Melting lead

You need old engineers. Moden engineers avoid the stuff.

Depends on your "lead". It's a metal that's more commonly encountered as alloys than as pure. Even slight impurity levels make a big difference to its properties, particularly melting point and hardness. The melting point of pure lead is about 325°, but common lead alloys may drop as low as 180°C. Lead is also soft, but a few percent of anything can make it 4 times as hard (Brinell numbers)

There's also the issue of "eutectic" behaviour. Lead alloys don't simply melt at one temperature - they have a "pasty" range where a solid and melt with different compositions can co-exist. At one composition (63/37 Sn/Pb) this range is zero and the alloy melts cleanly at single temperature (the lowest melting point for this alloy system). Outside this temperature, the "pasty" range can be as much as 100°C - essential for some soldering techniques, such as "wiped" joints. For lead alloys, the lower limit of this range stays at a fairly constant temperature and the upper limit increases to the values for the pure metals.

Roofing lead is handled as thin sheet, so needs to be ductile. It's one of the few grades that's reasonably pure, but even this will have

Linotype metal (old newspaper type) had 12% antimony and 4% tin in it, to make it particularly hard and also to control shrinkage on cooling, so that it could be moulded to shape easily.

Solder is effectively free of antimony (it's bad news in solders) but contains lots of tin. Famously this depends on purpose, from 20% tin for plumber's lead-working solders through to 50% tin for coppersmithing, 60% tin in electrical solders and modern lead-free solders that are practically all tin.

"Scrap lead" generally resembles wheel weights and other rubbish. It'll have about 3% antimony and a couple of percent of almost anything else; arsenic, bismuth, copper, tin. Scrap lead from batteries can have all sorts of rubbish in it (calcium, antimon, arsenic, selenium), some of which is most unpleasant (I'm told submarine batteries really shouldn't be handled hot except by the knowledgeable)

Yes, but there aren't that many of them. It's worse if you _over_ heat it. Generally the dross on the top is the stuff to watch out for

- dust from lead oxide, even when cold, is more of a hazard than metal vapour itself.

By far, the best thing is the electrically-heated bottom-pour pot used for bullet casting. This lets you draw clean melt off the bottom, away from the dross on top.

If you're top pouring, then almost any steel or iron vessel will work. Chinese supermarkets sell big ladles that work well (even for bronze). As a kid I used to re-cast my scrap lead in a ladle meld of steel scaff pipe stick-welded to a flat plate. You'll be wanting some tallow or rosin as a flux and a hardwood stick to scrape the dross clean away before pouring. Tallow is still available from electrical suppliers, but it's sold as a threading lubricant for conduit, not as a soldering flux.

Gosh Andy, I wish I'd said that ;o))

All I can add is the obvious - to do it out of doors or at the very least to avoid inhaling the vapour that will inevitably rise from the pot.

Oh, and I learned at the weekend that "old" lead (dunno how old, but probably pre war) is more ductile because it still has a trace of titanium and zinc which made all the difference, apparently. These days, the titanium is removed to make the oxides for white paint.

regards,

Kim Siddorn.

The usual impurities missing from "old" lead (pre-1943) are the nuclear isotopes. If you're making a lead castle as a screen for a counting chamber, then re-melting old lead is much better than freshly-smelted lead as it has a lower background count.

This is the same effect that led to the German fleet in Scapa Flow being "quarried" for their steel armour plate. It's WW1 steel, so it's radiologically clean, and because they were scuttled in shallow water they're relatively easy to access and aren't war graves.

I'm puzzled by Ti and Zn impurities making it ductile. I'd strongly expect _any_ impurity to make it less ductile (as a general principle for all alloys). Loss of ductility doesn't come about by mixing in a metal "that's brittle", it's because it's a mixture rather than a pure metal and that will always play havoc with tidy crystal lattices.

Ti is a fairly common impurity in metals, as it's a very common element. For pigment use though, it's the Ti oxide that's used. This is quite easily mined in the oxide form and there's no shortage of it. The high cost of Ti has always been due to the cost of smelting it - difficult and expensive, using the Kroll process. If there was any way to extract Ti from lead as a metal, then this would surely go as the valuable metallic Ti, not for conversion to the oxide pigment.

Zn is also a funny thing to find as an impurity. As anyone brazing or pickling brass can tell you, Zn boils (or usually sublimes) so readily that it's easily removed from an alloy just by heating it. Now this would be a little hot for mere lead, but even so Zn tends to disappear from where you want it, more readily than it shows up where you don't.

Lead sourcing has certainly changed from 100 years ago. Back then it was mined as primary lead, these days it's significantly supplied by recycling. I wouldn't be at all surprised that old lead is ductile, but I'd guess at that being because all modern lead is now likely to contain appreciable antimony from recycled hard lead, rather than a shortage of Ti or Zn.

Aye, they aren't digging it out of the ground from the countless mines around here anymore. Still plenty of lumps of Galena lying about of the spoil heaps though.