Help with making low-temperature melting alloys (newb)

[BarneyP]

Hi

Firstly, please be aware that I'm just a hobbyist and know next to nothing about metallurgy, so please be kind!

I've been experimenting with making low temperature alloys, in particular galinstan and Field's metal (Indium/Bismuth/Tin). Currently I'm just using a stovetop and stainless steel/cast iron cookware for melting the constituents, which is fine for Ga/In/Bi/Sn, though I'm going to start trying to incorporate copper and silver, for which I'll need something hotter. I'm hoping that the copper/silver amounts are small enough that a butane torch + a crucible may suffice, we'll see

My first problem is trying to remove the oxide layer which forms over the molten metals. So far, I've just stirred it into the molten metal, which doesn't seem to have greatly affected the alloys produced (their melting points are as expected and they show no visible tarnishing), but I think at least attempting to remove these impurities is probably a good idea.

I've read that borax might be used to absorb the oxides and that it should float to the top, but the only way I've read to remove it involves sulphuric acid, which cannot be bought in the UK without a license . So is there another approach?

Secondly, if I want to start melting higher melting point metals, can I avoid buying a furnace? I have an open fireplace - would a cast iron pan placed in the coals be sufficient for this - or do I just risk melting the pan?!

Many thanks

 

[moltenmetal]

Borax works as a flux only if it is above its melting temperature. It is likely too high in melting temperature to be a good flux for these alloys.

"Dross" (floating oxides) can be skimmed using a spoon.

A wood fire can take things to orange heat- yellow heat if you add a blower to the coals. DO NOT do that in your home fireplace obviously! Melting things in a pan obviously risks pick-up of any metals in the pan metal which are soluble in your mixture. I use mullite fire assay crucibles- they're meant to be disposable and used in mining assay labs, so they're very cheap, but they actually survive many, many firings and melts before they must be discarded.

A fired furnace isn't that hard to make- tons of stuff on YouTube of various designs- and will give you far more controlled performance. An electric kiln is harder to make, but for modest temperatures isn't too hard to make either.

My own melting furnace is a simple box made by stacking insulating firebricks on top of a hard refractory firebrick tile, tying the bricks together with a couple wraps of stainless steel wire, and wrapping the whole thing with a batt of mineral fibre insulation. The lid consists of two firebricks wired together with a hole drilled in the centre. The burner is a piece of 3/4" stainless steel pipe with a squirrel cage fan at one end (with variable speed drive) and a port drilled into the side to fit a valve connected to a barbeque propane tank with regulator. It can bring 5 pounds of brass up to melting temperature in about 15 minutes. I do have a thermocouple to measure temperature. Definitely for outdoor continuously attended use only, and of course with proper PPE.