here is the info on the mentioned website:
Sawdust is another material that has been used as a flux for bullet metal, and it has the advantages of both the previous classes of bullet fluxes (it has often been employed in conjunction with oil, but all that does is make fluxing smokier and smellier). Sawdust is also a sacrificial reductant that reduces tin, thereby returning it to the melt (again, reduction is the opposite of oxidation). It also has many building blocks (lignin's, tannins, gallates, etc.) that bind to oxidized metals. Lead, tin and antimony are fairly easy to reduce back to the metallic state, others are not so easy.
Of particular interest to the bullet caster are calcium, aluminum and zinc -- all of which are difficult to reduce and all of which cause casting problems if present in any significant amount (they muck up the surface tension of the alloy and prevent the alloy from filling out the mould properly). As the sawdust chars, it can be thought of as a kind of activated carbon. Both the lignin's of the original sawdust and the oxygenated sites of the activated carbon are very effective at binding metal ions like calcium, aluminum and zinc. Thus, the advantage of sawdust is that it does both jobs, returning the tin to the melt and removing the problematic impurities. Sawdust has the added benefit of being free.
I generally use walnut sawdust, left over from my grip-making activities. Different types of wood are known to give rise to grades of activated carbon with different activities, but whether or not this would make any difference to the bullet caster I don't know (doubtful, any sawdust should work just as well, and some, like cedar, redwood and pine smell awful purty!). A heaping tablespoon is just about right for a 10 lb pot, stirred in thoroughly to begin with and then left in place. Halfway through the pot, the lead-pot is stirred again, this time the activated carbon (dross) is removed. By leaving the charred sawdust on the melt for the first half or so of the casting session, a barrier is formed to slow down the oxidation of the tin, and by removing the charred dross before reaching the bottom, the sequestered impurities are removed before they can sneak through the bottom-pour spout and possibly cause inclusions.
- Glen E. Fryxell
Reprinted on lasc.us by permission of the authors and Sixguns.com
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