CAST GOLD ALLOYS

Seminar By

Dr. Balamurugan

Postgraduate Student

DEPARTMENT OF CONSERVATIVE DENTISTRY & ENDODONTICSSRI RAMACHANDRA DENTAL COLLEGE AND HOSPITAL

CHENNAI

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CONTENT

INTRODUCTION

HISTORICAL PERSPECTIVE ON DENTAL CASTING ALLOYS

DESIRABLE PROPERTIES OF CASTING ALLOYS

CLASSIFICATION OF DENTAL CASTING ALLOYS

IDENTIFICATION OF ALLOYS BY PRINCIPLE ELEMENTS

GOLD

KARAT AND FINENESS

ALLOY COMPOSITION AND TEMPERATURE

PROPERTIES:

MELTING RANGE

DENSITY

STRENGTH

HARDNESS

ELONGATION

HEAT TREATMENT OF HIGH NOBLE METAL ALLOYS

SOFTENING HEAT TREATMENT

HARDENING HEAT TREATMENT

CASTING SHRINKAGE

ALLOYS FOR ALL-METAL AND RESIN-VENEER RESTORATIONS

HIGH NOBLE ALLOYS FOR METAL-CERAMIC RESTORATIONS

MATERIAL CHOICE FOR A CAST RESTORATION

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INTRODUCTION

The physical properties of a tooth are the result of its

complex composite structure and the manner in which it is

supported within the bone by the periodontal ligament. The

enamel-dentin complex displays a brilliant marriage of hard

tissues with differing rigidity. Enamel, the harder mantle,

with the modulus of elasticity approaching 100 GPa, is

supported by a base of dentin, with a modulus of elasticity

of 14 to 28 GPa.

Intact, tooth is ideally suited for physiological function;

this involves dynamic change in its form and, with normal

function, the dentition wears without loss of vertical

dimension, strength or efficiency. When a tooth looses both

its enamel and dentin its physical integrity as well as its

biological integrity is compromised. Major requirement

when replacing lost tooth structure is to replace form and

function.

Plastic restorative materials are satisfactory in small

lesions but, as a problem of restoration of form and function

becomes more extensive, the need to employ rigid materials

increases in an attempt to reinstate the original anatomy

and function of the tooth.

Many materials are used for the construction of rigid

restorations. There are a number of sound reasons for the

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selection of noble metals and their alloys for the restoration

of teeth. Principally they resist oxidation and are not

attacked by acids. Seven metals meet this definition: Gold,

Platinum, Palladium, Rubidium, Ruthenium, Osmium and

Iridium. However, only the first three of these are used in

dentistry and they inert properties are of great value in the

hostile environment of the mouth.

HISTORICAL PERSPECTIVE ON DENTAL CASTING

ALLOYS

The history of dental casting alloys has been influenced

by three 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 acknowledge 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, fixed

partial dentures, and removable partial denture frame

works. Because pure gold did not have the physical

properties required of these dental restorations, existing

jewelry alloys were quickly adopted. These gold alloys were

further strengthened with copper, silver, or platinum.

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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: Vickers hardness

number (VHN) between 50 and 90), Type II (medium: VHN

between 90 and 120), Type III (hard: VHN between 120 and

150), and Type IV (extra hard: VHN 150).

At that time, some tarnish tests indicated that alloys

with a gold content lower than 65% to 75% tarnished too

readily for dental use. In the following years, several patents

were issued for alloys containing palladium as a substitute

for platinum. 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 not

known that in gold alloys, palladium is added to counter act

the tarnish potential of silver.

In the late 1950s, a breakthrough occurred in dental

technology that was to influence significantly the fabrication

of dental restorations. This was the successful veneering of

a metal substructure with dental porcelain. Until that time,

dental porcelain had a markedly lower coefficient of thermal

expansion than did gold alloys, making it impossible to

attain a bond between the two structural components. It was

found that adding both platinum and palladium to gold

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would lower the alloy’s coefficient of thermal contraction

sufficiently to ensure physical compatibility between the

porcelain veneer and the metal substructure. The melting

range of the alloy was also raised sufficiently to permit firing

of the porcelain onto the gold-based alloy at 1040C (1900F)

without deforming the metal substructure. 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 metal for dental use.

DESIRABLE PROPERTIES OF CASTING ALLOYS

Cast metals are used in dental laboratories to produce

in inlays, onlays, and crowns, conventional all metal bridges,

metal-ceramic bridges, resin-bonded bridges, Endodontic

posts, and removable partial denture frameworks. The

metals must exhibit biocompatibility, ease of melting,

casting, brazing (or soldering) and polishing, little

solidification shrinkage, minimal reactivity with the mold

material, good wear resistance, high strength and sag

resistance (metal-ceramic alloys), and excellent tarnish and

corrosion resistance. Generally, conventional type II and

type III gold alloys represent the standards against which

the performance of other casting alloys is judged.

CLASSIFICATION OF DENTAL CASTING ALLOYS

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Several brands of crowns and bridge alloys are

currently available that are designed for all metal crowns,

bridges, onlays and inlays that are described according to

American Dental Association (ADA) Specification No.5 as

Types I through IV. In the past, this specification referred to

gold-based alloys. Since 1989, ADA approved casting alloys

can have any composition as long as they pass the tests for

toxicity, tarnish, yield strength, and percentage of

elongation. The minimum values for yield strength and

percent elongation determine whether an alloy is classified

as Type I (soft: for restorations subject to very slight stress

such as inlays), Type II (medium: for restorations subject to

moderate stress such as onlays), Type III (hard: for high-

stress situations, including onlays, crowns, thick veneer

crowns, and short- span fixed partial dentures), and Type IV

(extra hard: for extremely high stress states, such as

endodontic posts and cores, thin veneer crowns, long span

fixed partial dentures, and removable partial dentures).

Mechanical property requirements of

American Dental Association Specification No.5

Alloy Type

Yield Strength (Mpa) (0.1%

offset)

Maximum elongation

(%)

Annealed Hardened Annealed Hardened

I (soft) 140 Maximum None 18 None

II (medium) 140-200 None 18 None

III (hard) 200-340 None 12 None

IV (extra hard) 340 500 10 2

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In 1984 the ADA proposed a simple classification for

dental casting alloys. The three categories are: high noble

(HN), noble (N), and predominantly base metal (PB). Many

manufacturers have adopted this classification to simplify

communication between dentists and dental laboratories

technologists. This system lacks the potential to discriminate

among alloys within a given category that may have quite

different properties. The dental casting alloy classification is

useful for estimating the relative cost of alloys because the

cost is dependent on the noble metal content as well as on

the alloy density.

Alloy classification of the

American Dental Association (1984)

Alloy Type Total Noble Metal Content

High noble metal alloy Contains 40 wt% Au and 60 wt% of the

noble metal elements (Au + Ir + Os + Pt + Rh +

Ru)

Noble metal alloy Contains 25 wt% of the noble metal elements

Predominantly base metal

alloy

Contains < 25 wt% of the noble metal elements

The alloys that are used for metal-ceramic restorations

can be used for all metal (or resin-veneered) restorations,

whereas the alloys for all-metal restorations should not be

used for metal ceramic restorations. The principle reasons

that alloys for all-metal restorations cannot be used for

metal-ceramic restorations are that the alloys may not form

thin, stable oxide layers to promote bonding to porcelain;

their melting range may be too low to resist sag deformation

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or melting at porcelain firing temperatures; and their

thermal contraction coefficients may not be close enough to

those of commercial porcelains.

Classification of Alloys for All-Metal Restorations, Metal-

Ceramic Restorations and Frameworks for Removable Partial

Dentures

Alloy Type

All-Metal Metal-CeramicRemovable

Partial Dentures

High Noble Au-Ag-Cu-Pd Au-Pt-Pd Au-Ag-Cu-PdMetal-ceramic alloys Au-Pd-Ag (5-12 wt%

AgAu-Pd-Ag (>12 wt% AgAu-Pd (no Ag)

Noble Ag-Pd-Au-Cu Pd-Au (no Ag) Ag-Pd-Au-CuAg-Pd Pd-Au-Ag Ag-PdMetal-ceramic alloys Pd-Ag

Pd-CuPd-CoPd-Ga-Ag

MARZOUK’S CLASSIFICATION

Class I

These are gold and platinum group based alloys in

accordance with the ADA specification #5. They are type I,

II, III and IV gold alloys.

Class II

These are low gold alloys, with gold content less than

50%. Some may contain as little as 5% gold.

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Class III

These are non-gold palladium based alloys.

Class IV

Nickel chromium based alloys.

Class V

Castable, moldable ceramics.

IDENTIFICATION OF ALLOYS BY PRINCIPLE ELEMENTS

As a result of several alternate alloy systems, an

understanding of their composition is vital, in view of

differences in formulations and the resulting properties.

Thus, the crown and bridge, metal-ceramic, and removable

partial denture alloys are classified according to not only

function but also according to their composition. When an

alloy is identified according to the elements it contains, the

components are listed in declining order of composition,

with the largest constituent first followed by the second

largest constituent. An exception to this rule is the

identification of certain alloys by elements that significantly

affect physical properties or that represent potential

biocompatibility concerns, or both.

GOLD

Pure gold is a soft, malleable, ductile metal that has

rich yellow colour with a strong metallic luster. Although

pure gold is the most ductile and malleable of all metals, it

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ranks much lower in strength. Gold melts at 1064C and has

a density of 19.32 g/cc. The density depends somewhat on

the condition of the metal, whether it is cast, rolled, or

drawn into wire. 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 causes gold

to be extremely brittle. Mercury in small quantities also has

a harmful effect on its properties. The addition of calcium to

pure gold improves the mechanical properties of gold used

for gold foil restorations.

Air or water at any temperature does not affect or

tarnish gold. Gold is not soluble in sulfuric, nitric, or

hydrochloric acid. However it readily dissolves in

combination of nitric and hydrochloric acids to form the

trichloride of gold. It is also dissolved by few other chemicals

such as potassium cyanide and solutions of bromine or

chlorine.

Because gold is nearly as soft as a lead, it must be

alloyed with copper, silver, platinum, and other metals to

develop the hardness, durability, and elasticity necessary in

dental alloys.

KARAT AND FINENESS

Traditionally the gold content of a dental alloy has been

specified on the basis of karat or fineness. Karat refers to

the parts of pure gold in 24 parts of an alloy. For example,

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24-karat gold is pure gold, whereas 22-karat gold is alloy

containing 22 parts of gold and 2 parts of other metals.

Fineness describes gold alloys by the number of parts

per 1000 of gold. For example, pure gold has fineness of

1000, and 650 fine alloy has a gold content of 65%. Thus,

the fineness rating is 10 times the gold percentage in an

alloy. An alloy that is three-fourths (75%) pure gold is 750

fine. Fineness is considered a more practical term than

karat.

ALLOY COMPOSITION AND TEMPERATURE

In each phase diagram the horizontal axis represents

the composition of the binary alloy. For example, the

horizontal axis represents a series of binary alloys of gold

and copper ranging in composition from 0% gold (or 100%

copper) to 100% gold. The composition can be given in

atomic percent (at%) or weight percent (wt%). The atomic

and weight percent compositions of the binary alloys can

differ considerably. For example, for the Au-Cu system

shown (A), an alloy that is 50% gold by weight is only 25%

gold by atoms. For other systems, such as the Au-Pt system

(F), there is little difference between atomic and weight

percentages. The difference between atomic and weight

percentage depends on the differences in the atomic

masses of the elements involved. The bigger the difference

in atomic mass, the bigger the difference between the

atomic and weight percentages in the binary phase diagram.

From a marketing and sales standpoint, most alloy

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compositions are given in weight percentages because the

weight percentages of gold are higher for this massive

element. However, the physical and biological properties of

these alloys relate best to atomic percentages. Therefore it

is important to keep the difference between atomic and

weight percent in mind when selecting and using noble

dental casting alloys. Alloys that appear high in gold by

weight percentage may in reality contain far fewer gold

atoms than might be thought.

A second aspect of the phase diagrams that deserves

attention is the liquidus and solidus lines. The y-axes show

temperature. If the temperature is above the liquidus line

(marked L), then alloy will be completely molten. If the

temperature is below the solidus line (marked S), then the

alloy will be solid. If the temperature lies between the

liquidus and solidus lines, the alloy will be partially molten.

The distance between the liquidus and solidus lines varies

among systems. For example, the temperature difference

between these lines is small for the Ag-Au system, and

varies considerably with composition for the Au-Cu system.

From a manipulative standpoint, it is desirable to have a

narrow liquidus-solidus range because the alloy should be in

the liquid state as little time as possible before casting.

While in the liquid state, the alloy is susceptible to

significant oxidation and contamination. If the liquidus-

solidus line is broad, then the alloy will remain at least

partially molten longer before it can be cast. The

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temperature of the liquidus line is also important and varies

considerably among alloys and with composition. For

example the liquidus line of the Au-Ag system ranges from

962-1064C (C) but the liquidus line of the Au-Pd system

ranges form 1064-1554 C. It is often desirable to have an

alloy with a liquidus line at lower temperatures because the

method of heating is easier, fewer side reactions occur, and

shrinkage of the alloy is generally less of a problem.

PROPERTIES

MELTING RANGE

Dental casting alloys do not have melting points, but

they do have melting ranges because they are mixtures of

elements rather than pure elements. The width of the

solidus-liquidus melting range is important to the

manipulation of the alloys. The solidus-liquidus range should

be narrow to avoid having the alloy in a molten state for

extended times during casting. If the alloy spends a long

time in the partially molten state during casting, there is

increased opportunity for the formation of oxides and

contamination.

The Au-Ag-Pt alloys, have wider ranges, which makes

them more difficult to cast without problems.

DENSITY

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Density is important during the acceleration of the

molten alloy into the mold during casting. Alloys with high

densities will generally accelerate faster and tend to form

complete castings more easily. Alloys with high densities

generally contain higher amounts of denser elements such

as gold or platinum. Thus the Au-Ag-Pt alloys and Au-Cu-Ag-

Pd-I alloys are among the most dense of the casting alloys.

STRENGTH

Strength of alloys can be measured by either the yield

strength or tensile strength. Although tensile strength

represents the maximum strength of the alloy, the yield

strength is more useful in dental applications because it is

the stress at which permanent deformation of the alloys

occurs. Because permanent deformation of dental castings

is generally undesirable, the yield strength is a reasonable

practical maximum strength for dental applications. For

several alloys, such as Au-Cu-Ag-Pd-I, II, and III, the

formation of the ordered phase increases the yield strength

significantly. For example, the yield strength of the Au-Cu-

Ag-Pd-II alloys increases from 350 to 600 Mpa with the

formation of an ordered phase. For other alloys, such as the

Au-Ag-Pt and Ag-Pd alloys, the increase in yield strength is

more modest in the hardened condition. The Pd-Cu-Ga alloy

do not support the formation of ordered phase because the

ratio of palladium and copper are not in the correct range

for ordered phase formation.

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The yield strengths of these alloys range from 320-

1145 Mpa (hard condition). The strongest alloy is the Pd-Cu-

Ga alloy with a yield strength of 1145 Mpa. The other alloys

range in strength from 320-600 Mpa. These latter yield

strengths are adequate for dental applications and are

generally in the same range as those for the base metal

alloys, which range from 495-600. The effect of solid-

solution hardening by the addition of copper and silver to

the gold or palladium base is significant for these alloys.

Pure cast gold has a tensile strength of 105 Mpa. With the

addition of 10-wt% copper (coin gold), solid-solution

hardening increases the tensile strength to 395 Mpa. With

the further addition of 10-wt% silver and 3-wt% palladium

(Au-Cu-Ag-Pd-I), the tensile strength increases to about 450

Mpa and 550 Mpa in the hard condition.

HARDNESS

Hardness is a good indicator of the ability of an alloy to

resist local permanent deformation under occlusal load.

Although the relationships are complex, hardness is related

to yield strength and gives some indication of the difficulty

in polishing the alloy. Alloys with high hardness generally

will have high yield strengths and are more difficult to

polish. The values for hardness generally parallel those for

yield strength. In the hard condition, the hardness of these

alloys ranges form 155 kg/mm2 for the Ag-Pd alloys to 425

kg/mm2 for the Pd-Cu-Ga alloys. More typically the hardness

of the noble casting alloys is around 200 kg/mm2. The Ag-Pd

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alloys are particularly soft because of the high concentration

of silver, which is a soft metal. The Pd-Cu-Ga alloys are

particularly hard because of the high concentration of Pd,

which is a hard metal. The hardness of most noble casting

alloys is less than that of enamel (343 kg/mm2), and

typically less than that of the base metal alloys. If the

hardness of an alloy is greater than enamel, it will tend to

wear the enamel of the teeth opposing the restoration.

ELONGATION

The elongation is a measure of the ductility of the alloy.

For crown and bridge applications, the value of elongation

for an alloy is generally not a big concern because

permanent deformation of the alloys is generally not

desirable. However elongation will indicate whether the alloy

can be burnished. Alloys with high elongation can be

burnished with out fracture. In the hardened condition, the

elongation will drop significantly. For example, for the Au-

Cu-Ag-Pd-II alloys, the elongation is 30% in soft condition

versus only 10% in hardened condition. In the soft condition,

the elongation of noble dental casting alloys ranges from 8

to 30%. These alloys are substantially more ductile than the

base metal alloys, which have elongation on the order of 1-

2%.

HEAT TREATMENT OF HIGH NOBLE METAL ALLOYS

Gold alloys can be significantly hardened if the alloy

contains a sufficient amount of copper. Types I and II alloys

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usually do not harden, or they harden to a lesser degree

than do Types III and IV alloys. The actual mechanism of

hardening is probably the result of several different solid-

state transformations. Although the precise mechanism may

be in doubt, the criteria for successful hardening are time

and temperature.

Alloys that can be hardened can, of course, also be

softened. In metallurgic terminology the softening heat

treatment is referred to as solution heat treatment. The

hardening heat treatment is termed age hardening.

SOFTENING HEAT TREATMENT

The casting is placed in an electric furnace for

10minutes at a temperature of 700C (1292F) and then it is

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. The tensile strength, proportional

limit, and hardness are reduced by such a treatment but the

ductility is increased.

The softening heat treatment is indicated for structures

that are to ground. Shaped or otherwise cold worked, either

in or out of the mouth. Although 700C is an adequate

average softening temperature, each alloy has its optimum

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temperature, and the manufacturer should specify the most

favorable temperature and time.

HARDENING HEAT TREATMENT

The age hardening or hardening heat treatment of

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 200C (400F) and 450C (840F).

The manufacturer specifies the proper time and

temperature.

Ideally, before the alloy is given an age-hardening

treatment, it should be subjected to a softening heat

treatment to relieve all strain hardening, if it is present, and

to start the hardening treatment with the alloy as a

disordered solid solution. Otherwise there would not be a

proper control of the hardening process, because the

increase in strength, proportional limit, and hardness and

the reduction in ductility are controlled by the amount of

solid-state transformations allowed. The transformations, in

turn, are controlled by the temperature and time of the age-

hardening treatment.

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Because the proportional limit is increased during age

hardening, a considerable increase in the modulus of

resilience can be expected. The hardening heat treatment is

indicated for metallic partial dentures, saddles, bridges, and

other similar structures. For small structures, such as inlays,

a hardening treatment is not usually employed.

The yield strength, the proportional limit, and the

elastic limit are all measures of essentially the same

property. This property reflects the capacity of an alloy to

withstand mechanical stresses without permanent

deformation. In general, the yield strengths increase when

progressing from Type I to Type IV alloys. Age hardening

substantially increases the yield strength.

The hardness values for noble metal correlate quite

well with the yield strengths. Traditionally, hardness has

been used for indicating the suitability of an alloy for a given

type of clinical application.

The elongation is a measure of ductility or the degree

of plastic deformation an alloy can undergo before fracture.

A reasonable amount of elongation is essential if the clinical

application requires deformation of the as-cast structure,

such as is needed for clasp and margin adjustment and

burnishing. Age hardening reduces the elongation, in some

cases significantly. Alloys with low elongation are brittle

materials and fracture readily if deformed.

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CASTING SHRINKAGE

Most metals and alloys, including gold and the noble

metal alloys, shrink when they change from the liquid to the

solid state. Such a consideration is very important in dental

casting procedure. For example, if a mold for an inlay is an

accurate reproduction of the missing tooth structure, the

cast gold inlay is an accurate reproduction of the missing

tooth structure, the cast gold inlay will be too small by the

amount of its casting shrinkage.

The shrinkage occurs in three 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 inherent in its change from the

liquid to the solid state: and 3) the thermal contraction of

the solid metal that occurs down to room temperature.

The first mentioned contraction is probably of no

consequence, because as the liquid metal contracts in the

mold, more molten metal can flow into the mold to

compensate for such shrinkage. The values for the casting

differ for the various alloys presumably because of

differences in their composition. It has been shown, for

example, that platinum, palladium, and copper all are

effective in reducing the casting shrinkage of an alloy. The

value for casting shrinkage of pure gold closely approaches

that of its maximal linear thermal contraction.

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In general, it is apparent that the values obtained for

the casting shrinkage are less than the linear thermal

shrinkage values, even though the casting shrinkage as

obtained included both the solidification shrinkage and

thermal shrinkage. This condition can be accounted for by

two logical assumptions, 1) when the mold becomes filled

with molten metal, the metal starts to solidify at the walls of

the mold because the temperature of the mold is less than

that of the molten metal; and 2) during initial cooling, the

first layer of metal to solidify against the walls of the mold is

weak, and it tends to adhere to the mold until it gains

sufficient strength as it cools to pull away. When it is

sufficiently strong to contract independently of the mold, it

shrinks thermally until it reaches room temperature.

The important consideration is that the thermal

shrinkage of the first weak solidified layer is initially

prevented by its mechanical adhesion to the walls of the

mold. During this period, it is actually stretched because of

its interlocking with the investment material. Thus, any

contraction occurring during solidification can be eliminated.

Also, part of the total thermal contraction can be eliminated,

with the result that the observed casting shrinkage is less

than might be expected on the basis of the possible stages

of he shrinkage.

Because the thermal contraction as the alloy cools to

room temperature dominates the casting shrinkage, the

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higher melting alloys tend to exhibit greater shrinkage. This

must be compensated for in the casting technique if good fit

is to be obtained.

Linear Solidification Shrinkage of Casting Alloys

Alloy Casting shrinkage (%)

Type I, gold base alloy 1.56

Type II, gold base alloy 1.37

Type III, gold base alloy 1.42

Ni-Cr-Mo-Be alloy 2.3

Co-Cr-Mo alloy 2.3

ALLOYS FOR ALL-METAL AND RESIN-VENEER

RESTORATIONS

In 1927, the Bureau of standards established gold

casting alloy Types I through IV according to dental function,

with hardness increasing from Type I to Type IV.

But based on the 1989 revision of specification No.5 by

the ADA, the following four alloy types are classified by their

properties and not by their compositions:

Type I (soft)- small inlays, easily burnished and subject

to very slight stress

Type II (medium)- inlays subject to moderate stress,

including thick three quarter crowns, abutments, pontics,

and full crowns

Type III (hard)- inlays subject to high stress, including

thin three –quarter crowns, thin cast backings, abutments,

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pontics, full crowns and denture bases, and short-span fixed

partial dentures. Some Type III gold alloys usually can be

age hardened, especially those containing at least 8-wt% of

copper.

Type IV (extra hard)- inlays subject to very high

stresses, including denture base bars and clasps, partial

denture frameworks, and long span fixed partial dentures.

The compositions of these alloys are usually based on a

majority of either gold or silver; gold alloys can be age

hardened by an appropriate heat treatment.

Composition Range (weight percent) of Traditional

Types I to IV Alloys

Alloy

TypeMain Elements Au Cu Ag Pd

Sn, In, Fe,

Zn, Ga

I High noble (Au base) 83 6 10 0.5 Balance

II High noble (Au base) 77 7 14 1 Balance

III High noble (Au base) 75 9 11 3.5 Balance

III Noble (Au base) 46 8 39 6 Balance

III Noble (Ag base) --- --- 70 25 Balance

IV High noble (Au base) 56 14 25 4 Balance

IV Noble (Ag base) 15 14 45 25 Balance

Metal

CeramicHigh noble (Au base) 52 --- --- 38 Balance

Types I and II alloys are often referred to as inlay

alloys. The development of modern direct and indirect tooth-

coloured filling materials has virtually eliminated the use of

types I and II gold alloys. Traditional Types III and IV alloys

are generally called crown and bridge alloys, although Type

IV alloys also are used occasionally for high-stress

applications such as removable partial denture frameworks.

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Physical Properties of Some Modern Noble

Metal Dental Alloys

Alloy

Type

Main

Elements

Melting

Range

Density

(g/cm2)

Yield Strength

‡ Hardness

(VHN)

Present

Elongation(Mp

a)(psi)

IHigh noble

943-960C 16.6 103(15,000

)80 36

IIHigh noble

924-960C 15.9 186(27,000

)101 38

IIIHigh noble

924-960C 15.5 207(30,000

)121 39

Noble 843-916C 12.8 241

(35,000

)138 30

Ag-Pd noble1021-1099C 10.6 262

(38,000

)143 10

IVHigh noble

921-943C 15.2 275(40,000

)149 35

High noble871-932C 13.6 372

(54,000

)186 38

Noble 930-1021C 11.3 434

(63,000

)180 10

Metal

ceramic* High noble 1271-1304C 13.5 572

(83,000

)220 20

HIGH NOBLE ALLOYS FOR METAL-CERAMIC

RESTORATIONS

The disadvantage of dental porcelain as a restorative

material is its low tensile strength and shear strength.

Although porcelain can resist compressive stresses with

reasonable success, substructure design does not permit

shapes in which compressive stress is the principal force.

A method by which this disadvantage can be minimized

is to bond the porcelain directly to a cast alloy substructure

made to fit the prepared tooth. If a strong bond is attained

between porcelain veneer and the metal, the porcelain

25

veneer is reinforced. Thus, the risk of brittle fracture can be

avoided or, at least, minimized.

To fabricate this restoration, a metal substructure is

waxed, cast, finished, and heat-treated. A thin layer of

opaque porcelain is fused to the metal substructure to

initiate the porcelain-metal bond and mask the colour of the

substructure. Then dentin and enamel porcelains,

sometimes referred to as body and incisal porcelains, are

fused onto the casting, shaped, stained to improve the

aesthetic appearance, and glazed.

The original metal-ceramic alloys contained 88% gold

and were much too soft for stress-bearing restorations such

as fixed partial denture. 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. It was found that the

bond strength of the porcelain to this type of alloy was less

than the cohesive strength of the porcelain. So if 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 alloy, the porcelain –metal bond

strength improved three fold. Iron also increases the

proportional limit and strength of the alloy.

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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 the porcelain itself.

Metal-ceramic alloys also fall into one of the three general

categories- high noble, noble, or base metal. In spite of

vastly different chemical compositions, all the alloys share

atleast 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; and 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 and

the melting range of alloys; that is, the higher the melting

temperature of a metal, the lower is its thermal expansion.

This fact is important in formulating metal-ceramic alloys for

different dental porcelains.

Gold-based Metal-Ceramic alloys- PFM alloys containing

more than 40 wt% gold at least 60 wt% of noble metals

(gold plus platinum and palladium and /or the other noble

metals) are generally classified as high noble.

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Gold-platinum-palladium alloys - these alloys 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 fixed partial

dentures should be restricted to three-unit spans, anterior

cantilevers, or crowns.

Gold-Palladium-Silver Alloys- These gold-based alloys

contain between 39% and 77% gold, up to 35% palladium,

and silver levels at high as 22%. The silver increases the

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 discoloration of porcelain. Alloys of this type

must be used with porcelains that have low coefficients of

thermal contraction to avoid the development of axial and

circumferential tensile stresses in porcelain during the

cooling part of the porcelain firing cycle.

MATERIAL CHOICE FOR A CAST RESTORATION

Several years ago the choice of an alloy for a cast

restoration was simple, insofar as there was no other choice

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than the four types of gold alloy. At the present time,

however, the choice is made difficult not only due to the

various alloys available to the profession, but also to the

availability of cast ceramics. It is the dentist’s duty both to

understand the properties of the cast materials used, and to

prepare the teeth and manipulate the materials to get the

most out of them. It is also our professional responsibility to

inform the patient of the advantages and possible

drawbacks of the material used and the measures required

by the dentist and patient to prevent any shortcomings from

making the restoration a failure.

Without doubt, gold alloys maintain several advantages

primarily because the technique for fabricating a cast

restoration in a gold alloy has been preferred over the years.

This does not mean that newer materials are not suitable for

oral use. It is fair to say that they still have a long way to go

to arrive to the current status of gold alloys.

Gold alloys are usually indicated when the casting has

lengthy margins with the possibility of marginal

discrepancies, even with the most meticulous technique.

The burnishability of these types of gold alloys makes it easy

to adjust these margins after fabricating the casting.

When using a class I alloy for a single tooth restoration,

if the casting is going to be subjected to normal and above

normal type of loading, type III gold alloys are the ideal

alloys to use. Type II gold alloys are used only in smaller

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castings and in areas of normal and less than normal

loading.

Type I gold alloys are seldom used. They are indicated

for use in areas with no direct occlusal loading, e.g., class V

and III restorations. The softness of the alloy makes it easy

to adapt the margins properly at these locations. Type IV

gold alloys are rarely used in single tooth restorations, but

may be indicated if the casting will be carrying an

attachment for a partial denture, or if it is a part of a long

splint, i.e., the casting will be subjected to unusually high

loading situations.

In evaluating class II alloys present clinical data reveal

that there is little different between them and class I alloys.

The major difference is in their tarnish and corrosion

resistance, especially when the gold content gets lower than

40%. Therefore, they can be used in lieu of high gold alloys

in areas with low corrosion activity, but the patient should

be

30

Phase diagrams for binary combination of A, copper and gold, B, copper and palladium, C, silver and gold, D, silver and palladium, E, palladium an gold, F, gold an platinum. Atomic percentages are shown along the bottom of each graph; weight percentages are shown along the top. L= liquidus, S=solidus.

advised of what to expect. The tooth preparation and cast

fabrication are no different than those for class I alloys, and

these alloys are definitely much less expensive than class I

alloys.

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