Gold Casting Alloys

Overview

ADA Classification (ADA Specification No. 5 / ISO 1562)

By Noble Metal Content (Current System)

By Function: Type I - IV (Original & Current System)

High Gold Alloys

Type I - Soft (Inlay Gold)

• Use: Small inlays (Class III, V) not subjected to significant stress. Easily burnishable.
• Gold content: Highest of the four types
• Noble metal content: ~83%
• Key property: High ductility, low hardness
• Not heat-treatable

Type II - Medium (Inlay Gold)

• Use: Inlays under moderate stress, thick ¾ crowns, abutments, pontics, full crowns, soft saddles
• Noble metal content: ~79%
• Key property: Better mechanical properties than Type I but less ductile
• Not heat-treatable

Type III - Hard (Crown & Bridge Alloy)

• Use: Inlays under high stress, thin ¾ crowns, thin cast backings, pontics, full crowns, denture bases, short-span FPDs
• Typical composition: Au 75%, Ag 11%, Cu 9%, Pt ~3.5%, Pd balance, Zn & Ga trace
• Hardness: 120-150 VHN | Tensile strength: 360 MPa | Yield strength: 331 MPa | Elongation: ~5-39%
• Heat-treatable: Yes - age hardening possible
• Linear casting shrinkage: ~1.42%

Type IV - Extra Hard (Crown & Bridge Alloy)

• Use: High-stress areas - crowns, long-span bridges, partial denture frameworks, bars and clasps, saddles
• Gold content: Lowest of the four types (<70%), with highest percentages of Ag, Cu, Pt, Pd
• Key property: Highest yield strength and hardness; lowest ductility
• Heat-treatable: Yes - most responsive to hardening heat treatment
• Provides the rigidity needed to resist flexure in long-span prostheses

Types I & II are collectively called "Inlay Gold Alloys." Types III & IV are collectively called "Crown & Bridge Alloys."

Low Gold and White Gold Alloys

Low Gold Alloys

• Reduced gold content compared to traditional high-gold alloys
• Copper, silver, and palladium substituted to maintain color and compensate for reduced gold
• Higher hardness and yield strength than high-noble counterparts
• Reduced corrosion resistance due to higher Cu and Ag content

White Gold Alloys (Ag-Pd Alloys)

• White/silver colored; predominantly silver-based
• Contain ≥25% palladium to confer nobility and tarnish resistance
• May also contain copper and small amounts of gold
• Properties: copper-free Ag-Pd ≈ Type III; with ≥15% Cu ≈ Type IV
• Casting temperatures similar to yellow gold alloys
• Limitation: lower density and tendency to dissolve oxygen when molten - requires precise temperature control

Role of Alloying Elements

Heat Treatment of Gold Alloys

1. Softening Heat Treatment (Solution Heat Treatment)

• Method: Place casting in furnace at 700°C for 10 minutes, then quench in water
• Effect: Increases ductility; reduces tensile strength, proportional limit, and hardness
• Indication: Before grinding, reshaping, or cold working the restoration (in or out of the mouth); before burnishing margins; before hardening treatment

2. Hardening Heat Treatment (Age Hardening / Precipitation Hardening)

• Method: Heat to ~400°C, then slow cool (or hold at temperature)
• Effect: Increases hardness, yield strength, proportional limit, and modulus of resilience; reduces ductility
• Mechanism: Cu-Au ordering within the solid solution (ordered Au₃Cu and AuCu phases form)
• Indication: Partial dentures, saddles, FDPs, clasps - anywhere rigidity is required

Important: Type III/IV alloys should be softened first, then hardened - never hardened from the as-cast state directly.

Desirable Properties of Dental Casting Alloys

1. Biocompatibility
2. Ease of melting and casting
3. Ease of brazing (soldering)
4. Ease of polishing
5. Minimal solidification shrinkage
6. Minimal reactivity with mold material (investment)
7. Good wear resistance
8. High strength
9. Excellent corrosion and tarnish resistance
10. Porcelain bonding capability (for metal-ceramic alloys)

Summary Table