Since antiquity, pure gold has been ametal of special intrinsic and practicalvalue. From the dawn of civilization untilthe present time, man has utilized gold insome form or other in the practice ofdentistry.
In its pure form, gold has the unusualability to cohere to it self at roomtemperature. Gold is quite soft, and in acompressible form, increments can bewelded by pressure into a solid metalmass.
Another unique feature about gold is itinertness. Under most conditions it doesnot 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; Incipient carious lesions Class I and pits Class V Class III Erosions Repair of defective cast margins
Advantages: No tarnish and corrosion Insoluble in oral cavity Good adaptation to cavity walls High wear resistance
Gold Foil ( RobertWoffendale 1795)Oldest of all products standard No. 4 Gold foil4 x 4 inch sheets weight 4 grains (0.259g) and0.51µm thickNo. 3 weight 3 grains – 3 µm thick.
Cohesive - NoncohesiveGold: Gold attracts gases and any adsorbed gas film prevents intimate atomic contactrequired for cold welding- Manufactures supply free of surface
NON COHESIVE FOIL:Majority are provided with an adsorbedprotective gas film like ammonia-Prevents premature cohesion of sheetsthat may come into contact. Volatile filmis removed by heating to restore thecohesive character of the foil.
Gold Foil Cylinders:Formed by rolling cut segments ofNo. 4 foils into a desired width- 3.2mm, 4.8mm, 6.4mm using modifiedNo.22 tapestry needle.
Preformed Gold Foils: Cylinders and ropes are nowavailable in preformed shapes madefrom No.4 foil which has been‘carbonised’ or corrugated• Historical interest Great Chicago fire of1871
Platinised Gold Foil: It is a laminated structure can be produced in one of the 2 ways.4. 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 anelevated temp. well below the melting point ofgold, it causes self diffusion betweenparticles where they are in contact.
Mat Gold ( Rule 1937) Is a crystalline, electrolyticallyprecipitated gold form that is formedinto strips .Preferred ease in buildingup the internal bulk, more easilycompacted within and adapted to theretentive portion of the preparedcavity.
Disadvantages: Because it is loosely packed it isfriable and contain numerous voidspaces. 2- material technique mat iscovered with a veneer of foil.
Mat foilSandwich of electrolytic precipitatedgold powder between sheets of No.3gold foilPurpose – Try to eliminate the needto veneer the restoration with a layer offoilNo longer Marketed
Alloyed electrolyticprecipitateThe newest forms of electrolyticprecipitate electro alloy RV is alloyedwith calcium. Calcium content of thefinished product is about 1% Purpose is to produce strongerrestoration by dispersion strengthening
Powdered Gold:1st used in early 1960’sGold powder was enclosed in No. 3gold foil .maximum particle size is 74µm Avg 15 µm
The atomized and chemicallyprecipitated powders are1st mixed with a soft wax to form pelletsThese wax – gold pellets wrapped withfoil
Removal of SurfaceImpurities:With the exception of non cohesivegold the DFGs are received by thedentist in a cohesive condition.During storage and packaging they areexposed to the atmosphere
It is necessary to heat the foil or pelletbefore it is carried into the preparedcavity. It is calledAnnealing, heat treatment, degassing amore appropriate term is desorption. Itis essential to achieve a cohesive mass.
Chamois finger tips should be worn by thedentist to protect the gold fromcontamination.
Proper desorption is a matter of heatinglong enough at a temperature that removesgases and in case of powdered gold burnsaway the wax.Temperatures in the range of 600-13000Fmust be producedUnder heating should be avoided because itdoes not adequately remove the impurities
Overheating should be avoided becauseexcessive sintering causes the material tobecome more stiffer less ductile anddifficult to condense.- Too Long time- Too high temperatureTemperatures below 3150C are inadequate
The Gold can be heated intwo ways1.Flame HeatingFlame annealing, held The alcohol flamenear the top of the is transparent; theflame, flame from the pellets is visible
2.Electric Annealing Pellets are being heated on a tray
When heating in bulk on a tray, excessiveamounts should be avoided, since thedifficulties arising from prolongedheating can arise, from repeated heatingas well.
Handle the pieces with stain less steel wirepoints that will not contaminate the gold.Flame desorption occurs when the goldsegment has exhibited a dull red glow.
The fuel may be alcohol or gas butalcohol is preferred because there is lessdanger of contamination.Alcohol must be pure methanol orethanol without colorants or additives.
Advantages of flame desorption3. Ability to select a piece of appropriate size.4. Desorption of only these pieces used.5. Less exposure to contamination.6. 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 GOLDCondensation is the procedure used tocondense and harden gold inside the cavitypreparation2 main process6. Welding7. Wedging
Welding:Forming atomic bonds between pellets orlayer as result of condensation.
Wedging refers to the pressurizedadaptation of the gold form within thespace between the tooth structure wallsthat have been slightly deformedelastically.
Straight points are cut in the preparedcavity and the 1st pieces of foil arewedged into these areas
COMPACTION: Can be done by:3. Hand mallet.4. Pneumatic vibratory condensers.5. Electrically driven condensers.
a. Nichrome wire used for carrying gold;b. Pneumatic condenserc. Detachable condensing tins
PHYSICAL PROPERTIES OF COMPACTED GOLD1. Strength and Hardness:BHN of pure gold is 25. Duringcondensation its hardness rises to 75.Tensile strength rises from 19000 psi to32000 psi.Yield strength rises from 0 to 30,000 psi
3) Density:Density of pure gold is 19.3. Density ofwell compacted direct gold is 16.5 cm3The difference is due to the presence ofporosity and voids4. Melting point of Gold-1063O C
5.Tarnish and Corrosion Resistance:Is good, there is very little marginalleakage between the filling and cavitywalls, if the compaction is good.6. It dissolves in aqua regia7. high thermal conductivity (.710cal /sec
6 Biocompatibility: It is biocompatible. Producesonly a minimal pulpal response, ifcompacted properly. The technique,however, does involve a certain amount oftrauma to the tooth and its supportingtissues.