The Big Think

As part of my ongoing efforts to learn new things and develop unique ways to poison, damage, or otherwise permanently maim myself (see: flight training, woodworking, trebuchet building, potato gun construction, and a few other things I’m not going to post here!), I decided to add to my list of Maker Skills by learning about metal alloys and home forging.

When we were in Ireland I picked up a few Medieval soldier molds from Prince August. PA makes vulcanized rubber molds so the hobbyist can craft their own toy soldiers from heavy metals. They also sell the hobby metal for around $13 for a 1/2 lb ingot (which will make 6-10 figures in the 28mm line). I bought an ingot and made a few soldiers with my new molds and I was very pleased with the results, but at that cost it was prohibitive to make very many soldiers. You can buy lesser-quality metals that will make passable figures, but the less you spend the less detail your models have, the more problems you have casting and, most importantly, the expensive molds have a much shorter lifespan. This is due to the fact that the cheaper casting metals have a higher and higher percentage of just lead, and since lead by itself has such a high melting and casting point, repeated castings have the tendency to slowly soften and melt the mold, until eventually there’s very little detail left. The higher cost casting metals add elements like antimony, tin, and bismuth to lower the melting temperature, increase the flow rate, and improve the detail of the final model. Interestingly, bismuth (Bi) is one of the very few elements that actually expands slightly as it cools. When you put a bit of Bi in your metal and then pour it into a mold, the metal pushes out into the recesses of the form and picks up the details much better than lead alone. As you can imagine, Bi is more expensive than cheap lead, or even tin, and this is part of the reason that really good casting metal is so expensive when you buy it in pre-made ingots.

So, naturally, I decided to make my own.

First question: where to get lead? You can buy it online for a couple bucks per pound, but I thought of something better: tire weights. A little Googling revealed that tire weights are 98% lead and 2% antimony (to increase the final strength of the lead as well as to add better flow characteristics). I went to a local tire shop and asked nicely for some used tire weights, expecting to get a pound or two. They ended up giving me a medium sized box with about 30 lbs of used weights!

The next step was to melt it all down safely. I went to Wal-Mart and picked up a $10 electric hotplate and a small cast iron skillet and set up a workstation in the garage. I realize at this point that my parents are probably having conniptions imagining me breathing lead vapors and getting singed by liquid metal, but since lead has such a bad reputation I did some research before I started messing with it. It turns out that plain lead in solid form isn’t all that dangerous. Hobby stores sell lead figures, Prince Albert sells a 90/10 Lead/Tin mix for melting, and tire store guys handle lead weights all day long with their bare hands, so obviously just having it around isn’t going to kill you. There’s a whole bunch of gun enthusiasts who melt down pure lead and cast their own bullets and some of them have been doing it for 50 years and more. It turns out that it’s the lead dust and lead vapors that can cause health problems. So I was very careful setting up a workspace in the garage that would 1. minimize lead dust, 2. ventilate everything very well, and 3. avoid lead vapors by not over-heating the liquid lead. I also wore a respirator, apron, long sleeves and jeans, and even sacrificed a pair of leather gloves to be my “lead gloves” that will get used for nothing else (ditto the cast iron skillet!). And I’m very careful to not touch the contaminated parts of the gloves with my bare hands as I put them on. Overall I feel like I have a very good system that keeps my exposure to the stuff at a bare minimum. Oh, and I’m obsessive about washing my hands whenever I take the gloves off. I’m really careful about all this.

So I set up the cast iron skillet on the burner and cranked it over to high. I dumped a bunch of tire weights in and waited about 45 minutes for the burner to heat up. 1100 watts is just barely enough to get the skillet up to the required 621 degrees F that is required to turn the solid tire weights into a dirty silver pool. Tire weights have the little clip on them to hold them to the tire and since this clip is steel and melts at a much higher temperature I had to use a long pair of tongs to extract the clips from the molten pool (very carefully!). I kept placing new weights into the pool and extracting the clips until I got a very hot, very yucky looking pool of the lead/antimony mix. the next step was to “flux” the pool by putting something into it to attract the impurities. Candle wax is the suggested substance so I stripped off a few shavings of an old candle and dropped them in. I stirred it a bit with a long ladle and most of the crud gathered at the top and I was able to pull it out. Next I very carefully poured the liquid into some small stainless steel condiment cups that I had bought from Wal-Mart for a buck. Once the ingots cooled (about 20 minutes) I was able to pop them out of the cups and stack them up. I now have about 8 lbs of mostly pure lead/antimony in 1lb ingots. Neat!

But what to do now? Just remelting these ingots and pouring them into my mold will eventually damage the molds, so I have to make a lead/tin/bismuth/antimony alloy that most closely matches the formula of the expensive Prince Albert Model Metal (56% Lead / 9% Tin / 35% Bismuth). I got online and found a place that will sell me mostly pure Tin and Bismuth in one pound ingots. I’m ordering a pound of each today. From here it will be a simple matter to weigh my lead ingots, calculate the proper amount of tin and bismuth, and remelt everything into new ingots! Nifty. I’m probably going to mess with the ratios a bit to try and optimize my metal usage (read: I’m cheap and don’t want to have to buy too much bismuth). It’ll be okay with less Bi, but the melting temp of the final alloy will be higher the less Bi I add. I’m planning using this chart to come to some sort of compromise. I’ve already worked up a spreadsheet with the ratios and final price per pound of the resultant alloy. Suffice it to say, it’s much cheaper to make this stuff yourself than to buy it- that is, if you don’t include your time, but hey this is fun and I’m not worried about taking the time if I learn something.

So the end result of this will be 7 lbs or so of really good model casting metal for a fraction of the cost of buying it new, and I will have learned a bit about alloying metals.

One more word about safety: I got some weird looks from passers-by yesterday, and my neighbor James came down the street to say hi and said “whenever I see you wearing a respirator I know to stay a long way away,” which I thought was hilarious. I’m going to extremes to make sure I stay safe with this, especially since lead poisoning is cumulative, but I figure a few days spent carefully experimenting won’t cause any lasting harm especially since there are home hobbyists who have melted lead on their kitchen stoves for 50 years with no reported problems, and the black hands of the tire workers tell me that handling it repeatedly for years hasn’t killed them. So don’t worry, mom, I’ll be safe. I’m the paranoid type when it comes to personal safety. It’s also safe for the neighborhood kids as long as I keep an eye on the hotplate and warn them away if they get too curious.

So expect to see some pics of the new Medieval figures sometime. Who knows… they may even populate a castle.