DM 2 - Precious alloys Flashcards
What is a cohesive gold foil restoration?
essentially tiny pieces of gold which are placed into a cavity which is often first lined with an oxyphosphate lining material, which allows the first piece of gold to effectively key into the cavity. The cavity is incrementally filled up, taking hours to do a simple occlusal restoration, by applying previously treated gold foil into the cavity, applying pressure with a spring loaded mallet which bonds at the point of contact, then a subsequent tiny pieces of gold foil is cold welded or malleted into position
What technique was used to place cohesive pure gold restorations?
cold welding - pressure applied and metallic bonds at point of contact
Why was the gold foil used in cold welded restorations heat treated first?
heated up to 250 degrees to drive off grease
How thick was the gold foil used in cohesive gold restorations?
1 um thick
What were the pros and cons/key points about pure gold restorations?
- time consuming
- no cement lute
- okay in protected cavities (e.g. buccal)
- rigidity and elasticity insufficient in high stress situations
- overworking at placement can work harden
What are the 4 types of traditional casting gold alloys?
- Type 1 low strength
- Type 2 medium strength
- Type 3 high strength
- Type 4 extra high strength
What is Type 1 low strength casting gold alloy used for?
castings subject to slight stress e.g. inlays
What is Type 2 medium strength casting gold alloy used for?
castings subject to moderate stress e.g. inlays/onlays
What is Type 3 high strength casting gold alloy used for?
high stress situations e.g. onlays, thin cast backings, pontics, full crowns
What is Type 4 extra high strength casting gold alloy used for?
for castings thin in cross section e.g. saddles, bars, clasps, crowns, bridges and partial denture frameworks
In traditional gold casting alloys how is the gold content expressed?
gold content expressed as:
- carat - parts by weight of gold in 24 parts of alloy
- fineness - parts by weight of gold in 1000 parts of alloy
What is the carat and fineness of an alloy with 75% gold?
- 18 carat
- 750 fine
What type of traditional casting gold alloy has the highest % composition of gold?
Type 1 - the weakest (85%)
What is the % gold in type 1 casting gold alloys?
85%
What is the % gold in type 2 casting gold alloys?
75%
What is the % gold in type 3 casting gold alloys?
70%
What is the % gold in type 4 casting gold alloys?
65%
What is the % of silver in casting gold alloys type 1-4?
1 - 11%
2 - 12%
3 - 14%
4 - 9%
What is the % of copper in casting gold alloys type 1-4?
1 - 3%
2 - 10%
3 - 10%
4 - 15%
What is the % of platinum/palladium in casting gold alloys type 1-4?
1 - 0%
2 - 2%
3 - 5%
4 - 10%
What is the % of zinc in casting gold alloys type 1-4?
1% for all
Summary of % composition in traditional gold casting alloys:
Summary of % composition of tradition gold casting alloys:
How do the properties of gold casting alloys types 1-4 differ?
as you move from type 1-4:
- increase in:
- hardness
- proportional limit
- strength
- decrease in:
- ductility
- corrosion resistance
Why, as gold content decreases, does hardness increase?
solution hardening - formation of solid solutions with gold
What properties are conferred by silver in gold alloys?
slight strengthening effect and counteracts reddish copper tiny
What properties are conferred by copper in gold alloys?
increases strength and lower MP. If content >16% alloy will tarnish
What properties are conferred by platinum/palladium in gold alloys?
increase both strength and MP
What properties are conferred by zinc in gold alloys?
acts as a scavenger. When used up this function is taken over by copper. This is undesirable as impairs physical properties of the casting if copper is used up this way. Practically always have some fresh allow in the button being melted
What gold alloy types can be heat treated?
- possible for types 3 and 4
- not possible for types 1 and 2
What does heat treatment of gold alloy types 3 and 4 result in?
results in further hardening by precipitation hardening of Ag/Cu and Au/Cu systems
Why can gold alloy types 1 and 2 not undergo heat treatment?
due to lack of silver and copper that is required for the necessary atomic diffusions
How is heat treatment of gold alloy types 3 and 4 achieved?
- melting alloy and cooling it - new crystal structure
- cooling takes time, length of time influences the structure
- slowly to room temp causes premature precipitation and order hardening
How is premature hardening of heat treated gold alloys avoided?
- casting cooled rapidly from excess of 600 degrees
- red colour fo sprue region of casting indicated the temp is over 600 degrees
- once the red colour is lost through cooling the casting should be plunged into cold water
- results in fine grain structure of the casting
- disintegrates investment material, therefore easier to remove
If the gold alloy undergoing heat treatment contains platinum/palladium, how is it cooled?
- cold water cooling risks coring
- therefore heat to 700 degrees for 10 minutes then quench
- known as homogenisation heat treatment
- aim is to eliminate coring
Why were low gold alloys developed?
developed to use less gold due to cost of gold
What are low gold content alloys?
- normally 45-50% gold but could be down to 10%
- high palladium
- “white” in colour
What are silver palladium alloys?
- primarily silver and palladium
- little/no gold
How are low gold alloys classified?
in the same way as conventional gold casting alloys, into the 4 types, utilise same casting equipment
What are the benefits of low gold alloys?
- good clinical performance
- relatively low in cost
What is the density of silver-palladium alloys cf. gold alloys?
lower density compared to gold alloys
Why does the density of silver-palladium allows affect castability?
requires casting equipment to exert greater force than for traditional gold casting alloys
also prone to dissolve oxygen causing porosity
What is the conductivity of silver-palladium alloys cf. gold alloys?
lower ductility than conventional gold alloys
