Stovetop Metalcasting

or

Revenge of the Whacked-Out Psychedelic Plastic Medieval Knight on Quaaludes


Melting metal with a "portable heating range"

If you want to melt metal without going through all the rigamarole of charcoal and blowers and whatnot, you can use an electric burner. This can actually be done indoors, if you're careful!

First Try

My first try involved a home-made stove made from a 1500-watt 240-volt burner, but run on 120 volts. Since current is proportional to voltage, and power is proportional to the square of current, halving the voltage quartered the power, and 375 watts was just not enough to melt anything.

First Melt

I switched to a Hamilton Beach 750-watt 120-volt "Fifth Burner Portable Heating Range". This did the trick nicely! I melted a handful of post-1982 (copper coated zinc) pennies. Once the zinc melted, I dissolved a bit of aluminum into it, to produce an alloy similar to the various ZA alloys. ZA-27 (71% Zinc, 27% Aluminum, and 2.2% Copper) is on par with cast iron for strength, but melts at a much lower temperature and is only two-thirds as dense.
Gray Cast Iron ZA-27 ZA-12 ZA-8
Compressive Yield Strength 65,000 psi 42,400 psi 33,400 psi 36,500 psi
Ultimate Tensile Strength 40,000 psi 60,900 psi 39,900 psi 54,200 psi
Density 0.258 lb/in³ 0.181 lb/in³ 0.218 lb/in³ 0.228 lb/in³
Melting Point 2795°F 709-903°F 711-810°F 707-759°F

I say the alloy I made is only similar to ZA-whatever because I didn't measure anything - this run was just to see if it was possible at all. To get a near perfect ZA-27 alloy would require $1.99 in post-1982 pennies, one pre-1982 penny, and 100 grams of aluminum. This assumes that the copper on the pennies won't oxidize, which is a stretch - to make sure there's enough copper, I'd add two pre-1982 pennies.

The pennies took about 15 minutes to melt from a cold start, and the aluminum took perhaps another 15 minutes to dissolve. The I turned the burner off and let the blob cool.

A Note on Pennies

U.S. pennies before 1982 were made from copper, with a bit of zinc. They weigh 3.1g each. In 1982, pennies started being made out of zinc, with a copper coating, and weigh 2.5g each. You can tell them apart very easily not only by the date on them, but by the sound they make when dropped on a hard surface. I found that they were easily identified by throwing them at a glass bottle - the copper pennies would "tingggggggggg!" while the zinc pennies would "tonk!" How are those for technical terms?

Pennies can be cleaned very easily by soaking them in a mixture of vinegar and salt. Just make sure they're DRY before you add them to a skillet of molten metal.

A Note on Aluminum

It may freak out some people to think that aluminum (melting point 1220°F) can be alloyed with other metals (such as zinc, melting point 785°F) at temperatures far below it's melting point. Just remember that the aluminum is not melting, it's dissolving. Like how table salt (sodium chloride, melting point 1427°F) will dissolve in water (melting point 32°F).

Second Melt

My second melt with this setup is better documented. I melted 295 zinc pennies and 70 grams of aluminum. I tried to add two copper pennies, but they didn't dissolve at all. Even so, it resulted in a nice pour.

Here are my notes:

5:52

Heating started. Calculated 260g of aluminum and 20g of copper required to make ZA-27

295 pennies × 2.5g each = 737.5g

737.5g × ( 27 ÷ 71 ) = 260gnote

6:02 Burner glowing, stinks, trace of smoke. There is NO WAY that I'm going to fit 260g of aluminum in the skillet.
6:26 Pennies starting to melt
6:40 Still not melted. The burner is cycling on and off - it didn't do that yesterday.
6:49 With the cover off, I can feel the heat on my face - with the cover on, I can't. Guess I'd better leave it on.
6:52 This is ridiculous. A whole hour and it still isn't melted! Experiment suspended.
7:07 Experiment resumed. The burner is now on full blast, and the thermostat is in my hand. The burner cool enough to handle, skillet too hot to touch, metal solidified.
7:10 Burner glowing faintly again.
7:16 Burner glowing well. Metal still not remelted.
7:25 Melting! About back to where I was at 6:52.
7:33 Metal is quite liquid. A few pennies not yet melted.
7:38 Almost all melted. Lots of crud.
7:40

Added 70g of aluminum. This should make ZA-8. Pennies totally melted.

737.5g × ( 8 ÷ 92 ) = 62gnote

7:45 Metal VERY liquid. Less crud than before. Did it dissolve?
7:50 All the aluminum is submerged.
7:52 Added 2 copper pennies.note
7:54 Aluminum mostly dissolved.
7:58 Everything seems to be dissolved.
8:00 POUR! Neither copper penny dissolved. Lots of zinc still in the skillet - foamy(?) consistency. Did not pour out with the liquid. Had JUST enough liquid to fill the mold.

After everything had cooled, I examined the weird foamy zinc-stuff left in the skillet - I suspect that all the crud I saw at 7:38 actually sank and formed this mess. The two pennies were still there, covered in zinc, but not dissolved at all. I should have given them a lot more than 8 minutes.

The Casting

The real purpose of this experiment was to see if it was possible to make and cast zinc-based using an electric burner at the workbench. However, since I was casting something anyway, I figured I'd cast something useful.

The casting I made was for a 2" angle plate, from a pattern I made out of 3/8" plywood, filleted with Bondo, and painted to seal it against moisture. I put NO draft on the pattern at all, but it still came out of the sand nicely.


Making a mold for a 2" angle plate

Safety Measures

I didn't ram the mold, I just pushed the sand into place with my thumbs, since the sand seemed a bit too moist. I was hoping (successfully, it seems) that the uncompressed sand would be more permeable to the steam generated when the hot metal met the moist sand, preventing steam bubbles in the casting, or worse, a steam explosion. I protected myself quite effectively with a Plexiglas "blast shield" that I could stand behind as I poured. Only my right arm would be exposed, and it was protected by a heavy leather glove and a 4" PVC pipe "bracer". I also wore a full-face welding mask with only the clear glass in place, and under that, I wore safety glasses! Nothing like overkill - I must have looked like some kind of whacked-out psychedelic plastic medieval knight on Quaaludes! (I couldn't figure out how to take a picture of myself.) As it turned out, the mold didn't explode or steam or even hiss when I poured - quite anti-climactic.


The Blast Shield

One final safety measure was keeping a fan in the window to make sure no zinc fumes accumulated. Zinc fumes can cause a flu-like condition known variously as "Fume Fever", "Zinc Fever", and "Zinc Shakes". (Drinking large amounts of milk is supposed to prevent Zinc Fever - something about the calcium - but I've no experience with that and wouldn't trust it.) At no point in this process did fumes become a problem, though, as the metal didn't get hot enough to create much vapor. If you see white smoke (zinc vapor), you're in trouble.

Results

The casting came out pretty well - there wasn't much crap in the metal, and what little there was I suspect is actually loose grains of sand. There was a big shrink in it, but that isn't because of the metal or method.


Wow.

Zinc is a lot cheaper in bulk - in penny form it comes out to $1.81 a pound, versus about 50¢ a pound from the scrap yard.

Furthermore, pennies oxidize something awful as they're heated, resulting in lots of crap getting mixed with the metal. While I just dumped the pennies into the skillet and put them on the burner, I should probably have first melted a "heel" of a few dozen pennies, and then added the other pennies by submerging them in the molten metal - this would have kept oxygen away from the copper while the pennies melted.

I weighed the casting, including the sprue, at 640 grams (256 pennies). Under water, it weighs 540 grams, so it displaces 100 cm³. (This was the perfect opportunity to teach my daughter about the story of Archimedes shouting "Eureka!" and running naked through the streets of Syracuse - she got a kick out of that.) The density is therefore 6.40 g/cm³, or 0.23 lbs/ci (all numbers to two significant figures). ZA-8 should have a density of 0.228 lbs/ci, so I came pretty damn close!

The pennies and the aluminum together weighed 810 grams (737.5g + 70g = 807.5g, 810g to two significant digits). Subtracting out the 640 grams for the casting means that the weird foam-like stuff left in the skillet should weigh 170 grams - in fact, it weighs 150 grams. I wonder where the other 20 grams are? I suppose it could just be experimental error.

All in all, this was a very successful experiment, yielding exactly the results I wanted with very little messing around. I have no doubt that I'll be using this method for metalcasting quite a bit in the future. Now to start collecting pennies...

References

Back


© 2003 W. E. Johns