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I just looked on the All-Clad website (faq 7 and faq 13). It says that because the MC2 has a brushed aluminum alloy exterior, it mush be hand washed. Why is the dishwasher bad for it? It's just soap and water (and sometimes rinse aid) right? Is it the rinse aid that does something to the pot/pan? Is it because the jets of the dishwasher too strong for the pot?

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up vote 8 down vote accepted

The usual reason given is that Aluminum will react with the alkalis in dishwasher detergent and discolour.

Automatic dishwasher detergent has a lot of stuff in it.

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I can answer you with first hand experience and a picture. Your lovely dark gray finish will become light gray, streaked, and hideous. I'll never buy this style of pot again, it is just too useful to be able to dishwash them sometimes. alt text

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Ouch. I don't even want to imagine what it must have been like opening up the dishwasher door and seeing that. – Aaronut Oct 21 '10 at 14:02
Really hard to argue with that picture. Ouch. – Marti Oct 21 '10 at 14:54
Damn. I would hardly believe that's an All-Clad pot if not for the stamped handle. – hobodave Oct 21 '10 at 17:35
Yeah; to be clear that is probably 5 times through the dishwasher. Once it was ugly the first time, I sorta figured, what the heck. – Michael at Herbivoracious Oct 22 '10 at 3:38
Wow. I've washed it a couple of times and so far it doesn't look that bad...but I'm not going to do it anymore! Yikes. – milesmeow Oct 25 '10 at 3:53

Dishwashing fluid isn't a simple soap; it's much more aggressive. A good portion of it is sodium hydroxide (lye) which will attack the protective layer on aluminum surfaces (aluminum oxide), allowing for significant corrosion to occur.

All aluminum will form this oxide layer in air, but it can also be intentionally grown via anodization and colored to form an attractive finish. The dissolution of the anodization is why Mike's pots look so bad now.

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Like all aluminium they will discolour in the dishwasher, but will still be safe to use. One site I saw stated that the discolouration can be avoided by removing before the rinse cycle, which suggests to me it's the softeners rather than the detergent that does the damage.

It's worth noting that very acidic foods or prolonged cooking can also stain aluminium. Ours has a dark stain where the water came up to while steaming a Christmas pudding and I've seen some ruined by jam making.

NB in the past some people thought aluminium released into food during cooking was hazardous but this has since been disproven.

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