Originally Posted by
DukeInFlorida
So, if I may........ Here are the physics and mechanics for using sawdust and other products when melting down raw lead materials.
1) Raw lead melts at about 700 degrees or so. It varies with other alloyed metals in the mix. So, tin and antimony, the normal additional alloy metals might increase the melting temp somewhat.
ZINC - Shudder in horror, melts at a much higher temperature, about 1200 degrees. It will ruin a batch of lead alloy. It's the reason to pre-sort out zinc wheel weights, if that is what you are melting down. IF you overfill your melting pot, and have a few zinc weights at the bottom, where they might see 1200 degrees, the pot and that batch will be ruined. I always start melting my batches with just a few handfuls of raw lead, and then when that melts, I stir in some more. I'm always terrified of some kind of zinc product being at the bottom of a very full smelting pot.
2) Lead is extremely dense. It is the most dense thing in the pot of molten lead. Toss in a steel bolt and it will float. The sand and other impurities also want to float to the surface, but the overall density of the molten lead prevents them from rising through that dense lead.
3) Stirring in sawdust, and especially soft pine or other resinous plant fibers creates pathways through the lead for th impurities to travel along, making their way to the top where they can be skimmed off. I usually use a good handful of soft pine shavings (horse bedding) or sawdust, and then stir it in vigorously, stirring all the way to the bottom of the pot. You are trying to create eddy current pathways for the junk to make it through the dense lead. Right when the pine starts to char, and before it burns into ash, I skim it all off the top, along with the sand, and other impurities. I do that three or more times, depending on how nasty the raw lead was.
4) I then switch over to paraffin wax, old candles from yard sales, they are always cheap. Pieces about the size of a walnut are good. Toss that on the top of the molten lead, and stir, again towards the bottom. Be ready for the wax, with a low burning point, to flare into flames quickly. The melted wax creates even finer pathways through the dense lead, allowing the finer grit to make its way to float on the top. I do the wax a couple to three times, and stir until the wax completey burns off. I allow the lead to tell me when it's done. When constant stirring and scraping on the bottom of the pot no longer loosens the grey dust stuff to the surface, I know I am ready to pour out into ingots.
5) Watch out for gold colored stuff which might float to the surface. That is typically TIN coming out of solution with the lead. Sure sign that you are overheating the lead in the pot. Turn down the heat, and stir that gold stuff back into solution. Tin helps the lead FLOW into tight corners in your mold.
6) Ditto for blue or purple stuff rising to the top. That's antimony coming out of solution due to over heating the lead. Antimony is added to make the lead harder. Mix that back into solution, and lower the heat.
You always want the lead to just barely stay molten during the "fluxing" process. Over heating it risks having tin and antimony come out of solution, and wastes propane or whatever you are using as a heat source.
Finally, while I am ladling out alloy into ingot molds (I like using cupcake tins), I use the back of the ladle to clear a very shiny path through the lead, and at the end of opening up that shiny area, I dip the ladle into that, to get my lead. I eventually, even after careful cleaning and fluxing, will start to get a bunch of gritty grey crap on the surface again. I stop making ingots and clear that stuff out of the pot. I don't want any of that in my ingots.
Remember, any grit you leave in the alloy is like sandpaper scraping through the bore of your barrel. I don't want to wear out barrels like that, and hope you don't either. I hope my explanation of how I do it might help others.