What is denture casting porosity and how can it be avoided?

• A. Porosity refers to color variation in the casting.
• B. Porosity is voids within the metal casting; avoid by proper burnout, venting, investment slurry moisture control, and degassing.
• C. Porosity is surface roughness of the casting; avoid by polishing.
• D. Porosity is micro-cracks from cooling; avoid by slow cooling.

Answer: (B)

Porosity in a denture casting means voids or air pockets trapped inside the metal as it solidifies. These pockets form when gases generated during pouring and cooling aren’t able to escape properly, often because moisture or contaminants are present in the investment, the investment isn’t adequately vented, or the molten alloy isn’t degassed. The result is a weaker casting with visible porosity defects that can compromise fit and strength. The way to prevent this is to follow a proper burnout that thoroughly removes moisture and organics, design and provide adequate venting so gases can escape during casting (including appropriate sprue design and vents in the invest), control the moisture content of the investment slurry, and degas the molten alloy to remove dissolved gases before pouring. Porosity is not color variation, surface roughness, or micro-cracks from cooling; those are different defects or characteristics.