WHEN FOUR commemorative stamps honoring American folk art debut this Friday in Ypsilanti, Michigan, there will be some artisans back in Washington who have a reason of their own to be celebrating.

They're the printers at the Bureau of Engraving and Printing who, after almost a year of trying, have succeeded in reproducing fine lacework on a set of stamps.

The stamps, says Postal Service spokesman Hugh McGonigle, will "feel like a tiny piece of lace."

It wasn't easy achieving that effect.

The four 22-cent stamps, designed by Libby Thiel of Bryans Road, Maryland, feature four pieces of bobbin lace made by members of the Great Lake Lacers Group, a Michigan organization that has lobbied for years to have the hobby honored by the Postal Service.

The service finally agreed in 1986 and announced the stamps would be a part of the folk-art series that has celebrated Pueblo Indian pottery, quilts, duck decoys and wooden figurines.

But approving the idea for a stamp, as is often the case, was the easy part. Reproducing the intricate bobbin lace patterns without losing their fine detail was the hard part.

The Bureau of Engraving considered various printing proposals from photo offset process based on photographs of the lace to embossing, or raising, the lace design into the paper.

Earlier this year, the bureau came up with a printing process that met its production standards as well as those of the Postal Service and its Citizens' Stamp Advisory Committee.

The process, said to have been used only once before on a U.S. stamp, calls for a combination of offset and intaglio, or engraved, printing. First two shades of blue are being applied to the stamps via the offset process, one for the lettering, another for the background.

The crucial process, printing the lace itself over the blue background by the intaglio process represented "the first time a stamp or stamps have been manufactured with white, intaglio printing over a dark, offset-printed background," according to the Postal Service.

Other processes "probably could have produced a straight, reasonably acceptable stamp," says McGonigle. The bureau, however, would probably end up with large quantities of unacceptable stamps, considered flawed because of the strong likelihood that some stamps would reproduce the lace accurately but others would not.

The combination process, used to overlay a small white star on a 1968 six-cent commemorative celebrating the Hemisfair in San Antonio, Texas, was the solution.

It won't be hard to judge if the process works with lace. If the threads in the lace are visible, thank Gary Chanconas, Thomas Hipschen, Edward P. Archer and John Wallace Jr., the engravers who painstakingly etched the lacework. Gary J. Slaght did the lettering.

The Postal Service has come up with a solution to waste caused by stamp collectors who tend to buy the corners of high-priced stamp sheets more rapidly than they buy the remaining 96 stamps.

Many collectors want blocks of four stamps from the corners because they come with so-called "plate numbers" designating the specific printing plates used in making the stamp.

The problem is what to do with the remaining 96 stamps. The solution, effective this month with the appearance of the new $5 Bret Harte stamp, will be printing the high-priced stamps in "miniature sheets" of 20.

Each sheet will bear four plate numbers, leaving only four single $5 stamps to sell after selling off the plate-number blocks.

The Harte stamp, part of the Famous Americans series, will not be available in sheets of 100, as most regular series stamps have been.

Collectors desiring Lacemaking first-day cancellations have until September 13 to send envelopes with the stamps affixed to Customer-Affixed Envelopes, Lacemaking Stamps, Postmaster, Ypsilanti MI 48197-9991. The service will affix stamps for 22 cents each for collectors sending addressed covers to Lacemaking Stamps, Postmaster, Ypsilanti MI 48197-9992.

Requests for the Bret Harte stamp first-day cancellation should be sent to Customer-Affixed Envelopes, Bret Harte Stamp, Postmaster, Twain Harte CA 95383-9991. The stamps will be applied at a price of $5 each at Bret Harte Stamp, Postmaster, Twain Harte CA 95383-9992. The requests must be postmarked by September 24.

Bill McAllister is a member of The Post's national staff.