In a catalogue I saw a special metal plate that removes tarnish from silverware on contact. How does it work (if it does)?

It does indeed work. There are several incarnations of these "magic" plates available from catalogues, usually for $19.95 plus enough shipping and handling charges to transport the road company of "Aida." The "special" metal plate is nothing but aluminum or magnesium. You can remove tarnish from your silver flatware and serving pieces just as easily with an aluminum pan, some salt and baking soda, and a little know-how. Here's the know-how.

Silver doesn't tarnish by oxidizing, the way copper and brass do, for example; it tarnishes by "sulfidizing." That is, it's not the oxygen in the air that reacts with the silver; it's sulfide gases that react to form a film of black silver sulfide.

Sulfide gases are unpleasantly odoriferous. They get into the air from three food-related sources: gas ranges and ovens, the cooking of cabbage and similar vegetables and human gaseous emissions following the digestion of the latter.

The utility companies deliberately put a small amount of a smelly sulfur compound (a mercaptan) into the natural gas that we burn in our ranges, water heaters and furnaces. They do that because natural gas itself (methane) is odorless, and without the smelly stuff we wouldn't be able to detect a leak. When the mercaptan burns along with the methane, sulfurous gases get into the air.

Cabbage, cauliflower, broccoli, brussels sprouts and their ilk are notorious for smelling up the house when cooked. They contain sulfur compounds that when heated break down into sulfurous gases, including the worst-smelling sulfide and one of the worst-smelling of all chemicals, hydrogen sulfide. Egg yolks also contain sulfur compounds that change into hydrogen sulfide with time and heat. That's why eggs tarnish silver spoons.

In passing, it should be noted that sulfides are not to be confused with sulfites, which are very different chemicals used in processed foods for bleaching and to prevent oxidation. Many people, especially asthmatics, are sensitive to sulfites, and the U.S. Food and Drug Administration requires specific labeling of products that contain them.

But back to your silverware. Whether it's sterling silver (92.5 percent pure) or silver plated, here's how to remove the tarnish without any rubbing or expensive polishes.

First, wash the silverware in warm water with a dishwashing detergent (oil or grease will prevent the chemical genies from getting at the tarnish). Then lay your tarnished silver pieces in an aluminum pie pan or aluminum-foil roasting pan big enough so that each piece of silver will be totally immersed when you add water. Now boil enough water to cover the pieces in the pan and add a couple of teaspoons each of salt and baking soda--not baking powder--for each quart (the amounts aren't critical). Stir to dissolve the chemicals and pour the solution into the pan that contains your silver.

That's all. In a very few minutes the tarnish will disappear "right before your eyes," "as if by magic," "with no harsh chemicals" and "no messy polishing," to quote the catalogues. It really is rather spectacular to watch.

Now I suppose you want to know how it works. But if you don't, meet me after the following paragraph.

How It Works

Silver atoms hold onto their electrons more tightly than aluminum atoms do. If silver is placed in contact with aluminum, the silver will want to grab electrons away from the aluminum. But it can't, because electrons are negatively charged and electrical balance must be maintained; an equal amount of negative charge must flow in the opposite direction, from the silver to the aluminum. Well, the sulfide part of the silver-sulfide tarnish is negatively charged. So if we offer it a nice, warm salt bath in which to swim from the silver to the aluminum it will do so. Result: the silver gets the electrons it wants and swaps its sulfides for them, leaving pure silver metal.

You can even see the aluminum pan turn black with the sulfides that it took from the silver. (Discard the pan after use.)

A few details: The hotter the bath is, the faster the reaction will go, so you might even want to keep the pan of water with your silverware on the stove over a low flame. Only those parts of the silverware that are submerged will be cleaned. If a piece is partly exposed, turn it after a while to clean the rest of it. The purpose of the salt is to make the water electrically conducting, so that the charged sulfides can swim from the silver to the aluminum. Many common chemicals, including borax, boric acid or even laundry detergent would do just as well, but the catalogues will be glad to sell you bottles of a mysterious "activator" for $7.50 a pound. The baking soda helps to keep the aluminum from becoming covered too soon with reaction-slowing sulfide.

You don't even need to buy an aluminum pan, by the way. Just line a Pyrex or other ovenproof glass baking dish with aluminum foil. (But don't heat it directly on the stove.) Aluminum is aluminum, after all. And it's only one of several metals, including zinc, iron and magnesium, that can steal sulfides away from silver. Magnesium works even faster than aluminum and that's what at least one of those commercial "magic" plates is made of.

The Bad News

Now for the bad news. You will be slightly disappointed when you see the clean silverware, because it won't be as bright and shiny as you had expected; it will be rather milky-looking. That's because the silver sulfide tarnish layer wasn't glossy-smooth, so neither is the silver metal that it reverted to. But a little rubbing with a soft cloth should restore the shine. Yes, that's a bit of work. But hey, you'd also have to do that after using one of those miracle plates. They don't tell you that.

More bad news: The process only removes tarnish. It doesn't leave behind a tarnish-inhibiting protective layer, as many commercial silver polishes do. Your silver will tarnish sooner than if it had been polished with a commercial cleaner.

So don't cook cabbage for a while, and don't eat beans.

Robert L. Wolke is professor emeritus of chemistry at the University of Pittsburgh and the author of "What Einstein Didn't Know--Scientific Answers to Everyday Questions." Send your food or cooking questions to wolke@pop.pitt.edu.