melting with an "oxidizing-reducing" atmosphere?

ArtMetal
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Posted by Rob Frink on May 31, 1999 at 12:43:08:

In referense to my natural gas crucible furnace...I'm not sure what an oxidizing atmosphere is. I usually adjust the blast gate for the maximum roar and brightest flame, but today I had some problems and I'm trying to run them down.

During a heat of some new everdur, I noticed a slightly greenish flame coming out of the exhaust hole of the furnace. When looking into the combustion chamber the heat looked hotter than I've ever noticed before. So I decided to leave the fuel/air mixture alone and pulled the crucible @ about 2100 F.

After skimming, the molten metal had a slippery rainbow film on the surface that slowly churned and glistened. I've never seen this before. The molds were poured and the crucible was returned to the furnace and recharged. The molds immediatedly showed signs of trouble. The Molten metal in the sprue seemed to swell and formed a slight mushroom shape. Normally they shrink and colapse. When I shook out the molds (green sand) the castings were covered with tiny little defects that looked like little broccili heads in the heavy sections. Thin castings from this pour were beautiful. The thick castings were scraped.

The next 2 pours was remelted gates/risers and sprues from a previous bronze purchase from a different supplier. No problems.

So....here's my questions:

Could the swelling be from a gassy melt?

i) caused by improper fuel/air mixture?

ii) caused by poor ingot metal

What was the glassy rainbow appearance of the molten metal?

The lab certifications from the ingot supplier show 0% phosphorous and I don't deoxidize with anything.

The material is everdur: 94.66 copper, .88 manganese, 4.25 silicon

Any feedback would be greatly appreciated.

Thanks,

-Rob


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