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I've been using a stainless steel sheet pan and an aluminum sheet pan for baking/roasting for the last year (new to cooking). I used the steel one regularly, so it accumulated a lot of brown stains. Eventually I started lining it with aluminum foil until I found a way to clean it. On a friend's advice, I got steel wool, and with some vigor, dish soap, and time, I scrubbed all that grime off. Also scrubbed off the little bit of grime on my aluminum pan.

Felt pretty proud of myself until I noticed that the fingers of my dishwashing gloves were stained DARK GRAY. I used wet paper towels and wiped the steel wool, the steel pan, and the aluminum pan, and it appears the gray color is mostly coming off the steel pan. A little bit of gray is also coming off the aluminum pan.

What on earth is this stuff? Is it no longer safe to use either of these sheet pans for cooking? If not, can I make it safe or do I have to toss them?

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  • It's very likely that the "brown stains" you were seeing are just seasoning. The fats from the things you roast bake onto the surface much like you'd season a carbon steel or cast iron skillet. This is actually a very desirable state, I've got some twenty year old pans that are flat out black at this point and nothing sticks to them anymore. I still use newer pans and parchment paper for pastries, but if I'm gonna bake a batch of turkey parm, I'll grab those dirty-looking black pans every time. I know I'll get a nice crisp bottom crust and they'll just slide off the pan.
    – LightBender
    Commented Apr 15, 2021 at 18:25

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It is oxidised aluminium (Al2O3). This is formed by oxidising with the oxygen in the air to form the aluminium oxide. Al2O3 forms a very thin (4 nm) impermeable layer on your Al, which prevents further oxidation and appears as a dull grey on the surface. Polished Al is shiny and can be highly reflective.

Al2O3 appears as a dark grey/black colour when you scrape it and some of the underlying Al off the sheet with your steel wool. It is likely that your "steel" sheets are actually aluminized steel, as the steel itself is not rust-resistant. The aluminium coating on the steel prevents corrosion from the air by forming the aforementioned oxide layer.

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  • So is that oxide stuff safe to ingest? Or can I just wipe that stuff off?
    – BatWannaBe
    Commented Apr 16, 2021 at 2:57
  • @BatWannaBe I would avoid eating it as much as possible. Aluminium is toxic at high doses (>40mg/kg body mass/day), so cumulative doses wouldn't be a great idea. You can wash most of the loose stuff off with a cloth and soapy water, just do it until the cloth no-longer turns black. Just line your cookware with baking paper/sheet and you should be fine.
    – bob1
    Commented Apr 16, 2021 at 3:12
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Never use steel wool on an aluminum pans or put in the dishwasher. It damages the surface and as others have said, provides fresh O2 to form oxides. Use scrubby sponges or brushes after soaking if necessary and most importantly, use parchment paper all the time. Buy it in 1/2 sheet pan sized sheets that fit ‘cookie sheets’ perfectly or cut to size. Everything gets cooked on parchment. Makes cleanup easy.

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