"Science"

Funny colors

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I turned this pot purple.

Funny colors

 

Copper changes color when it’s heated, but you may not see it unless you flip your pot over and look at the underside. Copper is wonderful at distributing heat but it still experiences high temperatures at the point where your heating surface touches it. I have a gas cooktop, so the bottom of my copper pots develops a colorful pattern where the flame tips reach the surface.

Mac Kohler of Brooklyn Copper Cookware wrote about this phenomenon on Medium in a post called “Meet Your Metal.” He calls this heat pattern the “bruise” and describes how it forms as copper atoms crystallize into grains that refract light differently. According to Mac, cooler temperatures form smaller grains of copper that refract light in the cooler, redder color tones, while hotter temperatures form larger grains that appear yellower. I call that brassy color the “yellow flag,” as it signals to me that the copper is getting hotter than it needs to be.

So I don’t know what the heck I did to this stewpot to make it turn into a kaleidoscope all up and down the sides. Color changes like this don’t usually reach all the way to the sides of the pan, though, because copper disperses heat so well; there’s a lot of yellow flag on the sides, so I must have heated it quite hot. It’s also possible that there was copper polish on it that reacted to the heat.

Funny colors

 

The bottom is a riot of colors. You can see the bloom where the gas flames were, and lines from the iron grid plates from my stove where the heat was lessened.

The lid is magenta.

Funny colors

 

These colors don’t affect the pan at all, and you might find this appearance to be more beautiful than plain copper. (I’ve seen handcrafted metal jewelry intentionally treated to produce this effect.) But this is more extreme than on my other pots and pans and after some consideration I decided it was worth cleaning off. (I also felt this would make for some good before and after photos.)

I used my favorite copper cleaner, Bistro.

Funny colors

 

Here’s a photo after I cleaned off just one part of the lid to show the dramatic difference.

Funny colors

 

The pot and lid cleaned up just fine, but it took a little more time and effort than run-of-the-mill tarnish. Bistro is a powerful cleaner and I let the chemicals do most of the work, but it took some massaging around the rim where the discoloration was most chaotic. (I think there must have been some kind of surface polish or chemical on the pot that reacted like this, and that took effort to remove. No harm done to the copper though.)

I don’t take copper cleaning lightly — the cleaner is an abrasive, and every time I use it I am scratching the copper a bit. Bistro (and Copperbrill) have very fine-grained abrasives and do the least scratching, but still, it’s not as mirror-fine a surface as it was when it came from the factory (or the retinner). I am not a perfectionist about my working copper but I also do not intentionally scratch it if I can help it. But in this case it was worth it.

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