A fluxing question

[jamesp81]

Pretty new to this but been doing some reading and have learned some things. Parrafin wax will reduce oxidized tin back into the melt but won't create slag to skim off. Commercial fluxes like Marvelux will bring impurities out as slag but also depleted tin.

I was was reading LA Silhoutte club's website and they recommend sawdust:

"Sawdust doesn’t really qualify under theformal definition of “flux” as it doesn’t produce a fusible slag, but it does verycheaply and very effectively accomplish the three primary goals that we set outfor cleaning up bullet metal. Reduce, remove and protect, sawdust does it all!"

I don't understand this. If the impurities don't come out as slag, how can you skim them off?

[JWFilips]

My Favorite Explanation from Geargnasher:


Boolit casters typically use dirty, contaminated scrap
from whatever source we can scrounge up, and that
sort of stuff needs special attention to make the best
boolits. Clean alloy like nuclear medicine shields or
foundry alloy doesn't require as much cleaning.
We need to do three things to our alloy: Clean, reduce
oxides, and flux.
Cleaning is accomplished by mechanical action,
stirring and skimming dirt, sand, steel clips, rust
flakes, copper jackets, whatever. A slotted spoon is
adequate.
Oxide reduction is next. We need to deal with the
oxide dross formation on top of the metal, whether in
smelting pot, or a freshly-melted casting pot full of
clean ingots. Boolit alloy dross is very rich in valuable
tin, so we need to turn it back into useable metal
rather than skim and toss it. The opposite of oxidation
is a chemical process called "reduction", so if we
induce a reduction/oxidation reaction on top of the
metal, we can save the scum. Combustion is a redox
reaction. Anything that will burn will trade electrons
with the oxidized metal, sort of "stealing" the oxygen
and freeing the tin and other metals from the scum so
they go back into the alloy. Grease, wax, oil, sawdust,
anything like that will work to reduce oxides, and if
your alloy is clean of other contaminating metals
waxes work fine for this job.
Now, about Fluxing. This is the part that seems to
confuse everyone. If your alloy came from wheel
weights or other dirty scrap, it likely contains a bunch
of other metals that don't cast very well and mess up
the flow, or FLUX, of the alloy. This makes it tough to
cast good boolits. Things we want to get rid of are
zinc, aluminum, iron, calcium, and a few others. Since
what we want to get rid of is all pretty much more
difficult to reduce than lead, tin, and antimony, we
can remove it through adsorption. With a "d". Things
that work really well at removing the oxides of
contaminating metals are molten borate glass and the
carbohydrates in wood. Wax won't do it. The problem
with borates (such as Marvellux) is that they don't
reduce any of the oxides at all, including tin, they just
adsorb them and remove them from the alloy. If you
want to save your tin/antimony/bismuth/lead oxides,
use sawdust because it saves the good stuff and
adsorbs the bad stuff so it can be skimmed and
thrown away with the ash when it has finished
burning.
So again, sawdust, being a hydrocarbon, will also
reduce tin/lead/antimony oxides we want to save
while adsorbing the remainder of the junk we want to
remove and capturing it in the ash. Two for one, so to
speak. Resiny, pine sawdust, particularly sappy yellow
pine, is one of the best reducant/fluxes I have ever
used because the resin is such a fine and quick
sacrificial reducant, quickly reducing the good stuff so
it won't get adsorbed, but leaving the oxidized trash
metals for the carbon to soak up as the wood chars.
Sawdust and ash cannot get below the surface of the
melt and cause problems unless you drag it down
there physically so that it gets trapped below the
surface tension of the alloy at the bottom of the pot.
Carrying ash down there on the end of a fresh ingot, a
handful of sprues, or by scratching around on the
bottom of the pot with a wooden stick are the
principle ways of getting ash junk on the bottom
where it will migrate to the spout and cause inclusions
in the boolits. Use common sense and it won't
happen. A wood stick is the bee's knees for scraping
all the stuck, baked dross off the sides of the casting
pot, it reduces oxides on contact.
Gear

[44man]

Wood works good but I ladle cast so must keep the surface clean.
Watch some commercial flux, they suck water and rust the pot fast.
When smelting, keep the melt at 600° and remove all that floats before fluxing.

[Krieger82]

Beem using sawdust for ages.

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[lightman]

When I'm smelting I use sawdust. At least 2 times and sometimes 3 times. Stirred in, sides and bottom scraped, fumes set on fire, the whole works. When I'm casting, I use wax. Old bullet lube, candle wax, paraffin, something of that sort.

[country gent]

When smelting I use fine wood chips ( pet bedding) and paraffin. I get the melt up to temp throw in a couple handfuls of the bedding and when it starts to charr a 1/4 of a sheet of canning paraffin. I then stir in a specific pattern scraping sides and bottom and bringing the lead up thru the flux and the flux down thru the melt. I do this twice. I then skim the crud off bring back to temp and a 3rd time with just paraffin, just because I can. This gives me a very clean consistant batch of 400-500lbs from my smelting pot. When casting I use beeswax and maybe it theres a reason some wood chips but no where near the amounts in the big pot. Just enough to re blend every 100 or so casts.

[Krieger82]

That is true, I have used alox in the past.

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