Harry O
12-12-2007, 02:53 PM
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.
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.