How homogeneous is an alloy?

I have been dabbling on testing an alloy strength with pencils. Now i just picked a Lee tester which I'm sure will provide me with hours of fun refining my ability to measure and testing different theories. As I was greeting into setting my samples I was wondering; how homogeneous is a homemade alloy? Say we grab some wheel weighs, measure some shotgun shot for added antimony, add some tin for better flow and to combat brittleness, flux and mix here and there; at the end of the day how even is the alloy from first ingot/bullet to the last one cast an hour later? Would the different metals have a tendency while on the pot to float out of solution into a separate layer? How about when they cool on the ingot? I know you shouldn't test the top but how much precipitation happens that renders the bottom significantly different than the middle, or sides, or top?

Would the different metals have a tendency while on the pot to float out of solution into a separate layer? Depends on temp of the mix. Tin will float to the top until all the metals are in the liquidus zone. At the cooling zone (thin layer) the % of an additive will vary but it stabilizes. It is recommended to NOT test ingots. For our purposes, no-one has shown the alloy changes from the first to the end of the pouring session.

How homogeneous is an alloy? = Very

The metal doesn't separate out when melted.


The following is from Glen Fryxell's, "From Ingot to Target, A Cast Bullet Guide for Handgunners" (chapter 8) :
"Gravity segregation. Some folks like to think that if you don’t stir your lead pot, the components will separate according to their differing densities. Nope. They’re all soluble in the melt and Brownian motion insures that they stay well mixed. No gravity segregation will take place in your lead pot. A little test you can perform for yourself at home: dissolve a teaspoon of salt in a glass of water. Stir it thoroughly and set it aside overnight. Table salt is over twice as dense as water (just like lead is approximately twice as dense as either antimony or tin). If homogenous solutions spontaneously gravity segregated, then this one would do so just like the hypothetical segregating lead alloys. Let this salt-water stand undisturbed over night, then carefully remove a spoonful from the surface of the glass and taste it, is there still salt in that surface water?"

In addition to what Petrol and Powder says, Antimony and Tin form a complex (SbSn, like a molecule) which is very soluble in lead.

Technically speaking, if you have oxidation on the surface of the melt and skim it off rather than reducing it back in, you are removing the component metals from the pot, and since the rate of oxidation varies with the metal, there may be a very, very small change in composition towards the end if you are repeatedly removing dross instead of using a cover layer. Practically, it a very minor effect - if I remember correctly, BNE actually analyzed first and last pours for changes and didn't measure any.

That oxidation on the surface at high temperatures is tin which, if skimmed off, will affect the composition of the alloy and is one of the challenges we face in order to keep our alloy consistent while casting. It can be kept in the alloy by reduction using a lipid (I like beeswax) and kept from oxidizing with flux (I like sawdust). There are few things more beautiful than the shining surface of a beloved alloy peering through a top layer of charring sawdust! Brings me to tears of joy just to think of it! So...no, not all alloys under all circumstances are homogeneous but we take action to optimize consistency. Read Glenn Fryxell's articles especially the one about the Sn/Sb system and saturation, best of luck!

The oxidation may be tin but how significant can that be? I mean, on a 10 pound pot I could have 6 ounces of tin. What's the weight of that tin stain on the top of the molten alloy? A few grains?

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Originally Posted by Don1357

The oxidation may be tin but how significant can that be? I mean, on a 10 pound pot I could have 6 ounces of tin. What's the weight of that tin stain on the top of the molten alloy? A few grains?

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Good question and as stated above by kevin c , the amount lost is so small that for practical purposes, it is insignificant. If you were to run the pot very hot and continuously remove that oxidized layer, you might have a greater affect on the total alloy during the use of that pot, but even then I doubt it would be significant.

I think it was BNE who tested this somewhat. Cooked some lead alloy for several hours and there was no change in the tin content. The tin oxidizes faster than the lead. So, if you keep skimming the surface, you'll probably see a change. But, once that skin is formed on the top, no further oxidization can take place and that skin isn't enough to make a difference.

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I have often wondered this myself. I seldom cast with anything other than straight wheel weights. But when I add in other things I stir it with a spoon before and during fluxing.

I stir during casting with some flux and bees wax just to be sure I get the best results possible.

Your melt will be darned near 100% homogeneous! Just stir occasionally and flux (as you are supposed to be doing anyway) to reduce the Sn back in and you will have the same alloy from top to bottom. The metals we mess with mix perfectly at the temps we use. I would not worry about any striation or layering at all.

And has been proven many time over these past several years, hardness is not really as critical as once touted by the experts. I cast 90% of my boolits (223 thru 45cal) at 10-12 and then PC them for perfect clean barrel, no leading shooting.

banger

This is a very interesting thread. I guess I never thought about an alloy changing in the pot while casting.

Seems like even if it did, the differences you'd probably get from individual pour to individual pour would make more difference than any alloy separation that would occur during the time it takes to empty a pot.