Simple and Comprehensive account on Noble metals

1. Dr. Dina Mostafa
Dental Casting Alloys
Dr Ambreen Azam
MSc Dental Materials (UK)

2. Learning Objectives:
Discuss Requirements of dental casting
alloys
To classify dental casting alloys
To discuss gold alloys in terms of
Composition, Classifications & Properties

3. General Ideal Requirements
Functional
requirements
Working
requirements

4. Functional Requirements
 High yield strength: to resist permanent deformation
 High modulus of elasticity: to resist elastic deformation
 High ductility: to facilitate burnishability
 High resilience: to absorb energy of elastic deformation
 High toughness: to absorb energy of fracture
 High cantilever bending strength: to resist cantilever bending
 High transverse strength: to resist transverse bending
 High fatigue strength: to resist cyclic loading
 High impact strength: to absorb energy of fracture under
sudden load
 High hardness: to resist scratching
 High sag resistance: to resist plastic deformation during firing of
porcelain
1- Mechanical Properties:

5.  High Tm: to resist sag
 High modulus of elasticity to increase the fracture resistance of
porcelain
 The coefficient of thermal expansion & contraction must match that
of porcelain
 Must be able to form surface oxides
 Must have surface roughness
 Must not discolor porcelain
2- Compatibility with porcelain

6. Working Requirements:
1- Ease of casting 2- Ease of soldering
3- Ease of burnishability

7. Dr. Dina Mostafa
Noble Metal Casting
Alloys
Gold Alloys

8. Gold Restorations
Direct Indirect
Gold Foil Gold Alloys

9. Direct Filling Gold
 Two pieces of pure gold are pressed together, metallic bonds are
formed at their point of contact
 Gold is welded together, without the application of heat. This
property of cold welding is utilized when building up a pure gold
filling
 Used in the form of a very thin gold sheet or ‘foil’, approximately
0.001 mm thick
 Condensation or ‘plugging’ of the gold may be done by hand or with
an automatic mallet

10. Direct Filling Gold
 Mechanically similar to Soft gold or type 1 gold alloy for casting
 Perfectly corrosion resistant
 Does not require an adhesive cement in well supported cavities
 Used where there is little or no opposing force in non stress bearing
areas
 Time, high cost and inability to be used in load bearing areas limit
there use.

11. Indirect Filling Gold
 Type 1… for casting subjected to very light stress…Inlay
 Type II…Medium strength inlays and onlays
 Type III….High strength… Onlays, thin cast backings, pontics, full
crowns and saddles
 Type IV…High stress and thin in cross section…Saddles, bars,
clasps crown bridges and partial denture frame works.

13. Noble Metals Family

14.  The six metals of the platinum group are: platinum,
palladium, iridium, rhodium, osmium and ruthenium,
along with gold these are called the noble metals.
 Ruthenium (Ru), rhodium (Rh), and palladium (Pd) are
called the light group.
 The osmium (Os), iridium (Ir), and platinum (Pt) are
referred to as the heavy group.
 The noble metals, together with silver (Ag) are sometimes
called precious metals.
Noble Metals Family

15. Noble Metals Family
 Standards for ‘dental gold casting alloys’ require a noble metal
content of at least 75%
 ISO standard requires that of the minimum noble metal content
of 75%, at least 65% shall be gold and remaining noble metal
components shall be platinum group metals such as platinum,
palladium, iridium, ruthenium or rhodium
 The most commonly used is palladium

16. General constituents of Noble Metal
alloys:
Elements Role
Gold
• Resist tarnish & corrosion
• Yellow color
• Tm = 1063°C
• specific gravity = 19.3
• Weak, Soft, ductile & malleable

17. Platinum • Resist tarnish & corrosion
• Lighten gold color
• Tm = 1769°C
• specific gravity = 21.37
• Tough, ductile & malleable
Palladium • Resist tarnish & corrosion
• White, darker than Pt
• Tm = 1552°C
• specific gravity = 11.4
• Ductile & malleable
Greenish color of silver
Redness of copper

18. Iridium,
Ruthenium
and
Rhodium
50ppm
• Refining the grain size of
gold alloys
• White color
• Tm = 2440°C
• specific gravity = 22.4
• Strong, hard and brittle

19. Silver
• White color
• Tm = 961°C
• specific gravity = 10.4
• Strong, hard, ductile &
malleable
Pure sliver occludes oxygen
in the molten state which is
evolved during solidification.
As a result, small pits,
porosity and rough casting
surface develop. This
tendency is reduced when 5%
to 10% copper is added to the
silver.

20. Copper • Strength & hardness to gold
• Red color
• Tm = 1083°C
• Ductile & malleable
Zinc • Improve castability & fluidity
of the alloy
• blue white
• Tm = 420°C
• Scavenger

21. Indium • Soft
• gray color
• Tm = 156°C
• Used as a replacement of zinc
Tin • Produce oxides helping in the
bonding of porcelain
• White
• Tm = 232°C
• hardening effect with pt & pd
Produce oxides
helping in the
bonding of porcelain

22. Iron
• 1% with porcelain to
produce ppt hardening
Pt3Fe

23. Classification of Gold Alloys
1. According to gold content
2. According to mechanical properties
3. According to alloy description

24. Classification of gold alloys
 I- According to gold content:
Carat Fineness
The carat of an alloy is the
parts of pure gold in 24 parts of
alloy.
The fineness of a gold
alloy is the parts per
thousand of pure gold.
This classification is not suitable as they refer to the amount of gold only and do not determine the
other metals which may have a pronounced effect over the properties.

25. II- Classification According to Mechanical
Properties
Type Gold Copper
Type I (soft) 87 4
Type II (medium) 76 8
Type III (hard) 70 10
Type IV (extra hard 65 15

26. Properties of Gold Alloys:
Yield strength
MPa
Vickers
hardness
number
Elongation
%
Type I soft 100 - 120 50 - 90 35
Type II medium 150 - 180 95 - 120 30
Type III hard S 200
H 310
S 120
H 170
S 35
H 20
Type IV extra
hard
S 275
H 510
S 150
H 250
S 30
H 8

27. Modulus of elasticity: 75 -105 GPa
Melting temperature: 870˚C –
1050˚C
Density: 15.2 – 16.8gm/cm3

28.  III- According to alloys description:
Description by color
Description based on low
gold content - Economy
gold
White gold (economy or
low gold)
Yelow gold
• Minimum 75% noble
metals.
• Gold 42-58%.
• 1% Pd for each 3%Ag
to counteract the
tarnish prop. of Ag.
• Ag/Cu ratio must be
balanced.
Predominantly Au
Pt, Pd ›5% & Ag

29. Solid State Reaction
 Annealing
 Homogenization
 Precipitation Hardening (AgCu system)
 Heat treatments of gold-copper system (AuCu
system)

30. Alloy heat treatments
Practical considerations
 On melting the alloy for casting a new crystal structure is created as
the metal ‘freezes’ inside the mould
 The gold alloy castings are quenched before they cool to the range
of temperatures within which heat hardening takes place
 If allowed to cool slowly premature precipitation and order hardening
will take place
 Casting is left in mould and rapidly cooled from above 600ºC
 The mould is then plunged into cold water in order to chill the metal
quickly and cause disintegration of the mould forming fine grain
structure

31. Alloy heat treatments
Practical considerations
 The casting is then cleaned and when platinum or palladium are
present a homogenization heat treatment may be carried out to
remove coring
 This involves heating to 700ºC for 10 minutes, then quenching
 Any minor adjustments, such as bending of clasps, etc., are made
at this stage whilst the alloy is still in the softened state. If
adjustments are made, a low temperature stress relief anneal
should be carried out

32. Alloy heat treatments
Practical considerations
 The hardening heat treatment may then be carried out with type 3 and
type 4 alloys by heating the casting to above 450ºC and allowing it to
cool slowly until its temperature has dropped to about 200ºC, then
quenching.
 The procedures are designed such that hardening by both
precipitation and ordering can occur.
 In practical hardening heat treatments are rarely performed for type III
and infrequent for Type IV alloys.
 Properties of the as cast material are adequate for most purposes
 Following hardening the casting is repolished and, in the case of a
denture, the teeth are added

33. Heat treatment
Softening heat treatment
(Homogenization)
10 min.
700 0C
Quenched in water
disordered S.S.S.
(F.C.C.)
lower strength, hardness,
high ductility.
Hardening heat treatment
(Order hardening/Age hardening)
15-30 min.
200-450 0C
Quenched in water
super lattice (ordered F.C.T)
in (disordered F.C.C)
Higher strength, hardness,
lower ductility than disordered
SSS

34. Heat treatment of gold-copper
alloy (Au/Cu)
AuCuAuCu3

35. N.B. Type III and type IV can be
age hardened (heat treated) into
soft and hard forms.

36. Classification of Dental Casting Alloys
Alloys
High
Noble
metals
Noble 60%
Gold 40%
Predominantly
Base metals
Noble less than 25%
Noble metals
(other noble
metals)
Noble 25%

37. Other Noble Metal Alloys
Silver Palladium
Palladium Silver
60% 30%
+ Tin Indium Gallium
70% 25%

38.  Problems with their use:
 Lower density in AgPd alloys which requires greater centrifugal force
for casting.
 AgPd properties same as Type III gold
 Lower ductility that results in lower burnishability.
 Their tendency to gas absorption during melting and casting. The gas
absorbed will come out and cause porosity. Therefore they need
specialized equipment for melting and casting.
N.B: In general it should be kept in mind that the higher the gold content,
the better is the fit and marginal adaptation of the restorations.

39. References
 McCabe Applied dental materials 9th Edition
 Phillips science of dental materials 12th Edition.