Precious metal alloys /orthodontic courses by Indian dental academy

PRECIOUS METAL
ALLOYS
INDIAN DENTAL ACADEMY
Leader in continuing Dental Education
www.indiandentalacademy.com

 “ALL NOBLE METALS ARE PRECIOUS BUT
ALL PRECIOUS ARE NOT NOBLE METALS”
INDIAN DENTAL ACADEMY
LEADER IN CONTINUING DENTAL
EDUCATION

INTRODUCTION
 Noble metals:
• gold ,
• platinum,
• palladium,
• rhodium,
• ruthenium,
• iridium,
• osmium,
• and silver

 These are called noble metals as they do not form
oxides.
 Gold and palladium do not oxidise at any
tempratures

 Rhodium has a very good resistance to oxidation
at all temps.
 Osmium and ruthenium forms volatile oxides,
and palladium and iridium form oxides in
temperatures ranges from 400 to 600 to 1000
degrees centigrade respectively

 In 1971 the gold came into market and due to the
increasing price of that gold made it to be
replaced with palladium after that base metal
alloys had replaced these noble metal alloys
completely.
 The use of palladium in cars as a catalyst
increased its cost and demand.

ALLOYS
 . The alloys are compounded to produce
properties most acceptable for their intended
dental applications, such as simple inlays,
bridges, removable cast restorations, solders, (or)
wrought wire forms

HISTORICAL PERSPECTIVE ON DENTAL
CASTING ALLOYS
 The history of dental casting alloys has been
influenced by 3 major factors:
 1.The technologic changes of dental prosthesis.
 2.Metallurgic advancements; and
 3.Price changes of the noble metals since 1968.

 Taggart’s presentation to the New York
odontological group in 1907 on the fabrication of
cast inlay restorations often has been
acknowledged as the first reported application of
the lost wax technique in dentistry
 The inlay technique described by Taggart was an
instant success.

 It soon led to the casting of complex inlays such
as onlays, crowns, FPD, and RPD frame works.
 Because pure gold did not have the physical
properties required of these dental restorations,
existing jewellery alloys were quickly adopted.
These gold alloys were further strengthened with
copper, silver (or) palladium

 In 1932, the dental materials group at the
National Bureau of Standards surveyed the alloys
being used and roughly classified them as Type 1,
2, 3 and 4.

 By 1948, the composition of Dental Noble metal
alloys for cast metal restorations had become
rather diverse, with these formulations, the
tarnishing tendency of the original alloys
apparently had disappeared. It is now known that
in gold alloys, palladium is added to counter act
the tarnish potential of silver.

 The base metal removable partial denture alloys
were introduced in the 1930s. Since that time,
both Nickel-chromium and cobalt-chromium
formulations have become increasingly popular
compared with conventional Type IV gold alloys

 by 1978, the price of gold was climbing so
rapidly the attention focused on the noble metal
alloys. To reduce the precious metal content yet
retain the advantages of the noble metals for
dental

PROPERTIES OF NOBLE METALALLOYS
 GOLD :
 Pure gold is a soft, malleable, ductile metal that does
not oxidize under atmospheric conditions and is
attacked by only a few of the most powerful oxidizing
agents.
 It has a rich yellow colour with a strong metallic
luster.
 Although it is the most ductile and malleable of all
metals, it ranks much lower in strength.

 The pure metal fuses at 1063°C, which is only
20° below the melting point of copper (1083°C).
 Small amounts of impurities have a pronounced effect
on the mechanical properties of gold and its alloys.

 The presence of less than 0.2% lead will cause
gold to be extremely brittle.
 Mercury in small quantities also has a harmful
effect on its properties.

 Gold is nearly as soft as lead, with the result that in
dental alloys, coins, and articles of jewellery it must
be alloyed with copper, silver, platinum and other
metals to develop the necessary hardness, durability
and elasticity.
 The specific gravity of pure gold is between 19.30 and
19.33, making it one of the heavy metal.

 Air (or) water at any temperature does not affect (or)
tarnish gold.
 Gold is not soluble in sulfuric, nitric (or)
hydrochloric acids

PALLADIUM
 Palladium is not used in the pure state in
dentistry, but it is used in many dental alloys,
combined with either gold (or) silver.
 It is cheaper than platinum and since it imparts
many of the properties of platinum to dental
alloys, it is often used as a replacement for
platinum.

 Palladium is a white metal some what darker than
platinum.
 Its specific gravity is 11.4 (or) about half that of
platinum.

 It is a malleable and ductile metal with a melting
point of 1555°C, which is the lowest of the
platinum group of metals.
 The metals has the quality of absorbing (or)
occluding large quantities of hydrogen gar when
heated. This can be an undesirable quality when
alloys containing palladium are heated with an
improperly adjusted gas air torch.

IRIDIUM, RUTHENIUM, AND RHODIUM
 Small amounts of iridium are some times present
in dental alloys, either as impurities combined
with platinum (or) as additions to modify the
properties.
 As little as 0.005% (50 ppm) is effective in
refining the grain size of cast gold alloys.

 Ruthenium produces a similar effect.
 Iridium is a hard metal that is quite brittle, white
with a high sp. Gravity of 22.42 and an
exceptionally high melting point estimated to be
about 2440'C

SILVER
 Silver is malleable and ductile, white, the best-
known conductor of heat and electricity, and
stronger and harder than gold but softer than
copper.
 It melts at 960.5'C, which is below the melting
point of both gold and copper.

 It is unaltered in clean, dry air at any temperature
but combines with sulfur, chlorine and
phosphorous, (or) vapors containing these
elements (or) their compounds.
 Foods containing sulfur compounds cause severe
tarnish on silver.

 Pure silver is seldom employed in dental
restorations because of the black sulfide
formation on the metal in the mouth, although it
is used extensively for small additions to many
gold alloys.

 Addition of small amounts of palladium to silver
containing alloys prevents the rapid corrosion of
such alloys in the oral environment.

KARAT AND FINENESS OF GOLD
 For many years the gold content of gold alloys
has been described on the basis of karat, (or) in
terms of fineness, rather than by weight %.
 The term karat refers to the parts of pure gold
in 24 parts of an alloy.

 For many years the gold content of gold alloys
has been described on the basis of karat, (or) in
terms of fineness, rather than by weight %.
 The term karat refers to the parts of pure gold
in 24 parts of an alloy.

 Fineness describes gold alloys by the number of
parts per 1000 of gold. For example, pure gold
has a fineness of 1000, and 650 fine alloy has a
gold content of 65%. Thus the fineness rating is
to timer the gold % in an alloy.

CLASSIFICATION OF DENTAL CASTING
ALLOYS (OR) DENTAL GOLD ALLOYS
 Type I
 Type II
 Type III and
 Type IV

 Type I (Soft): These alloys are limited to use in
inlays that are subject only to slight stress during
mastication

 Type II (Medium): These medium alloys can be
used for all types of cast inlays and onlays.

 Type III (Hard): These alloys are most
acceptable for crowns, thin 3/4th crowns, and
anterior and posterior bridge abutments, which
should not be cast from the softer and weaker
Type I and Type II alloys.

 Type IV (Extra hard): These alloys are designed
to have sufficient strength and adequate
properties for cast removable partial dentures
with clasps, precision cast fixed bridges and ¾
crowns, which are not subjected to hard working
(or) burnishing operations.

 Composition:
 The composition of the gold casting alloys
that meet the requirements of ADA Sp. No. 5 are
given in the Table below:

TYPE GOLD PLATINUM PALLADIUM
I 81-83% - 0.2-4.5%
II 76-78% - 1-3%
III 73-77% - 2-4%
IV 71-74% 0-1% 2-5%

 It is apparent from the Table that there is some
reduction in gold content when a comparison is
made between Type I and IV alloys.
 An increase in copper content is observed as
the gold content is decreased.
 An increase in the zinc content also occurs in
Type IV alloys.

 Platinum is rarely added to Type 1 gold alloys,
but a small amount of palladium is always added
to all 4 types.

PROPERTIES OF GOLD CASTING ALLOYS
Type
Vickers hardness number
(Kg/mm2) Softened
Yield strength, (0.1% Offset) MPa Elongation
Minimum Maximum
Softened
minimum
Hardened
minimum
Softened
minimum
Hardened
minimum (%)
I 50 90 None None 18 None
II 90 120 140 None 12 None
III 120 150 200 None 12 None
IV 150 None 340 500 10 2

 From the above table it may be seen that as the
hardness increases from Type I to Type IV, the
yield strength and tensile strength values are also
increased, and the elongation generally
decreased.

 Soft alloys have a higher degree of elongation
and a relatively greater quality of ductility than
the alloys of higher hardness values

 Melting range:
 its between 920-960
 The melting range of the alloy is important for
selecting the type of investment and type of heating
source needed
 Density:
 It indicates the number of dental castings made from
an unit weight of the metal.
 Gold alloys are lighter than pure gold

 The Castability of the alloy is also effected by the
density
 Hardness:
 this indicates the ease of with which these alloys can
be cut , ground , and polished
 Gold alloys are generally more user friendly

 Elongation:
 it indicates the ductility of the material
 Alloys with low elongation are brittle

 Modulus of elasticity:
 It indicates the stiffness/ flexibility of the metal
 Tarnish and corrosion resistance:
 gold alloys are resistant to tarnish and corrosion
 Casting shrinkage:
 It is less than 1.25 and 1.65% when compared to base
metal alloys

 This shrinkage occurs in 3 stages
1. Thermal contraction of the liquid material
2. Contraction of the metal while changing from liquid
to solid state.
3. Thermal contraction of the solid metal as it reaches
room temperature

 Biocompatibility: def :ability of a material to
elicit an appropriate biological response in an
given application in a body
 gold alloys are relatively compatible
 Casting investment: gypsum bonded investments
are used for gold alloys because of their low
fusion temperature

FUSION TEMPERATURES OF DENTAL
GOLD CASTING ALLOYS
METAL (or) ALLOY TYPICAL FUSION TEMPERATURE (°C)
Type I 1005-1070
Type II 900-970
Type III 875-1000
Type IV 875-1000

 The fusion temperatures are important factors in
choosing the type of investment to be used.
 Alloys having fusion temperatures higher than
about 1100°C should not be cast into calcium
sulfate bonded investment

 In 1984 the ADA proposed a simple classification
of dental casting alloys.
ALLOY TYPE TOTAL NOBLE METAL CONTENT
High noble metal
Contains  40 wt% AV and  60% wt of the
noble metal elements (Au+Ir+OS+Pt+Rh+Ru).
Noble metal Contains  25% of the noble metal elements.
Predominantly base metal Contains < 25 wt% of noble metal elements.

 Many manufacturers have adopted this
classification to simplify the communication
between dentists and dental laboratory
technologists.

 CLASSIFICATION OF ALLOYS FOR ALL
METAL RESTORATIONS METAL,
CERAMIC RESTORATIONS AND FRAME
WORKS FOR REMOVABLE PARTIAL
DENTURES

Alloy Type All-Metal Metal-Ceramic Removable Partial Dentures
High noble
Au-Ag-Cu-Pd metal ceramic
alloys
Au-Pt-Pd
Au-Pd-Ag (5-12 wt% Ag)
Au-Pd-Ag (>wt 12% Ag)
Au-Pd (no Ag)
Au-Ag-Cu-Pd
Noble
Ag-Pd-Au-Cu
Ag-Pd
Metal ceramic alloys
Pd-Au (no Ag)
Pd-Au-Ag
Pd-Ag
Pd-Cu
Pd-Co
Pd-Ga-Ag
Ag-Pd-Au-Cu
Ag-Pd

HEAT TREATMENT OF HIGH NOBLE AND
NOBLE METALALLOYS:
 Gold alloys can be significantly hardened if the
alloy contains a sufficient amount of copper.
 Types I and II alloys usually do not harden, (or)
harden to a lesser degree than do the types III and
IV alloys.

 The actual mechanism of hardening is probably
the result of several different solid - solid
transformations

 SOLUTION HEAT TREATMENT: Alloys that
can be hardened can of course, also be softened.
In metallurgical terminology the softening heat
treatment is referred as
 AGE HARDENING: The hardening heat
treatment is termed so

SOFTENING HEAT TREATMENT
 The casting is placed in an electric furnace for 10
minutes at a temperature of 700°C (129°F) and
then it is quenched in water
 The tensile strength, hardness and proportional
limit are reduced by such a treatment but the
ductility is increased

 The softening heat treatment is indicated for
structures that are to be ground, shaped (or)
otherwise cold worked, either in or out of the
mouth.


HARDENING HEAT TREATMENT:
 The age hardening of the dental alloys can be
accomplished 'in several ways. One of the most
practical hardening treatments is by "Soaking"
(or) aging the casting at a specific temperature for
a definite time, usually 15 to 30 minutes, before it
is water quenched.

 The aging temperature depends on the alloy
composition but is generally between 200°C and
450°C.
 Ideally, before the alloy is given an age-hardening
treatment, it should be subjected to a softening
heat treatment to relieve all strain hardening

 The hardening heat treatment is indicated for
metallic partial dentures, bridges, and other
similar structures.

CASTING SHRINKAGE:
 Most metals and alloys, including gold and noble
metal alloys, shrink when they change from the
liquid to the solid state
 The values for the casting shrinkage differ for the
various alloys presumably because of differences
in their composition

Alloy Casting shrinkage (%)
Type I, Gold base 1.56%
Type II, Gold base 1.37%
Type III, Gold base 1.42%
Ni-Cr-Mo 2.3%

SILVER PALLADIUM ALLOYS:
 These alloys are white and predominantly silver
'm composition but have substantial amounts of
palladium (at least 25/o) that provide nobility and
promote the silver tarnish resistance. They may
(or) may not contain copper and a small amount
of gold.

 The copper (Cu) free Ag-Pd alloys may contain
70% to 72% silver and 25% palladium and may
have physical properties of a Type III gold alloy.
 Other silver-based might contain roughly 60%
silver 25% palladium and as much as 15% or
more of copper and may have properties more
like a Type IV gold alloy.

 Because of the increasing interest in aesthetics by
dental patients, a decreased use of all metal
restorations has occurred during the past decade
 The use of metal ceramic restorations in posterior
sites has increased relative to the use of all metal
crowns and onlays.

HIGH NOBLE ALLOYS FOR METAL
CERAMIC RESTORATIONS
 The original metal ceramic alloys contained 88%
gold and were much to soft for stress bearing
restorations such as FPD. Because there was no
evidence of a chemical bond between these alloys
and dental porcelain, mechanical retention and
undercuts were used to prevent detachment of the
ceramic veneer

 Using the stress bond test, it was found that the
bond strength of the porcelain to this type of alloy
was less than the cohesive strength of the
porcelain itself
 This mean that if the failure occurred in the
metal-ceramic restoration, it would most probably
arise at the porcelain metal interface.

 By adding less than 1% of oxide forming
elements such as iron, indium, and tin to this high
gold content alloy, the porcelain metal bond
strength was improved by a factor of 3. Iron also
increase the proportional limit and strength of the
alloy.

 This 1% addition of base metals to the gold,
palladium and platinum alloy was all that was
necessary to produce a slight oxide film on the
surface of the substructure to achieve a porcelain-
metal bond strength level that surpassed the
cohesive strength of porcelain itself.

THE HIGH NOBLE ALLOYS FOR METAL
CERAMIC RESTORATIONS ARE:

GOLD-PLATINUM - PALLADIUM ALLOYS
 These9alloys have a gold content ranging up to
88% with varying amounts of palladium,
platinum and small amounts of base metals.
 Some of these alloys are yellow in colour. Alloys
of this type are susceptible to sag deformation
and FPD's should be restricted to 3-unit spans,
anterior cantilevers (or) crowns

Gold-Palladium-Silver Alloys:
 These gold based alloys contain between 39%
and 77% upto 35% palladium and silver levels as
high as 22%.
 The silver increases thermal contraction
coefficient but it also has a tendency to discolour
some porcelains

Gold-Palladium Alloys:
 A gold content ranging from 44% to 55% and a
palladium level of 35% to 45% is present in these
metal ceramic alloys, which have remained
popular despite their relatively high cost.

 The lack of silver results in a decreased thermal
contraction coefficient and the freedom from
silver discolouration of porcelain.

 Alloys of this type must be used with porcelains
that have low coefficients on to avoid the
development of axial and circumferential of
thermal contraction to avoid the development of
axial and circumferential tensile stresses in
Porcelain during the cooling part of the porcelain
firing cycle.

NOBLE ALLOYS FOR METAL
CERAMIC RESTORATIONS

PALLADIUM BASED ALLOYS
 Palladium - Silver alloys.
 Palladium Copper alloys:
 Palladium - Cobalt alloys:
 Palladium- gold alloys:
 Palladium - Gallium - Silver and Palladium -
Gallium – Silver – Gold alloys

PALLADIUM BASED ALLOYS
 Noble palladium based alloys offer a compromise
between the high noble gold alloys and the
predominantly base metal alloys
 The price per ounce of a palladium alloy is
generally one half to one third that of a gold
alloys.

 The density of a palladium based alloy is midway
between that of base metal and of high noble
alloys

Palladium - Silver Alloys:
 Pd-Ag alloys were introduced widely in the late
1970s. This alloy type was introduced to the U.S.
market in 1974 as the first gold free noble metal
available for metal ceramic restorations

 Pd-Ag alloys enjoyed wide spread popularity for
a few years after they were introduced, but their
popularity has declined some what in recent years
because of their tendency to discolour porcelain
during firing.

 The compositions of Pd-Ag alloys fall within a
narrow range: 53% to 61% palladium and 28% to
40% silver
 Tin (or) Indium or both are usually added to
increase alloy hardness and to promote oxide
formation for adequate bonding of porcelain. In
some of these alloys, the formation of an internal
oxide rather than an external oxide has been
reported.

 Other palladium alloys contain 75% to 90%
palladium and no silver and were developed to
eliminate the greening problem some of the high
palladium alloys develop a layer of dark oxide on
their surface during cooling from the degassing
cycle, and this oxide layer has proven difficult to
mask by the opaque porcelain.

 Other high palladium alloys such as the Pd-Ga-
Ag-Au type seem not to be plagued by this
problem.
 The replacement of gold by palladium raises the
melting range but lower the contraction
coefficient of an alloy. Increases the silver
content tends to lower the melting range and
raises the contraction coefficient

 Because of their high silver contents compared
with the gold based alloys, the silver
discoloration effect is most severe for these
alloys. Gold metal conditioners or ceramic
coating agents may minimize this effect.

 The low specific gravity of these alloys (10.7-
11.1) combined with their low intrinsic cost
makes these alloys attractive as economical
alternatives to the gold based alloys.

PALLADIUM-COPPER ALLOYS:
 This alloy is comparable in cost to the Pd-Ag
alloys
 Because of their low melting range of
approximately 1170°C to 1190°C, these alloys are
expected to be susceptible to creep deformation
(Sag) at elevated firing temperatures

 These alloys contain between 74-80% palladium
and 9-15% copper
 One should be aware of the potential effect on
aesthetics of the dark brown (or) black oxide
formed during oxidation and subsequent
porcelain firing cycles

 Care should be taken, to mask this oxide
completely with opaque porcelain and to
eliminate the un-aesthetic dark band that develops
at metals porcelain junctions.
 The Pd-Cu alloy have yield strengths upto
1145MPa.

 Elongation values of 5% to 11% and hardness
values as high as some base metal alloys.
 Thus, these alloys would appear to have a poor
potential for burnishing except when the marginal
areas are relatively thin.

PALLADIUM-COBALT ALLOYS:
 This alloy group is comparable in cost to the Pd-
Ag and Pd-Cu alloys. They are often advertised
as gold free, nickel-free, beryllium-free, and
silver-free alloys.
 . The reference to nickel and beryllium indicates
that these alloys, as is true with the other noble
metals, are generally considered biocompatible.

 This Pd-Co group is the most sag resistant of all
noble metal alloys.
 The noble metal content (based on palladium)
ranges from 78% to 88%.
 The cobalt content ranges between 4 and 10wt%
over commercial alloy contains 8% gallium.

 An example of typical properties of a Pd-Co alloy
is as follows:
 Hardness 250DPH
 Yield strength 586MPa
 Elongation 20% and
 Modulus of elasticity 85.2Gpa

 Although these alloys are silver-free,
discolouration of porcelain can still result because
of the presence of cobalt

PALLADIUM-GALLIUM-SILVER AND PALLADIUM-GALLIUM-SILVER-GOLD ALLOYS
 These alloys are the most recent of the noble
metals. This group of alloys was introduced
because they tend to have a slightly lighter
coloured oxide than Pd-Cu (or) Pd-Co alloys and
they are thermally compatible with lower
expansion porcelains.

 The oxide that is required for bonding to
porcelain is relatively dark, but it is somewhat
lighter than those of the Pd-Cu and Pd-Co alloys
 The silver content is relatively low (508 wt%)
and is usually inadequate to cause porcelain
“greening”.

GREENING:
 It is believed that the colloidal dispersion of the
silver atoms entering body and incisal porcelain
or the glazed surface from vapour transport or
surface diffusion may cause colour change
including green, yellow-green, yellow-orange,
orange, and brown hues

 . One theory that has been proposed for this
greenish yellow discoloration, popularly termed
"Selling" is that the silver vapour escapes from
the surface of these alloys during firing of the
porcelain, diffuses as ionic silver into the
porcelain and is reduced to form colloidal
metallic silver in the surface layer of porcelain.

 Porcelains with higher sodium contents are
believed to exhibit a more intense discoloration
because of more rapid silver diffusion in sodium
containing glass
 This hypothesis is based on observations of
greater discoloration in lighter shades of
porcelain and in porcelain with lower opacifier
contents and higher sodium content.

 The intensity of discolouration ( chroma ) usually
increases in the cervical region because surface
diffusion of silver from marginal metal provides a
higher localised silver concentration.

 This greening is more in:
 1.high silver content alloys
 2.lighter shades multiple firing procedures
 3. higher temperatures
 4. body porcelain in direct contact with the alloys
vacuum firing cycles
 5. and certain brands of porcelain

 Claire Manaranchea, Helga Hornbergerb.
“A proposal for the classification of
dental alloys according to their resistance
to corrosion”.
 The purpose of this study was to establish a
method to compare and classify dental alloys in
relation to their resistance to corrosion

 Results.
 High gold alloys had a similar polarization curve
than gold. The same effect was observed for Pd–
base alloys, their curves were similar to the one
of palladium.
 The ions released during chemical corrosion
were non-precious metallic ions.
 Thereby Ni–Cr alloys were found to release the
most ions.

 Au–Pt alloys showed the highest release of ions
compared with other precious alloys but low
compared with Ni–Cr.
 Electrochemical corrosion was more aggressive
than chemical corrosion and every type of
elements was etched,
 the higher the precious metal content, the higher
the resistance to corrosion of the alloy.

 N. Silikas a, P.L. Wincott b, D. Vaughanb,
D.C. Wattsa, G. Eliadesc,∗
“Surface characterization of precious
alloys treated with thione metal primers”
 Objectives. To characterize the effect of two
thione metal primers with phosphate groups
on the surface morphology and composition
of two noble Prosthodontic alloys.

 Results.
 After Alloy Primer treatment, Polarised Light
Microscopy revealed a crystalline phase
dispersed in an amorphous phase on both the
alloys tested.
 MP demonstrated a fibrial arrangement with the
most dense structure found on the Hi–Pd alloy.
Fourier-transform infrared micro spectroscopy
failed to clearly resolve the presence of S H
peaks on alloy surfaces.

 Moreover, NH and P S peaks were identified
denoting the presence of original thione
tautomers.
 In both primers, phosphates were detected in a
dissociative state ( PO3 2−).

CONCLUSION
 Though these noble metals are costlier than base
metal alloys their properties like biocompatibility
gained them a lot of importance in dentistry
 So one should have a through knowledge of these
precious metal alloys

 REFERENCES:
1. ANUSAVICE- PHILLIPS SCIENCES OF
DENTAL; MATERIALS- 19TH EDITION
2.DENTAL MATERIALS PROPERTIES
AND MANIPULATION –CRAIG OBREIN
POWERS 5TH EDITION
3. E.C. COMBE NOTES ON DENTAL
MATERIALS, 6TH EDITION

4. Claire Manaranchea, Helga
Hornbergerb. “A proposal for the
classification of dental alloys according
to their resistance to corrosion”.
5. N. Silikas a, P.L. Wincott b, D.
Vaughanb, D.C. Wattsa, G. Eliadesc,∗
“Surface characterization of precious
alloys treated with thione metal primers”

Precious metal alloys /orthodontic courses by Indian dental academy

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