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metals and alloys used in prosthodontics
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METALS AND ALLOYDR. RITESH SHIWAKOTI
MScD PROSTHODONTICS
Ductility: the ability of a material to withstand permanent
deformation under a tensile load without rupture; ability of a
material to be plastically strained in tension.
Malleable : capable of being extended or shaped with a
hammer or with the pressure of rollers.
Vickers hardness number: (VHN) a measure of hardness
obtained with a diamond pyramidal indenter with a square
base and an angle of 136. VHNis proportional to the ratio of
the applied load to the area of the indentation. Devised in the
1920’s by engineers at Vickers, Ltd. in the United Kingdom
Modulus of elasticity : in metallurgy, the coefficient found by
dividing the unit stress, at any point up to the proportional
limit, by its corresponding unit of elongation (tension) or
strain. A ratio of stress to strain. As the modulus of elasticity
rises, the material becomes more rigid
Tensile stress : the internal induced force that resists the
elongation of a material in a direction parallel to the direction
of the stresses
Proportional limit : that unit of stress beyond which
deformation is no longer proportional to the applied
Corrosion resistance :A material's ability to resist deterioration caused by exposure to an environment .
Elongation at break also known as fracture strain, is the ratio between changed length and initial length after breakage of the test specimen. It expresses the capability of a material to resist changes of shape without crack formation
Sag resistance: the resistance to flow of a one-inch plug of uncured sealant after being placed on a vertical plane.
yield strength : the strength at which a small amount of permanent (plastic) strain occurs, usually 0.1% or 0.2%, and most frequently measured in MPa or psi
INTRODUCTION
In dentistry, metals represent one of the three
major classes of materials used for the
reconstruction of damaged or missing oral
tissues. Although metals are readily
distinguished from ceramics and polymers.
An opaque lustrous chemical
substance
that is a good conductor of heat and
electricity and, when polished, is a
good reflector of light.
- The Metals Handbook
(1992)
An alloy is a substance with metallic
properties that consists of two or more
chemical elements, at least one of
which is a metal.
The Metals Handbook (1992)
PROPERTIES
Exhibits a luster
Good thermal and electrical
conductors
High fracture toughness (KIc)
Ductile
Malleable
The Metals Handbook (1992)
Highly resistant to corrosion and
oxidation (Noble metals e.g. gold,
iridium, osmium, palladium, platinum,
rhodium, and ruthenium)
Most metals have a “white”
appearance (e.g., aluminium, silver,
nickel, palladium, tin, and zinc.) Non
white metal e.g. Gold and Copper
The Metals Handbook (1992)
CLASSIFICATION
HEAVY METALS
LIGHT METALS
NOBLE METALS
( HEAVY METALS)
DUCTILEBRITTLE
HIGH MELTING
www.bpc.edu
ALLOYS
A crystalline substance with metallic
properties that is composed of two or
more chemical elements, at least one
of which is a metal.
Pure metals are rarely used in
dentistry because they are weaker
than they are when mixed with other
metals.
APPLIED DENTAL MATERIALS –McCABBES AND WALLS
CLASSIFICATION
ADA CLASSIFICATION OF THE DENTAL CASTING ALLOY:
• High noble alloys “precious metals”
◦ at least 60% noble. 40% of which is gold. The remaining 40% is base metal
Noble alloys(semiprecious)◦ at least 25% noble (no gold requirements). 75%
base metal
Base metal alloys ◦ Less than 25% noble
National Bureau of Standards
Type I (Soft, Vickers hardness number
[HV] between 50 and 90)
Type II (Medium, HV between 90 and
120)
Type III(Hard, HV between 120 and
150)and
Type IV (Extra Hard, HV ≥150).
APPLIED DENTAL MATERIALS –McCABBES AND WALLS
ALLOY TYPES BY
DESCRIPTION
A) CROWN AND BRIDGE ALLOYS
1. Noble metal alloys:
i)Gold based alloy -type III and type IV gold alloys , low gold alloys
ii) Non-gold based alloy-Silver -palladium alloy
2. Base metal alloys:
i) Nickel-based alloys
ii) Cobalt based alloys
3. Other alloys:
i) Copper-zinc with Indium and nickel
ii) Silver-indium with palladium
B) METAL CERAMIC
ALLOY
. Noble metal alloys
for porcelain
bonding:
i) Gold-platinum -
palladium alloy
ii) Gold-palladium-
silver alloy
iii) Gold-palladium
alloy
iv) Palladium silver alloy
v) High palladium alloy
2. Base metal alloys
for porcelain
bonding:
i) Nickel -chromium
alloy
C) REMOVABLE
PARTIAL DENTURE
ALLOY
Although type-IV noble
metal alloy may be
used, majority of
removable partial
framework are made
from base metal
alloys:
1. Cobalt-chromium alloy
2. Nickel-chromium
alloy
3. Cobalt-chromium-
nickel alloy
4. Silver-palladium
alloy
5. Aluminum -bronze
Alloy type All-metal Metal-ceramic Removable partial
dentures
High noble Au-Ag-Cu-Pd Au-Pt-Pd Au-Ag-Cu-Pd
Metal ceramic alloys Au-Pd-Ag (5-12wt%
Ag)
Au-Pd-Ag (>12wt%Ag)
Au-Pd (no Ag)
Noble Ag-Pd-Au-Cu Pd-Au (no Ag) Ag-Pd-Au-Cu
Ag-Pd Pd-Au-Ag Ag-Pd
Metal-ceramic alloys Pd-Ag
Pd-Cu
Pd-Co
Pd-Ga-Ag
Base Metal Pure Ti Pure Ti Pure Ti
Ti-Al-V Ti-Al-V Ti-Al-V
Ni-Cr-Mo-Be Ni-Cr-Mo-Be Ni-Cr-Mo-Be
Ni-Cr-Mo Ni-Cr-Mo Ni-Cr-Mo
Co-Cr-Mo Co-Cr-Mo Co-Cr-Mo
Co-Cr-W Co-Cr-W Co-Cr-W
Al bronze
Classification of alloys for All-Metal restorations, metal ceramic restorations, and
frameworks for removable partial dentures.
ALLOY TYPE BY MAJOR ELEMENT: Gold-based, palladium-based, silver-based, nickel-based, cobalt-based and titanium-based .
ALLOY TYPE BY PRINCIPAL THREE ELEMENTS: Such as Au-Pd-Ag, Pd-Ag-Sn, Ni-Cr-Be, Co-Cr-Mo, Ti-Al-V and Fe-Ni-Cr.
(If two metals are present, a binary alloy is formed; if threeor four metals are present, ternary and quaternary alloys,respectively, are produced and so on.)
ALLOY TYPE BY DOMINANT PHASE SYSTEM: Single phase [isomorphous], eutectic, peritectic and intermetallic.
HISTORY AND ITS
PRESPECTIVETHE LOST-WAX PROCESS(1905)
"investment casting", "precision casting",
or cire perdue in French
Presented by Taggart
First used in dentistry for Inlay fabrication
A duplicate metal structure is cast from wax
model or a copy of the wax model
Examples: Taggart cast inlay (1907)
: Cave of the Treasure in Israel
http://en.wikipedia.org/wiki/Lost-wax_casting
Use of Gold made it more expensive
Performance compromised
Not good aesthetics
APPLIED DENTAL MATERIALS –McCABBES AND WALLS
Co-Cr replacement to Gold
(1933)
Used as base metal for removable partial
denture
Lighter in weight
Greater stiffness(elastic modulus)
Cheaper than gold alloy
PHILLIPS DENTAL MATERIALS
Porcelain-Fused-to-Metal
Process (1959) platinum and palladium were added into
gold alloy
Lowered the coefficient of thermal
expansion
Increased biocompatibility with porcelain
Thermally compatible metal-porcelain was
made
PHILLIPS DENTAL MATERIALS
The Gold Standard (1971)
Response to the increasing price of
gold introduced following changes:
1. Gold was replaced with palladium.
2. Palladium eliminated gold entirely.
3. Base metal alloys with nickel as the
major element eliminated the
exclusive need for noble metals
The United States abandoned the gold
standard in 1971.
APPLIED DENTAL MATERIALS –McCABBES AND WALLS
The Medical and Dental
Devices Act (1976)
Dental industry became under control of the
FDA
Dental alloy for prosthesis was classified as
passive implants
Manufactures were required to have a
quality system
-Organizations engaged in preparing standards for dental
materials ( George Corbley Paffenbarger )
Desirable Property of Dental
casting alloy Biocompatible : The material must
tolerate oral fluids and not release any
harmful products into the oral
environment.
Should have good corrosion
resistance
Good tarnish resistanceAPPLIED DENTAL MATERIALS –McCABBES AND WALLS
Non-allergic
Good aesthetics
Must have adequate thermal properties to tolerate
Melting range should be low enough to form smooth surface with the mould walls
Economic and easily available
APPLIED DENTAL MATERIALS –McCABBES AND WALLS
NOBLE METALS
Periodic table of the element shows
eight noble metals gold, the platinum
group metals (platinum, palladium,
rhodium, ruthenium, iridium, osmium),
and silver.
GOLD
Oldest dental restorative materials
Used since 4000 years
Phoenician used gold wire to bind
teeth
Etruscan and then roman used gold
strip to make fixed bridges
Atomic number is 79
Au – symbol
Gold content:Traditionally the gold content of dental
casting alloys have been referred to in terms of:
Karat FinenessKarat:It is the parts of pure gold in 24 parts of alloys.
For Eg: a) 24 Karat gold is pure goldb) 22 Karat gold is 22 parts of
pure gold and remaining 2 parts of other metal.
The term Karat is rarely used to describe gold content in current alloys.
Fineness:Fineness of a gold alloy is the parts per
thousand of pure gold. Pure gold is 1000 fine. Thus, if ¾ of the gold alloy is pure gold, it is said to be 750 fine
OCCURANCE
The metal occurs often in free
elemental (native) form, as nuggets or
grains in rocks, in veins between two
rocks and in alluvial deposits. Less
commonly, it occurs in minerals as
gold compounds, such as
with tellurium as calaverite, sylvanite
and krennerite
PROPERTIES
Dense
Soft
Ductile
Most malleable : 1 gram can be
beaten into 1 square meter
Lustre
Most corrosion resistant
USES
Dental wires to support mobile tooth
by Romans , Greeks , Arabs (5th
century BC to 11th century AD)
Reduce and fix fractured mandible
during world war
Filling teeth with gold foils
Inlay and onlay
Crown and bridges
GOLD CASTING ALLOYS:
ADA specification No. 5 classify dental gold casting
alloys as:
1. High Gold Alloys Type I
Type II
Type III
Type IV
2.Low Gold Alloys
3. White Gold Alloys
INLAY GOLD ALLOY
CROWN AND BRIDGE ALLOY
CASTING GOLD ALLOYS
TYPE 1 GOLD ALLOY
Soft
Strength – 50 to 90 HV
Casting subjects to very slight strength
COMPOSITION
Gold (Au) 85%
Silver (Ag) 11%
Copper (Cu) 03%
Platinum/ palladium (Pt/Pd) XX
Zinc (Zn) 1%
PROPERTY
Hardness (VHN) 50–90
Modulus of elasticity (GPa) 80
Tensile strength (MPa) 250
Proportional limit (MPa) 120
Elongation at break (%) 35
Melting range (ºC) 950–1100
USES
Inlay ( class 1, III , V cavities)
Electroforming
Gold foil for direct restoration
Telescopic crown
TYPE II
COMPOSITION
Gold (Au) 75%
Silver (Ag) 12%
Copper (Cu) 10%
Platinum/ palladium (Pt/Pd) 02
Zinc (Zn) 1%
PROPERTY
Hardness (VHN) 90-120
Modulus of elasticity (GPa) 80
Tensile strength (MPa) 340
Proportional limit (MPa) 200
Elongation at break % 25
Melting range (ºC) 900-980
USES
These are used for conventional inlay or
onlay restorations subject to moderate
stress, thick three quarter crowns,
pontics and full crowns. These are
harder and have good strength.
TYPE III
COMPOSITION
Gold (Au) 70%
Silver (Ag) 14%
Copper (Cu) 10%
Platinum/ palladium (Pt/Pd) 05
Zinc (Zn) 1%
PROPERTY
Hardness (VHN) 120 -160
Modulus of elasticity (GPa) 85
Tensile strength (MPa) 360
Proportional limit (MPa) 290
Elongation at break % 20
Melting range (ºC) 900-1000
USES
Inlays subject to high stress and for
crown and bridge in contrast to type I
and type II
TYPE IV
COMPOSITION
Gold (Au) 65%
Silver (Ag) 09%
Copper (Cu) 15%
Platinum/ palladium (Pt/Pd) 10%
Zinc (Zn) 1%
PROPERTY
Hardness (VHN) 150-230
Modulus of elasticity (GPa) 100
Tensile strength (MPa) 750
Proportional limit (MPa) 500
Elongation at break % 8
Melting range (ºC) 870-950
USES
These are used in areas of very high
stress, crowns and long span bridges. It
has lowest gold content of all four type
(Less than 70%) but has the highest
percentage of silver, copper, platinium
and Palladium. It is most responsive to
heat treatment and yield strength but
lowers ductility.
Comparative properties of
casting gold alloys
Hardness , Proportional limit , Strength
Type IV > Type III > Type II > Type I
Ductility and Corrosion resistance
Type I > Type II > Type III > Type IV
LOW GOLD CONTENT ALLOY
SILVER PALLADIUM ALLOY
Contains no gold
25% palladium
Contains small quantity of copper, zinc,
indium
Low density that affects the castability
Low ductility,
less corrosion resistance
Property Ag/Pd Gold (type 3)
Hardness (VHN) 120–220 120–160
Modulus of elasticity (GPa) 80–95 85
Proportional limit (MPa) 250 290
Elongation at break (%) 3–25 15–25
Melting range (ºC) 900–1100 900–1000
Density (g cm−3) 11–12 15–16
The main function of metal-ceramic
alloys is to reinforce porcelain, thus
increasing its resistance to fracture.
METAL CERAMIC ALLOYS
REQUIREMENT
1.They should be able to bond with
porcelain
2.Its coefficient of thermal expansion
should be compatible with that of
porcelain
3.Its melting temperature should be
higher than the porcelain
4.It should not stain or discolor
porcelain
The Gold-Platinum-Palladium (Au-Pt-Pd) System:
This is one of the oldest metal ceramic alloysystem. But these alloys are not used widelytoday because they are very expensive.
COMPOSITION
Gold – 75% to 88%
Palladium – Upto 11%
Platinum – Upto 8%
Silver – 5%
Trace elements like Indium, Iron and Tin for porcelain bonding.
Gold-Palladium-Silver (Au-Pd-Ag) System:
These alloys were developed in an attempt to overcome
the major limitations in the gold-platinum-palladium system
(mainly poor sag resistance, low hardness & high cost)
Two variations on the basic combination of gold, palladium
and silver were created and are identified as either high-silver
or low-silver group.
Composition (High Silver Group):
Gold – 39% to 53%
Silver – 12% to 22%
Palladium – 25% to 35%
Trace amount of oxidizable elements are added for porcelain
bonding.
Composition (Low Silver Group):
Gold – 52% to 77%
Silver- 5% to 12%
Palladium – 10% to 33%
Trace amounts of oxidizable elements for porcelain
bonding.
Gold-Palladium (Au-Pd) System:
This particular system was developed in an attempt toovercome the major limitations in the Au-Pt-Pdsystem and Au-Pd-Ag system. Mainly-
-Porcelain discoloration.
-Too high coefficient of thermal expansion &contraction.
COMPOSITION
Gold – 44% to 55%
Gallium – 5%
Palladium – 35% to 45%
Indium & Tin – 8% to 12%
Indium, Gallium and Tin are the oxidizable elementsresponsible for porcelain bonding.
Palladium-Silver (Pd-Ag) System
This was the first gold free system to be introduced in theUnited States (1974) that still contained a noble metal(palladium). It was offered as an economical alternativeto the more expensive gold-platinum-silver and gold-palladium-silver (gold based) alloy systems.
Composition: (available in two compo.)
1. Palladium – 55% to 60% Silver – 25% to 30%
Indium and Tin
2. Palladium – 50% to 55% Silver – 35% to 40%
Tin (Little or no Indium)
Trace elements of other oxidizable base elements arealso present.
BASE METAL ALLOYS-Nickel based
-Cobalt based
Alloys in both systems contain chromium as the second largest
constituent.
A classification of base metal casting alloys
Base metal
Casting alloy
Removable
Partial denture
Co-Cr
Co-Cr-Ni
Ni-Cr
Co-Cr-MoSurgical
Implant
Ni-Cr
Fixed
Partial denture
Cobalt-chromium alloys
These alloys are also known as ‘satellite’
because they maintained their shiny,
star-like appearance under different
conditions.
They have bright lustrous, hard, strong
and non-tarnishing qualities.
The chemical composition of these
alloys specified in the ISO Standard
for Dental Base Metal Casting is as
follows:
Cobalt main constituent
Chromium no less than 25%
Molybdenum no less than 4%
Cobalt + nickel + chromium no less
than 85%
COMPOSITION
Cobalt - 55 to 65%
Chromium - 23 to 30%
Nickel - 0 to 20%
Molybdenum - 0 to 7%
Iron - 0 to 5%
Carbon - upto 0.4%
Tungsten, Manganese, Silicon and Platinumin traces
The main purpose of the chromium is
to further harden the alloy by solution
hardening and also to impart corrosion
resistance.
Silicon – Increases ductlity
Molybdenum and beryllium refine the
grain structure and improve the
behaviour of base metal alloys during
casting
Carbon – controls the brittleness and
ductility
These alloys are also known as
‘satellite’ because they maintained
their shiny, star-like appearance under
different conditions
The Cobalt-Chromium alloys have
replaced Type IV gold alloys because
of their lower cost and adequate
mechanical properties. Chromium is
added for tarnish resistance since
chromium oxide forms an adherent
and resistant surface layer.
PHYSICAL PROPERTY
lighter in weight – lesser density
8 to 9 gms/cm3.
Fusion temperature: The casting temperature
of this alloy is considerably higher than that
of gold alloys. 1250oC to 1480oC.
A.D.A. specification No. 14 divides it into
two types, based on fusion temperature
(which is defined as the liquidus
temperature)
Type-I (High fusing) – fusing temperature greater
than 1300oC
Type-II (Low fusing)– fusing temperature lower
than 1300oC
Mechanical Properties:
Yield strength: It is higher than that of gold alloys.710Mpa (103,000psi).
Elongation: Ductlity is low ranges from 1 to 12%.
These alloys work harden very easily, so care mustbe taken while adjusting the clasp arms of the partialdenture
Hardness: 432 VHN.
Thus, cutting, grinding and finishing is difficult.
Modulus of elasticity: 22.5103Mpa. Due to high modulus of elasticity casting can be made more thinner, thus decreasing the weight of the R.P.D. Adjustment of clasp is not easy.
Tarnish and corrosion resistance: Formation of a layer ofchromium oxide on the surface of these alloys preventstarnish and corrosion in the oral cavity.
Solutions of hypochlorite and other compounds that arepresent in some denture-cleaning agents will causecorrosion in such base metal alloys.
Even the oxygenating denture cleansers will stain suchalloys.
Therefore, these solutions should not be used for cleaningcobalt-chromium base alloys.
Casting Shrinkage: The casting shrinkage is much
greater than that of gold alloys (2.3%), so limited
use in crown & bridge.
The high shrinkage is due to their high fusion
temperature.
Porosity: As in gold alloys, porosity is due to
shrinkage and release of dissolved gases which is
not true in case of Co-Cr alloys.
Porosity is affected by the composition of the alloys
and its manipulations.
APPLICATIONS:
1. Denture base
2.Cast removable partial denture framework.
3. Surgical implants.
4. Car spark plugs and turbine blades.
Nickel-chromium (Ni-Cr) System
The major constituents are nickel and chromium, with
a wide array of minor alloying elements.
The system contains two major groups:
-Beryllium free (class 1)
-Beryllium (class 2)
Of the two, Ni-Cr-Beryllium alloy are generally
regarded as possessing superior properties and
have been more popular
The chemical composition of these alloys
specified in the ISO Standard for Dental
Base Metal Casting Alloys:
Nickel main constituent
Chromium no less than 20%
Molybdenum no less than 4%
Beryllium no more than 2%
Nickel + cobalt + chromium no less than 85%
NICKEL-CHROMIUM
BERYLLIUM FREE
ALLOYSComposition:
Nickel – 62% to 77% Chromium –
11% to 22%
Boron, molybdenum, Niobium,
columbium and tantalum (trace
elements).
Advantages
1. Do not contain beryllium which is
harmful to technician and patient
2. Low cost
3. Low density means more casting
per ounce
Disadvantages
1.Cannot use with Nickel sensitive patients.
2.Cannot be etched. (Cr doesn’t dissolve
in acid)
3. May not cast as well as Ni-Cr-Be alloys
4.Produces more oxide than Ni-Cr-Be
alloys.
NICKEL-CHROMIUM-BERYLLIUM ALLOY
Composition:
Nickel – 62% to 82%
Chromium – 11% to 20%
Beryllium – 2.0%
Numerous minor alloying elementsinclude aluminum, carbon, gallium, iron,manganese, molybdenum, silicon, titaniumand /or vanadium are present
Advantages
1. Low cost
2. Low density, permits more
casting per ounce.
3. High sag resistance
4. Can produce thin casting
5. Poor thermal conductor
6. Can be etched to increase
retention
Disadvantages1.Cannot use with nickel sensitive patients
2.Beryllium exposure can harmful to technicians and
patients.
3. Proper melting and casting is a learned skill.
4. Bond failure more common in the oxide layer.
5. High hardness (May wear opposing teeth)
6. Difficult to solder
8. Difficult to cut through cemented castings
TITANIUM
Titanium is called “material of choice” in dentistry.
This is attributed to the oxide formation property
which forms basis for corrosion resistance and
biocompatibility of this material. The term 'titanium'
is used for all types of pure and alloyed titanium.
PROPERTY
-Resistance to electrochemical degradation
- Biological response
-Relatively light weight
-Low density (4.5 g/cm3)
-Low modulus (100 GPa)
-High strength (yield strength = 170-480 MPa; ultimate strength = 240-550 MPa)
-Passivity
-Low coefficient of thermal expansion (8.5 x 106/°C)
-Melting & boiling point of 1668°C & 3260°C
USES
Commercially pure titanium is used for
dental implants, surface coatings,
crowns, partial dentures, complete
dentures and orthodontic wires
STEEL Steel is an alloy of iron and carbon in which
the carbon content is less than 2%.
Carbon content makes it brittle
One phase consists of a very dilute solid
solution of carbon in iron (up to 0.02% C),
called ferrite. The other phase is a specific
compound of iron and carbon with formula
Fe3C, called cementite. The mixture of
ferrite and cementite is termed pearlite
Eutectic refers to the behaviour of an alloy of two mutually insoluble metals during crystallization.
Alloys with greater concentrations of carbon are called hypereutectoid alloys and those with smaller carbon contents, hypoeutectoidalloys.
The hypereutectoid alloys contain relatively greater amounts of cementite while the hypoeutectoid alloys contain greater amounts of ferrite.
Cementite is a very hard, brittle material whilst ferrite is softer and more ductile
SATINLESS STEEL
In addition to iron and carbon the stainless
steels contain chromium which improves
corrosion resistance. This is achieved by
the passivating effect in which the
chromium exposed at the surface of the
alloy is readily oxidized to form a tenacious
surface film of chromic oxide.
Nickel is also present in many stainless
steels. It contributes towards corrosion
resistance and helps to strengthen the alloy.
sufficient quantities of these two metals are incorporated, the austenitic structure remains even at room temperature.
One of the most commonly used stainless steels contains 18% chromium and 8% nickel (termed 18/8 stainless steel).
18/8 stainless steels are used in applications where heat hardening is not necessary, for example, for noncuttinginstruments, wires and occasionally as denture bases
When smaller quantities of chromium and
nickel are incorporated into steel it is
possible to produce an alloy which has
adequate corrosion resistance but which
can be hardened by heat treatment., 12%
chromium and little or no nickel. is capable
of forming a martensitic stainless steel.
This type of alloy is commonly used to
construct cutting instruments and probes.
Stainless steel denture bases Formed from very thin pressed/rolled
sheets of wrought stainless steel.
The wrought stainless steel sheets have high values of modulus of elasticity and proportional limit. This enables sufficient rigidity to be achieved with a very thin sheet of material.
Conducts heat through metallic plates thus ensuring the patient retains normal reflex reaction to hot and cold
1. THE JOURNAL OF PROSTHETIC DENTISTRY VOLUME 94
NUMBER 1
2. Glossary of prosthodontic terms – 2005
3. Phillips science of dental materials 11th edition – Aunacavice
4. The Use of Gold in Dentistry-J. A. Donaldson
5. Gold Alloys, Uses and Performance-Helmut Knosp
6. APPLIED DENTAL MATERIALS –McCABBES AND WALLS
7. Internet