This document discusses impression materials used in dentistry. It defines impression materials as those that accurately record oral tissues to create a negative reproduction called an impression, from which a positive model is made. It classifies impression materials based on use (single tooth, partial denture, complete denture) and setting behavior (elastic vs non-elastic). Specific hydrocolloid materials discussed include agar and alginate. Agar and alginate compositions, manipulations, properties, and applications are summarized.

1. Impression materials
(requirement and classification(
Dr. Waseem Bahjat Mushtaha
Specialized in prosthodontics

2. Definition:
*The function of an impression material is to
accurately record the dimensions of the oral
tissues and their related structures.
*The impression gives a negative
reproduction of these tissues.
*The positive reproduction is called a model
or cast.

3. Requirements:
1(Good dimensional accuracy.
2(Adequate flow properties.
3(Sufficient mechanical strength not to tear or
permanently deform during removal.
4(Suitable setting time
5(Ease of manipulation and reasonable cost.
6(Acceptability to the patient.
7(Safety “non toxic or irritating”
8(Compatibility with die and cast materials

4. Classification:
*According to uses:
I- Single tooth impression:
1(Rubber 2) copper band + compound
II- Partial denture:
1(Primary Alginate
2(Secondary : a) Alginate b) Rubber c)2 pieces impression
III- complete denture:
1(Primary : a) compound b) Alginate
2(Secondary : a) Rubber b) Zinc oxide Eugenol c) plaster of parise

5. *According to behavior of material after
setting:
1-Non – Elastic Impression:
1(Plaster of Paris 2) compound 3) Zinc oxide
eugenol 4) waxes
2-Elastic Impression:
1(Alginat 2) Agar, Agar 3) Rubber

6. Hydrocolloid impression materials
A colloid must be distinguished from a solution and
suspension. A solution is a homogenous mixture.
For example, in an aqueous solution the solute
exists as small molecules or ions in the solvent. In
contrast to this, a suspension is heterogeneous,
consisting of particles of at least sufficient size to
be seen microscopically, dispersed in a medium.
Thus a suspension is two-phase system.

7. Colloids fall between these two extremes.
They are heterogeneous, ( 2 phase systems),
like suspensions, but the particle size of the
dispersed phase is smaller, usually in the
range 1-200 nm. However, it is not always
possible to distinguish between a colloid
and a solution on the one hand and a colloid
and suspension on the other.
When the dispersion medium of a colloid is
water, it is termed a hydrocolloid.

8. Colloid may exist in the sol and gel state. In
the sol state, the material is a viscous liquid.
A sol can be converted into a gel- a material
of gelatin-like consistency, due to
agglomeration of the molecules of the
dispersed phase, to form fibrils, or chains of
molecules, in a network pattern. Theses
fibrils enclose the dispersion medium, for
example water.

9. A sol may be converted into a gel in one of
two ways:
1(By a reduction in temperature: such
processes are reversible, since on heating, a
sol is formed again; an example of this is
agar. In such a gel the fibrils are held
together by Van Der Waals forces.
2(Other materials can form a gel by a
chemical reaction, which is irreversible (for
example, alginates(

10. The strength or toughness of gel depends on:
1(The concentration of fibrils – the greater the
concentration, the stronger the material
2(The concentration of fillers – inert powders can
be added to a gel to render it less flexible.
A gel can lose or take up water or other fluids. Loss
of water can occur by evaporation. Syneresis can
also occur; this happened when the gel molecules
are drawn closer together, for example by
continuation of an setting reaction. As a result a
fluid exudate appears on the surface of the gel.
Uptake of water is called imbibition.

11. In the use of hydrocolloids for dental
impression, the material is inserted in the
mouth in sol state, when it is sufficiency
fluid to record detail. No gelation should
have occurred at this stage. It is removed
from the tissues after the gel is formed,
when it exhibits elastic properties.
Clearly evaporation form, and imbibition by
the gel should be avoided, as the former is
associated with shrinkage and the latter
results in expansion.

12. Agar
( Composition(
Constituent Approximate percentage Function
Agar 14 colloid
Borax 0.2 strengthens the gel, but
retards the setting of
dental stone model
materials
Potassium sulphate 2 To accelerate the setting of
stone
Water 83.8 Dispersion medium

13. Manipulation
a) The material is supplied in sealed containers to prevent evaporation of
water.
b) It is brought to a fluid state by heating the tube in boiling water in a
processing unit for 10-45 minutes.
c) After boiling, the material can be stored at 65ºC for up to 8 hours
d) It is important that the entire tube of material is softened
e) The material is extruded onto metal trays with mechanical interlocking.
f) The filled tray is placed in a tempering bath at 45ºC for 2 minutes,
before insertion into patient’s mouth.
g) With water-cooled trays, the agar material set quickly.
A higher temperature is required for the conversion of gel to sol than for
reverse reaction.

14. Properties
1(Accuracy:
*The material can be suffiently fluid to record fine
details if it has been correctly manipulated.
*The first material to set is that which is in contact
with the tray (contrast with alginates) since this is
cooler than the tissues. Thus the material in contact
with the tissues stays liquid for the longest time, and
can flow to compensate for any inaccuracy due to
dimensional changes, or to inadvertent movement of
the tray.

15. *The set material can be withdrawn over
undercuts. The adhesion of agar to metal is
poor, so perforated trays are used.
*Models should be cast up immediately from
agar impressions, to avoid the possibility of
the evaporation or imbibition. If this is not
possible, it has been claimed that stability is
achieved by storage in 2% K2SO4 solution
or humidor.

16. *The compatibility with cast materials depends on the
chemicals in the impression material. Without an accelerator
for the setting of stone (e.g. K2SO4) a soft surface may be
obtained.
2(These materials are non-toxic and non-irritant.
3(Their setting time is rather slow, unless efficient cooling is
achieved.
4(Tear resistance is poor.
5(Their shelf-life is adequate. The material can be reused and
can be sterilized. Loss of water, with an increase in viscosity
of the sol, may occur. Water can be added if required.
6(Can be sterilized by immersion for 10 minutes in dilute
aqueous solutions of sodium hypochlorite or glutaraldehyde.

17. Applications
These materials may be used to some extend
for prosthetic impressions, and in crown
and bridge work. Agar can be employed in
the laboratory, for model duplication, since
they can be re-used many times, due to the
reversible nature of the reaction.

18. Alginates
(composition and setting(
Constituent approx.% function
Sodium potassium 12 react with ca2+ to give calcium
gel
Slowly soluble calcium salt caso4 12 releases ca2+ to react with
alginate
Trisodium phosphaste 2 reacts with ca2+ to give
ca3(po4)2 to delayed gel
formation
Filler (diatomaceous earth) 70 increases cohesion of mix and
strengths gel
Silicon fluorides small quantity improves surface of stone model
Flavouring agents small quantity makes materials more acceptable
to patient
Chemical indicator small quantity changes color with PH change, to
indicate different stages in
manipulation, e.g. violet color
during spatulation, pink when
ready to load the tray, white when
ready for insertion into the mouth.

19. On mixing the powder with water a sol is formed, and the alginate, the
calcium salt and the phosphate begin to dissolve. The following
reaction occurs to form an elastic gel of calcium alginate:
NaAlg +CaSO4 Na2So4 + CaAlg………..1
Only the outer layer of each particle of sodium alginate dissolves and
reacts.
However, the mixing and tray loading procedures. This is obviously
undesirable, since the material should deform plastically, not
elastically, on insertion into the mouth. Gel formation is delayed by
trisodium phosphate, which reacts with calcium sulphate to give a
precipitate of calcium phosphate, as following:
2Na3Po4 + 3CaSo4 Ca3(Po4)2 + 3Na2So4…………2
This latter reaction does not contribute any elastic properties to the
material.
Reaction 2 occurs in preference to 1, no substantial quantity of calcium
alginate gel is formed until the trisodium phosphate is used up. The
manufacture can therefore control the setting time of this product by
adjusting the quantity of this constituent.

20. Manipulation
The following points should be observed in order to
obtain the best results:
1(The container of powder should be shaken before
use to get an even distribution of constituents.
2(The powder and water should be measured, as
directed by the manufacture. On brand of powder
has been supplied in water soluble sachets, which
help to ensure a uniform consistency of mix.

21. 3(Room temperature water is usually used,
slower or faster setting times can be
achieved, if required, by using cooler or
warmer water respectively.
4(Retention to the tray is achieved by one or
both of two means:
a) Perforated tray
b) An adhesive such as molten sticky wax or
methyl cellulose.

22. 5(There should be vigorous mixing by spreading the
material against the side of the bowel (one
minute(
6(An alginate impression should be displaced
sharply from the tissue- this sudden displacement
ensure the best elastic behavior. The impression is
removed about 2 minutes after set.
7(On removal from the mouth, the impression
should be:
a) wash with cold water to remove saliva.
b) Converted with a damp napkin to prevent
syneresis.
c) Cast up as soon as possible, preferably not more
than 15 minutes after taking the impression.

23. Properties
1(Alginate are sufficiently fluid to record fine detail
in the mouth.
2(During setting of the material it is important that
the impression should not be removed. The
reaction is faster at higher temperatures and so the
material in contact with the tissues set first any
pressure on the gel due to movement of the tray
will set up stresses within the material, which will
distort the alginate after its removal from the
mouth.

24. 3(The material is sufficiently elastic to be
withdrawn over undercuts, tearing of the
impression material may occur with sever
undercuts.
4(Alginate are not dimensionally stable on
storage, because of evaporation.
5(Compatibility with plaster and stone can be
good.
6(The materials are non toxic and non irritant,
their taste and odor are usually acceptable

25. 7(The setting time depends on the
composition and temperature of mixing
8(The alginate powder is not stable on
storage in the presence of moisture.
9(Difficult to sterilize, spray disinfection
diminish the sharpness of surface detail,
while immersion in solutions adversely
affects dimensional accuracy.

26. Application
These materials are not generally used for
impression for inlay, crown and bridge
work, but are applied with great success for
prosthetic and orthodontic purposes.
Alginates are dimensionally less stable than
the elastomer.

27. Elastomeric Impression Materials
Chemically there are 4 kinds:
1(polysulphid
2(Condensation polymerizing silicone
3(Additional polymerizing silicone
4(Polyether

28. 1-Polysulphide rubber impression
materials:
1(The first rubber impression material.
2(Didn’t have major changes during storage
that agar and alginate have.
3(Impression was much stronger and more
resistant to tearing than agar and alginate.
4(Rubber could be electroplated therefore
metal die as well as gypsum models could
be prepaired.

29. Chemistry of the polysulphides
Alternative names: rubber-base, mercaptan,
Thiokol.
Composition:
The materials supplied as two past:
1(The base past
2(The reactor past

30. 1(paste base contains:
*80%low molecular weight organic polymer
containing reactive mercaptan gp (-SH(
*20%reinforcing agents (titanium oxides- zinc
sulphate, copper carbonate, or silica.
2(The reactor past ( accelerator or catalyst past(
*Lead dioxide (PbO2) this causes
(polymerization and cross-linking), dioxide
using it result in paste being dark brown to
dark gray.

31. *Sulphur
*An inert oil (dibutyle or dicotyle phthalate(
To make past.
*Other catalyst system use copper hydroxide,
organic peroxides (not used since they are
volatile(.
N.B. the viscosity of the material is controlled
by the molecular weight of the mercaptan and
by the selection of the reinforcing agents.

32. Setting
The –SH groups can be oxidised by PbO2, giving S-S
linkages, as follows:
R-SH+PbO2+HS-R R-S-S-R+PbO+H2O
This linear polymer contains approximately 1mol% of
branches to provide enough pendant mercaptan groups
as chain cross-linking sites. This polymer is usually
cross-linked with an oxidizing agent such as lead
dioxide. It is the lead dioxide that gives polysulfide its
characteristic brown color. During the condensation
reaction of the lead dioxide with the SH groups of the
polysulphide polymer.

33. Two phenomena occur:
1(Chain- lengthening polymerization from the
reaction with the terminal-SH groups
2(Cross-linking from the reaction with the pendant-
SH groups.
Because the pendant groups compose only a small
percentage of the available –SH groups, initially,
the polymerization reactions result in chain
lengthening, which causes the viscosity to
increase. The subsequent cross-linking reactions
tie the chains together, forming a three-
dimensional network that confers elastic
properties to the material.

34. The polymerization reaction of polysulfide
polymer is exothermic, the amount of heat
generated depends on the amount of total
material and the concentration of initiators.
Moisture and temperature exert a significant
effect setting of polysulfide impression
material. The condensation reaction by-
product is water. Loss of even this small
molecule from the set material has a
significant effect on the dimensional
stability of the impression.

35. Silicon rubber impression materials
Were developed to over come the
disadvantages of polysulphides which are:
1(Objectionable odor.
2(Staining clothes by the lead dioxides.
3(Long setting times.
4(Fairly high permanent deformation.

36. Composition
1(Base:
-Paste contains low molecular weight silicon
liquid dimethyl siloxane which has
terminal reactive “ OH” gp.
-Reinforcing agents “ silica”
1(Proper consistency to the mix.
2(Stiffness to the set rubber.

37. 2(Accelerator:
Usually supplied as a liquid but sometimes as a paste by
the use of thickening agent tin octate and an alkyl
silicate such as artho -ethyl silicate.
3(Silicone pastes supplied:
Consistency controlled by a) concentration of reinforcing
agents.
b) Molecular weight of dimethyle siloxan
*light…………………….. 35% reinforcing agent
*regular………………….
*heavy-bodies………mol.w
*putty………………mol.w……..75%

38. Setting reaction
1(Dimethyl siloxan + ortho-ethyl silicate + tin octate
silicone rubber + ethylac.
2(The multifunctional ethyl silicate produces a cross-linked
structure that partly accounts for the low value of
permanent deformation and flow of silicon rubber.
3(Ethylac (by product) evaporates gradually reasonable for
shrinkage during 24 hrs after setting.
4(Setting reaction is more sensitive than polysulphid to
moisture and heat-increase either of them the setting and
working time( normally the silicon setting and working
times are shorter than polysulphides(

39. Addition type
1(Silane – containing siloxane + vinyl-terminal siloxane +
chloroplatinic acid silicone rubber (by volatile byproducts)
minimal dimensional changes during polymerize action
2(In Temp the rate of reaction and shorten the setting time.
3(If hydroxyl gp. Present in the addition silicon
Side reaction occurs with the result of H2 gradually released from
the set. Impression bubbles in the gypsum models prepared
after less than 1hr often impression taken. Overcome by:
1(Control the presence of hydroxyl gp.
2(Put H2 absorber such as palladium

40. Polyether rubber impression
materials
It offers mechanical properties than
polysulphides and dimensional changes the
condensation silicone. However it possesses
limitations such short working time and
high stiffness.

41. Composition and setting reaction
1(Base:
1(Lower molecular weight polyether contain ethyleneneamin terminal
gp.
2(Plasticizers
3(Filler
2(Catalyst:
Aromatic sulphonate acid ester these terminal gps reacted together by the
action of catalyst to form cross-linked mol.w.rubber.
2(Plasticizer
3(Filler
Polyether + sulphonic ester cross linked rubber

42. Visible light-cure impression
material
1(Polyether urethanedimethacrylate resin
which visible light-cure photo initiator and
photoaccelerater added.
2(The silicon dioxide filler has a refractive
index close to that of the resin in order to
provide the translucency for maximum
depth of cure.

43. Manipulation
a) Uniform mixing is required. Some products are
now produced with an automatic mixing system,
in which the two pastes are mixed as they are
extruded together through a spiral in the nozzle of
a syringe.
b) Addition silicones are very sensitive to
contamination, especially by contact with traces
of:
1-other silicon products
2-sulphur compounds from disposable latex gloves.

45. c) Retention of the material on the tray is achieved
by use of an adhesive such as a rubber solution.
d) Ideally there should be a uniform thickness (2-3
mm) of the impression materials.
e) Numerous techniques of impression taking have
been reported. Two step impression (use of a fluid
past in a putty impression) can lead to dimensional
distortion and problems of the adhesion between
the two materials. A single-step technique is
usually favored, either with two mixes of material
of different viscosity, or with a single mix of
elastomer.
f) Set elastomers should be displaced sharply from
the tissues to ensure elastic behavior.

47. Properties
a) Accuracy:
1-the fluidity of these materials depends largely on their
composition. Some polysulphides are supplied in a range
of viscosities, for example, light bodes for injection by a
syringe, and medium and heavy bodies for use on a tray.
Some materials are supplied with a diluent to enable the
operator to alter the viscosity of the mixed material a
required. Silicones are also supplied with a range of
consistencies including putty type and lower viscosity
materials. In general elastomers can record fine detail.
Unmixed elastomers are usually psudoplastic in nature.

48. 2-there is a small contraction on setting of
these materials, because of polymerization
shrinkage. Contraction also occurs on
cooling the impression from the mouth to
room temperature. The coefficient of
thermal expansion of these materials is in
the order:
polyether > silicon > polysulphide
The magnitude of the thermal shrinkage is
reduced by the adhesion of the material to
the tray.

49. 3-theses materials are sufficiently elastic to be
withdrawn over undercuts, and are usually tougher
and less likely to tear than the alginates. Polyether
materials are stiffer than the other elastomers,
therefore are the most difficult to withdraw over
undercuts.
4-on storage, contraction can result from further
polymerization of the material. Evaporation of
volatile by products from a condensation reaction
(.e.g. alcohol) is another source of shrinkage. The
dimensional stability of polyethers may be poor in
the presence of moisture. In terms of dimensional
stability, addition silicones are the best, followed
by polyether, then polysulphide

50. 5-these materials are in general compatible
with model and die materials, though they
can cause a small degree of softening of a
dental stone surface. In early silicones
evolution of hydrogen from materials
containing an organo -hydrogen siloxane
caused pitting of stone surfaces. Present day
silicones either do not use this system, or
contain a compound to react with hydrogen
as soon as it is generated. Theses
impression materials can be electroplated to
give metal coated dies

51. Other properties:
1-in general these materials are non-toxic and
non-irritant as set. However, unmixed
pastes can be irritant and produce tissue
reactions. The odor and taste of some paste
containing lead dioxide is not pleasant.
2-the setting time depends on the
composition of the material (for example,
the quantity of reactors) also, the presence
of water and high temperatures accelerate
the setting of polysulphides.

52. 3-in term of tear energy:
A- Higher viscosity materials are often better than
lower viscosity products.
B- polysulphides are better than silicones, but
distortion can occur instead of tearing.
C –polyethers are similar to regular grade silicones.
4-Stability on storage of the unmixed materials is not
always ideal, some of the reactors are unstable
over a two-year period, but keep better if stored in
a refrigerator.
5-Elastomers can be sterilized by immersion in
aqueous hydrochloride or glutaraldehyde
solutions, short-term immersion does not have any
significant effect on dimensional stability.

53. Applications
The chief use of elastomers is in impressions
for inlays, crowns and bridges, or for partial
dentures when the undercuts are so sever
that alginate would tear on removal from
the tissues. Because of their expense, these
materials are not frequently used
impressions requiring large quantities of
material.

54. Non-elastic impression materials
1(Plaster of Paris
2(compound
3(Zinc oxide eugenol
4(waxes

55. 1(Plaster of Paris
Chemistry:
The main constituent of the impression plaster
is calcined calcium sulphat hemihydrate. On
mixing with water this reacts to form a rigid
mass of calcium sulphate dihydrate.
During the setting an expansion may be
demonstrated, this is substantially reduced
by the addition of potassium sulphate.

56. As this accelerates the setting too much, borax is added to
slow down the process. Another additive is alizarin red,
this imparts a pink color to the impression, enabling it
to be distinguished from the cast that is prepaired from
it. A flavoring agent may also be present.
The additives may be blended with the plaster powder, or
may be supplied as an aqueous solution to the mixed
with plaster. In the later case a suitable solution
contains: 4% potassium sulphate, 0.4-1% borax ( the
precise value being chosen to give a desirable setting
times; this may vary to give a desirable setting time,
this may vary for different batches of plaster) and
0.04% alizarin red. This solution is called “AE” or
‘anti-expansion’ solution.

57. Some impression plasters may also contain:
*Additive such as gum tragacanth, to improve
the cohesive properties of the mixed plaster.
*Starch, incorporated so that the set plaster
will disintegrate due to the swelling of the
starch when boiling water is poured over it
this facilitates removal of the impression
from the cast model. Materials containing
starch are called “soluble plasters’

58. Manipulation
The plaster should be mixed with water or an
AE solution in the ratio of 100gm to 50-
60ml. Care must be taken to ensure that the
mix is free of air bubbles, since they may
appear on the surface of the impression
leading to inaccuracy.

59. Properties
a) Accuracy:
1-Plaster is excellent at recording fine detail since
the mixed material is very fluid when insertion
into the mouth.
2-The dimensional changes on setting are small, due
to anti-expansion additives.
3-If there are undercuts present, the plaster
impression will fracture on removal from the
mouth.

60. 4(On storage of the plaster impression, the
dimensional changes are small, though a small
degree of drying shrinkage may occur
5(Before preparing a cast in plaster or dental stone,
the plaster impression must be treated with a
separating agent. An alginate mould seal, a
varnish or waterglass or soap solution may be
used.
6(Impression plasters are non-toxic. However they
may be unpleasant for the patient, because they
produce a dry sensation in the mouth.
7(The setting time can be precisely controlled by
use of the appropriate quantities of additives.
8(Plaster is stable on storage over a long time,
provide it is kept in a sealed container.

61. Impression composition
(compound(
Constituents and applications:
These materials are generally composed of a mixture
of natural resins (e.g. colophony and shellac
andor waxes), filler (soapstone or talc) and
lubricants (stearic acid or stearin). They are
thermoplastic ( they soften when heated and
harden when cooled, without the occurrence of a
chemical reaction.

62. The available materials may be classified into types:
a) Type 1. lower fusing materials:
1-For recording prosthetic impressions, such as
preliminary impressions of edentulous patients,
supplied in sheets about 4-5 mm thick
2-Peripheral seal materials
3-Supplied in stick from for cobber band impression
for inlays and crowns and marginal additions to
special tray
b) Type II. Higher fusing materials, used as tray
materials, which are sufficiently rigid to support
other impression materials

65. Manipulation
a) For prosthetic impressions the composition is
heated in a water bath at 55-60C. Since the
material has a low thermal conductivity it must be
immersed in the water bath for sufficient time to
ensure complete softening. However, if it is left
too long, some of the constituents may be leach
out into the water bath, so altering the properties
of the material. If the composition is kneaded in
the water bath, water will become incorporated in
the material, and act as a plasticizer.

66. If the composition is too cool, it will not flow
properly in the mouth, if its too hot, the
material becomes sticky and can burn the
patient’s mouth. In all cases the water bath
should be lined with a napkin, otherwise the
material will adhere to the bath.
b) For copper band impressions, e.g. for
inlays and crowns, the stick of composition
is heated in a flame. If overheating occurs,
some of the constituents may be volatilized,
with a consequent alteration of properties of
the material.

67. Properties
a) Accuracy:
1-In general, this material, though plastic on insertion into
the mouth, is not sufficiently fluid to record all the fine
detail of the mouth.
2-Impression composition has a high coefficient of thermal
expansion, thus, on cooling during setting, there is
considerable shrinkage. This can be minimized to some
extent in prosthetic impressions by heating the surface of
the set material in a flam and retaking the impression.
Since only a small quantity of composition is contracting,
the actual magnitude of the contraction is small. Shrinkage
also occurs on cooling from mouth temperature to room
temperature (about 1.5% by volume(.

68. 3-Impression composition will distort on removal
over undercut areas.
4-Dimensional changes can occur on storing the
impression in the laboratory. Stress can be set up
with the material, particularly if is manipulated or
deformed when it is not fully softened.
Subsequently distortion can occur due to relief of
these stresses, particularly if the impression is left
for some time, in a warm atmosphere, before
casting up the model.
5-These materials are compatible with cast and die
materials.

69. b) Other properties:
1-Non-toxic and non irritant
2-Hardness in a reasonably acceptable time
in the mouth.
3-shelf-life is vary adequate, but changes in
shellac may cause deterioration over a long
period.

70. Zinc oxide- eugenol and similar
pastes
a) The zinc oxide is supplied in paste form.
This is achieved by the addition of an oil
( e.g. olive oil, light mineral oil or linseed
oil). The oil acts as a plasticizer for the
material. Hydrogenated resin can also be
incorporated, it quickens the setting and
makes the impression paste more cohesive.

71. b) The eugenol contains talc or kaolin as a
filler, to form a paste.
c) Either or both pastes may contain
accelerators, such as zinc acetate.
d) At least one proprietary paste contains a
substitute for eugenol. This is a carboxylic
acid, which can react with zinc hydroxide
(possibly formed by hydrolysis of zinc
oxide) to form a salt as follows:
Zn(OH) +2RCOOH (RCOO)2Zn + 2H2O

72. Manipulation
The two paste are provided in contrasting
colors. The correct proportions ( usually
equal lengths of the two) are mixed together
on a slab or mixing pad with a flexible
spatula until a homogeneous color is
obtained.

75. Properties
a) Accuracy:
1-these impression materials are sufficiently
fluid to record the fine detail in the mouth.
2-there is probably little or no dimensional
change associated with the setting process.
3-the set material is not elastic, so will not
record undercuts.

76. 4-the set material appears to be stable on storage in
the laboratory.
5-impression pates are compatible with dental stone
cast materials. The past can be removed from the
stone by softening it in water at 60C
Other properties:
1-these materials are non-toxic, but those containing
eugenol can be irritant, giving a tingling or
burning sensation to the patient and leaving a
persistent taste, which some people may regard as
unpleasant.

77. 2-The paste can adhere to tissues, so the lips
of the patient are usually coated with
petroleum jelly.
3-The setting time is usually satisfactory if
the manufacture’s instructions are follow
and the correct ratio of paste used. The
presence of water, and an increase in
temperature, both reduce the setting time.
4-The shelf-life of these materials is
satisfactory.

78. Application
This material is generally used in thin sections
(1-2mm) as a wash impression. A zinc
oxide eugenol impression can be taken
using a close-fitting special tray, or in an
existing denture, particularly one that is to
be relined.

79. Impression waxes
Waxes, sometimes in combination with resin of low melting
point, can be used as impression materials. Theses
materials differ from impression composition, in that they
sometimes flow at the mouth temperature. In contrast to
the zinc oxide-eugenol paste, they don’t set by chemical
reaction. Difference combinations of waxes and resins can
be blended to give a range of the materials for different
techniques. However, theses materials are not used
frequently. A cast should be prepared immediately from
such an impression to avoid distortion.