Casting Porosity: Causes, Prevention and Repair

Casting porosity is any void left in metal as it freezes. Gas porosity forms when gas is trapped in the solidifying metal; shrinkage porosity forms when metal contracts and no liquid metal is left to fill the space. Shrinkage points to sprue design and temperatures, gas points to burnout, investment and melting practice, and isolated pits can often be laser-welded while widespread or structural porosity usually means recasting.

Telling the two types apart

The descriptions below follow a defect study by Timothy Donohue and Helmut Frye, written with Platinum Guild International. The same patterns apply to gold and silver.

	Gas porosity	Shrinkage porosity
Shape	Smooth-walled, nearly spherical cavities	Spongy or jagged voids, sometimes showing dendrites
Distribution	Small pores in groups, larger ones isolated; can occur anywhere	Concentrated in the last section of the casting to freeze
Typical location	Often just below the surface until polishing opens it	Heavy sections and the area near the sprue attachment
Root cause	Gas from the metal, the mold materials, or air trapped by turbulence	Improper gating or casting parameters
Where the fix lies	Burnout, investment, melting practice	Sprue design, flask and metal temperatures

Dendrites are the branching, tree-like crystals that grow as an alloy solidifies. In a shrinkage void the surface looks frosty or crystalline because those crystal tips are exposed. A clean, round bubble with a smooth lining is the signature of gas.

A useful clue: porosity that turns up in the same place on every casting of a model is almost always a feeding problem, not a gas problem.

What causes gas porosity

Donohue and Frye group gas sources into three families.

Gas from the mold

Incomplete burnout leaves wax residue and carbon in the cavity, and these turn to gas when hot metal arrives. Investment that comes out of burnout gray rather than chalky white is the warning sign.

The investment itself can also produce gas. In a 1999 Santa Fe Symposium paper, G.M. Ingo and colleagues reported that the thermal decomposition of calcium sulfate, the binder in gypsum investment, generates sulfur dioxide and causes gas porosity in gold alloys. They found that decomposition in real investments starts at a lower temperature than in pure calcium sulfate, close to the casting temperatures of some gold alloys, and that metal oxides such as zinc oxide and copper oxides lower it further. Overheated flasks, overheated metal and oxide-laden scrap all push the mold toward that reaction.

Gas from the metal

Molten metal picks up oxygen and other gases during melting, especially under an oxidizing torch flame. Dirty scrap adds more: Stuller warns that investment residue carried over on remelted metal causes long-term porosity problems. Each remelt also burns off some of the alloy’s zinc deoxidizer, as Romanoff notes, which is one reason suppliers recommend a minimum of 50% fresh metal.

Gas trapped mechanically

Air is swept into the metal when the mold is not permeable enough to vent it or when turbulent gating churns the stream.

What causes shrinkage porosity

Eddie Bell, writing for the World Gold Council, puts the volume lost by silver and gold alloys during solidification at 5 to 6%. The casting has to draw replacement metal from the sprues as it freezes. If a feed sprue, or a thin section between the sprue and a heavy section, freezes first, the heavy section is cut off and the shrinkage stays inside it as voids.

Stuller calls improper spruing the leading cause of shrinkage porosity. The usual culprits:

• Feed sprues that are too small or flattened
• Sprues attached to a thin section instead of the heaviest one
• Choke points where the gate narrows before it meets the piece
• Designs with abrupt changes from heavy to thin sections
• A flask so cold that sprues freeze before the piece does

Prevention checklist

Sprue design

• Attach the feed sprue at the heaviest cross-section of the piece.
• Use round sprues and never make them smaller than necessary. Bell cites the rule that a feed sprue’s cross-section should be 70–150% of the attachment point.
• Flare or fillet the gate where it meets the piece; avoid choke gates.
• Make the button and tree weigh at least as much as the castings, as Stuller recommends.

Investment and burnout

• Weigh powder and water and stay within the manufacturer’s ratio.
• Burn out fully, to clean white investment, without exceeding the investment’s maximum temperature.
• Hold the flask at casting temperature for at least an hour so the core matches the kiln.

Metal and melting

• Use at least 50% fresh metal, and clean all investment and oxide from sprues and buttons before reuse.
• Melt with a neutral or reducing flame, flux, or protective gas, and do not overheat. Stuller suggests 100–200°F above the melting temperature for vacuum casting and 50–125°F for centrifugal.
• Match flask temperature to the piece: Stuller’s ranges for karat gold run from 800–1000°F for heavy, simple pieces to 1100–1300°F for fine, detailed work.

Records

• Log flask temperature, metal temperature and scrap ratio for every flask. Bell recommends a simple test grid of flask and metal temperatures, grading each result as acceptable, repairable or reject, to find the best settings for a given pattern.

Spotting porosity before setting

Porosity found after the stones are in costs far more than porosity found on the tree. A practical inspection routine:

1. Examine each casting under magnification after devesting and pickling, before any polishing. Look hardest at sprue attachment areas, heavy sections, prong bases and the underside of heads.
2. Pre-polish, then inspect again. Polishing can open subsurface gas pores, and it can also drag metal over small pits and hide them for a while.
3. Watch the cuts while seating stones. Pits exposed in a seat wall or prong bearing mean the metal holding the stone is compromised. Stop and assess before setting.

Repair options

Situation	Usual approach	Notes
A few isolated pits in a non-structural area	Laser weld with matching filler	Clean the pit first so the weld fuses to sound metal
Pits found after stones are set	Laser weld	Avoids removing heat-sensitive stones, but protect the stones and work slowly
Porosity in prongs, the shank or a seat	Recast	The repair would sit exactly where strength is needed
Porosity throughout the piece	Recast after fixing the cause	Repeated welding rarely chases it all out

Bob Staley’s laser welding tips for Bench magazine are a sound starting point. He notes that most welding can be done with 30-gauge filler wire, recommends matching the filler to the alloy (for white gold, a palladium white gold wire), and describes cutting a groove in thicker areas and collapsing the sidewalls of the host metal into it. In a Ganoksin article by Suzanne Wade, manufacturer Robert Aletto points out that a laser can close a pit near a set stone without exposing the stone to torch heat.

Two older fixes are best avoided. Burnishing a pit shut closes the surface without filling the void below, and the pit tends to reopen with wear or the next polish. Filling with solder leaves a color mismatch, and torch heat can make trapped gas expand into blisters.

Whatever the repair, the cause is still in the process. If nothing changes in the sprue design, burnout or melting practice, the next flask will show the same defect.

Safety

Casting work carries the hazards described in our lost-wax casting primer: silica dust from investment, burnout fumes, and molten metal, which call for a fitted respirator, local exhaust ventilation and shaded eye protection. When laser welding, use the machine’s built-in eye protection and interlocks exactly as the manufacturer intends, never defeat them, and extract the fumes produced at the weld.

Questions

Can porosity be polished out?

Only when it is very shallow. Polishing removes metal, so it often exposes more pores beneath the first ones.

Does vacuum casting prevent porosity?

It helps with fill and lets air escape through the investment, but it cannot fix a feeding problem. Shrinkage porosity still appears if the sprues freeze before the piece does.