing
flow
Air incorporated during
mixing
Irregularly shaped
voids
Material too cool or grainy Moisture or debris on
tissue
Rough or chalky
stone model
Inadequate cleansing of
impression
Excess water or potassium
sulfate solution left in
impression
Premature removal of die
Improper manipulation of
stone
Air-drying the impression
before pouring
Inadequate cleaning of
impression
Excess water left in
impression
Premature removal of
model
Leaving models in
impression too long
Improper manipulation of
stone.

88. 88
Distortion Impression not poured
immediately
Movement of tray during
gelation
Improper removal from
mouth
Use of ice water during
initial stages of gelation
Premature removal from
mouth
Impression not poured
immediately
Movement of tray
during gelation
Premature removal
from mouth
Improper removal
from mouth

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Advantages
Easy to mix and manipulate.
Minimum requirement of equipment.
Flexibility of set impression.
Accuracy if properly handled
Low cost
Comfortable to the patient.
It is hygienic, as fresh material must be used for each
impression
It gives a good surface detail even in presence of saliva.

90. 90
Disadvantages
Cannot be electroplated so metal dies not
possible.
It cannot be corrected.
Distortion may occur without it being obvious if
the material is not held steady while it is setting.
Poor dimensional stability- it cannot be stored for
long time.
Poor tear strength.

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References
1) Science of dental materials Anusavice
(10th edition)
2) Textbook of restorative Dentistry Craig
3) Basic Dental Materials
J.J.Manappallil
4) Journal of Oral Rehabilitation
1993/20/379
1990/17/303
5) Journal of Prosthodontic Dent.
1982/47/172
1986/55/1
1992/67/873
1992/67/535
6) Can Dent Assoc Journal
1989/55/545
7) Internet sources

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