Why Do Copper Pots Turn Purple/Brown With Flame?

[jyl]

I've been cooking with French copper pots and pans lately.

The pots get discolored by the flame, turning brown and purple.

I use something called "Twinkle" which literally rinses off the discoloration and the pots are gleaming again. So no big deal.

But - somebody knows this I bet - why does the copper get discolored?

[nostatic]

just to piss you off.

[M.D. Holloway]

If you burn copper, the flame will be green. The metal turns due to oxidation. Oxidized metal absorbs at a different wave length than in its transistion 2 or 3 state.

[Don Plumley]

Originally posted by nostatic
just to piss you off.

LOL!

I'm sure your doctoral advisor at Caltech is proud...

[azasadny]

Use Bar Keepers Friend (liquid), it will remove the discoloration and prevent it (somewhat). We use it on our All Clad stainless steel and copper pots and pans.

[jyl]

OK, thanks Lube - and no thanks to Todd.

So, heat accelerates oxidization and oxided copper is not, well, copper colored.

Buy all the different colors? I just cleaned off the saute pan that I used tonight, just on the bottom alone there were bright purple streaks, dull brown blotches, and the edge of the pan was a brassy yellow.

Copper conducts heat very well, right? So wouldn't the whole bottom be more or less the same temperature, and then why doesn't it turn more or less the same temperature?

I mean, if the whole thing would turn shiny purple, that would be alright.

[M.D. Holloway]

Well, it doesn't...just to piss you off!


Actually, if you heated it evenly the color would have a better chance of being uniform.

There is a corrosion test which is done on lubricating oils to understand how well they can protect against oxidation and corrosion.

You take pure copper strips and expose them to the oil and you can even heat them or put them in an enviromental chamber. The strips are taken out after a while and compared to a standard which has a bunch of various copper strips in various states of oxidation / corrosion. The 1A is pristine copper. It goes done to a 6C which is a nasty brown. It goes from a copper red to a green to a blue purple to a darker purple to a dark brown - runs the spectrum.

[jyl]

What, you mean my copper pots don't heat evenly!

Interesting, I am going to do some experiments. I have various copper pots, from fairly thin ones that have shiny brass handles and are more for show, to really thick ones that I can barely lift with one hand and are all go. I'm going to observe how they discolor and I may learn some ways to tell even-heating pots from the other kind.

Thanks, Dr. Lube!

[jyl]

Art, I'll look for Bar Keepers Friend. Thanks.

[jdm61]

You are tempering the pots. Steel does the same thing when heated. It goes from a light straw yellow color eventually to deep purplish blue, depending on how hot it gets and how long it stays at that heat. Whe it reaches transformation temperatures, it develops hard scale.

[M.D. Holloway]

jyl - you may also want to see what the difference in flame height and burner size effect has as well as the difference between gas v electric. Also, keep in mind the pots may not be 100% copper and chances are they are not. They may have another metal(s) alloyed in as well as being cladded to other metals. The whole thing is pretty interesting.

One other thing you could do is look at the heat of transfer. Set the pot on the burner and turn it on to t specific flame height and measure the temp of the surface using an IR temp gun (if you want I will send you one but you have to promise to send it back someday!). Measure the heat in the different zones and record how quickly the pan surface will heat. Compare the results to other pans, flame height and burner size then compare it to the color of the copper.

Share the data with me and we can co-write a paper!

(dumber things have been published!)

[jyl]

I have a IR temp reader (use it as a quickie thermometer in the darkroom) and when I find it in this endless unpacking I will try it.

I've noticed the pan that discolors the most and the most unevenly is a thin, tinned, "for show" saute pan (made in Brooklyn NY). The more recently-acquired thick French copper seems to discolor less, and more evenly.

But I must cook, and investigate, further. I'll be 250 lbs when all this is done, but anything for science.

[nostatic]

You should probably try some therapy with the pots. They probably feel unloved, due to being taking from the smelter too soon. Maybe Rolf them or do some deep-alloy massage.

[svandamme]

i hate copper
and silver too for that matter

and you would too if you had to clean and polish 40 coffee pots
and 200 sets of forks, knives, cosmetic service plates and whatnot in a fancy restaurant once a week, for a year straight...

[jyl]

I am thinking when my kids get a bit older, that's how they'll earn their allowances . . .