De-oxidize my mortar & pestle, please.

December 18, 2007 11:56 AM

I have a gorgeous cast-iron mortar and pestle, that I use a lot for cooking. Unfortunately, it has some rust, especially on the bottom, probably from when it's washed and left to dry still slightly wet on its bottom. How should I: a) get rid of the present rust. b) avoid future rust.

Pics here. You can clearly see the rust on the bottom.

posted by signal to Technology (23 answers total)

a: Scrub with steel wool to remove rust.

b: Stop leaving it to dry still slightly wet on its bottom, and start drying it thoroughly.
posted by dersins at 12:02 PM on December 18, 2007

Response by poster: dersins: the thing is, I'm not usually the one who washes it. Is their some sort of treatment (oil, heat, etc.?) that would make it less rust-prone?
posted by signal at 12:03 PM on December 18, 2007

Bake it for a bit after washing, so it doesn't stay wet long enough to matter?
posted by Su at 12:04 PM on December 18, 2007

Or, yes, oil it the way you're supposed to an iron skillet. I'd figured there was some reason you weren't doing this already.
posted by Su at 12:04 PM on December 18, 2007

Response by poster: Another thing, won't the steel wool scratch it?
posted by signal at 12:05 PM on December 18, 2007

This page has a nice instructional section on how to care for and clean your cast iron including a nice rub with a neutral oil.
posted by inconsequentialist at 12:10 PM on December 18, 2007

No, steel wool won't harm it. You can heat it after oiling to form a more durable coating, but even that will be vulnerable to washing with soap. You should rely, as often as possible, on an abrasive of some sort, followed by a wipe with a dry paper towel. You need to train whoever is doing the inappropriate washing to treat it correctly.
posted by jon1270 at 12:15 PM on December 18, 2007

Don't bake it, that will make it rust faster. If it is allowed to dry thoroughly and treated with olive oil after each washing, it should stop rusting.
posted by 517 at 12:15 PM on December 18, 2007

2nd the olive (or canola, or any other kind of cooking) oil...do the bottom too.
posted by dendrite at 12:18 PM on December 18, 2007

Are you grinding spices? I use mine mostly for grinding dry spices and just give it a quick wipe with a damp towel. I've occasionally fully washed it, and then I make sure to dry it.
posted by OmieWise at 12:26 PM on December 18, 2007

Oh, and don't put any oil in the inside, this may well lead to a gummy mess.
posted by OmieWise at 12:26 PM on December 18, 2007

Yeah, putting oil on the inside will eventually cause a concretion of gross stuff on the inside.

A little rust is good for you anyway. I wouldn't worry about it. And while steel wool may indeed scratch it, as long as you aren't entering your mortar into a beauty contest, who cares?
posted by Sukiari at 12:28 PM on December 18, 2007

Start washing it yourself, and dry it throughly.
posted by yohko at 12:45 PM on December 18, 2007

Steel wool won't hurt it a bit. I would not "season" a mortar and pestle the way I would a skillet -- there's no need. It'll get gunky. If all you are doing is grinding dry spices, then I wouldn't ever wash it, I'd just wipe it with a damp cloth and turn it over until I needed it again.

Or if you MUST wash it, then dry thoroughly, or pop it in the oven for a minute.

Exposed iron is going to rust, period -- there's no way to stop this (OK, I guess you could enclose it in Lucite or store it in an airless void, but that seems a bit extreme).

Keep it dry and scrape the rust off with steel wool, or if it's really bad, scour it with a grinding bit using a Dremel tool, and then make sure your kids tell their kids how to do that, so they can use it too.

I've got a cast-iron griddle that dates back at least three generations that I still use a couple of times a month.
posted by BitterOldPunk at 12:53 PM on December 18, 2007

Response by poster: I mostly use it for spices and herbs, and fresh basil.

Sukiari: "as long as you aren't entering your mortar into a beauty contest, who cares?"

I care. The whole point is to have beautiful things to cook with.
posted by signal at 1:18 PM on December 18, 2007

You could paint the bottom flat black.
posted by electroboy at 2:13 PM on December 18, 2007

OK, I know we all know better, but I feel compelled to mention that painting the interior of a device used to grind stuff up is simply idiotic.

Unless you like eating paint.
posted by BitterOldPunk at 3:02 PM on December 18, 2007

B.O.P., the rust is on the outside, which is presumably where the paint would go. Not to say that I think paint would be a good idea.
posted by Jahaza at 3:11 PM on December 18, 2007

The inside looks just fine, as I'd expect from a device whose main function involves grinding on that surface. To get rid of the rust on the underneath, scrub it with steel wool soaked in vegetable oil, then wipe as much oil off the cast iron as you can with paper toweling, then bake the mortar in the oven until it goes dark underneath. Basically you're seasoning the underneath just like you'd season the inside of a cast iron pan. Do keep the inside (the grinding surface) oil-free though.
posted by flabdablet at 3:34 PM on December 18, 2007

Something else you might try (for the underneath only) is rust converter. You can get this from hardware stores. It's basically just phosphoric acid, and it reacts chemically with wet iron oxide (rust) to form iron phosphate, which is a flat blue-black and fairly durable. Use it on the bottom only.

But the main thing is changing the washing and drying technique that gets used on this thing. If you wet cast iron and then leave it sitting around to dry, it's just going to rust. That's what it does.
posted by flabdablet at 3:37 PM on December 18, 2007

OK, I know we all know better...

It helps to read the question and look at the pictures.
posted by electroboy at 1:53 PM on December 19, 2007

That's a great idea, electroboy! I'll keep it in mind. It also helps to read the responses, one of which was my detailed answer to the question.

But the next time you want to snark at me in AskMe for making a tangential suggestion, just send me a MeFiMail, so we won't derail the thread when I tell you to go fuck yourself.

Happy holidays!
posted by BitterOldPunk at 10:27 PM on December 19, 2007

Your suggestion is duly noted and filed appropriately. I'll also point out that your detailed answer was a much less succinct version of the very first answer.
posted by electroboy at 11:35 AM on December 20, 2007