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Stains on my polished bronze plaque

2007

I HAD 15 SILICONE BRONZE, 6X10X1 OVAL PLAQUES CAST FROM A PATTERN THAT I HAD MADE. WHEN I GET TO THE FINAL POLISHING, USING Fabulustre [affil links] AND A FLANNEL WHEEL THESE STAINS APPEAR ON THE 1" HIGH SIDE OF THE PIECE AND ON THE 1" WIDE BORDER THAT RUNS ALL THE WAY AROUND THE TOP. THESE SPOTS ARE ABOUT THE SIZE OF A THUMBPRINT,REDDISH IN COLOR. NOT A SOLID REDDISH SPOT BUT MORE OF A SWIRLING LOOK. I HAVE REGROUND THE SURFACE AND REPOLISHED BUT THEY STILL COME BACK. HAVE POLISHED 7 PIECES AND EACH ONE HAS THE SAME PROBLEM. THERE ARE 5-7 SPOTS ON EACH PIECE AND THEY SEEM TO BE IN THE SAME GENERAL AREA ON EACH ONE. I SUSPECT SOME TYPE OF CONTAMINATION OR PERHAPS THE METAL WAS NOT HOT ENOUGH WHEN POURED. ANY THOUGHTS OR SUGGESTIONS WILL BE GREATLY APPRECIATED.

THANKS FOR THE HELP, DEAN ARRINGTON

DEAN ARRINGTON
NEW PODUCT DESIGNER - PHOENIX, Arizona

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The spots on the casting may be porous areas of shrinkage porosity. Check the sprue and gating system on the casting as it may be the cause of the shrinkage porosity. Flat pieces with raised areas can be troublesome to cast as the heavier areas will solidify last. Attaching the ingates to the heavier areas of the casting may be helpful as it will allow progressive solification from the thin to thick areas and avoid the shrinkage porosity.

Good luck & Best regards,

Jim Sivertsen
Refining & Alloys - Alden, New York
January 4, 2008