NEXT TIME YOU'RE rummaging around in the cupboards for some battered old thing to mold the mousse in, consider this: the Duke of Wellington in his prime owned over 230 separate molds, all tin-lined copper. They were designed so the lobster mousse could look like a lobster, the tomato aspic like an ocean wave and the plum pudding like a series of gothic arches.

Of course the Duke of Wellington had entertainment needs that went slightly beyond those of the average late-20th-century American. And his collection of molds ended up in the Brighton Museum in England, where they are studied by scholars like Miller-Wescott's David Miller, a metalsmith turned mold manufacturer.

The molds produced in Salem, Mass., by Miller and his former metalsmithing professor at Massachusetts College of Art, John Wescott, are renditions in plastic of 18th- and 19th-century designs, mostly European. In interpreting the antique molds for late-20th century use, Miller and Wescott had to rein in some of the earlier versions' excessive height because as Miller points out, "people want to eat the product." Eighteenth and 19th-century molded foods served more as table decoration than as food, so they were laced with extra gelatin or other tougheners to make them not only stand up but also endure.

Although in 20th-century home life, presentation is not what it was in 19th-century official England, a pint of sherbet presented in a graceful scallop shape looks better sitting on the dinner table than a dripping and grubby cardboard container does. A series of different-flavored sherbets layered in a tall fluted mold and served sliced with a creamy creme anglaise sauce takes things a step further.

The difference between a good mold and a bad mold, today as then, is in the fineness of its detail and in its durability. Most of the molds on the market today are watered-down versions of classic earlier designs. They form the basic shape--fish, say, or melon--but without the exquisite detail that graced earlier versions. And cheap, light molds batter and bend easily, after which their usefulness as molds is ended.

Molds in some materials, if they are heavy enough, can be used for both hot foods, such as cakes and puddings, and cold foods, such as molded ice creams or aspics. (You may not find much use for a salmon-shaped cake, however.) Heavy tin-lined copper molds--not the flimsy, fake-copper versions--are fine for all techniques, have the weight of history behind them, produce beautiful finished dishes, look lovely when they are not in use, are very difficult to find and cost an arm and a leg when you can find them (well, $50 to $100 or more).

If you're looking for a mold to be used with very clear and shiny creations like aspics, remember that bumps and ridges on the inside of the mold will reproduce themselves exactly in the finished product. And, the smoother and shinier the inside of the mold, the more readily it will release the food that's molded in it. (If you are making a cake in a detailed mold and hope to get it out whole, experts recommend greasing the mold, then dusting it with finely ground, dry bread crumbs.)

Miller-Wescott's plastic molds are fine in their detail and graceful in their design, and they are smooth and shiny on the inside. Obviously, they can only be used with cold foods. Their cost is around $10, and they come in 10 designs, from the simple "Europa," a classic tall fluted design, to the complex "Nuremberg," created, according to the accompanying brochure, from a Baroque copper original used in a famed Nuremberg dollhouse. The erudite little brochure is not only interesting in its historical detail but also useful--a combination not often found in promotional literature.

Molds of tin-washed steel can be used for hot or cold foods. They are widely available, but interesting designs outside of the usual kugelhopf shape are difficult to find. Many tin-lined steel molds have a seam which, if you are a perfectionist, can mar a finished mousse or aspic, and which, if they are not well constructed, can eventually allow leakage.

Miller-Wescott's plastic molds are available directly from them at 19R High Street, Salem, Massachusetts 01970, telephone (617) 745-1786, or locally from La Cuisine in Alexandria. For heavy copper molds in interesting designs, try La Cuisine or Bridge Kitchenware at 214 E. 52nd St., New York, N.Y. 10022, telephone (212) 688-4220. Both Bridge and La Cuisine will mail, and La Cuisine puts out a catalogue.