If you've never heard about it before, you'll probably think silver solder is nothing more than a special solder for use on silver. But it's a lot more than that. It's one of the most useful repair materials you can have in your home. Why? Because it makes strong repairs on a variety of metals including iron, steel, brass, stainless, copper and, of course, silver. And because it's very simple to use.

The stuff isn't cheap - after all, it is an alloy of silver. But once you have it, you'll be able to fix things you'd normally throw away, or take to a welding shop. So chances are very good that the silver solder will pay for itself in a short time.

For example, last summer the throttle lever on my lawnmower broke. I fixed it in five minutes, and saved the cost of a new one, plus the bother of ordering the replacement and waiting for it to arrive. When the handle came off our saucepan, I had it back on in minutes. When a visitor broke the wire frames on his glasses, I had him seeing again in the wink of an eye.

Some hardware stores carry silver solder, but usually the low-temperature type, with a tensile strength around 10,000 psi. The really strong silver solders - tensile strength over 60,000 psi - are a little harder to find. You can probably get some at a welding supply shop, but the easiest source is a mailorder house. The Brookstone Co., 125 Vose Farm Rd., Peterborough, N.H., 03458, sells five feet of 1/16th-inch wire for about $8. Besure to get some flux, and be sure it's specifically for use with silver solder.

An ordinary household propane or butane torch will work, but even better is one of the new torches that burn MAPP gas. Most hardware stores have them. They burn much hotter than ordinary torches, which makes silver soldering go faster and easier.

Use silver solder like ordinary solder. Parts to be joined should fit together snugly; you may have to bend or file the parts slightly to get a good fit.

Cleanliness is important, too. Wash parts to be joined, then sandpaper or steel wool the mating surfaces just enough to produce bright, bare metal. Once you get the sur faces clean, don't touch them. Next, apply a thin coating of the flux to both surfaces. Place the parts together snugly and clamp or provide some other means of keeping them in contact. If the parts move during soldering the joint will fail.

Apply heat to the work. If one part is much larger than the other, apply most of the heat to that part; otherwise, the smaller part will heat up much faster than the larger. As you apply the torch, keep an eye on the flux. The pasty white flux will bubble at first, then turn clear and resemble molten glass. Next it will begin to flow and spread out. When that happens, apply the silver solder wire to the work, using the hand that's not holding the torch. The work should not be hot enough to melt the solder. If not, remove the solder and continue to heat the work for a few more seconds. Then reapply the solder.

It's very important to let the work, not the direct flame, melt the solder. This will make sure that the solder penetrates the parts being joined, producing the strongest possible joint. One other tip: just like ordinary lead/tin solder, silver solder will flow toward heat. So you can lead the molten solder anywhere you want to go, even uphill. Just apply the torch wherever you want the solder to flow.

Once the solder flows into all parts of the joint, remove the solder wire and the torch, and let things cool down. Don't touch the work again until you're sure the solder has solidified. Chip away any of the remaining glasslike flux and the job is done.