Precious and Low Gold Alloys Flashcards
3 Types of gold used in dentistry?
- Pure gold.
- Traditional casting gold alloys.
- Alloys with noble metal content of at least 25% but less than 75% (low gold casting alloys, silver palladium alloys).
Alloys with noble metal content of at least 25% but less than 75%?
- Low gold casting alloys.
- Silver palladium alloys.
What is the cohesive gold foil technique?
- Cavity lined with an oxyphosphate lining material.
- Tiny pieces of gold placed into a cavity and pressure applied with SPRING LOADED MALLET.
- Cavity is incrementally filled up - takes MANY HOURS.
What technique is used for cohesive gold foil technique?
COLD WELDING.
- Pressure applied.
- Metallic bonds form at point of contact.
- Prior to placement heat to 250μm - drives off grease.
- Foil is 1μm.
What did patients complain about with cohesive gold foil technique?
Soreness of PDL due to pressure being applied with spring loaded mallet.
2 applications for cohesive gold foil technique?
- Occlusal.
- Buccal pit of molar.
5 disadvantages of pure gold in dentistry?
- Time consuming
- No cement lute.
- Ok in protected cavities (class I or buccal pit).
- Rigidity and elasticity insufficient in high stress situations.
- Overworking at placement can cause work hardening.
4 types of traditional casting gold alloys?
- Type 1: Low strength - casting subject to SLIGHT stresses (ex. inlays).
- Type 2: Medium strength - castings subject to MODERATE stresses (ex. inlays/ onlays).
- Type 3: High strength - high stresses (ex. onlays, thin cast backing, pontics, full crowns).
- Type 4: Extra high strength - castings THIN IN CROSS SECTION (ex. saddles, bars, clasps, crowns, bridges and partial denture frameowrks).
What are two ways in which gold content is expressed?
- Carat: parts by weight of gold in 24 parts of alloy.
- Fineness: parts by weight of gold in 1000 parts of alloy.
What does an alloy with 75% gold mean in carat and fineness?
- 18 carat.
- 750 fine.
6 elements present in gold casting alloys?
Gold, silver, copper, platinum, palladium, zinc.
What happens to the percentage of gold content when going from Type 1 to Type 4 traditional casting gold alloys?
- At strength increases, gold content DECREASES.
What happens to the percentage of silver content when going from Type 1 to Type 4 traditional casting gold alloys?
Stays about the same (some fluctuation).
What happens to the percentage of copper content when going from Type 1 to Type 4 traditional casting gold alloys?
As strength increases, copper content INCREASES.
What happens to the percentage of platinum/ palladium content when going from Type 1 to Type 4 traditional casting gold alloys?
As strength increases, platinum/palladium content INCREASES.
What happens to the percentage of zinc content when going from Type 1 to Type 4 traditional casting gold alloys?
Stays the same (1).
What are the changes in properties when we move from traditional casting gold alloy 1 to 4.
Increase in:
- Hardness.
- Proportional limit.
- Strength.
Decrease in:
- Ductility.
- Corrosion Resistance.
Why does hardness increase as gold content decreases?
Due to SOLUTION HARDENING - formation of solid solutions with gold.
What is the effect of silver in gold alloy (2)?
- Slight strengthening effect.
- Counteracts reddish copper tint.
What is the effect of copper in gold alloys (3)?
- Increases strength.
- Lowers MP.
- If content higher than 16% alloy will TARNISH.
What is the effect of platinum/palladium in gold alloys (2)?
- Increase strength.
- Increase MP.
What is the effect of zinc in gold alloys (1)?
- Acts as a SCAVENGER (forms complex with OXYGEN).
What happens if zinc is used up in the gold alloy? How can this be prevented?
- Zinc acts as a SCAVENGER.
- When used up its function is taken over by COPPER. This impairs the physical properties of the copper.
- Prevent by always having fresh alloy in the button.
What types of alloys can be heat treated?
- Types 3 and 4
What happens when type 3 and 4 alloys are heat treated? How does this occur?
further HARDENING by PRECIPITATION HARDENING of Ag/Cu and Au/Cu systems.
What happens when type 1 and 2 alloys are heat treated? How does this occur?
- CANNOT be heat treated.
- Due to lack of silver and copper required for the necessary atomic diffusions.
How are heat treatments achieved? What influences the structure?
- Melting alloy and cooling it –> formation of a NEW CRYSTAL STRUCTURE.
- LENGTH OF TIME alloy is allowed to cool to room temperature influences the structure.
What happens when an alloy is allowed to cool SLOWLY to room temperature?
- Premature precipitation and order hardening.
How can premature precipitation and order hardening be avoided?
- Casting cooled rapidly from excess of 600 (red color).
- Once red color lost through cooling, do a COLD WATER PLUNGE.
How do we know the casting has reached above 600 celsius?
- RED color of sprue region of casting.
What are the two effects of doing a cold plunge once the red color has been lost?
- Disintegrates investment (helps remove casting).
- Results in FINE GRAIN structure of casting.
What is coring?
Zones of concentration of a metal (of constituents) which can result in CORROSION.
What is the effect of eliminating coring?
Improves the CORROSION RESISTANCE of the metal.
What may happen after cold water cooling in alloys containing Pt/Pd? How can this be avoided?
- risks CORING.
Avoid by HOMOGENISATION HEAT TREATMENT: - Heating to **700* for 10 minutes.
- Quenching (rapid cooling).
What is the process and aim of homogenisation heat treatment?
- Heat to 700C for 10 minutes.
- Quenching.
- Aim is to ELIMINATE CORING (in alloys containing Pt/Pd).
What are the main contents (2) and color of low gold content alloys?
- Normally AU 45%-50% but could be down to 10%.
- High PALLADIUM.
- WHITE in color.
What are the main contents (2) of silver palladium alloys?
- Ag and Pd.
- Little/ no Au.
How are low gold alloys classified?
- As for conventional gold casting alloys, classified into 4 TYPES.
3 advantages of low gold casting alloys?
- Can utilize same casting equipment as gold.
- Low cost (relative to conventional gold casting alloys).
- Good clinical performance.
Why is the fact that silver palladium alloys have a lower density compared to gold alloys?
Lower density means the alloys requires casting equipment that exerts a GREATER FORCE than for a conventional gold casting alloy.
4 disadvantages of silver-palladium alloys?
- Lower density compared to gold alloys (affects CASTABILITY).
- May dissolve oxygen - POROSITY.
- Lower ductility than conventional gold alloys.
- Care must be taking in their use.
