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1. NOBLE METAL ALLOYS
INTRODUCTION
For casting of dental restorations, it is necessary to combine various
metals to produce alloys with adequate properties, for dental applications.
These alloys are produced largely from gold combined with other noble metals
and certain base metals to produce properties most acceptable for their intended
dental applications such as inlays, onlays, bridges, removable cast restorations
etc.
The noble metals are those elements with a good metallic surfaces that
retain their surfaces in dry air.
The 6 metals of the platinum group are  platinum, palladium, iridium,
rhodium, osmium, ruthenium, and along with gold, are called noble metals.
The history of dental casting alloys has been influenced by three major
factors:
1) The technologic changes of dental prosthesis.
2) Metallurgic advancements.
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 Jaggart was an instant success.
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2.  It soon led to the casting of complex inlays such as onlays, crowns, fixed
partial dentures and removable partial denture frameworks.
 Because pure gold did not have the physical properties required or these
dental restorations, existing jewellery alloys were quickly adopted.
 These gold alloys were further strengthened with copper, silver, or platinum.
 In 1932, the dental materials group at the National Bureau of Standards
surveyed, the alloys being used and roughly classified them as:
Type I – Soft-VHN-50 to 90.
Type II – Medium  VHN of 90 to 120.
Type III – Hard  VHN of 120 to 150.
Type IV – Extra Hard  VHN ≥ 150.
 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.
 By 1978, the price of gold was climbing so rapidly that attention focused on
the noble metal alloys – to reduce the precious metal content, yet retain the
advantages of the noble metals for dental use.
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3. Desirable properties of casting alloys:
Cast metals used in dental laboratories must exhibit the following
properties:
1) It should be biocompatible.
2) Easy to melt.
3) Easy to carry out casting, brazing (soldering) and polishing.
4) Little solidification shrinkage.
5) Minimal reactivity with the mold material.
6) Good wear resistance.
7) High strength and sag resistance (metal ceramic alloys).
8) Excellent tarnish and corrosion resistance.
Classification of Dental Casting Alloys
According to ADA specification No.5, the castings alloys can be
classified as:
1) Type I (Soft) small inlays, easily burnished and for restorations subject
to very slight stress such as inlays.
2) Type II (Medium)  Inlays subject to moderate stress including onlays,
thick ¾ crowns, thin cast backings, abutments, pontics and full crowns.
3) Type III (Hard)  Inlays subject to high stress including thin ¾ crowns,
thin cast backings, abutments, pontics, full crowns denture bases and
short span fixed partial dentures.
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4. 4) Type IV (Extra hard)  Inlays subjected to very high stresses including
denture base bars and clasps, partial dentures, frameworks and long-
span fixed dentures, full crowns are often made for this type high
stresses such as endodontic posts & core.
The development of modern direct tooth coloured filling materials has
almost eliminated the use of type I- II alloys. Types I and II alloys are often
refined to as “Inlay alloys. Types III and IV are generally called “Crown &
bridge” alloys.
In 1984, the ADA proposed a simple classification for dental casting
alloys:
Alloy Type Total Noble Metal Content
1) High noble metal.
2) Noble metal.
3) Predominantly base metal.
Contains≥ 40wt%Au+≥60wt% of noble
metal elements (Au+IS+Os+Pd+Rh+Ru).
Contains ≥25wt% of the noble metal
elements.
Contains <25wt% of the noble metal
elements.
According to Marzouk
Type I
Type II
1) Class I – Gold and platinum group based alloys
Type III
Type IV
2) Class II  Low gold alloys (gold content <50%).
3) Class III  Non-gold palladium based alloys.
4) Class IV  Nickel-chromium based alloys.
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5. 5) Castable moldable ceramics.
Other Classifications:
1. Yellow golds  Yellow coloured
 Gold content > 60%
2. Low gold / economy gold.
 Usually yellow coloured.
But with gold content 60%
3. White Golds  White coloured
 Gold content > 50%
CLASSIFICATION OF ALLOYS FOR ALL-METAL RESTORATION,
METAL CERAMIC RESTORATIONS, AND FRAMEWORKS FOR
REMOVABLE PARTIAL DENTRUES
Alloy Type All-Metal Metal-Ceramic Removable Partial
Denture
1) High Noble Au-Ag-Cu-Pd
Metal-Ceramic alloys
Au-Pt-Pd
Au-Pd-Ag (5-12wt%Ag)
Au-Pd-Ag (>12wt%Ag)
Au-Pd (Zn Ag)
Au-Ag-Cu-Pd
2) Noble Ag-Pd-Cu
Ag-Pd
Metal-Ceramic alloys
Pd-Au (Zo Ag)
Pd-Au-Ag
Pd-Au
Pd-Cu
Pd-Co
Pd-Ga-Ag
Ag-Pd-Au-Cu
Ag-Pd
Note : The principle reasons that alloys for all-metal restorations cannot be
used for metal-ceramic restorations are that: 1) the alloys may not form thin,
stable oxide layers to promote bonding to porcelain, 2) their melting range may
be too low to resist sag deformation or melting at porcelain firing temperatures
their thermal contraction co-efficients may not be close rough to those of
commercial porcelains.
Ingredients of noble metal alloys : The most important element in dental
gold alloys are gold, copper, silver, platinum metals and zinc).
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6. Gold
1) Gold is primarily responsible for  Deformability (ductility)
 Ranks lowest in strength.
 Characteristic yellow colour with a strong metallic luster.
 Density (sp.gravity)  19.3g/cm3.
 Tarnish resistance.
 Fusion temperature – 1063°C.
 Not soluble in sulphuric, nitric or hydrochloric acids.
Lowest density necessitates more force in centrifugal casting to
compensate for lower wt/vol.
However, lower density will allow more restorations per unit wt which
can be economical to some extent lowest density necessitates more force in
centrifugal castings to compensate for the lower wt/vol. High density enables
case of castings. Alloys for dental use should be atleast 16K resistance.
2) Platinum : May be added to:
1. Strengthen the alloy.
2. Raise the fusion point (1755).
3. Import rigidity, nobility, hardness.
4. Whiten the alloy.
5. Sp gravity 21.37
6. Also malleable and ductile.
Its coefficient of exp is close to that of porcelain to prevent buckling of
the metal or fracture of porcelain during changes in temperature.
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7. 3) Palladium  Serves the same functions but is much less expensive than
platinum.
Disadvantages – Palladium hydrogen gas because gold containing palladium
may be more porous when cast  SP.Gr  11.4 melting point – 1555°C.
4) Iridium, Ruthenium and Rhodium
Trace amounts of these metals are added as “Grain Refiners” Melting
point  960, 5°C  below the melting point and both gold and copper as little
as 0.005% is sufficient to refine the grain size. Grain refiners produce smaller
grains.
Fine grained alloys have smaller grains compared to coarse-grained
alloys which relatively larger grains.
Fine-grained alloys are generally more stronger and more ductile than
coarse grained alloys.
Indium – can also act as a savenger for the alloy during cast procedure.
Can also serve to increase the tarnish and corrosion.
5) Silver also contributes to hardness and strength of the alloy. Although it
mimics gold in its deformability effect, it adversely affect the mobility.
In other words it lowers tarnish resistance.
- Food containing sulphur compounds cause severe
tarnish on silver.
- Silver serves to balance the red colour given by
copper.
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8. - Adding small amounts of palladium to silver
containing alloys prevents the rapid corrosion of such alloys in the
oral environment. Also occludes appreciable quantities of O2 in the
molten state. Porosities or rough casting surface can be prevented by
adding 5-10% Cu.
6) Copper : Contributes strength and hardness, but decreases the mobility
of the alloy, i.e. it decreases the tarnish and corrosion resistance because
the maximum content should not exceed 16%.
1) Gives the alloy and reddish appearance.
2) Lowers, the fusion temperature.
Zinc : (Present only in low percentages, around 0.5%) acts as a deoxidizer and
reduces the oxygen content. (Because O2 released during solidification results
in porosity).
Karat and Fineness
Karat  is the traditional unit expressing gold content in an alloy. Karat refers
to the parts of pure-gold in 24 parts of an alloy.
- Pure gold is 24 karat.
- 22K gold is an alloy containing 22 parts pure gold
in and 2 parts of other metals.
Fineness  Fineness is the percentage of gold multiplied by 10 because a 24
karat alloy would have a fineness of 100 x 10, or 1000 and a 12-karat alloy
would be 500 fine (500f).
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9. Heat Treatment of Gold Alloys
Gold alloys can be significantly hardened if the alloy contains a
sufficient amount of copper (at least 8 wt/copper).
 Heat treatment is a process of healing a metal to improve its properties.
An alloy can be subjected to – Hardening heat treatment also “age
hardening”, softening heat treatment also referred to as “solution heat
treatment”.
“Softening Heat Treatment
 Casting is placed in an electric furnace for 10 minutes at a temperature of
700°C (1292°F) and then quenched in water.
 During this period, all intermediate phases are presumably changed to a
disordered solid solution and the rapid quenching prevents ordering from
occurring during cooling.
Softening heat treatment reduces - Tensile strength
- Proportional limit
- Hardness
but improves – Ductility.
Softening heat treatment is indicated for, structure that are to be ground,
shaped or otherwise cold worked.
Hardening heat treatment – can be accomplished in many ways. One of
the most practical hardening treatments is by “soaking” or aging the casting at
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10. a specific temperature for a definite time  usually 15-30 minutes before it is
water quenched.
 The aging temperature on the alloy composition but is generally between
200°C (400°F) and 450°C (840°F).
 Age hardening improves- Strength (yield).
- Proportional limit.
- Hardness.
 Age hardening is indicated for metallic partial dentures, saddle bridges and
other similar structures.
For small structures such as inlays, a hardening heat treatment is not
usually employed.
 Ideally before the alloy is given an age- hardening heat treatment, it should
be subjected, 1) to a softening heat treatment for 2 reasons: to reline all strain
hardening, if it is present, 2) to start the hardening treatment with the alloy as a
disordered solid solution.
Casting Shrinkage:
Most metals and alloys including gold and the noble metal alloys shrink
when they change from the liquid to the solid state.
The shrinkage occurs in 3 stages:
1) The thermal contraction of the liquid metal between the temperature to
which it is heated and the liquidus temperature.
2) The contraction of the metal from liquid to solid shape.
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11. 3) The thermal contraction of the solid metal that occurs down to room
temperature.
The values for casting shrinkage differ for the various alloys presumably
because of difference in their composition.
 It has been shown that platinum, palladium and copper all are effective in
reducing the casting shrinkage of an alloy.
The following table shows the linear solidification shrinkage of casting alloys:
Alloy
Type I, gold base 1.56
Type II, gold base 1.37
Type III, gold base 1.42
Base metals 2.3
(Ni-Cr-Mo-Be-Co-Cr-Mo)
Composition of Traditional I to IV alloys
Type Au Cu Ag Pl Sn, In, Fe, Zn, Ga
I
II
III
IV
83%
77
75
69
6
7
9
10
10
14
11
12.5
0.5
1
3.5
3.5
Balance
Balance
Balance
Balance
Silver-Palladium Alloys
 These alloys are white in colour.
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12.  Main component is silver, but small amounts of palladium (attract 25%) is
also present (Palladium  added to provide mobility and tarnish resistance).
 Copper may or may not be present and a small amount of gold.
 Copper-free silver-palladium alloys may contain 70-72% silver and 25%
palladium and may have physical properties of type III gold alloys.
 Other silver-based alloys might contain roughly 60% silver, 25% palladium
and as much as 15% or more of copper. These alloys have properties triangular
to Type IV gold alloy.
Disadvantages : Greater potential for tarnish and corrosion.
High Noble Alloys for Metal-Ceramic Restoration:
The original metal-ceramic alloys contained 85% gold and were much
too soft for stress-bearing restorations such as fixed partial dentures.
 There was no evidence of a chemical bond between these alloys and
dentinal porcelain.
 Therefore 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.
 So, stress was concentrated at 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.
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13. (Iron also increases the proportional limit, and strength of the alloy).
Because this 1% addition of base metals to the gold, palladium and
platinum alloy produced a slight oxide film on the surface of the substructure to
achieve a porcelain-metal bond strength level that surpassed the cohesive
strength of the porcelain itself.
This new type of alloy, with small amounts of base metals added
became the standard for the metal-ceramic restoration.
In spite of vastly different composition, these alloys share at least three
common features:
1) They have the potential to bond to dental porcelain.
2) They possess coefficients of thermal contraction compatible with those
of dental porcelains.
3) Their solidus temperature is sufficiently high to permit the application of
low-fusing porcelains.
The coefficient of thermal expansion tends to have a reciprocal
relationship with the melting point of alloys.
The higher the melting temperature of a metal, the lower its CTE.
The high noble alloys for metal-ceramic restorations are:
1) Gold-platinum-palladium alloys:
 Gold content ranges upto 88%.
 Varying amounts of palladium, platinum and small amounts of B metals.
 Colour – yellow
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14. Disadvantages :  Susceptible to sag deformation and FPD should be
restricted to 3-unit spans, amount cantilever or crowns.
2) Gold-palladium-silver alloys:
 Gold content ranges between 39% - 77% gold.
 Palladium – 35%.
 Silver – less than 22%.
Silver increases the thermal contraction coefficient.
Disadvantages : Silver present in this alloy can discolour same porcelains.
3) Gold-Palladium alloys
 Gold content ranges for 44% to 55%
 Palladium  35% to 45%.
These alloys have remained popular despite of their relatively high cost.
Disadvantage : The lack of silver results in freedom from silver distry.
The lack of silver results in a decreased coefficient thermal content
because these alloys must be used only with porcelains that have low CT
contraction to avoid the development of initial circumferential tensile stresses
in deny cooling point of ---------.
Noble Alloys for Metal-Ceramic Resins
According to ADA classification of 1984, noble alloys must contain at
least 25wt% of the noble metals but do not necessarily contain any gold.
 Noble palladium-based alloys after a compromise between the high-noble
gold alloys and predominantly base metal alloys.
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15.  Also, the price / ounce of a palladium alloy is generally one half to one third
that of a gold alloy.
 Density is midway between that of base metal and high noble alloys.
 Palladium-based alloys have a workability angular to gold and scrap value.
1) Palladium-silver alloys:
 These alloys were introduced widely in the late 1970s (to overcome some
disadvantages of early B.M. alloys e.g. : castability, porcelain bonding and
workability problems).
 In the recent years the popularity of these alloys has declined because of the
disadvantages with these alloys is it discolours porcelain during firing.
This discolour, is usually a greenish-yellow discolor and is popularity
termed as “Greening”. Greening occurs mainly because of the presence of
silver  Silver vapor escapes for the surface of these alloys during firing of the
porcelain.
Diffuses as ionic silver into the porcelain.
Finally reduced to colloidal metallic silver in the surface layer of
porcelain.
Not all porcelains are susceptible to silver discolour because some do
not contain the necessary elements to reduce the ionic silver.
 To eliminate the greening problem, palladium alloys with no silver were
developed.
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16.  These alloys contain 75% to 90% palladium.
 Some of these high palladium alloys develop a layer of dark oxide on their
surface during cooling from the degassing cycle.
 This oxide layer has proven difficult to mark by the opaque porcelain.
 Other high palladium alloys such as the Pd-Ga-Ag-Au type do not seem to
be plaqued.
 This alloy type was introduced to the U.S. market in 1974 as the 1st
free
noble metal available for metal ceramic restorations.
Composition :
Palladium – 53% to 61%.
Silver – 28% to 40%.
Small amounts of tin + indium or both are added to promote oxide
formation for adequate bonding of porcelain.
 In some alloys, the formation of an internal oxide layer rather than an ext.
oxide layer has been reported.
 Increasing the silver content tends to a lower the metal range and raises the
contraction coefficient of an alloy.
 Because of the high silver content, silver discolouration effect is most
severe for these alloys.
 However, gold metal conditioners or ceramic coating agents may minimize
this effect.
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17.  The low specific gravity of these alloys (10% to 11%) combined with their
low intrinsic cost makes these alloys attractive as economical alternatives to the
gold-based alloys.
Palladium-copper Alloys
This alloy type is comparable in cost to the Pd-Ag alloys because alloys
of this type are recent introductions to the dental market, little clinical
information is available on their long-term clinical success.
Disadvantages because of their low melting range of approx 1170°C to
1190°C, these alloys are susceptible to creep deformation (Sag) at elevated
firing temperatures.
Thus, one should exercise caution in using these alloys for long-span
fixed partial dentures with relatively small connectors.
Composition:
Palladium – 74% and 80%.
Copper – 9% to 15%.
Some may contain as little as 2% gold – this small amount of gold
serves no useful purpose.
 Porcelain discolouration due to copper is possible but does not appear to be
a major problem.
 There has also been some concern recently over the potential cytotoxic
effect of copper released intra-orally from certain Pd-Cu alloys.
 One should also be aware of the potential effect on aesthetics of the dark
brown or black oxide formed during oxidation and subsequent firing cycles.
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18.  Care should be taken by the technician to mask the oxide completely with
opaque porcelain and to eliminate the unaesthetic dark bond that develops at
metal-porcelain junctions.
 It is also important that technician ensure that a brown rather than a black
oxide is formed on the metal surface during oxidation treatment otherwise,
poor adherence to porcelain may result.
 Some of these alloys are technique sensitive with respect to casting.
 In some instances the molten alloy should have a thin oxide firm appearing
on its surface at the casting temperature.
 Some instructions specify that heating be maintained for an additional 7
seconds beyond the point at which a rolling motion of the alloys is observed.
Because of the lack of a specifically defined melt appearance, there may
be a tendency to overheat the alloy to eliminate this film.
This error would cause significant changes in the properties of the alloy
and a decrease in metal-porcelain bond strength.
 Underheating of the molten alloy is also possible because of the difficulty in
judging the proper melt appearance  this could result in incomplete castings
or rounded margins.
 Because alloys have a poor potential burnishing, except when the marginal
areas are relatively thin.
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19.  Although thermal incompatibility is not considered to be a major distortion
of ultrathin metal copings (0.1mm) has been occasionally reported. The exact
cause of this effect is not known. It could be :
- Because of metal-porcelain incompatibility stresses.
- High creep rate of these alloys at temperatures near
the glass transitions temperature of porcelain.
- Relaxation of elastic stresses due to solidification,
grinding and sandblasting.
This distortion of single unit / multiple unit castings could be overcome
by using thicker copings / connectors or changing the band of porcelain or by
using a different metal-ceramic system with more acceptable overall properties.
Palladium-Cobalt Alloys
This alloy group is comparable in cost to Pd-Ag and Pd-Cu alloys.
 They are often advertised as gold-free, nickel-free, beryllium free and silver
free alloys.
 Like many noble metals, these alloys have a fine grain size to minimize hot
tearing during the solidification process.
 This Pd-Cu group is the most sag resistant of all noble metal alloys.
Composition :
- Palladium – 78% to 88%.
- Cobalt – 4% to 10wt%.
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20.  One commercial alloy contains 8% gallium.
An example of typical properties of a Pd-Cu alloy is as follows:
Hardness – 250DPH.
Yield strength – 586 MPa.
Elongation – 20%.
Modulus of elasticity 85.2 GPa
Although these alloys are silver free, discoloration of porcelain can still
result because of the presence of cobalt. But this is not a significant problem.
 Failure of the technician to completely mask out the dark metal oxide colour
with opaque porcelain is a more common cause of unacceptable aesthetic
results.
 No metal casting agents are required to mask the oxide colour or to promote
adherence to porcelain.
 Like the Pd-Ag and Pd-Cu alloys the Pd-Co alloys generally tends to have a
relatively high thermal contraction coefficient. Hence would be more
compatible with high expansion porcelains.
Palladium-Gallium-Silver and Palladium-Gallium-Silver-Gold alloys
 These alloys are the most recent of the noble metals.
 This groups of alloys was introduced because they tend to have a slightly
lighter coloured oxide than the Pd-Cu or Pd-Co alloys.
 The silver content is relatively low (5-8wt%) and is inadequate to cause
porcelain greening.
20

21.  Since they have a low coefficient of thermal contraction they would be more
compatible with lower expansion porcelains (e.g. vita porcelains).
21

22. Conclusion
Numerous types of casting alloys have been used for the restoration of
teeth while gold-based alloys have played the major role for many years their
dominance has been challenged now by base metal system. The main reason is
that base metals are less expensive as compared to gold alloys. Unfortunately
these substitutes have been less than ideal in numerous way and one of greatest
questions about them is their biological compatibility. However, further
investigation need to be conducted to evaluate their biocompatibility.
However, the performance of any restoration is related to multiple
factors for ex: the design of the appliance, the skill and accuracy with which it
has been fabricated and the properties of the materials used.
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