That old black-silver teapot your grandmother left you may be worth hundreds - and even thousands of dollars.

The classic example is the old silver tankard made by Philip Syng of Philadelphia, who had made the inkstand used in the signing of The Declaration of Independence.

"Some idiot had soldered a spout to it," says William Pimble, silver smither and manager of the Universal Electro-Plating Co. of Georgetown.

Well, his company restored the tankard, took off the misplaced spout and recreated its original silver beauty. For a cost of $500 in workmanship, the tankard finally sold in Miami for $10,000. Before that, even in its ungainly state, it had been valued at $5,000.

Pimble, who is an expert silversmith, says the $500 price was for expertise and the long hours it took for a "terribly tedious job." He kept going back to it. After he had removed the misplaced spout, he had to cover the holes with silver so they would not show.

The tankard was brought to Pimble after he had restored two sterling silver teaspoons for the owner. He has a knack with old silver spoons, forks and knives which have been damaged in a disposal or some other modern conveniences. The price averages between $9 and $15 per piece.

He specializes in silver-plating hollow ware. He can replate coffee pots for $45; a teapot for $40, a cream and sugar for around $30 each. A butter plate averages between $32 and $35.

Replating a silver-plated tray averaging 23 x 18 inches, costs around $100. The cost is based on the square inch, at 9 1/2 cents per square inch. But both sides must be done so that makes the price run up.

The silver-plated pieces are immersed in a tank contining a silver solution and two electric terminals. A bar of silver is attached to the positive terminal; the piece to be silverplated goes on the negative one.

The electricity carries silver particles on piggyback, so to speak, and deposits them on the piece.

Depending on the length of time a piece is left in the liquid bath, it will be trippled can be restored by Universal for between $45 and $50. The company has made repairs for Blair House and the White House, too.

Anthony Facciolo of the Alexandria Metal Finishers, Inc. is a former lawyer. His company does work for the Smithsonian Institution as well as the White House and embassies.

He complains "there are not many craftsmen left. We get business from all over the country." He has great luck restoring brass chandeliers, too.

Art Pierdon, a former president of the American Electroplaters Society, who now acts as a consultant to the Alexandria Metal Finishers, used to specialize in restoring tea and coffee sets left by grandparents when he had his own shop in Washington.

John Ebner of the Sheffield Electro Plating Co. on M Street has restored many old tea and coffee sets at around $300 for a five-piece set, not including trays. Smaller repairs run as low as $15 to $30 for small pieces.

Like many of the others in the business, he complains that metal plating is becoming a lost art.

"We have so many customers, but the trouble is we cannot get help," says Ebner. "People don't want to learn the trade. A skilled man in this line of work is hard to find."

So great is the demand for silver-replating, there are 7,500 members of the American Electroplaters' Society, the professional organization for what is called a multi-billion-dollar business. The society has its headquarters in Winter Park, Fla. Its executive director, Howard Schumacker, said 2,000 delegates came to a convention here at the Washington Hilton recently from as far away as Australia, Japan, England, Germany and Russia. In the Washington-Baltimore area there are 100 members of the society.

Electroplating includes parts for cars, planes and rockets, as well as household silver and gold and jewelry.