PRESENTATION ON GOLD ALLOYS USED IN DENTISTRY. USEFUL FORALLBRANCHES OF DENTISTRY SPECIALLY PROSTHODONTICS AND CONSERVATIVE DENTISTRY. INCLUDES VARIOUS CATEGORIES OF ALLOYS AND THEIR ADVANTAGES, DISADVANTAGES, USES ETC.

1. GOLD ALLOYS
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2. metal
• A crystalline material that consists of positively
charged ions in an ordered, closely packed
arrangement and bonded with a cloud of free
electrons.
• This type of bond, called a metallic bond, is
responsible for many of the properties of
metals- electrical and thermal conductivity,
metallic luster, and (usually) high strength
2

3. Alloy
• A material that exhibits metallic properties and
is composed of one or more elements-at least
one of which is a metal.
• For example, steel is an alloy of iron and
carbon, brass is an alloy of copper and zinc, and
bronze is an alloy of copper and tin.
3

4. Types of metals:
Noble Metal – A metal that is resistant to
oxidation; includes gold, platinum, palladium and
the other platinum group metals.
Precious Metal – A metal that is relatively high in
cost; includes gold, platinum, palladium and silver.
Base Metal – A metal that readily oxidizes or
dissolves to release ions.
4

5. GOLD
• Soft, rich yellow color and a strong metallic luster
• Most malleable and ductile
• 0.2% lead – brittle
• Soluble in aqua regia
• Air or water at any temperature does not tarnish
gold
• Alloyed with copper, silver, platinum – increases
hardness , durability and elasticity
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6. PLATINUM
Bluish white metal
Tough, ductile and malleable
Hardness similar to copper
Higher melting point ( 1772°C) than porcelain
Coefficient of thermal expansion close to porcelain
Lighten the color of yellow gold based alloys
Common constituent in precision prosthetic
attachments
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7. PALLADIUM
White metal darker than platinum
Density little more than half that of Pt and Au
Absorbs hydrogen gas when heated
Not used in pure state in dentistry
Whitens yellow gold based alloys.
7

8. IRIDIUM, OSMIUM, RHODIUM,
ruthenium
• Grain refiners
• Improves mechanical properties and uniformity of
properties within alloy
• Extremely high melting point of Ir - 2410°C and Ru -
2310°C – serve as nucleating centers
• Osmium(Os) has a very high melting point, and is very
expensive, hence not used in dentistry.
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9. SILVER
Malleable, ductile; white metal.
Stronger and harder than gold, softer than copper.
Absorbs oxygen in molten state-difficult to cast
Forms series of solid solutions with palladium and
gold .
 Neutralizes reddish color of alloys containing
copper
Prone to tarnish and corrosion
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10. Alloying Elements in Dental Gold Alloys
Largest fraction for these alloys is Gold with lower amounts of silver and copper.
Silver - Has a slight strengthening effect and counteracts the reddish tint of
copper.
Copper - increase the strength and reduces the melting temperature.
 Amount that can be added is 16%.
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11. Platinum and Palladium – increases both the strength and
the melting temp.
Zinc – Act as scavenger during casting, preventing oxidation
and improves the castability.
Iridium, Ruthenium and Rhodium( <0.5 % )
Have very high melting temp and act as nucleating sites
during solidification, thus produce fine grain size  they are
the grain refiners.
11

12. The Amount of Gold in an Alloy is defined in one of two
ways :
1. Carat
 Pure gold has a carat value of 24.
 An Alloy carat is expressed in terms of the number of the 24th part
of Gold within it.
 An alloy with 50% of gold will be designated as 12 carat gold.
2. Fineness
 Pure gold has a fineness of 1000.
 18 carat gold is 750 fine ; 9 carat gold is 375 fine. 12

13. DESIRABLE PROPERTIES
Choice of alloy is governed by a number of Factors :
1) Cost
2) Biocompatibility of the Alloy
3) Its resistance to corrosion and tarnish.
13

14. 4) Mechanical Properties
a. Stiffness
b. Strength
c. Ductility
d. Hardness
 Higher the elastic modulus , the stiffer the structure would be
 this is a important consideration for long span bridges,
partial dentures and denture clasps.
 Alloys need to be very ductile and soft.
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15. 5) Ease of Casting
 Important to know the melting range and casting range of the alloy .
 Higher these are more problem in handling of alloys.
 Quality of fit may alter  casting shrinkage and cooling contraction of the alloy.
6) Density
 Casting are carried out in a centrifugal force casting machine.
 The higher the density of the alloy, the easier it is to force the air out of the
mould space completely with the alloy. 15

16. The ideal hardness of the gold alloy is that it
shall wear at the same rate as the teeth.
A filling material should be hard enough to
resist excessive wear but not so hard that it
does not wear evenly with the biting surfaces
of the teeth.
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17. CLASSIFICATIONS
I. Formerly, ADA specification No. 5 classified casting alloys as
:
Depending upon its content of gold and platinum group metals
a) Type –I
b) Type –II
c) Type – III
d) Type – IV
Noble metal content ranged from 83 to 75 wt% respectively and
all alloys were gold based.
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18. II. Current, ADA specification Classifies into 3 groups :
1) High Noble
2) Noble
3) Predominantly Base Metal
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Alloy Type Total Noble Metal Content
High noble metal Contains > 40 wt% Au and > 60% of the noble
metal elements
Noble metal Contains > 25 wt% of the noble meal elements (Au,
Pd, Pt)
Base metal
Contains < 25 wt% of the noble metal elements

19. Mechanical property requirements
proposed in iso draft
international standard 1562 for casting
gold alloys( 2002)
Alloy type Minimum yield
strength(0.2%) in MPa
Minimum elongation
after fracture (%)
Type 1 80 18
Type 2 180 10
Type 3 270 5
Type 4 360 3
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20. CLASSIFICATION OF CASTING
METALS FOR FULL METAL,
METAL CERAMIC AND
PARTIAL DENTURES.
Metal type All metal Metal ceramic Partial denture
framework
High noble Au –Ag –Pd
Au-Pd – Cu- Ag
Au-Pt-Pd
Au –Pd-Ag(5 –
12% ag)
Au-Pd-Ag(>
12% ag)
Au-Ag-Cu-Pd
Noble Ag-Pd-Au-Cu Au-Pd
Pd-Au
Pd-Au-Ag
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21. CLASSIFICATION BASED ON
ALLOY COLOUR
According to their color and principal element or elements
(Phillips, 1982).
• Yellow golds-
yellow color, with greater than 60% gold
content
• White golds-
white color, but with more than 50% gold content
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22. • Low (or economy) golds-
usually yellow colored, with less than 60% gold (usually 42% to
55%)
• High palladium –
white colored, with palladium the major component; may
contain small quantities of gold (2%) and a limited amount of either copper or
cobalt
Copper adds a reddish colour and silver lightens the red or yellow colour of
alloys .
22

23. 23

24. As the number of casting Alloys is immense it is possible to
subdivide each ADA composition group into several classes :-
Three Classes For High Noble Alloys –
1. Au-Ag-Pt  Typically consists of 78 wt% gold with roughly
equal amounts of silver and platinum.
2. Au-Cu-Ag-Pd-I  Alloys are typically 75wt% gold with Approx
10 wt % each of silver and Cu & 2-3 wt% palladium.
3. Au-Cu-Ag-Pd-II  have less that 60 wt% gold, with silver
content increased to accommodate the reduced gold content.
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25. HIGH GOLD ALLOYS
According to ANSI/ADA Specification No.5 for
Dental casting Alloys :
A) Type I - Soft (VHN 60 to 90)
B) Type II - Medium (VHN 90 – 120)
C) Type III - Hard (VHN 120- 150)
D) Type IV – Extra Hard (VHN 150 – 250)
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26. Type I alloys
 Are weak, soft and highly ductile
Useful in areas not subject to Occlusal stress
Do not harden by heat treatment.
Type II alloys
Used for inlays where burnishability of margins is important than
strength.
Type III alloys
Used in higher stress applications for inlays, onlays, three quarter
crowns and for fixed partial denture retainers and pontics.
Restoration design makes burnishability less important than strength.
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27. Type IV
Are strong , hard and non-ductile .
Used for high stress applications such as – partial dentures, cast
posts and long span bridges.
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28. [
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29. 29

30. Type %Au & Pt VHN Restoration
I (soft) 83 50-90 Inlay
II (medium) 78 90-120 Inlay/onlay
III (hard) 78 120-150
Onlay/Crown&Br
idge
IV (extra-hard) 75 150-250
Crown &
Bridge/RPD
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31. Medium & Low Gold Alloys
 Rise in the prices of noble metals in 1970’s stimulated
Manufacturers to produce new alloys with reduced gold content.
a) Medium Gold Alloys
 Gold content varying from 40 to 60 %.
 Palladium & silver contents were increased to compensate for
reduced gold content.
 Cu content range from 10-15%
 Pd is added to counteract the tendency of silver to tarnish.
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32. b) Low Gold Alloys
Gold content is 10-20%
Other elements –
a) Silver 40 to 60%
b) Palladium upto 40%
Due to reduced gold content, these alloys are white in
appearance.
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33. Properties
Medium Gold Alloys are used/ recommended for same
application as type III and type IV gold alloys.
• ductility tends to be lower than that of type IV gold alloys
• Their high yield stress makes them difficult to burnish.
Low Gold Alloys
• Tend to have lower mechanical properties than the medium gold
alloys.
• Recommended / used as an alternative to type III gold alloys.
• Less popular due to its WHITE colour.
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34. Strengthening Mechanism
 All alloy elements increase in the yield strength of the
Gold alloy by forming a solid solution.
Most effective strengthening mechanism is the addition
of Copper  “Order Hardening”
Carried out after homogenizing anneal at approx 700°
this ensures uniform composition through out the
casting.
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35. It involves reheating the alloy at 400°C & holding it at that
temperature for approx 30 min.
The Copper atoms arrange themselves in little ordered
clusters.
Cannot occur in type I & II gold alloys.
Type III and type IV show significant improvement in
strength.
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36. “Self Hardening”
For some alloys, the hardening process is to allow the
alloy to cool slowly on the bench rather than quenching
it immediately on casting.
Disadvantage – Not well controlled.
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37. HEAT TREATMENT
Two types of Heat Treatment :
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Softening
Heat
Treatment
Hardening
Heat
Treatment

38. The Softening Heat Treatment
• Also referred as “Solution Heat Treatment”
• The casting is placed in an electric furnace for 10 min at a temp of
700° C & then quenched in water.
• All intermediate phases are changed to a disordered solution
• Rapid quenching prevents ordering from occurring during cooling.
• Tensile Strength, Proportional Limit and Hardness  REDUCED
• Ductility  INCREASED 38

39. Hardening Heat Treatment
Also termed as “Age Hardening”
Soaking or aging the casting at a specific temp for a definite time ,
usually 15-30 min; before it is water quenched.
Aging temp depends on the alloy composition generally b/w 200°C
(392° F) & 450° C (842° F)
39

40. Alloys used in metal ceramic
restorations
Essential requirements:
• Potential to bond with porcelain
• Compatible coefficients of thermal expansion with that
of porcelain
• Sufficiently high melting points to resist sag deformation
during sintering of porcelain
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41. Thermal compatibility
• Porcelains have coefficient of thermal expansion between
13.0 and 14.0 X 10-6 and metal between 13.5 and 14.5 X 10-
6.
• The difference of 0.5 X10-6 in thermal expansion between
metal and porcelain causes the metal to contract slightly
more than does the ceramic during cooling after firing the
porcelain.
•
• This puts the ceramic under slight residual compression
which makes it less sensitive to applied tensile forces.
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42. • If the porcelain has much larger coefficient of
contraction than that of metal, tensile strength of
porcelain may be exceeded since of large tensile
stress causing crack propagation in porcelain veneer.
• If contraction coefficient of porcelain is much lower
than that of metal, failure level stresses can occur at
the metal porcelain interface.
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43. GOLD PLATINUM ALLOY
• Developed alternative to palladium alloys
• For full cast as well as metal-ceramic restorations.
 More prone to sagging, they should be limited to short span bridges.
 A typical composition is Gold 85%;
 Platinum12%;
 Zinc 1%;
 Silver (in few brands)
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44. • Good corrosion resistance
• Precipitation hardening due to formation of FePt3
• Optimum heat hardening for 30 min at 550 degree
centigrade.
• Costly due to high noble metal content
• Yellow in colour so better esthetics compared to
white alloys.
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45. GOLD PALLADIUM ALLOY
• Used for full cast /metal-ceramic restorations.
• Palladium - high melting temperature
- impart a white or gray color
- improves sag resistance
 These alloys usually contain indium, tin or gallium to promote an oxide layer.
A typical composition :
 Gold 52%;
 Palladium 38%;
 Indium 8.5%;
 Silver (in some brands).
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46. • Good corrosion resistance
• Solution hardening done
• Stronger, stiffer, more ductile compared to gold
platinum alloys
• Easy to cast and solder due to higher casting
temperature
• Less good esthethic results compared to gold
platinum alloys
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47. GOLD ,COPPER, PALLADIUM,
SILVER ALLOY
• Two types
Low silver (5 to 11.9%)
High silver ( more than 12%)
Greening of porcelain due to silver
Copper tends to cause sagging during porcelain processing
47

48. • Typical composition:
• Gold 72%
• Copper 10%
• Silver 14%
• Palladium 3%
• Good tarnish and corrosion resistance
• Poor esthetics due to white alloy and greening effect
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49. Finishing and polishing of gold
alloys
• Slow speed hand piece should be used
• Contour with carbide burs, green stones, or heatless stones.
• Finish with pink stones ( aluminum oxide) , or medium grade abrasive
impregnated rubber wheels and points
( brown and green)
• Apply fine abrasive- impregnated rubber wheels, cups and points .
• Apply Tripoli or rouge with rag or leather wheels
49

50. References :
• Craig’s Restorative Dental Materials – 12th Edition
• Phillips Science of Dental Materials – 11th Edition
• Gold in dentistry:alloys, uses &performance Article in gold
bulletin 2003
• Hardening of gold based alloys- JDR march 1977 56: 335-
345
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