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Hello, We had a very unusual reaction. An art student created a clay sculpture that was covered with pennies and fired in a kiln.

The newer zinc pennies reacted and formed long (4 to 6 inch) hollow spiral white tapered tubes, orange on the inside. Very odd looking. I could send photos. Have you heard of this? Why the tubes?

Question Date: 2013-01-25
Answer 1:

This is an interesting question. It looks like the zinc pennies underwent an oxidation reaction in the kiln. The white tubes are probably what is typically referred to as "calx" -- the white residual material left after a metal ore (such as zinc) is "roasted" to produce an oxide.

Regarding the tubes, I'd like to see pictures. Also, what temperature did you fire the kiln at? Is it the rotating kind? Did the student use a glaze?

Answer 2:

picture 1
picture 2
picture 3
picture 5

These are really cool pictures. To me, this phenomenon is somewhat reminiscent of "Pharaoh's Snakes" fireworks (see Youtube?). In the case of the sculpture, the zinc oxide wants to "get out" from the penny and as it does so these tubes form.

Why are the tubes hollow? To be perfectly honest, I'm not sure. Disclaimer: most of what I'm saying is "educated conjecture." But I have a couple of guesses. It could be a consequence of the fact that at the molecular level, zinc oxide exhibits crystalline structure. There have been experiments in the last decade or so that show that zinc oxide can grow in hollow tubes. However, the size of the tubes in these experiments was much smaller than what the pictures you sent look like. The hollow tubes could alternatively be the result of the zinc oxide pushing its way through the clay, which is porous. The size of the pores in clay, I think, are more consistent with the size of the tubes that formed (on the order of mm or so). And why are the tubes spiral? When I look at the pictures, the tubes look like they were pushed out one little "bit" at a time. It looks almost as if some zinc oxide might have formed, pushed its way out, more might have formed, and pushed its way out and so on, but very rapidly.

One interesting thing you might do if you have the time/resources is to try to reproduce this and see if the same thing happens; and if time and resources allow, maybe try it with the pennies embedded more deeply in the clay?

Sorry that I didn't give any really solid answers, but I hope this helps. It was interesting thinking about this question -- thanks for bringing it to our attention!

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