Fluxing Question

[blk43]

Several years ago I started using Pat's wood shavings for a flux. I'm very happy with the results but have a question.

After the initial flux I usually get a large tablespoon of sludge off the surface. Fifty bullets leater I flux again and may even get a larger quantity of sludge. I recall reading somewhere that some of the properties that surface are worth keeping but this stuff does not reabsorb into the mixture.

Can someone explain to me what this is and if I'm doing the right thing by skimming it off and getting rid of it?

Thanks, Bruce

[trixter]

Wow, I would like to be in on this one too. Am I wasting something or is it just 'slag'?

[geargnasher]

Bruce, if you're ladle-casting that can really speed up the surface oxidation.

Couple of things to point out here. First, there's oxide scum and there's slag. Any molten metal exposed to the air reacts with he oxygen in the air to form metal oxides on the surface, so there's no getting around the oxide scum formation unless you seal the metal surface with borate glass, sawdust ash, bentonite (kitty litter), or similar. Slag is impure junk that a good and true "flux" has pulled out of the melt. CFF, like any wood-based substance, both fluxes (isolates, absorbs, and removes impurities when you burn it and skim the ash) and reduces oxides (reduction/oxidation chemical reaction where the reducant sacrifices carbon to the metal and removes the oxygen from the scum, making it elemental metal again) both at the same time, but to make it really work as a FLUX you need to stir, stir, stir the melt or ladle the alloy up and pour over the CFF repeatedly to expose the metal to the flux. Otherwise, you're only purifying the metal on the surface of the melt. If you aren't fluxing the metal very well, then the flux will keep pulling stuff out a little at a time each time you flux. Realize that the oxide formation will continue to build on the surface as you cast and that's normal, in fact all you need to take care of that after you've fluxed the pot really well is some wax of some sort, the burning of the wax on the surface will reduce the oxides just fine without a lot of ash.

What you're probably remembering about the value of the stuff on top of the melt is that it is disproportionately rich in tin, if there's any tin in the melt to begin with. At pot temps higher than 750F tin really starts to cook out of the melt by oxidizing at the surface. That's why you want to flux the oxide scum back into the melt from time to time, or use a surface "sealer" to keep it from oxidizing in the first place. When a molten lead alloy containing tin is exposed to the air, the tin oxidizes first, in fact this is how tin makes the lead more "fluid", by flash-oxidizin on the surface of the flowing melt and protecting the lead from the air. Lead oxide is thicker and tougher than tin oxide, so the metal flows better due to the unique nature of tin oxidizing first. Unfortunately, this property can also deplete the tin content of your melt very quickly if you don't reduce it back into the melt.

So, I don't know if you're dealing more with dross, ash from the wood, or oxide scum, but hopefully from the above rambling you can figure it out.

Gear

[blk43]

Thanks Gear,

I am ladle pouring and probably am a little hot at 800 degrees. I try to stir about every 10 bullets. I'll try some kitty litter and see if I see any improvment in the quality of the bullets.

Bruce

[geargnasher]

Cut the pot temp to 700 and cast faster to keep the mould hot. Ladling increases the oxide scum formation exponentially vs. bottom-pouring because you're constantly exposing a large surface area of the melt to the air as you ladle it out and pour it back in. Reducing the pot temperature helps a bunch to slow the oxidation rate down.

Gear