Use of saw dust

[9.3X62AL]

Looking on here, folks. Lotta food for thought--THANK YOU.

I do use sawdust, both during smelting/blending and during actual casting sessions--95% of which is done with an RCBS bottom-pour furnace. My thoughts are that fluxing can't be "overdone", but the law of diminishing returns applies as well.

 

[fiver]

I keep the wood out of my casting pot.
if I want something on top of the pot I add kitty litter I quit doing that too and just leave a layer of oxides on the pot.
every now and then I wax it all back into the alloy well mostly,, I generally scrape about half of it off and save it in a bucket and re-melt it into the next batch of ww alloy I'm cleaning up.
it takes me forever to get enough to bother with.

at the end of my last session I emptied my my pot down to about 10 lbs left, so the next time I fire it up I can empty it out and give it a cleaning.
it's only been about 15 years since the last one so I guess it's about time.

 

[S Mac]

I also almost always use a ladle, and throw a bit of wax in and stir to try to reduce the tin back in. I never notice any diminished amount of dross afterwards, skim it off and carry on casting. Should I see less dross after the wax?

 

[Brad]

I don't find wax to do as well at eliminating the dross. Maybe I don't use enough?

Has anyone compared woods to see if one does better?

 

[358156 hp]

I quit using wood after doing it wrong and fouling the bottom of the pot, like many others. It does make sense that there is surface tension everywhere there is a surface (duh). I use wood for initial melts in my big dutch oven, but after that I use wax of some sort. Outside of that, I can't add anything to fivers post. Our procedures are so much alike that it's almost eerie.

 

[Ian]

That's not correct brad. The oatmeal look on top of the melt would more than likely be Sb, not tin.
.

I'm not so sure that it's always antimony trying to creep out of solution that causes that. If I cast 2/3 of a 20 lb pot and throw all the sprues back in as I go with no fluxing or reducing, I get about three heaping teaspoons of clumpy, grainy stuff accumulated on top. A simple wax reduction puts it all back in, which only tells me for certain that it is some sort of oxide accumulation, likely mostly tin-rich "skins" of culled bullets and sprues. When what I *think* might be antimony separating (no sprues put back in, usually casting at low pot temp with alloy like Hardball), it usually takes a load of sawdust to make it go back into solution.



On the subject of dross composition, I know some tests have been done on intentionally generated pot dross and IIRC it had the same content as the alloy from which it came. I don't believe those reports, and here's why: Tin oxidizes on the surface of molten ternary alloy instantly upon exposure to air. It is this very mechanism that causes tin to "reduce the surface tension" of the alloy and make it flow/cast better. Tin oxide skin is more flexible and thin than antimony or lead oxide skin and lets the melted metal flow like a lava field. Ever notice "tin tails" hanging from your bottom pour spout? That's the tin skin at work. So it follows that if you have a mostly tin or tin/antimony skin forming on the metal surface (and it takes a redox reaction to convert it, not heat), that "dross" will be disproportionately rich in tin compared to the base alloy and you will deplete tin by skimming the pot. Considering tin is usually only present in a few percent, it doesn't take much loss to change the alloy characteristics significantly during the course of a single-pot casting session. Probably not a big deal, but throwing back sprues as you go (I do this sometimes, as a volume expedient) can cause issues that may or may not need to be corrected by a little fluxing, stirring, and reduction.

Type metal vats had to be maintained with some sort of enrichment alloy (high in tin and antimony) to make up for the losses through skimming. I have in my possession a box of monotype letters which mostly test between about eight and 12 on the BHN scale. Obviously they were cast from various depleted alloys.

For what it's worth, I did my own testing of wheel weight plus 2% tin skimmings years ago by reducing the skimmings to a small, clean ingot and comparing specific gravity to that of the base metal. I don't remember the exact number, but I'm pretty sure the skimmings came in around 8.5 grams per CC using a graduated cylinder and a powder scale, draw your own conclusions as to their composition.

It's easy to get wrapped around the proverbial axle on these things, I tend to stick with the Bass Ackwards philosophy and go with what works for what I'm doing, which isn't always exactly the same technique for every casting session.

 

[Ian]

I don't find wax to do as well at eliminating the dross. Maybe I don't use enough?

Has anyone compared woods to see if one does better?

I've compared woods (I worked at a custom cabinet shop for a few years when attempting to retire from automotive service), and the ones that universally do the best are evergreen softwoods. If you want to eliminate the mosquitoes for a wide radius, Western Cedar is good for that. It's also good for making you very sick if you breathe too much of the smoke. Knotty, sappy, sticky yellow pine sawdust is hands-down the best thing I've ever used. Chainsaw shavings from green juniper trees (we're infested heavily with them around here) are also excellent.

If there is rosin in the wood, it seems to reduce clumpy oxides far better than any wax or oil, I think because it can physically penetrate the surface of the melt and mix with the stuff. Wax just floats on top and burns off before it can reduce more than just the few molecules of skin on the very top.

 

[Rick]

For what it's worth, I did my own testing of wheel weight plus 2% tin skimmings years ago by reducing the skimmings to a small, clean ingot and comparing specific gravity to that of the base metal. I don't remember the exact number, but I'm pretty sure the skimmings came in around 8.5 grams per CC using a graduated cylinder and a powder scale, draw your own conclusions as to their composition.

Back when all the XrF alloy testing was being done on the CB forum I had a full coffee can of used sawdust flux all from CWW+2% Sn. I got a couple of 5 pound ingots by melting it all down. This alloy tested exactly the same as CWW alloy it came from.
.

 

[Ian]

Interesting, Rick. I didn't use my ash can stuff, just collected the un-reduced scum and grainy stuff skimmed from the pot, then reduced all that at once with sawdust (and maybe some wax, I don't remember for sure). Maybe that's the difference.