Fluxing

[Harry O]

Back when I first started casting, I followed the directions in a number of places, including the part about “fluxing”. I fluxed every 15 minutes or so, first with wax, then (after complaints about the smoke and smell) Marvelux. I never understood fluxing, though.

From what I have read, the melt (particularly the tin part) forms oxides at the surface. That makes sense. It is necessary to mix the oxides back in from time to time. The problem was that I remembered when I was learning to weld that oxides usually have a much higher melting point than the parent metal. It is VERY hard to melt oxides and mix them into the melt. Could the weak flame from a bit of wax melt oxides so it could be mixed back in? It did not seem so with as much as I skimmed off the top and discarded.

So I checked the melting temperatures of the parent metals we use and their oxides:

Lead = 621º F.
Lead oxide = 1,630º F

Tin = 450º F
Tin oxide = 1,976º F

Antimony = 1,167º F
Antimony oxide = 1,213º F

To get tin oxide to melt and allow it to be stirred back in, the temperature would have to get to nearly 2,000 degrees F. I seriously doubt that is possible with the equipment we have.

Since I started casting with a hollow-base bullet and even now, at least half of what I cast is hollow-base, I had to crank up the heat all the way. That made it worse for the formation of oxides. I read in the Lyman manual to use Boric Acid in that case. It worked so well, I am now using it for all my casting. It is cheap, available in drugstores, and protects the surface of the melt from forming oxides.

I get the mould warmed up, dump the first tries back into the pot, flux it one last time, and then cover the surface with Boric Acid. Then I keep on casting until I need to replenish the mix. That also reduces the change in melt temperature. When I have to replenish the mix, I treat it just like I was starting from scratch again. Melt it until the thermostat shuts off, warm the mould, flux the mix, and cover the top with Boric Acid again. It works well with plain base moulds, too, even though I don’t need to go quite as hot. In either case, I don’t really flux while casting.

[leftiye]

I'm a little leary of boric acid as it will coat your pot with boric acid and you'll really enjoy getting it off. Just like Marvelux. Stearic acid is THE best oxide reducer (we incorrectly call these reducing agents fluxes) there is, mix it with a little rosin maybe (smells better and works better). Boric acid and Marvelux do cover the melt and keep oxygen off, and they do absorb dirt etc from the alloy (this is the correct meaning of fluxing), but they don't reduce oxides.

As for re integrating tin into your melt, the mechanism is to reduce the oxides chemically, not cook them back into the melt. Wood , sawdust, anything which burns and supplies carbon, and will steal oxygen from the oxides leaving the tin pure and letting it re alloy into the melt.

Anything that will get your mold hotter (can get too hot though) will make it possible to get those problem molds to cast well with a cooler metal. I use a mold heater, and set the mold in the heater while inspecting my booits when I need a hotter mold. Works wonders with hollow points and bases, and minies.

If you can work it into your technique, cover your metal with something to keep oxgen away from the metal. I use ashes, and crushed charcoal (just add more charcoal once in a while). Even if you use a ladle you can do this if you have one of the ladles with a hole type spout. Just make an opening in the charcoal and set the spout down into the opening. Bottom pour, this charcoal stirredwith a stick just about does away with fluxing.

Don't NEVER use anything to stir the melt except a stick, end of discussion. Best thang since mother's milk! It actually fluxes while down in the metal, and removes oxides, and dirt that otherwise will stay down in there forever. Solved my since forever problems with inclusions this way!

[floodgate]

HarryO:

The flux doesn't MELT the tin oxide; what it is is a material that gloms onto oxygen atoms more tightly than does the tin (or antimony or lead), leaving the purified (de-oxygenated) metal to return to the melt. Such "fluxes" are actually "reducing agents" and are used to extract and refine metals from oxide ores, slag, etc.

Another approach (the "kitty litter" one) floats a non-combustible layer on top of the melt that prevents oxygen from the air above from ever getting TO the molten metal.

Powdered charcoal briquets actually do BOTH at the same time.

Doug

[Harry O]

Thanks for the additional information. I had not thought of a chemical reaction. I don't know enough about Chemistry to tell what is going on anyway. I know that in welding, they say that we have to clean off the oxides and weld fast or protect it with gas (argon or other combinations) to prevent oxides from forming again while welding. No chemical reactions: just remove and protect.

The Boric Acid does the protecting for me when I cast. I have never had a problem with removing it. It is a little bit sticky, but as long as I get it while the pot is still hot (which I do), it is reasonably easy to remove. Compared to what benefits I get from it, it is worth the bother.

With hollow-base bullet moulds, I have never had a problem with getting them too hot. I usually have to get them even hotter. I often have to use a candle next to the pot to heat the center pin and sometimes the sprue plate. Even without that, I rest the bottom of the mould on the top of the melting pot while waiting for the sprue to solidify, in order to help keep the mould hot enough.

With plain base moulds, I don't have to run things that hot, but I usually try to keep them just below frosting temperature. This seems to reduce the weight spread, too, another benefit. Before I started to cast this hot, I cast 200gr bullets with most of them within a 3 to 4 grain spread (with a few stretching the spread out to 6 or 7 grains). Now, I can expect most of them within 2 grains (with a few stretching it out to 3 grains).

[Junior1942]

Try fluxing like I flux--not at all. I haven't for years, and my bullets turn out just fine. Try one pot with no fluxing and see what happens.

[Lloyd Smale]

Im with junior. i flux when i smelt wws or make up big batches of alloy but rarely when im casting. Try casting fluxing every 15 minutes and then try a batch not fluxing at all and id about bet your bullets if anything are more consistant without fluxing.
Try fluxing like I flux--not at all. I haven't for years, and my bullets turn out just fine. Try one pot with no fluxing and see what happens.

[Harry O]

To explain further, I no longer flux while casting. I do at the start, after everything is melted, but before I put Boric Acid on the top. I don't flux after that. It does seem to work for me.