Dental Metals and Alloys: Noble, Base Metal Casting Guide

Dental metals and alloys are behind gold crowns, partial denture frameworks and the copings under porcelain-fused-to-metal bridges. Pure metals are too soft for the mouth, so dentists use alloys, blended mixtures that combine strength, corrosion resistance and castability.

Alloys matter because the mouth is brutal: hot coffee, ice water, acidic foods, salty saliva and chewing forces up to 800 newtons repeated millions of times. An alloy must survive all this without corroding or breaking, while shrinking predictably so the casting fits the tooth.

Key Concepts

• Noble alloys: contain gold, platinum, palladium or silver; highly corrosion-resistant and tissue-friendly. Traditional crown gold is about 70 to 75 percent gold, hardened with copper, silver and platinum.
• Base metal alloys: nickel-chromium and cobalt-chromium; harder, stiffer and cheaper than noble alloys, but technique-sensitive to cast, and some patients are nickel-allergic.
• Casting shrinkage: molten alloy contracts on solidifying, typically 1.25 to 2.5 percent linearly; the investment mold expands by the same amount to compensate.
• Density: gold alloys about 15 to 19 grams per cubic centimeter; base metals about 8, affecting the crucible charge weight.
• Yield strength and hardness: soft Type I gold for small inlays, hard Type IV for long bridges and denture frameworks.
• Biocompatibility: noble alloys release almost no ions; nickel alloys release more, so nickel-free cobalt-chromium suits sensitive patients.

How It Works

1. The wax pattern is built on the model with a sprue channel attached.
2. It is invested in phosphate-bonded investment (for high-fusing alloys) and burned out, leaving a mold cavity.
3. Alloy is melted by torch or induction; base metals melt above 1300 degree Celsius, gold alloys around 900 to 1000 degree Celsius.
4. Centrifugal or vacuum force drives the molten metal into the mold.
5. The casting is divested, fitted on the die, finished and polished; porcelain is fused on if required.

Worked Example

A technician casts a gold crown. The wax pattern plus sprue weighs 0.45 g (density about 1 g per cm3, so volume about 0.45 cm3), and the gold alloy density is 15 g per cm3:

1. Alloy mass = 0.45 cm3 x 15 g per cm3 = 6.75 g.
2. A 2 to 3 g button is added so the sprue stays fed as the crown solidifies: total charge roughly 9 to 10 g.
3. In cobalt-chromium (8.3 g per cm3) the same crown needs only 0.45 x 8.3 = 3.7 g plus button, nearly half the weight, which is why base metal frameworks feel lighter.

Common Misconceptions

• Gold crowns are pure gold. Wrong. Pure gold is far too soft; dental gold is hardened with copper, silver, platinum and palladium.
• Base metal means low quality. Wrong. Cobalt-chromium is stronger and stiffer than gold, exactly what a denture clasp needs.
• Any investment suits any alloy. Wrong. Gypsum-bonded investments break down above about 700 degree Celsius; high-fusing alloys need phosphate-bonded investments.

Real-World Applications

• Full gold crowns remain the longest-lasting molar restoration, often serving 20 years or more.
• Porcelain-fused-to-metal crowns and long bridges use noble or base metal copings for strength.
• Partial denture frameworks are cast in cobalt-chromium for stiffness and thin, comfortable design.
• Implant abutments use titanium alloys for strength and bone-friendly surfaces.

Dental alloys, from high-gold noble formulations to stiff cobalt-chromium, are engineered to survive the mouth's heat, acid and chewing forces while casting to a precise fit. The lab matches each alloy's shrinkage with a compensating investment expansion, and charge weight follows from pattern volume and density. Well chosen and well cast, metal remains the durability benchmark.