Fluxing Zinc in the Backyard Foundry

According to the backyard foundry experts, the most useful casting metal that’s even easier to melt than aluminum is zinc. It melts around 420°C/800°F, so you can even melt it on a hot stove. It casts harder than aluminum for better wear, and zinc/aluminum alloys can approach cast iron in strength and durability. Sounds great! All you have to do is buy some nice clean zinc ingots, and… Whoa nelly, what?! Buy ingots? I’m way too cheap-@$$ frugal for that. We’re going to melt our own. Easy as pie, right? Turns out that melting zinc is easy. Cleaning it? Not quite so much.

Melting the Scrap

I had myself a nice big pile of zinc scrap metal, or rather, various scrap metal that I suspected had zinc in it somewhere. Zinc is popular for small die-cast items with fine details. I had some chrome-plated bathtub knobs, a pile of extra medals from a local foot race, cabinet door handles, and other miscellaneous pieces. Most of it had some kind of plating or coloring, but when I cut into it with a hacksaw, I could see a dark dull grey center. Looked promising.

I started to melt it in my cheap little tin-can environmentally-conscious recycled crucible. It was highly entertaining watching the zinc melt out of the faucet spout, leaving a paper-thin shell of chrome shaped like the spout, until I touched it (yes, with a stick, not my finger, duh) and it collapsed. Same thing with all the little medals; they were plated with something copper-colored and left little pieces of paper-thin shell behind after the center melted out.

However, the pool of zinc I got just didn’t act right. It seemed more like a sludgy pot of liquid with sand in it, rather than a clean pot of liquid like I got with the aluminum. Since much of the plating from the items went into the pool, I knew I had impurities. The sludgy feeling had to be that, just waiting for me to remove it.

I started scooping out the sludgy parts, trying to let the liquid zinc flow back into the crucible as much as possible. I added more scrap and scooped sludge, until I had put in everything. I poured it into my ingot mold, and then assessed what I had accomplished.

What I had accomplished was making a big pile of slag, three or four times the size and weight of the clean ingot. This was just plain wrong. I put it aside to think it over for a week or three.

More Heat

Perhaps I didn’t get it hot enough to separate cleanly. Aluminum separated better when hotter, so zinc might be the same. On my next melt, I put back in some of the big slag pile and cranked up the blower to blast that charcoal. By this time I had a better crucible, which my dad bought me from McMaster-Carr. It heated up and melted the zinc slag chunk, and after a while the zinc pool started to behave more like aluminum.

As it got hotter, the dross started floating to the top where I could scrape it off. The zinc was glowing nice and orange, and the dross was even turning into white whispy fluffy strings. It looked really fascinating. Enthralling. Mesmerizing. Uh oh. Wait a minute. Glowing orange? This must be hotter than 420°C/800°F. A lot hotter. White whisps floating up? Oh, $%#@&* bleep !!!!!

I remembered what I read about welding galvanized steel: Don’t. If you see white smoke, you are vaporizing the zinc coating into toxic fumes. I don’t know how hot I got the zinc, but I am sure those white wisps were vaporized zinc. Fortunately I didn’t seem to have inhaled anything or gotten sick from it, but so much for the “just get it hotter” approach. I put it aside again to think it over for another week or three.

Flux

Extrapolating again from aluminum, I figured I needed flux. Something to assist in the chemistry of the metal, separating the dross from the zinc.

Aluminum is fabulous for turning junk scrap into clean castable metal. When you add flux, the dross turns into a little pile of grey powder floating on top of the pool of silver liquid aluminum, and you just scoop it off. Very nice. For flux, you can use salts such as sodium chloride and/or potassium chloride, which are commonly sold in the big home improvement stores for winter ice melting and water softening.

Being a lower-temperature melt than aluminum, I figured zinc was going to need a different flux. My dad said he had fluxed molten lead using soap or candle wax. We tried that on the zinc, but it boiled or burned off before seeming to do any good. This wasn’t really a surprise since lead melts even lower than zinc. I also tried the sodium chloride and potassium chloride which I had used successfully on aluminum, but they didn’t seem to melt at zinc temperature.

Most of my research on zinc flux only came up with answers for big industrial processes, using zinc chloride. It sounds effective, but not readily available as a common household item that I could find. Further research led to a breakthrough. Some people said that you could make a lower-melting salt flux by combining sodium chloride, potassium chloride, and calcium chloride. You can melt them together into a single solution, and the resulting solution should dissolve into zinc at a low enough temperature to work as a flux.

Calcium chloride is available retail as a winter ice melter, sold as “Driveway Heat”. In my research I found conflicting opinions on what ratios to mix the three salts. I didn’t understand the chemistry enough to figure it out myself, so I tried equal parts of all three. I melted them together in a tin can to get a watery liquid and poured it into a muffin-pan mold. It cooled into thin disks of salt, which broke into chips.

I melted the zinc slag again, and finally got some different results. As I melted the flux chips into the zinc, stuff actually started to separate. The dross came up to the surface, and zinc formed into little balls and fell out of the dross pile back into the pool.

Unlike aluminum, where the dross forms as a grey/black powder, the zinc dross came up as a soggy black blob of mush. But it seemed to be working.

It took a lot of the flux chips to produce the soggy black mush dross, certainly much more flux than I had needed with aluminum. I don’t know if that is because zinc inherently needs more, or if my zinc was just more impure than any of my aluminum, or both. But I was able to re-melt the entire slag pile, and reclaim most of it as clean zinc.