The question From: GUEST,gillegan 15 Feb 08 - 06:15 AM raises the issue of there being two separate and distinct kinds of discoloration of aluminum pots.

The original description, of heavy black deposits in the bottom of the pot, suggests the "curing on" of residual oils and vegetable residues over a long period of use. Especially with hand washing, it's almost impossible to get all the surface oils removed at each washing. When the pot is used the next time, the residual organic scum on the surface is hardened, and eventually is converted to what amounts to a layer of nearly pure carbon. This is the same process intentionally used to cure cast iron cookware; but the carbon is "chemically compatible" with iron and can "bond" with the cast iron, while it's essentially just a layer on the surface for aluminum. (It may be difficult to tell that there's a difference when you're trying to get it off.)

"Cooked on carbon" is virtually inert to most chemical cleaning agents that can be safely used on aluminum, and removal is best done, if necessary, by abrasive methods. Martha Stewart can buy a new set of pots when they get dirty. You'll probably prefer to go ahead and use some grit, especially if "sentimental values" are involved.

The description by gillegan, of discoloration that appears from just leaving water standing in the pot overnight is more likely from chemicals in the water, reacting with or depositing on the aluminum surface. Discoloration of this kind can usually be removed most readily – if at all – using vinegar (about 1 tbsp per qt of water) or cream of tartar (a couple of tsp per qt) and heating. Other "chemicals" such as bicarbonate of soda etc may have similar effect, and there are lots of things one can try.

Quite likely, what we assume is cooked on carbon in Richard's case may be accompanied by some "chemical discoloring" due to long use with even the purest of common water supplies, so a combination of methods may be needed there, although abrasive attack sufficient to remove the carbon likely will get most of whatever else is present.

In gillegan's case the very rapid discoloration (overnight? - or was it really a couple of nights?) suggests a level of contamination in the water supply that would be likely to be well known by those in any area where it occurs, and local advice would likely turn up recommendations from those who've found what works with the local water. The vinegar or tartar treatment, of those commonly effective elsewhere, is probably most likely to work, but may not be effective for "peculiar local conditions."

AUS has a well-developed (on paper) water quality program¹ that is apparently aggressively implemented at least in some areas but local conditions may slip out of the norm. Less stringent standards are applied to "small systems" with up to about 1,000 users, and there appears to be significant use of "bore water" and rainfall catchment sources that may be contrived, constructed, and maintained by individual users. There are apparently some local areas where fairly high "trace materials" – especially minerals – may be present in locally accessed water; but I found no immediately accessible info on widespread areas where local water might be of particularly low/variant quality.

Some areas do appear to provide separate "drinking water" supplies with a second and independent "lawn and laundry water" supply. In an area where this is done, using only the "drinking water" supply for cooking would be advisable. There appear to be areas where local "point of dispensing" filtration is recommended, but they're not identified in what I found.

If there is significant concern about the water in the new location being "different" than what you're used to, the Water Made Clear: A consumer guide to accompany the Australian Drinking Water Guidelines 2004 may give you clues as to who to ask. (See the "Further Information section near the end.) The local water provider quite likely can supply a recent "analysis" of what they're giving you, if that's applicable. If you're on an "independent supply" you should be able to identify an appropriate testing agency to find out what you're using, and periodic retesting would probably be a good idea.

¹ National Water Quality Management Strategy: This site identifies some rather large "methods" documents that describe "how it's supposed to be done," but give little detail about specific water sources and common inclusions. A local supplier is more likely to have the information you really want; but this document may give you some clues on how to contact the right people.

John