What sorta problem here? Some sort of contamination?

[Twmaster]

I rounded up another big pile of pewter things this weekend and as I was bored I'd decided to make some ingots.

All was going well until I put parts from picture frame into the melt. It left an odd dross as it melted.

Temp was below 600F and the pot was near full at about 10 pounds. Casting outside, 33F windy felt cold.

After pouring the ingots into the mini-muffin mold I use for pewter all seemed well. Nice shiny surface. Seemed to take a long time to solidify. Once it did solidify and began to contact as it cooled it left a very weird cavity in the resulting ingot. Also the area surrounding the cavity looks like a honeycomb.

http://www.twmaster.com/stuff/shoot/reload/cast/pewterproblem.jpg

The cavity is easily 5/16" deep and varies from 1/4" to 3/8" wide. The muffin is a bit more than 1-3/4" wide overall. The remainder of the ingot is nice and shiny and seems to have filled out the mold nicely.

Any ideas?

There was a very small amount left in the pot to which I added a pewter plate and an old tea pot made of 'Britannia metal' (english pewter)

The ingots from that pot came out like usual. Satin finish with nearly no contraction.

[Twmaster]

After talking with a longtime friend whose been casting since a child he thinks it's bismuth causing that. The cheapo picture frame I melted in is the likely culprit.

[el34]

Too bad there's not an XRF app.
I wonder- if it's bismuth- if it's too much to be usable. Do you have a hardness tester?

[Twmaster]

Whatever is in there seems to be easily beaten by dilution. Per my comments the subsequent pot of pewter, which used about an inch worth of remains from the suspect alloy, was diluted by a couple of pounds of pewter and it seems to be OK.

It started to snow so my casting activities had to be cut off for the evening. Else I'd have tried to mix it into 10:1 alloy.

There is a fellow on here who was gracious enough to test some shot for me. I wonder if he's up to the task of testing this stuff?

[scottfire1957]

I had one batch make cavities like that. No "crust" appearance though. I just figured my temp was what caused it.

[jsizemore]

Temp at least 100F too high.

[williamwaco]

I have seen it.
Don't know what causes it.

Still makes good bullets

[Forgetful]

Bismuth causes expansion during cooling. A little bit in your alloy (less than 1%) negates most alloy shrinkage. If you're using a 20:1 or 10:1 ratio and the boolits are dropping slightly too small, add a little Bi and they'll drop a little bigger.

I've had zinc ingots do that, due to the surface area solidifying before the core, and breaches the surface. Use a torch to keep the top molten until the core solidifies... That's how you negate inclusions in zinc pours. Lead alloy will do that too, but a big enough sprue helps provide a liquid source during the shrink, to draw metal from. Actually, all metals will do that, and is based on how deep of a pour you make. If you stopped pouring sooner so you wouldn't get a muffin-top, you probably won't have this issue.

The wind apparently stripped the heat out of the muffin-top and didn't allow the core temperature to propagate upwards at a normal rate, making a hot-pocket.