gearnasher 02-19-2013, 08:45 PM (
https://castboolits.gunloads.com/arc.../t-184987.html)
Murphy, there is a lot of misinformation given already here as to what exactly does what, so I'm going to try to clear it up for you with scientific facts confirmed with the experience of a whole bunch of us.
OK, here's the deal, as figured out a long time ago by a Ph, D. chemist named Glen Fryxell. There's three things we do with our dirty scrap alloy to process it: There's cleaning dirt out, there's reverting the oxide scum back into useable, elemental metal (called "reducing", the opposite chemical reaction to the process of "oxidizing", which is what makes the scum in the first place), and there's FLUXING, which is removing the dissolved metals we boolit casters consider impurities because they impede the flux, or "flow" of the metal when we try to cast things like boolits out of it.
To get the dirt out, it usually just requires a good scraping, stirring, and skimming because it will float to the top. Unless the dirt is denser than lead, like Uranium, but I haven't seen much of that in my boolit metal.
Then there's getting that tin-rich oxide scum to go back in rather than skimming it and tossing it. This is where the "reduction" of oxides takes place, and that requires only heat and unburned hydrocarbons. You can use wax, grease, oil, tallow, butter, margarine, tree bark, grass, sap, hair, ground coffee, cereal grains, rosin, vaseline, etc. You get the idea. The hydrocarbons react with oxidized metal to make carbon dioxide, water (vapor), and DE-oxidized boolit metal. There will be a tiny bit of ash unless you can make the reaction in your open pot stoichiometric, which you can't, and it doesn't matter. Just skim the pinch of grey powder and call it good.
Now, there's FLUXING. This is the part that is mostly misunderstood. A chunk of candle was DOES NOT constitute flux. Marvakrap does, but it has a lot of drawbacks, and it doesn't REDUCE OXIDES at all, neither does kitty litter. Kitty litter doesn't even flux, but I digress. Back to fluxing. Most lead scrap has all sorts of evil junk in it like calcium, iron, aluminum, zinc, cadmium, you name it. We want that stuff GONE but want to keep antimony, arsenic, copper, and tin, along with any trace gold, silver, or copper that might be in there. The thing that does it all is CARBOHYDRATES, and sawdust has plenty. Yes, you can flux with potato chips, if you don't mind the salt corroding things. But you said you hate the stuff I call Marvakrap, so that rules out salty snacks for flux.
Since Rosin and sap makes an excellent sacrificial reducant, and carbohydrates do too, sappy pine shavings make some of the very best flux/cleaner/reducant to be had anywhere at any price.
But in order to get the fullest effect you must expose as much of the metal to the smoldering sawdust as you can so it can soak up that nasty calcium, aluminum etc. etc. while also sitting on the top and keeping oxides reduced. Oxides float, and putting the sawdust on top takes care of reducing them back into the alloy, but to really get the bad junk out you have to expose all the metal in the pot to it. To do this when smelting, first I toss in some wax to reduce the oxides out of the clumpy, dirty junk floating on top after getting all the scrap melted and skim all that off. Then I put about an inch of chainsaw debitage on top (the bar oil residue helps some too) and let it start charring really good. Then I take my big ladle and scrape the heck out of the bottom and sides of the pot and get it stirred up good, then start bringing up big ladlefulls of alloy and drizzling it through the smoldering sawdust layer. I do this over and over again until the sawdust is just a grey ash, which I skim off and toss, then add another layer and repeat. If there is a lot of the junky metals in there, you will notice a big difference in the ash from the first fluxing to the second or third fluxing. The ash from the first fluxing will be more like burnt aluminum foil and later fluxing ash more like fine powder.
That's it! After cleaning, fluxing, and ingotizing, you may not need to flux any more. When casting, if you got the bad junk out during the smelt, all you need to do is keep the oxides reduced, if even that. Kitty litter is good for sealing the surface of the casting melt from oxygen on a bottom-pour setup or to insulate and keep heat in during cold-weather casting, but in and of itself does nothing to absorb impurities or reduce oxides. During casting, you can use wax to keep the oxide scum reduced, stir with a wooden stick, or do what I do and float about a quarter inch of sawdust on top of my casting pot, because as it burns to ash it seals the surface just like kitty litter does, only is easier to clean out and less dusty. The charring sawdust layer, while actively charring and before it turns to ash, is also a great reducant for the flash-oxide that forms on sprues, so if you toss them back in the pot as you cast it will help keep the excess scum caused by that process from forming. You might skim and refresh the sawdust periodically if you wish while casting. Otherwise, grease/wax/oil will keep the oxides at bay.
Hope that sheds a little light,
Gear