Why does it take forever to reduce my stocks and soups?

I once had a mutual fund that reduced my stocks in no time at all. But I suppose you mean meat and fish stocks on the stove.

Your stocks may not reduce enough for the same reason that some people don't reduce enough: they don't try hard enough. Water can be every bit as frustrating a substance as fat, in that it is much harder to get rid of than you'd expect. To boil off even a small amount of water it requires a lot more heat than you might imagine.

I'll assume that you have done everything properly to produce a most flavorful soup or stock. For a chicken stock, for example, you have rinsed the bones and carcass, placed them in a pot of cold water with chopped vegetables, garlic, peppercorns and a bouquet garni, brought it all quickly to the boil, simmered it gently for 3 to 4 hours while skimming frequently and then strained, seasoned and de-fatted it. All the complex chemical reactions that produce a richly flavored broth have therefore had ample opportunity to work their magic.

But then you taste it and the flavor is good, but too thin. You have to remove some of the water to concentrate the flavors. (You could have used less water in the first place, of course, but it's too late now.) There's nothing to do but simmer and simmer, waiting for that maddeningly simple-sounding cookbook phrase to become a reality: "reduce to two-thirds." It does indeed seem to take forever.

Here's why.

Getting the Vapors

Water molecules stick quite tightly to one another, so it takes a lot of work to separate them from the liquid and send them off into the air as vapor. In order to boil off a pint of water, that is, to convert it from liquid to vapor after it is already at the boiling point, your range burner must pump more than 250 calories of heat energy into it. That's the amount of energy a 125-pound woman would use up in climbing stairs nonstop for 18 minutes. Just to boil off one pint. 'Nuff said?

You can, of course, turn up the burner to add the necessary heat more rapidly, but only after you have strained and de-fatted the stock. Until then, boiling, as opposed to gentle simmering, would have muddied up the liquid by breaking up solids into tiny pieces and fat into tiny, suspended globules. Or you can transfer the liquid to a wide, shallow pan. The more surface area the liquid has, the more of it is exposed to the air and the faster it can vaporize.

How Much Heat, How Fast?

Back when you put the bones and carcass into the cold water, you quite properly turned up the burner full blast because you wanted to bring the water rapidly to a boil before turning the heat down to a simmer. That brings up the question of just how much heat a stove-top burner can put out at its maximum setting. There is a lot of confusion out there about range-top burners and amounts of heat.

Heat, as you know, is a form of energy. For nutritional purposes we measure it in calories, but range and oven manufacturers use the engineers' way of measuring heat, in Btu. A Btu, which stands for British thermal unit, is defined in an absolutely insane way, but it turns out to be equal to one-fourth of a nutritional calorie. So, for example, the 250 calories that you needed to boil off that pint of water from your chicken stock is equal to 1,000 Btu. A candle contains the potential of generating some 5,000 Btu of heat, but it releases its heat slowly over a period of several hours, and that's why you can't reduce your stock over a candle flame. I know that you've always wondered about that.

Range burners and ovens are therefore rated according to how fast they can pump out heat, in Btu per hour at their top settings. The source of confusion is that the burners' ratings are not amounts of heat; they are the rates at which they pump out heat.

Most home gas or electric range burners can produce from 9,000 to 12,000 Btu per hour. The gas burners in restaurant kitchens are capable of putting out twice as much heat per hour, because their gas-supply pipes are bigger and there are generally several concentric burner rings instead of just one. Chinese restaurants doing high-temperature wok cooking have broad gas burners that spew out heat like a dragon with a mouthful of habanero peppers.

Remember that to boil off that pint of water from your stock required 1,000 Btu of heat? Well, using your 12,000-Btu-per-hour burner it should take one-twelfth of an hour or five minutes, but you know it takes a lot longer than that. The reason is that most of the heat emitted by the burner is wasted. Rather than going directly into the liquid in the pan, most of it goes into heating up the pan itself and the surrounding air. Put two different pots of food on two identical burners set at identical levels and they will heat and cook quite differently depending on their shapes and sizes, what they're made of, how much and what kinds of foods they contain, and so on. That's why you have to keep your eye on the pot and continually adjust the burner for every specific situation. Ain't cooking fun?

Silver--Beware

In my last column I told how an aluminum tray or aluminum foil can be used to clean silverware electrochemically. Several readers have pointed out correctly that this method removes not only tarnish but also the dark, etched designs that are deliberately applied by some manufacturers of fine silverware. Recalling his unfortunate experience at the age of 14, Henry Brown of Great Falls, Va., warns, "Don't use this method if you don't want a scolding from your mother."

According to John B. Wright of J. A. Wright & Co., manufacturers of Wright's Silver Cream, the aluminum method is best used on old, heavily tarnished, almost-black silver that resists all other efforts. It should be followed by a jeweler's rouge polish such as Wright's to remove the milky haze that I mentioned in the column and to smooth the silver surface to a bright, mother-lovin' shine.

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.