Podcast
Questions and Answers
What is the consequence of having a burnout temperature that approaches 700° C during casting?
What is the primary purpose of alloying gold with metals like copper and silver?
Why must the coefficient of thermal expansion of the alloy be higher than that of the veneering ceramic?
Which of the following acids is NOT involved in dissolving gold?
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What is typically used instead of a gypsum-bonded investment when the burnout temperature is near 700° C?
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Which high-noble alloy typically consists of 78 wt% gold along with equal amounts of silver and platinum?
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What is a characteristic of Au-Cu-Ag-Pd-I alloys in terms of their composition?
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Which of the following alloys has gold content that is only 20 wt%?
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What determines the color of casting alloys?
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Which noble alloy is composed of little or no gold, with about 75 wt% palladium?
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What is the primary compensation for the reduced gold content in Au-Cu-Ag-Pd-III alloys?
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Which alloy retains a light yellow color due to the interaction of indium and palladium?
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What is a characteristic of Ag-Pd alloys regarding their gold content?
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Study Notes
High Noble Alloys
- Three classes of high-noble alloys:
- Au-Ag-Pt alloys (78 wt% gold, equal amounts of silver and platinum)
- Au-Cu-Ag-Pd alloys with a gold content greater than 70 wt% (Au-Cu-Ag-Pd-I) (75 wt% gold, 10% silver and copper, 2-3% palladium )
- Au-Cu-Ag-Pd alloys with a gold content of 50% to 65% (Au-Cu-Ag-Pd-II).
- These alloys are typically used for casting and porcelain-metal alloys.
- Base metals (tin and indium) form an oxide layer to promote bonding with veneers.
Noble Alloys
- Four classes of noble alloys:
- Au-Cu-Ag-Pd alloys (Au-Cu-Ag-Pd-III) with 40 wt% gold.
- Au-Ag-Pd-In alloys with 20 wt% gold, 40% silver, 20% palladium, and 15% indium.
- Pd-Cu-Ga alloys with 75 wt% palladium and equal amounts of copper and gallium.
- Ag-Pd alloys with 70 wt% silver and 25 wt% palladium.
- Palladium content makes these alloys considered noble according to ADA specification.
Alloy Properties
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Color:
- Alloys with over 10 wt% palladium are white (Pd-Cu-Ga and Ag-Pd alloys).
- Other alloys are yellow, with the exception of Au-Ag-Pd-In alloys which are light yellow due to palladium interaction with indium.
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Solubility:
- Gold is not soluble in sulfuric, nitric, or hydrochloric acids.
- Gold is readily dissolved in aqua regia (18 vol% nitric and 82 vol% hydrochloric acids).
- Gold is also dissolved by potassium cyanide and solutions of bromine or chlorine.
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Melting Range:
- Alloys have melting ranges instead of specific melting points.
- Wide melting range can lead to oxide formation and contamination during casting.
- Burnout temperature should be at least 500°C below liquidus temperature.
Ceramic-Metal Restorations
- Consist of a cast metallic framework with ceramic layers baked on top.
- Coefficient of thermal expansion of the alloy should be slightly higher than the veneering ceramic to ensure compression after cooling.
- This compression improves resistance to crack propagation in the restoration.
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Description
Test your knowledge on high noble and noble alloys, including their classifications and properties. This quiz will cover various alloy compositions and their applications in casting and metal bonding. Challenge yourself to understand the differences and similarities between these alloys.
