INTRODUCTIONINTRODUCTION INTRODUCTIONINTRODUCTION

Since antiquity, pure gold has been a

metal of special intrinsic and practical

value. From the dawn of civilization until

the present time, man has utilized gold in

some form or other in the practice of

dentistry.

In its pure form, gold has the unusual

ability to cohere to it self at room

temperature. Gold is quite soft, and in a

compressible form, increments can be

welded by pressure into a solid metal

mass.

Another unique feature about gold is it

inertness. Under most conditions it does

not tarnish, corrode or stain.

Its chief disadvantages are its color, high

thermal conductivity(53.20), and technical

difficulties in forming a dense restoration.

It has one of the highest densities of all

elements (19.3 g/cm3)

Indications;Indications;

Incipient carious lesions

Class I and pits

Class V

Class III

Erosions

Repair of defective cast margins

Advantages: Advantages:                                     

No tarnish and corrosion

Insoluble in oral cavity

Good adaptation to cavity walls

High wear resistance

DisadvantagesDisadvantages

Colour

Manipulation

Thermal conductivity

They are Classified into three categories:

I. Foil

A. Sheet

1. Cohesive

2. Noncohesive

B. Ropes

C. Cylinders

D. Laminated foil

E. Platinized foil

II. Electrolytic precipitate

A. Mat gold

B. Mat foil (mat gold plus gold foil)

C. Gold – calcium alloy

III Powdered gold (encapsulated gold

powder)

Gold Foil ( Robert Woffendale 1795)

Oldest of all products standard No. 4 Gold foil

4 x 4 inch sheets weight 4 grains (0.259g) and

0.51m thick

No. 3 weight 3 grains – 3 m thick.

Cohesive -

Noncohesive Gold:

Gold attracts gases and any adsorbed gas

film prevents intimate atomic contact

required for cold welding

- Manufactures supply free of surface

contaminants i.e. inherently cohesive.

COHESIVE FOIL

NON COHESIVE FOIL:

Majority are provided with an adsorbed

protective gas film like ammonia

-

Prevents premature cohesion of sheets

that may come into contact. Volatile film

is removed by heating to restore the

cohesive character of the foil.

Gold Foil Cylinders:

Formed by rolling cut segments of

No. 4 foils into a desired width

- 3.2mm, 4.8mm, 6.4mm using modified

No.22 tapestry needle.

Preformed Gold Foils:

Cylinders and ropes are now

available in preformed shapes made

from No.4 foil which has been

‘carbonised’ or corrugated

• Historical interest Great Chicago fire of

1871

Platinised Gold Foil: It is a laminated structure can be

produced in

one of the 2 ways.

1. 2 sheets of No.4 pure gold foil and a

layer of pure platinum foil sandwiched

between there can be hammered until

the thickness of No.4 sheet is obtained.

2. Layers of platinum and gold can

be bonded together by cladding

process during the rolling

operation.

Objective of adding platinum:

Increases hardness and wear

resistance

Electrolytic Precipitate:

Crystalline gold powder formed by

electrolytic precipitation.

Powder is formed by sintering at an

elevated temp. well below the melting point of

gold, it causes self diffusion between

particles where they are in contact.

Mat Gold ( Rule 1937)

Is a crystalline, electrolytically

precipitated gold form that is formed

into strips .Preferred ease in building

up the internal bulk, more easily

compacted within and adapted to the

retentive portion of the prepared

cavity.

Disadvantages:Disadvantages:

Because it is loosely packed it is

friable and contain numerous void

spaces.

2- material technique mat is

covered with a veneer of foil.

Mat foil

Sandwich of electrolytic precipitated

gold powder between sheets of No.3

gold foil

PurposePurpose – Try to eliminate the need to veneer the restoration with a layer of foil

No longer Marketed

Alloyed electrolytic Alloyed electrolytic precipitate precipitate

The newest forms of electrolytic

precipitate electro alloy RV is alloyed

with calcium. Calcium content of the

finished product is about 1%

Purpose is to produce stronger

restoration by dispersion strengthening

Powdered Gold:

1st used in early 1960’s

Gold powder was enclosed in No. 3

gold foil .maximum particle size is 74

m Avg 15 m

The atomized and chemically

precipitated powders are

1st mixed with a soft wax to form pellets

These wax – gold pellets wrapped with

foil

Removal of Surface Removal of Surface Impurities:Impurities:

With the exception of non cohesive

gold the DFGs are received by the

dentist in a cohesive condition.

During storage and packaging they are

exposed to the atmosphere

It is necessary to heat the foil or pellet

before it is carried into the prepared

cavity. It is called

Annealing, heat treatment, degassing a

more appropriate term is desorption. It

is essential to achieve a cohesive mass.

Chamois finger tips should be worn by the

dentist to protect the gold from

contamination.

Proper desorption is a matter of heating long enough at a temperature that removes gases and in case of powdered gold burns away the wax.

Temperatures in the range of 600-13000F must be produced

Under heating should be avoided because it does not adequately remove the impurities

Overheating should be avoided because

excessive sintering causes the material to

become more stiffer less ductile and

difficult to condense.

- Too Long time

- Too high temperature

Temperatures below 3150C are inadequate

The Gold can be heated in two ways

1.Flame Heating

Flame annealing, held near the top of the flame,

The alcohol flame is transparent; the flame from the pellets is visible

2.Electric Annealing

Pellets are being heated on

a tray

When heating in bulk on a tray, excessive

amounts should be avoided, since the

difficulties arising from prolonged

heating can arise, from repeated heating

as well.

Handle the pieces with stain less steel wire

points that will not contaminate the gold.

Flame desorption occurs when the gold

segment has exhibited a dull red glow.

The fuel may be alcohol or gas but

alcohol is preferred because there is less

danger of contamination.

Alcohol must be pure methanol or

ethanol without colorants or additives.

Advantages of flame desorption

1. Ability to select a piece of appropriate size.

2. Desorption of only these pieces used.

3. Less exposure to contamination.

4. Less danger of over sintering

DISADVANTAGES OF ELECTRIC ANNEALING

- Pellets may stick together, if the tray is

moved.

- Air currents may effect the uniformity of

heating

- Difficult to anneal appropriate amounts of

gold.

- Oversintering.

- Greater exposure to contamination.

- Size selection among the pieces of

desorbed gold is limited.

COMPACTION OF DIRECT FILLING COMPACTION OF DIRECT FILLING

GOLDGOLD

Condensation is the procedure used to

condense and harden gold inside the cavity

preparation

2 main process

1. Welding

2. Wedging

Welding:

Forming atomic bonds between pellets or

layer as result of condensation.

Wedging refers to the pressurized

adaptation of the gold form within the

space between the tooth structure walls

that have been slightly deformed

elastically.

Straight points are cut in the prepared

cavity and the 1st pieces of foil are

wedged into these areas

COMPACTION:COMPACTION:

Can be done by:

1. Hand mallet.

2. Pneumatic vibratory condensers.

3. Electrically driven condensers.

a. Nichrome wire used for carrying gold;

b. Pneumatic condenser

c. Detachable condensing tins

1. Strength and Hardness:

BHN of pure gold is 25. During

condensation its hardness rises to 75.

Tensile strength rises from 19000 psi to

32000 psi.

Yield strength rises from 0 to 30,000 psi

PHYSICAL PROPERTIES OF COMPACTED GOLD

3) Density:

Density of pure gold is 19.3. Density of

well compacted direct gold is 16.5 cm3

The difference is due to the presence of

porosity and voids

4. Melting point of Gold-1063O C

5.Tarnish and Corrosion Resistance:

Is good, there is very little marginal

leakage between the filling and cavity

walls, if the compaction is good.

6. It dissolves in aqua regia

7. high thermal conductivity (.710cal /sec

/cm2/(0C/cm)

6 Biocompatibility:

It is biocompatible. Produces

only a minimal pulpal response, if

compacted properly. The technique,

however, does involve a certain amount of

trauma to the tooth and its supporting

tissues.