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I want to remove a layer of 2 mm thick carbon black on a copper bowl on the outside from hanging over wood and coal fire. I have tried different solvents, but the carbon seems to be very resistive.

Which formula solves carbon black without solving copper? The bowl is made from pure copper. It will melt at roughly 1000 degree Celsius and has a high heat capacity.

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    $\begingroup$ Carbon in any form is pretty tough to dissolve. What have you tried? And what can the bowl endure? Can it be heated, scratched or sonicated? One possibility is to heat it in an oxygen atmosphere to a high temperature (below the melting point of copper, of course). $\endgroup$ – Nicolau Saker Neto May 20 '13 at 0:31
  • $\begingroup$ I doubt that Jonas knows what "carbon black" is. From his description that crust must be rather rigid, and is not carbon black. He should describe how that crust was deposited. $\endgroup$ – Georg May 21 '13 at 18:53
  • $\begingroup$ Well the heating I suggested (if it works) can't be over a regular fire, otherwise the soot from the fire will just add to the layer. It should also oxidise the surface of the copper to $\ce{CuO/Cu_2O}$, though I imagine that's happened by now anyway. Also, at first I assumed the bowl was from a laboratory and that you had access to lab equipment, but you may be asking for a home solution. That complicates things. Scraping the layer off is the solution involving the least complex instrumentation, though it's a lot of work and may damage the surface of the bowl slightly. $\endgroup$ – Nicolau Saker Neto May 23 '13 at 13:16
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    $\begingroup$ Why isn't polishing a viable solution? $\endgroup$ – bobthechemist Apr 20 '14 at 12:30
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By far the safest solution is to physically abrade the surface until you have worn away everything that isn't copper. This is slow and requires some sort of grinding agent but has the upside that you are not likely to injure yourself.

There is a chemical solution that will work but it requires access to some dangerous chemicals and is extremely hazardous even in a well-equipped lab. It is insane to try anywhere else as you won't have adequate safety equipment.

The best way to remove carbon black from anything is to use a mixture of concentrated sulfuric acid and hydrogen peroxide. This is often used (but also often highly discouraged because of the danger involved) in laboratories to clean sintered glass filters contaminated with things like activated carbon. In the case of a copper bowl, the solution will remove the carbon but will also etch the copper surface (it is sometimes used as an etchant to remove copper from printed circuit boards).

The mixture of sulfuric acid and hydrogen peroxide is sometimes known as Piranha solution of reasons that are obvious when you have seen it work.

I would not encourage anyone without good chemical safety training and access to very good safety equipment to work with the mixture. And even then they should work only with small amounts and in a fume hood behind a shield and while wearing eye protection, strong gloves, and a fireproof coat.

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  • $\begingroup$ I have access to a well equipped lab so it should be possible to stick to the safety rules. But I fear, that the H2O2 will ruin the copper. $\endgroup$ – Jonas Stein Apr 21 '14 at 17:16
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Elemental carbon tends not to dissolve in solvents due to its very stable structure, but it will react with oxygen to form carbonates/carbonites, so the heating of the bowl in atmospheric oxygen (as mentioned in a previous comment) could be a possible solution.

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All these materials are hazardous by skin contact and inhalation. The are toxic and evil. One possibility is soaking overnight in auto supply store generic carburetor cleaner. Another is overnight soak in automatic transmission fluid. Gloves, goggles, disposable activated charcoal painter's mask. If you get anything on a belt or your shoes, you throw them out. Contaminated clothing must be quickly removed. Wash skin with soap and water - twice.

http://www.youtube.com/watch?v=bRiZOTTel34
http://www.break-free.com/?location=/products/index.asp

If anything works, wash the cleaned bowl in a good dishwashing detergent solution - twice. There must be no residual product odor.

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When a cooking utensil used over a campfire becomes encrusted on the outside, it's because the contents of the utensil keep it relatively cool. Simply heating an empty container (below its melting point!) should oxidize off the carbon-based material. (Probably creosote)

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  • $\begingroup$ I will give it a try as soon I get a large gas torch. The bowl has 60 cm diameter, but it might work. $\endgroup$ – Jonas Stein Apr 21 '14 at 17:11
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Hot steam above $\mathrm{250~^\circ C}$ can erode carbon black, $\mathrm{600~^\circ C}$ or higher is better.

Activation/Oxidation: Raw material or carbonized material is exposed to oxidizing atmospheres (oxygen or steam) at temperatures above $\mathrm{250^\circ C}$, usually in the temperature range of $\mathrm{600–1200~^\circ C}$. See https://en.wikipedia.org/wiki/Activated_carbon

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  • $\begingroup$ @A.K. please note that the proper formatting for units should be preceded a space. $\endgroup$ – M.A.R. Jan 9 '16 at 18:19
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The situation is not clear.

If your carbon is pure ( why would it be?) then burning down with flame would be the easiest and cleanest solution. Practically nothing solves pure carbon, except melt iron.

If your carbon is not pure, but e.g. you burned some food on the surface, cooking with sodium-carbonate generally help.

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I would try chlorinated water from a swimming pool mix. Leaves are broken down somewhat in pool water. I tried it on an electronic circuit with carbon shorts on the substrate and it cleared the carbon current leak between the copper tracks. Be sure to monitor the contact time and wash it off with water.

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