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I have a practical question: today I have to clean the oven, and it doesn't have self cleaning burn down mode, nor an official scraper I can use...

It's full of charred grease and sugar. I managed to remove 98% of the charred grease with cold grease remover. But it seems that reagent has no effect on the remaining 2%, that's black fully charred carbon or sugar. I also tried vinegar, but it doesn't have any effect on it either.

I don't want to scrape it because I fear that I would scratch the oven.

So which reagent can I use to remove those pesky carbon deposits without damaging the oven, and what will be the chemical reaction?

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    $\begingroup$ The charred stuff shouldn't be pure carbon: it consists of some organic "goo" mixed with carbon particles. In a labratory one might use aggressive bases like sodium butanolate mixed with sodium hydroxide but that might be too corrosive for your oven. Why don't use oven cleaner? It's active component is $\ce{NaOH}$ and so might work. But you should be careful if your oven contains aluminum parts as $\ce{NaOH}$ will destroy those. $\endgroup$ – Philipp Sep 6 '13 at 18:14
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    $\begingroup$ @Philipp I used the 'Well done st moritz cold action oil and grease remover'. The bottle says it contains 25% $\mathrm{KOH}$ along with many red warning signs. I washed down 98% of the stuff with it. But it has no effect on the remaining 2%, so is either pure carbon, or something that's not originating from grease, maybe charred sugar? $\endgroup$ – Calmarius Sep 7 '13 at 8:07
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    $\begingroup$ Hmm, you could try going higher with the $\ce{KOH}$ concentration or use $\ce{KOBu}$ but that might be too much. The only things that I heard of that might be able to dissolve carbon are $\ce{CS2}$ or maybe $\ce{CCl4}$ but you definetly don't want them anywhere near your oven. $\endgroup$ – Philipp Sep 7 '13 at 11:50
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    $\begingroup$ @Philipp I read in the Wikipedia that hot carbon can react with water: $\mathrm{C + H_2O \rightarrow CO + H_2}$. So would heating the oven to the highest possible setting, and spraying water on the carbon deposits be enough to start this reaction? (I'll need good ventilation...) $\endgroup$ – Calmarius Sep 8 '13 at 7:49
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    $\begingroup$ @Calmarius If I remember correctly $\ce{C + H2O -> CO + H2}$ happens at something between 800°C and 1100°C (which cannot be achieved with a conventional oven). Even if the numbers aren't correct you'd have to flow water steam over the carbon residue, which won't be easy in a conventional oven. I'd suggest you to carefully scratch it or follow Philipp's suggestion. $\endgroup$ – Alex Sep 9 '13 at 6:43
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As a disclaimer, this is for information only. Anyone using this information assumes all associated risks. The manufacturer's label should be followed for any cleaning products.

The exact chemical composition of the goo is not precisely known. It is likely either tar or creosote existing as some form of hydrocarbons. These hydrocarbons are tangled up to the point it is extremely difficult to dissolve. The temperature required to burn this material is likely too high for anything short of a ultra high temperature industrial furnace.

A good way to clean it is to use oven cleaners which take advantage of the saponification reaction. Saponification will loosen the material; however, some scrubbing and/or scraping will still be involved.

For safety when oven cleaning be sure to have good ventilation. Wear gloves, eye and face protection, as well as not having skin exposed. Caustic cleaning compounds and vapors from the cleaning are hazardous.

Most oven cleaners have multiple components.

  • A caustic solution causes a saponification reaction. The saponification reaction is commonly used to make soap from saponifiable oils. Because the oils in the material to be cleaned are likely saponifiable, the saponification reaction makes the material much more likely to dissolve in water.
  • A foaming component creates a foam. When foam bubbles pop, the popping action works to break up the tangled bonds.
  • A compound may be included to ensure the entire solution will stick to a non-horizontal surface.
  • Another compound may be included to seal the solution so as to prevent oxidation. Oxidation may interfere with the saponification.

The trick to getting oven cleaners to be effective, in my experience, is in the timing. Timing is important because the contents of tars are not exactly known, so the softening reaction is brief.

  1. A warm surface is easier to clean; however, a hot surface will dry the cleaner before it has a chance to work.

  2. The oven should not be powered when attempting any cleaning.

  3. When any oven cleaner is first applied, it is important to leave the cleaner alone and give it time to work. Ten minutes is usually sufficient.

  4. Once dissolution occurs there is a brief window when the tar or creosote can be easily removed with a nylon brush or sometimes even a paper towel.

  5. When the materials dissolved, vapors which were previously trapped in the tars may be released. These released vapors are not good to breathe.

  6. Sometimes it is necessary to get deeper layers of tar. The cleaner can be reapplied up to three times before the solution dries if necessary. Each time allow it to go undisturbed for ten minutes. (In my experience, by the fourth application, cleaning the tar is simply not worth the effort.)

  7. If too much time passes and the solution dries, then the tar is more difficult to clean. The caustic solution can also damage metal components.

  8. Rinse the oven of all cleaner when done. Caustic residues can damage the oven if left on the surface.

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