Problems casting slugs from wheel weight

257Roberts

257Roberts

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I have just cast my first slugs using a new bottom pour electric pot. I have been casting for more than 3 years now using a cast pot and laddle and have never had this problem before.

The wheel weight I am using was previously smelted, cleaned and poured into ingots. During my casting session the surface of the metal in the new pot kept skimming over at all the heat settings that I tried. I kept removing the dross and found that it was a very hard metal and I could not get it remelted.
The more I kept skimming off the dross/metal the more it kept forming.

I use a small cast iron frying pan on a propane stove for smelting and making my ingots and I could not get the skimmed metal remelted.

I believe that it could be the tin surfacing from the wheel weight as the more I cast, the slugs became more frosty looking.

Anybody out there with the same problem or any suggestions?

I have not tried the pure lead ingots that I have for other slugs so I do not know if this will be a problem with lead.
 
Did you flux the molten lead with wax or whatever you use for fluxing? Is it possible you didn't flux because it was already clean material. It is possible that without fluxing you are just separating the various parts of the alloy. Hard to tell without being there.
 
I flux with bees wax. The metal stays clean and when stirred the skim does not mix. I also thought that so I did reflex my metal about half way through my casting session. And that did not help.
Good thought though.
 
Tin melts at a lower temperature than does lead, so the dross would not be tin. I doubt that wheelweights have any tin to speak of in them anyway. However, they do contain some Antimony, which is a higher melting point metal. Without tinning properly, the antimony will tend to separate out and leave you with a softer lead than the original wheel weights. Regards, Eagleye.
 
Thanks for your responses.
Eagleye got me thinking. I looked up the density and melting points of tin, lead, antimony and zinc.

Melting Point
Antimony 1166 deg. F.
Lead 621.5 deg. F.
Tin 449.5 deg. F.
Zinc 787.34 deg. F.

Weights are as follows.
Antimony 6.684 g/cubic cm.
Lead 11.34 g/cubic cm.
Tin 7.31 g/cubic cm.
Zinc 7.13 g/cubic cm.

My thought are that the antimony is coming to the surface as Eagleye suggested. It has a higher melting point and would be hard to remelt and would be harder when cooled. during my next casting session I will take some temperature readings and keep you informed.
Eagleye if you are out there when you refer to tinning do you mean fluxing?
Thanks again.
 
You will always get a layer of oxide (mostly lead oxide) forming on the surface of any melt, however it will form much faster at high temperature. My suggestion - don't worry about it too much. Start with well-fluxed and clean alloy, after the layer of oxide forms it will insulate the rest of the melt from further oxidation. This is a self-limiting thing. If you keep skimming you are only succeeding in making lots of lead oxide - dangerous stuff.

Best way to limit oxide formation is to cast a lower temperature. One other option is to put a layer of powdered carbon over the surface of the melt. Just crush a charcoal briquette and sprinkle it over the melt.

Tin, antimony or any other component in an alloy won't "rise to the top". This is a fairly persistent myth of casting. Once a metal has been alloyed it is basically "dissolved". Think of sugar in water.
 
Thanks PEI. As in my original post I was not having this problem with my cast iron pot, it just happened with a new electric pot I am using.
I plan to take some temperature readings with the new pot and compare to my old readings. Being a new toy I just had to give it a quick try to see how it worked when the melt began to skim. Thanks again for the info.
 
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