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I have some large cast iron hob grilles of an awkward shape, and want to clean the burnt-on fat from them using NaOH solution. I have two options

  • Use a small amount of a strong solution, with a brush or turkey baster to keep them wet, while they sit on a plastic cement-mixing board with a raised edge, or
  • Use a large amount of a weak solution, in a plastic dustbin or suitable trough, to immerse them either fully, or partially and turn them over

The former would require a lot of hands-on time, and lots of opportunity for splashes, all the worse for being of a strong solution. The latter I could just leave to sit for 24 hours, as long as I knew the solution was strong enough to work, or at least work fast enough. I don't really want to buy, or dispose of, large amounts of NaOH.

I've search around but only found the SV, Saponification Value, for various fats and oils. This will at least give me a lower bound for how much I need, if I can estimate the amount that's there.

I have the gut feeling that there is a threshold pH below which saponification won't occur. However, the pH of even 0.01M NaOH is around 12. So my gut says concentration won't matter, and that any practical concentration of NaOH will work, albeit just affecting the speed of reaction.

I have found sources that suggest that 2.5M (10% by weight) will work in 10 minutes. I would expect the speed to vary simply as 1/concentration.

Are my suppositions on threshold and speed correct? I don't really want to spend a day on tens of litres of a weak solution that doesn't work.

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    $\begingroup$ There is a commercially available product called "Oven Mate" that is KOH in polyethylene glycol gel which is great for this. You paint it on, leave it then rinse off. I use it on oven trays and interiors. Works well. $\endgroup$
    – Waylander
    Commented Dec 12, 2022 at 10:02
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    $\begingroup$ And there is also the possibility of using a propane torch to complete the combustion process. Then just brush off the ash. $\endgroup$
    – James Gaidis
    Commented Dec 13, 2022 at 3:21

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2.5 M might be a bit strong. 1.0 M is 40 g NaOH per liter of water, or around 4% solution.

Start with 1 % solution and leave it soak. As with oven cleaners, a little heat should help move things along, like around 150 F. No need to boil it.

I would check it every 2-3 hours. It might be done by then. A little dish soap, rubber gloves, and an abrasive pad should finish the job.

If you need to neutralize, some phosphoric acid from the local hardware store may be of use. Please be careful and wear eye protection. Caustic solution is very corrosive to skin as well.

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