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Porosity in castings

Hello all

I recently set up a casting studio after a long hiatus from making jewelry. I was given a Victor Oxy acetelyne torch with a standard pointed tip for melting metal, mostly silver. I am getting a lot of porosity in my castings. The last time I can the pieces looked almost like filigree. I used silver with a deox alloy. I believe that the problems are being caused by the acetelyne torch. Does anyone have any suggestions, should i use a rosebud torch tip? Is propane better for casting?

Sorry to hear about your problem…
Porosity can be caused bu many things. Without actually seeing the piece to examine it it is hard to narrow it down to just one thing. That said … first thing would be sprueing problem… not positioned correctly or not large enough to allow metal to flow before it starts to freeze.
Over heating can cause problems too. Bad investment mixture… too much water usually leaves streaks on the surface but not always.
Gas can react to platinum alloys in a negative manner…Platinum and dirty gas do not mix and your gas has a lot of carbon in it that will contaminate some alloys.
In my shop I only use propane gas as is a much cleaner burning gas. Not quite as hot as acetylene gas but very close… for what we work with in making jewelry.
Hope this helps…
keep a note book and write down every step and how much water to investment you use type of alloy and how it is sprued. That way you can use it for a reference when trying to eliminate a future problem.
Hope this helps. Good luck and enjoy the learning curve.

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Turbulence in metal movement as it enters the sprue, turns to the piece and even the piece itself can all cause porosity. Temps matter for both flask and metal. A good IR high temp reader can help you stay on target. A small filet between sprue and piece smooths action. Anywhere that metal moves before it gets to the piece, it needs to be a smooth a transition as possible. No wiggly patches of sprues (though this doesn’t bother pewter much since it cools slowly from its casting temp) Some say a square sprue will allow gas escape without affecting movement since the metal has surface tension. It is not traditional.

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A number of years ago this subject was discussed on this forum. One of the responses said that molten silver has an affinity for dissolving oxygen which manifests itself as porosity in the casting. The recommendation was to use the lowest oxygen flame as possible and don’t overheat the metal before casting. You can probably research the archives to find the postings.