Did I skim the tin?

[Iron Mike Golf]

Newbie caster here. I think maybe I skimmed out a bunch of the tin from my alloy. I started commercial cast boolits made from 92-6-2. I used the lube on the boolits to flux. During the casting session, I skimmed oxide, but did not flux again. The skimmate is metallic, not like the ash left over after fluxing.I re-melted sprues and reject boolits.

Later in the casting session, I saw the texture of the cooling sprues change to sort of lumpy and the sprues got real brittle. They'd break when they hit the towel. At the breaks, the metal was real grainy. I go heavy frost on my boolits and that happened all at once, not a gradual transition over a bunch of casts. The surface of the boolit is very grainy and rough.

Per the Lee Hardness Tester, original alloy hardness 24 hrs after casting is 18.6 BHN. The brittle alloy is 17.9 BHN. those boolits seem to be leading very badly. Boolits from the same mould give little leading (a few flakes and not impact on group size) after 100 rds. The brittle alloy lost accuracy after 25 rds or so.

Is this all consistent? Can I just re-melt those boolits and add back the skimmed stuff and flux? Should I add some solder to "top off" the tin?

Thanks.
Jeff

[cavalrymedic]

Well, I don't know why your alloy got brittle, but I assure you, it's impossible to seperate out the tin from the alloy. It's a suspension alloy and you need electrochemical processes to seperate the metals of your alloy. Skimming and fluxing and over.under heating etc will not result in alloy seperation.

[454PB]

It sounds like your problem is too much heat and casting too fast. Try reducing the temperature and/or casting slower. Anytime a sprue breaks when it falls, the mould is too hot. That grainy, galvanized appearance is another sign the melt and/or mould is too hot.

I strive for a "frosty" appearance, but yours has advanced too far.

[lwknight]

I doubt that you depleted enough tin in one session to make a big difference. I do think that you just got more proficient at the casting cadence and got your molds hotter than what is optimal.
Slow down a little and be mindful of the pot temperature. As the level goes down and time goes by , the pot gets hotter and hotter and that will also heat up your molds faster and make frosty brittle spru.

The frostyness is caused by slower cooling that allows the antimony crystals to be larger than quick cooled alloy.

[hydraulic]

you're casting too fast. Either slow down or use my method. I keep a rag in a tuna can soaked with water, and after the sprue freezes I lay the sprue plate on the rag to cool the sprue. This enables the sprue plate to cut the sprue instead of tearing it out of the bullet base. This is faster than waiting several seconds before cutting the sprue.

[Iron Mike Golf]

How long do you cool the sprue on the wet rag?

[runfiverun]

thats the art part.
for a 2-4 cav steel mold 3 seconds is probably good.
once i get my mold up to temp i turn my pot down,[about 15-20*] and monitor my cadence.
i rarely need the damp rag.
but i'm slow, and judge by opening the sprue by hand feeling for consistency.

[white eagle]

Think about this
as you lower the level of lead in your pot the temp of that reduced lead increases
so if you reduce the heat you will be in boolits again

[geargnasher]

Well, I don't know why your alloy got brittle, but I assure you, it's impossible to seperate out the tin from the alloy. It's a suspension alloy and you need electrochemical processes to seperate the metals of your alloy. Skimming and fluxing and over.under heating etc will not result in alloy seperation.

That simply isn't true. Tin oxidizes faster than the other metals in the alloy, so the oxide scum that forms on the surfaces will have a higher tin content than the alloy's percentage, by a large measure. I've reduced and analyzed dross and found WW metal with 2% tin added can yield as much as 80% tin in the dross.

That being said, I think the issue the OP was having is with an overheated mould and sprue plate. Overheated moulds can produce boolits that are softer than expected due to the much slower rate of chill vs. a cooler mould. Overheated sprues will look like aluminum sand clumped together, that's simply a result of the conditions under which it froze. I agree that there probably wasn't enough tin lost to be worried about at all.

Gear

[Ole]

Were you casting with a thermometer?

I know the lead in my Lee 4-20 pot tends to creep hotter as the pot gets emptied.

[Iron Mike Golf]

I think it was overheat. I re-melted the sprues and boolits and now cool with a wet towel as needed. I do cast with a thrmometer in the pot and was keeping the melt under 700.

[truckjohn]

I have really only had trouble loosing Tin when the lead temp was too cold and there was a lot of mixng... Haven't had that problem when running higher casting temps.

Glad you sorted it out... Casting too fast can overheat your mold pretty easily.... Brittle, grainy sprues are a giveaway.

Thanks

[cbunt1]

The 6/2/92 alloy you describe (Hardball) provides pretty clear and distinct feedback about when the mold's too hot...the "crumbly" sprues are classic, IME.

It's pretty forgiving about overall mold temperature, but once they're "too hot," that's that...not like wheelweights that lose that shininess before they frost...one cast is fine, next one's dull, and then FROST...right before the first dull one I get a crumbly sprue...

Sounds like you got it all worked out though. That alloy's a bit finicky, but it's my favorite alloy when I start serious production!

[Hanzerik]

Glad I saw this thread. Answers some questions I had. First casting session yesterday using my RD mold. I'm cast the 44 265gr bullets. At first I was having problems with fill-out so I turned the LEE pot up to "7". Then the bullets started to look good, but as I continued to cast the spruces started to just break off instead of a nice clean cut. Bullets started to become frosty but nicely filled out though. Guess I need to slow down, but it's kind of exciting to see the bullets drop out of the mold LOL. Guess I'll melt those back down andhwork slower.

[truckjohn]

I found that with Aluminum molds - you gotta hoof it at first to get the mold hot enough to cast right.... but then at some point - you gotta slow back down to maintain the mold and sprue plate temp.

In general, it seems like you can pretty well tell by the way the sprue cuts... It will go from requiring quite a bit of pressure to cut off to just popping right off.... Crumbly sprues aren't far past sprues cutting too easy...

One thing you can do to avoid skimming the Tin and Antimony right out - don't run your casting pot too cold.... A cold casting pot will produce really shiney, metallic looking dross... and that shiny, metallic dross *DOES* contain much higher levels of Tin and Antimony than powdery, yellow dross......

Thanks