For seminar in dental material

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2. Introduction
Definition
The various physical properties include:
Hardness.
Concept ofStress and Strain.
Viscosity
Creep and flow.
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3. • Color and color perception.
• Thermophysical properties.
• Introduction to tarnish & corrosion
• Types of corrosion
Conclusion
References
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4. Dental materials are the very basic of modern
dentistry.
 They form the vital integral upon which all
of dental procedures are formulated and
executed.
 Hence, it becomes imperative that every
dentist is equipped with a sound knowledge
of every aspect of these fascinating
materials.
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5.  Physical properties are based on
the laws of mechanics, acoustics,
optics, thermodynamics,
elasticity, magnetism, radiation,
atomic structure, or nuclear
phenomena.
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6.  Any appliance/ prosthesis / restoration
undergoes deformation due to various forces
of mastication
 Complete knowledge of physical properties of
material enhances:-
- effectiveness of prosthesis
- safety limitation
- longitivity of prosthesis
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7. • Property of being difficult to indent, cut
or scratch.
• Simply resistance to plastic deformation or
permanent deformation of material.
• Used as an index of the ability of a material
to resist abrasion or wear.
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The surface hardness tests commonly
used in dentistry are :
•BRINELL
•KNOOP
•VICKERS
•ROCKWELL
•BARCOL
•SHORE

9.  Oldest methods used to test metals and
alloys
 Small steel or tungsten carbide ball, typically
1.6 mm in diameter, subjected to a weight of
123 N and it remain in contact to specimen
for 30 sec
 Smaller the area of indentation, harder the
material and larger the BHN value.
 Ratio of the load applied to the area of the
indentation produced.
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10.  Also known as 136 degree
diamond pyramid test.
 The method is similar to BHN except that a
136 degree diamond pyramid shaped
indenter is forced into the material with a
definite load application.
 The indenter produces a square indentation,
the diagonals of which are measured.
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Vickers Hardness Test
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11.  It is suitable for brittle materials so it is used
for the measurement of hardness of tooth
structures and also of dental casting gold
alloys.
 A significant advantage of this test is that it
can be used for testing very small specimens
because the indenter tip is small.
 Also, because the load applied to the specimen
can be varied, the test can be used on materials
that are soft as well as on those that are hard.
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13.  Diamond is used as indenter having extended
pyramidal shape with the length to width ratio being
7:1 and respective face angles are 172 degrees for the
long edge and 130 degrees for the short edge.
 Materials with KN
Dentin-68
Gold Foil-69
Tooth Enamel-343
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•Used for thin plastic or metal sheets
•Advantage of this method is that materials
with a great range of hardness can be tested
by varying the test load.
•Disadvantage is that it needs very high
polished and flat surface.

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16. ROCKWELL HARDNESS TEST
 This was developed as a rapid method for
hardness determination.
 A ball or metal cone indenter is normally
used and the depth of the indentation is
measured with a sensitive dial micrometer.
 The indenter balls or cones are of different
diameters and load applications ( 60 to 150
kg)
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18.  The advantages of Rockwell system is that
hardness is read directly and it is good for
testing viscoelastic materials.
 The disadvantages are that a preload is
needed, greater time required and the
indentation may disappear immediately on
removal of the load.
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19. BARCOL HARDNESS TEST
 This method is used to study the depth of cure of
resin composites.
 The Barcol indenter is a spring loaded needle with
a diameter of 1 mm that is pressed against the
surface to be tested.
 If no penetration of the needle into the surface
occurs the scale reads 100.
 The reading on the scale decreases as the indenter
penetrates the surface.
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20.  Depth of cure of a resin composite is tested
by preparing specimens varying in
thickness from 0.5 to 6.0 mm.
 Then the top surface of a specimen is
activated by a light curing unit.
 The Barcol hardness of the top surface is
compared with that of the bottom surface.
 10% decrease in Barcol hardness of a resin
composite results in a 20% decrease in the
flexural strength.
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21.  This method is used to determine the
hardness of rubbers because the
indentation disappears after removal of
load.
 An instrument called Shore Durometer is
used in the rubber industry to determine
the relative hardness of elastomers.
 The indenter is attached by a lever to a
scale that is graduated from 0 to 100 units.
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Shore Hardness Test
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22.  If the indentor completely penetrates the
specimen, a reading of 0 is obtained, and if
no penetration occurs, a reading of 100
units results.
 The usual method is to press down firmly
and quickly on the indenter and record the
maximum reading as the shore hardness.
 The test has been used to evaluate soft
denture liners, mouth protectors and
maxillofacial elastomers.
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23.  Knoop and Vickers test are called
microhardness tests.
 The Brinell and Rockwell are macrohardness
test.
 Knoop & Vicker’s tests used loads less than
9.8N. The indentations are small and are
limited to a depth of less than 19 mm.
 They are capable of measuring hardness in
small regions of thin objects
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24. • When force applied on a body, internal
resistance to load is stress.
• The strain may be elastic /reverts back to
original form or plastic/ permanent
deformation.
• The value of stress which causes an object to
give way or break is called strength of
material.
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25.  Force induced by or resisting an external
force.
 Stress= Force per unit area
 Stress is equal and opposite
in direction to the load or
external force.
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Axial
Tensile
Compressive

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Non Axial
Shear
Torsion
Bending

28.  Can be defined as change in length per unit
length of the body when subjected to stress.
 Has no definite unit.
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29.  Strain can either be elastic or plastic
 Elastic strain is strain that totally disappears
once the external load that caused it is
removed.
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30.  Plastic strain is strain that permanently
remains once the external load that caused
it is removed.
 It occurs when the force applied to the
atoms moves them so far from their
equilibrium position that they do not return
to it once the force is removed.
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Stress vs. Strain Graph

32.  Resistance of a liquid to flow
 Viscosity is controlled by internal frictional
forces and is measured in poise (Mpa/sec) or
centipoise.
 Study of flow characteristics of materials and
deformation is rheology.
 The term rheology was coined by Eugene C.
Bingham.
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33. •The success or failure of a given material
may be as dependent on its properties in the
liquid state as it is on its properties as a
solid.
•Materials like cements and impression
materials undergo a liquid-to-solid
transformation in the mouth.
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34. •Gypsum products used in the fabrication
of models and dies are transformed from
slurries into solid structures
•Amorphous materials such as waxes and
resins appear solid but actually are
supercooled liquids that can flow
plastically under sustained loading or
deform elastically under small stresses.
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35.  To explain viscous nature of some materials,
shear stress v/s shear strain rate curve can be
plotted.
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NEWTONIAN FLUID:
- an ideal fluid
- Shear stress proportional to strain rate
-Straight line on curve
-Viscosity(η)= shear stress(τ)/strain rate (ε)
-Constant velocity.
PSEUDOPLASTIC FLUID:
-viscosity decreases with increasing strain rate,
until it reaches a nearly constant value.
e.g. Ketchup, blood, nail-polish

37. DILATENT FLUID
Viscosity increase with increasing stress.
The material become more rigid under
stress(disadvantage)
e.g.-Acrylic denture base material, sand in water
PLASTIC FLUID
Material behaves rigid until a minimum of stress is
applied ,then it starts behaving like Newtonian
fluid.
e.g.- clay suspension, composite material
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38. 4.CREEP AND FLOW
The term creep implies a relatively small
deformation produced by a relatively
large stress over a long period of time
whereas flow implies a greater
deformation produced more rapidly with
a smaller applied stress.
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39. Mujtaba Ashraf 39
•Creep is defined as a time dependent plastic
strain of a material under a static load or
constant stress.
•Because of its low melting range, dental
amalgam can slowly creep from a restored
tooth site under periodic sustained stress,
such as would be imposed by patients who
clench their teeth.

40. •The term flow, rather than creep, has
generally been used in dentistry to describe
the rheology of amorphous materials such as
waxes.
•The flow of wax is a measure of its potential
to deform under a small static load even that
associated with its own mass.
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41.  What is Color?
 Sensation induced from light of varying
wavelengths reaching eye.
 Cone cells of retina are responsible for
spectral sensitivity.
 Determined visually by measurement of
hue, chroma, and value.
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42. Hue is defined as the
particular variety of
a color, shade, or tint.
E.g. RED, GREEN
YELLOW
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43. The intensity of the
color, degree of
saturation of a
particular hue.
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44. Value is defined as the
relative lightness
or darkness of a color or
the brightness of an object.
• Value of 0 = black
• Value of 10 = white
Higher value =
lighter shade
Lower Value =
darker shade
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45. •Professor Albert H. Munsell
•Basic principles first published in 1905
•Hues are divide into 10 gradations:
yellow, yellow-red, red, red-purple,
purple, purple-blue, blue, blue-green, green
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46. 46Mujtaba Ashraf

47.  The Commission Internationale de l’Eclairage
(CIE),
an international
color research group
published the CIELAB
color system in 1976.
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•3-dimensional color system
• L* refers to brightness (0 to 100)
• a* represents red (+a*) vs. green (-a*)
• b* indicates yellow (+b*) vs. blue (-b*).
•When a* and b* are zero, the L value
represents the continuum of black to white

49. • Opacity is the measure of
impenetrability of visible light.
•Transparency- capable of transmitting light
•Translucency-Diffused transmission of visible
light
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•Metamerism-Phenomenon
in which the color of
an object under one type of
light appears to change
when illuminated by different
light source.
Different light sources interact
differently with different materials.

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•Fluorescence is the absorption of light by a
material and the spontaneous emission of
light in a longer wavelength.
•In a natural tooth, it primarily occurs in the
dentin because of the higher amount of
organic material.
•UV light is absorbed and fluoresced back as
light primarily in the blue end of the
spectrum.

52.  The color of teeth encompasses only a small
portion of the total color space.
 The color ranges of human teeth have been
measured by different researchers at different
times and using different methods and color
notation systems.
 All of the studies indicate that human teeth
are in the yellow-red to yellow portion of the
spectrum, they are relatively high in Value
(light or bright), and they have a relatively low
Chroma (not too much color intensity).
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53.  Shade guides are used in determining the
color of natural teeth so that artificial
substitute restorations will possess similar
color and esthetics.
 Clinical shade selection involves direct visual
comparison of the different color samples
that are present in a shade guide with the
natural teeth and determination of which one
best matches the teeth.
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The VITAPAN Classical shade guide introduced in
1956 still is widely used for shade matching in
dentistry.
It has16 shade samples

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•VITA SYSTEM 3D-MASTER introduced in 1998.
•It has 26 shades, divided into group 1to5.
•Tabs are marked using a number-letter-number combination.
•First number i.e. 1-5 represent Value
•Letter L, M, R represent Hue from yellowish to reddish
•The second number designates Chroma

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Tips for shade-taking:
• Ensure the tooth condition is appropriate (e.g. clean,
hydrated) for matching
• Tooth shades should be determined in daylight or under
standardized daylight lamps and not under operation lamps.
• Because eyes usually tire after 5 -7 seconds, it is
recommended to make a selection quickly.
• Avoid bright colors in the shade-taking environment, i.e.
no lipstick, tinted eyeglasses, no bright-colored clothes.

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•Consider the selection distance. A selection made
at one to three feet is generally more useful than
one made in close proximity to the teeth.
•Evaluate the patient’s natural teeth to determine
their color characteristics by looking at the cervical
aspect of the teeth.
•Evaluate prospective shade guide specimens one
at a time by holding them next to the tooth being
matched .

61. Thermal Conductivity :
• Heat transfer through solid substances is most
commonly occur by means of conduction.
• The conduction of heat through metals occurs
through the interactions of crystal lattice
vibrations and by the motion of electrons and
their interaction with atoms.
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62. • The thermal conductivity, or coefficient of
thermal conductivity, is the quantity of heat
in calories per second that passes through a
specimen 1 cm thick having a cross-sectional
area of 1 cm2 when the temperature
difference between the surfaces
perpendicular to the heat flow of the
specimen is 1K.
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According to the second law of
thermodynamics,
Heat flows from points of higher temperature
to points of lower temperature.
•Materials that have a high thermal conductivity
are called conductors.
•Materials of low thermal conductivity are
called insulators.

64. • The International System (SI) unit or measure
for thermal conductivity is watt per meter per
second per degree Kelvin (W x m¯1 x s¯1 x
K¯1).
• The higher the thermal conductivity, the
greater is the ability of the substance to
transmit thermal energy, and vice versa.
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65. • Compared with a resin-based composite that
has a low thermal conductivity,
heat is transferred more rapidly away
from the tooth when cold water contacts a
metallic restoration because of its higher
thermal conductivity.
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•This increased conductivity of the metal
compared with that of the resin composite
induces greater pulpal sensitivity, which is
experienced as a negligible, mild,
moderate, or extreme discomfort,
depending on previous tooth trauma and
the pain response of the patient.

67.  The coefficient of thermal expansion describes how
the size of an object changes with a change in
temperature.
 It measures of the fractional change in size per degree
change in temperature at a constant pressure.
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Clinical Significance
•Close matching of the coefficient of thermal expansion
(α) is important between the tooth and the restorative
materials to prevent marginal leakage.
•Opening and closing of gap results in breakage of
marginal seal between the filling and the cavity wall,
this breakage of seal leads to marginal leakage,
discoloration & hypersensitivity.

69. • It is a measure of the rate at which a body with
a no uniform temperature reaches a state of
thermal equilibrium.
• The thermal conductivity of zinc oxide-
eugenol is slightly less than that of dentin, its
thermal diffusivity is more than twice that of
dentin.
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•The square root of thermal diffusivity is
indirectly proportional to the thermal
insulation ability, whereas the thickness of
the cement base is directly related to its
benefit as an insulator.
•Thus the thickness of the liner is a more
important thermal insulation factor than the
thermal diffusivity

71. • Relevance of thermal diffusivity :
• In the oral environment, temperatures are not constant
during the ingestion of foods and liquids. Under such
conditions, thermal diffusivity is important.
• Typical values of thermal diffusivity in units of 10-4
cm2/sec are as follows:
• pure gold-11,800
• Amalgam-960
• Composite- 19-73,
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72. • Water-14
• Glass ionomer- 22
• Dentin-18-26
• Enamel- 47
• zinc phosphate cement- 30
• Thus for a patient drinking ice water, the low specific
heat of amalgam and its high thermal conductivity
suggest that the higher thermal diffusivity favors a
thermal shock situation more than that is likely to occur
when only natural tooth structure is exposed to the cold
liquid.
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73. • For any thermal insulator, tooth structure must be
present in sufficient thickness for insulating dental
cements to be effective. When the layer dentin
between the bottom of the cavity floor and the pulp is
too thin, the dentist should place an additional layer
of an insulating base
• The low thermal conductivity of enamel and dentin
aids in reducing thermal shock and pulpal pain when
hot or cold foods are taken into the mouth.
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74. • Tarnish is a surface discoloration of the
metal or even a slight loss of the luster.
• This surface discoloration is due to either,
the formation of hard and soft deposits on
the surface, or the formation of thin films of
oxides, sulfides or chlorides.
• Tarnish is often the first step of corrosion
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75. • Stain or discoloration arises from pigment –
producing bacteria, drugs containing chemicals
such as iron or mercury & absorbed food debris.
• Although deposits are the main cause of the tarnish
in the oral environment, surface discoloration may
also arise on a metal from the formation of thin
films, such as oxides, sulfides, or chlorides.
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76. • Corrosion is not only a surface discoloration but is a
disintegration of a metal by reaction with its
environment.
• Corrosion may occur due to moisture, atmosphere,
acid or alkaline solutions, & certain chemicals.
• Common e.g. rusting of iron, a complex chemical
reaction in which iron combines with oxygen in air &
water to form hydrated oxide of iron.
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77. • Various causes of tarnish & corrosion
• Eggs & other food contain significant amount of sulfur,
various sulfides (hydrogen or ammonium sulfide)
corrode silver, copper, mercury & similar metals present
in dental alloys.
• water, oxygen & chlorine ions present in saliva.
• various acidic solutions such as phosphoric , acetic, &
lactic acids at proper concentration & pH promote
corrosion.
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Classification of
corrosion
Chemical /
dry corrosion
Metal and
non-metals
Electrochemical
/wet corrosion
Dissimila
r metals
Heterogeneou
s surface
composition
Stress corrosion
Concentration cell
corrosion
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1) Non aqueous (dry) or chemical
corrosion:
•In which there is a direct combination of
metallic and non-metallic elements
•Electrolytes are absent
•e.g. oxidation, halogenations, or
sulfarization reaction.
•This type of corrosion is less susceptible
to occur in the mouth.
•Example, oxidation of metal surface
during soldering and heat treatment
procedures.

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2) Aqueous (wet) or electrolytic
corrosion:
Corrosion occurs in presence of water or
some other liquid electrolyte

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Types of electrolytic corrosion:
1) Galvanic cell corrosion.
2) Stress corrosion.
3) Concentration cell corrosion.

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Galvanic cell Corrosion
Galvanism is results of difference in potential
between dissimilar restoration in opposing or
adjacent tooth.

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Galvanic Shock
A pain sensation caused by electric current
generated by a contact between two
dissimilar metal forming a galvanic cell in
oral environment.

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Stress Corrosion
Degradation by the combined effect of
mechanical stress and corrosive
environment, usually in form of cracking.
Example, a metal which has been stressed
by burnishing produces the localized stress
in some part of structure
If stressed and under stressed metals are in
contact in an electrolyte the stressed area
will become anode and will corrode

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Concentration cell corrosion (difference in
composition of electrolyte)
A homogeneous metal or alloy can
undergo electrolytic corrosion where
there is a difference in electrolyte
concentration across the specimen.
Example; a metallic restoration which is
partly covered by food debris will differ
from that of saliva, and this can
contribute to the corrosion of the
restoration.

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87. • Certain metals develop a thin ,adherent , highly
protective film by reaction with the environment ,
such a metal is said to be passive.
• A thin surface oxide forms on chromium , a good e.g.
of a passivating metal, & stainless steel contain
sufficient amounts of chromium to passivate the
alloy.
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88. • Avoid using dissimilar metals.
• Patients should be warned against using household
bleaches for cleaning partial denture frameworks
or removable orthodontic appliances that are
alloyed with chromium.
• The surface of any dental restoration should be
smooth and lustrous. A polished, smooth surface
provides easier cleaning and prevents accumulation
of debris.
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89.  A proper knowledge of physical properties of dental
materials helps us in making correct choice for
various clinical restorations. This in turn increases
the durability and life span of the restoration.
 When tooth shade is selected using conventional
means, knowledge & skill of practitioner comes
into play.
 Technique based system provide dentist with
distinct advantage in creating highly esthetics ,
natural looking restoration
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90.  Phillips science of dental material 10th &
11th edition
 RESTORATIVE DENTAL MATERIALS – CRAIG
13th edition
 Dental Materials and Their Selection- 3rd
edition by William J. O'Brien
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