Porosity: The Facts!

The causes of porosity today are still shrinkage and gas, just as they were when I wrote my first article on porosity in 1976. However, most of the porosity that we see in jewelry castings today is shrinkage porosity, which results from the metal contracting during freezing and solidification.

Shrinkage vs. Gas Porosity

There is a distinct difference in appearance between shrinkage and gas porosity:

• Gas Porosity: Generally shows up on the surface of castings as irregularly shaped bubbles.
• Shrinkage Porosity: Appears as pinpoint or "wormlike" holes. In a raw casting, it may look like a dark spot on the inside or outside wall. Under magnification, it reveals a fernlike, crystalline structure known as dendritic growth.

Shrinkage porosity is often confused with gas porosity or "hot tears" (internal stresses), though all three usually occur in the heavier sections of a casting.

Common Causes of Shrinkage

1. Piece Design: Abrupt changes from heavy sections to thin sections.
2. Sprue Size: Sprues that are too small to keep molten metal feeding the area that is cooling.
3. Flask Density: Placing castings too close together in the flask, which prevents heat from escaping and slows the chilling process.

Pro Tip: Shrinkage is a feeding problem. If porosity appears in the same area consistently, the ring is fed improperly. If the location varies from tree to tree, the issue is likely inconsistent metal or flask temperatures.

Progressive Solidification

The ideal sprue should promote progressive solidification. This means the heaviest part of the casting is fed by a slightly heavier sprue, which is fed by an even heavier "tree trunk," leading back to the button. The goal is for the casting to solidify first, pulling metal from the still-liquid sprues as it shrinks.

Temperature Guidelines

Flask Temperature

I recommend keeping flask temperatures constant for everything in a particular oven. While a controller changes quickly, the interior of a flask takes 30 to 60 minutes to reach a new temperature.

The Four Casting Temperature Ranges

• Range A (Melting Point to +75°F): Often results in incomplete castings because the metal is too cold.
• Range B (+75°F to +150°F): Forms a complete casting, but "Cold Porosity" occurs where two streams of semi-fluid metal meet in the shank and fail to bond homogeneously.
• Range C (+150°F to +190°F): The Ideal Range. Metal is hot enough for streams to meet in a fluid state, creating a solid casting.
• Range D (Everything above Range C): Leads to excessive shrinkage in heavy parts because the metal remains liquid too long while cooling.

Hot Tears

Hot tears are caused by high internal stresses when a section cannot expand or contract during cooling. Solutions include:

• Designing models with gradual transitions rather than sharp bends.
• Adding sprues to heavy sections to minimize stress.
• Changing alloys; some alloys exhibit higher shrinkage rates than others.

Analyzing Common Failures (Diagram Guide)

• Turbulence (Diag 1 & 2): Avoid 90° sprue corners. Use flared entrances for smooth metal flow.
• Multiple Feeds (Diag 3): Sprue the heaviest portions of a casting directly to avoid thin areas freezing first and cutting off the feed.
• Investment Bubbles (Diag 5): Caused by poor vacuuming. Use a surfactant like Vacufilm to reduce surface tension on wax.
• Inclusions (Diag 6 & 7): White spots are usually investment particles; black spots are typically graphite flakes from an old crucible.
• Wax Shrinkage (Diag 8): "Sinks" in flat surfaces occur if the wax isn't held under pressure long enough. Hold the mold against the nozzle for 30–40 seconds.
• Improper Burnout (Diag 9): Rapid temperature climbs in the first hour cause wax to boil, breaking down the investment surface.

Managing Gas Porosity

Gas porosity occurs when molten metal absorbs oxygen or hydrogen. Prevention is key:

• Controlled Atmosphere: Melt in a vacuum and introduce Argon to create a protective blanket.
• Reducing Atmosphere: Use graphite crucibles to minimize gas absorption.
• Proper Burnout: Investment should be "chalk white." Residual ash acts as a barrier, trapping gas.
• Alloy Integrity: Use at least 50% fresh metal. Zinc acts as a deoxidizer but burns off with every re-melt.
• Moisture Control: Pre-heat crucibles and furnaces to eliminate absorbed moisture before casting.

Modern machines that utilize vacuum, argon, immersion thermocouples, and overpressure vibration offer the best conditions for a perfect, gas-free casting.