Porosity means small holes in a casting. They usually appear during polishing, after the gold and labour are already spent. The good news: there are two main kinds, and once you know which one you have, the fix is usually clear.
Gas porosity or shrinkage porosity?
| Gas porosity | Shrinkage porosity | |
|---|---|---|
| Looks like | Round bubbles, often near the surface | Tiny pinholes or worm-like holes |
| Where | Anywhere | Thick sections, where thick meets thin |
| Cause | Gas picked up by the metal or trapped in the flask | A section cooled last with no metal left to feed it |
| Main fix | Cleaner metal, dry crucibles, full burnout, vacuum | Sprue size and position, steady temperatures |
A quick test: are the holes in the same place on every tree? If yes, the problem is how that piece is fed by its sprue. If they move around, look at temperatures first. Shrinkage is the more common of the two, so check for it first.
How to fix shrinkage porosity
Metal shrinks as it turns solid. The casting should freeze from its thin, far ends back towards the sprue, so liquid metal keeps flowing in. If a thick part is cut off from that supply, it shrinks into itself and leaves holes.
- Attach the sprue to the thickest part of the piece, not to a thin edge.
- Make the sprue thick enough to stay liquid longer than the part it feeds.
- Avoid sharp corners where the sprue joins. A flared joint lets metal flow smoothly.
- Do not pack pieces too close together in the flask.
- Do not overheat the metal. One casting supplier suggests pouring roughly 85 to 105 °C above the alloy's melting point. Follow your alloy maker's data where you have it.
How to fix gas porosity
- Use enough fresh metal. One supplier recommends at least 50% fresh metal per melt.
- Keep scrap clean. Never re-melt scrap that contains solder.
- Dry the crucible before the first melt. See our guide to gold melting machine mistakes.
- Burn out completely. Ash left in the flask traps gas. Our burnout furnace guide explains the cycle.
- Cast under vacuum, so air is kept away from the molten metal.
Keep temperatures steady
Many porosity problems come from temperatures that change from flask to flask. The oven display changes quickly, but the inside of a flask can take 30 to 60 minutes to catch up. Cast too early and the flask is colder than you think. Keep one flask temperature for each oven load, and measure the metal temperature instead of judging by eye.
Where casting machines help
- Induction melting. Our induction melting machines melt without a flame, so no burner gases sit over the metal.
- Bottom pouring under vacuum. The IVC-1/2 pours from the bottom of the crucible under vacuum. Dirt and oxides float on top, so the cleanest metal enters the flask first.
- Vacuum pressure casting. Our vacuum pressure casting machine suits detailed work.
A machine cannot fix a badly sprued tree, but it removes many of the causes of gas porosity.
Checklist for your next casting
- Look at the holes: round bubbles or tiny pinholes? Same place or moving?
- Same place: change the sprue for that piece.
- Moving: check flask and metal temperatures.
- Check the fresh metal ratio and that no solder scrap went in.
- Check the crucible was dry and the burnout complete.
Where to see and buy it
These are the Semsons Tech machines for this job. The store links let you book online; the other links open the full specifications or a quotation request.
| Machine and where to buy | Key facts |
|---|---|
| Manual vacuum pressure casting machine Buy or book on our store | Induction melting with vacuum and gas pressure casting For gold, silver, brass and copper alloys |
| IVC-1/2 vacuum casting machine Full specifications Ask for a quote | Melts by induction and pours from the bottom under vacuum 1 to 2 kg of 24 kt gold, three phase 440 V, 7 kW |
Want to see vacuum casting in action? Book a demonstration or call +91 98216 12340.