Smelting 99.9% pure tin ingot at home, need some advice.

[Paul Loatman]

Hello,

 

I'm a watchmaker and i have a tin ingot which i would like to smelt in a cast iron skillet and then pour into something else, but i'm not sure what materials can be used to prevent the tin from sticking. I want to use something with a flat bottom because i'm using the tin as a polishing block and i would prefer a close to flat surface right after pouring so i don't have to do too much more work to get it in a usable state, and of course, i want to be able to remove the tin from whatever i pour it into to use the flat side. I'm looking around on Amazon.com and have found a baking pan of the appropriate size with a silicone based non-stick coating, obviously intended for food, but would the silicone coating help, hurt, or make no difference with the tin? If this sort of pan wouldn't help, could someone suggest a material that would work? I've heard that the old Corning Pyrex baking dishes/pans would work but the modern Pyrex glass is different from the old stuff and not designed to withstand high thermal shocks (like having molten metal poured onto it.).

 

I would appreciate any help, also i apologize if this might be in a wrong section of the forums, i wasn't entirely sure where to ask, thanks.

 

-Paul

Edited February 4, 2013 by Paul Loatman

[John Cuthber]

Tin melts at about 230C

Oven-proof cookware should be able to tolerate higher temperatures than that, and you only need it to work once.

You could probably use a little olive oil too as it has a smoke point higher than the melting point of the tin.

 

http://en.wikipedia.org/wiki/Smoke_point

 

I presume that it's the underside of the tin which you expect to be flat.

[Enthalpy]

If pure tin behaves like tin-lead eutectic, then it won't stick to aluminium within a reasonable time because of the alumina layer. Same with stainless steel. You can anodize both to thicken the oxide layer.

 

Though, I made unvoluntarily a good weld joint with tin-lead on aluminium. The liquid was hot, stayed in alu for hours, and had some soldering flux.

 

Ceramic would be my first thought. A strong sheet of Ptfe (Teflon) maybe, but it's expensive.

 

Pyrex: just pre-heat it to 230°C in an oven. No more shock then.

 

Other approach: instead of casting tin, machine it. A milling machine gives you at least 0.01mm flatness. Some school near to you that has one?

[John Cuthber]

I suspect that, from a watchmaker's point of view, 10µm may be a bit coarse.

I'm also amused by the idea of reversing the process for making float glass..

http://en.wikipedia.org/wiki/Float_glass

[Paul Loatman]

I suspect that, from a watchmaker's point of view, 10µm may be a bit coarse.

I'm also amused by the idea of reversing the process for making float glass..

http://en.wikipedia.org/wiki/Float_glass

 

One micron is about where i try to keep tolerances for escapements and my own gear cutters, but for this i just need something relatively flat, it actually needs to have a slightly rough surface so that the polishing compound stays on the surface, I can add that with a file, but one of the sides needs to be overall pretty much level so that i can make smooth strokes with whatever i'm polishing. This is a method for giving steel parts a "black polish" (essentially a mirror finish.). Never dealt with glass, but sounds interesting, i may try something similar if i ever get around to making a pocket watch crystal myself. Thanks for bringing up that topic, it may come in handy for me.

 

If pure tin behaves like tin-lead eutectic, then it won't stick to aluminium within a reasonable time because of the alumina layer. Same with stainless steel. You can anodize both to thicken the oxide layer.

 

Though, I made unvoluntarily a good weld joint with tin-lead on aluminium. The liquid was hot, stayed in alu for hours, and had some soldering flux.

 

Ceramic would be my first thought. A strong sheet of Ptfe (Teflon) maybe, but it's expensive.

 

Pyrex: just pre-heat it to 230°C in an oven. No more shock then.

 

Other approach: instead of casting tin, machine it. A milling machine gives you at least 0.01mm flatness. Some school near to you that has one?

 

This is good to know that it won't stick to stainless or aluminium, i can get an aluminium pan in the size that i need, the stainless pans on Amazon all seem to be too big though. I can actually mill the ingot myself as it is, but the reason for smelting it into a pan is so that i can have a larger surface area for polishing, a lot of people (watchmakers) will try polishing on a 2 inch square block of tin or zinc and it will work, eventually, but the work is so much quicker (and uniform) to have a polishing surface in the shape of a disk at 6-8 inches for watch sized parts, and larger for clocks. One thing i've learned is that the largest machines and tools are used to make and finish the smallest parts in the big Swiss factories, the extra size usually equates to rigidity, and rigidity is key for precision.

 

I'll look into Ceramic, and maybe try the pyrex, although i'd go for the aluminium first.

 

I appreciate the post, that clarifies a few things for me.

 

Tin melts at about 230C

Oven-proof cookware should be able to tolerate higher temperatures than that, and you only need it to work once.

You could probably use a little olive oil too as it has a smoke point higher than the melting point of the tin.

 

http://en.wikipedia.org/wiki/Smoke_point

 

I presume that it's the underside of the tin which you expect to be flat.

 

The olive oil is a good idea, i will definitely try that. Yes, i want the underside to be flat, i'm sure the top side will be relatively flat, but i know it will sink slightly and it would be much quicker to just pour it onto a flat pan so that i don't have to do much else to get that side working, i expect a file should be enough as long as the pan is actually flat, otherwise i'll have to mill it flat then file it.

 

I'll probably try a few small tests first to see how it all goes before melting the whole ingot. I appreciate the posts fella's.

 

Thanks,

 

-Paul

[Enthalpy]

Intuitively, I'd try pre-heated Pyrex before aluminium, as it must be flatter and smoother. Just put a tin catcher below in case the Pyrex breaks.

 

Because liquid metals are reactive (tin less so, sure) I'd first make a limited trial with olive oil - in case some carbide forms even below the oil's smoke point. But I may be horribly wrong.

 

10µm is coarse milling. This is the overall flatness; roughness is far better, like 2µm being very rough to the finger and 0.3µm a medium finish. The operation is so quick that you could find a milling machine at some neighbour university or company - or a turning machine if the roughness can be concentric.

 

Presently my preferred material: cake tins made of silicone rubber. Usable at 350°C in air. Very smooth surface, no adherence, easily removed thanks to flexibility. They may need some additional support with the heavy tin.

Got 600 hits on eBay.co.uk for:

cake tin silicone

including flat ones like items 150979423481, 170981202374, 251135007693 (ID = 8"), 380433105724 (9" * 5") and many more. Ask for flatness and smoothness. Other sources as well.

 

Maybe you could melt the tin in the mold directly, if using a cooking oven. It depends on where the tin oxide goes, hopefully it floats. Then tin will have enough time to outgas every bubble from the mould's bottom.

[Paul Loatman]

Intuitively, I'd try pre-heated Pyrex before aluminium, as it must be flatter and smoother. Just put a tin catcher below in case the Pyrex breaks.

 

Because liquid metals are reactive (tin less so, sure) I'd first make a limited trial with olive oil - in case some carbide forms even below the oil's smoke point. But I may be horribly wrong.

 

10µm is coarse milling. This is the overall flatness; roughness is far better, like 2µm being very rough to the finger and 0.3µm a medium finish. The operation is so quick that you could find a milling machine at some neighbour university or company - or a turning machine if the roughness can be concentric.

 

Presently my preferred material: cake tins made of silicone rubber. Usable at 350°C in air. Very smooth surface, no adherence, easily removed thanks to flexibility. They may need some additional support with the heavy tin.

Got 600 hits on eBay.co.uk for:

cake tin silicone

including flat ones like items 150979423481, 170981202374, 251135007693 (ID = 8"), 380433105724 (9" * 5") and many more. Ask for flatness and smoothness. Other sources as well.

 

Maybe you could melt the tin in the mold directly, if using a cooking oven. It depends on where the tin oxide goes, hopefully it floats. Then tin will have enough time to outgas every bubble from the mould's bottom.

 

I actually already order the aluminium cake pan, it's heavy gauge stuff, and not flexible, has a very flat bottom, so it should be fine. I was thinking it would be fairly thin and flexible and therefore end up with an un-even bottom during manufacture or through transit, until i looked around and found that they're designed to be quite thick for more even heating (for bakers). I'm going to try a few small tests before melting the whole ingot so i'll see if it works or not. I saw the silicone pans when i was searching, but it seemed like it may be too soft for 4 pounds of tin, i wasn't sure how well it would hold up to the sudden temperature either.

 

If it needs a bit of milling, that'll be fine, i can do that myself, but looking at the pan, i don't think it will. I tried checking the flatness with a machinist square and it's very flat, there's a slightly detectable dip right in the center which i will just mill away or possibly file away.

 

I'll let you know how it goes. Thanks for the post.

 

-Paul

[Paul Loatman]

Just wanted to let everyone know that this turned out fine. The aluminium sort of warped when i poured the tin in it, so it was no longer flat, it was close, but the dip that was in the pan reversed itself and created a dip in the tin instead, so i had to mill quite a bit more than i was expecting, but it wasn't that bad.

 

For anyone else who wants to do something similar; the tin melted very easily over the stove in a cast iron skillet, and it machines very easily, similar to aluminium but a bit gummier, no need for special tooling, HSS works fine.

 

Thanks for the help everyone.

 

-Paul

 

[StringJunky]

That looks like a good job done.

[Enthalpy]

Congratulations, and thanks for sharing!