After considerable detective work, the Pierpont Morgan library has mounted a one-man show that 50 years ago would have distressed the artist, embarrassed some art historians and lowered the value of a number of collections.

The show collects about 75 works of the "Spanish Forger," who worked well into the 20th century but was so successful in concealing himself that even the name he carries is most likely in error. It is doubtful that the Spanish Forger was Spanish.

In the early years of this century, when artists in Paris laid the groundwork for the startling changes in painting that influenced the world, the Spanish Forger was also active - creating fake 15th-and 16th-century French and Flemish works.

It is even possible, although highly unlikely, that he used his skills painting his own works as well as the forgeries, and is known as a legitimate artist under his real name.

Paris was the center of the art world, and it also had a thriving school of forgery before the Spanish Forger came on the scene. His skill, his success and the volume of his work make him special.

The detection of the Spanish Forger's works has been associated with the Morgan Library since 1930, when its first director, Belle de Costa Greene, was shown a painting of the Betrothal of St. Ursula.

Count Umberto Gnoli was seeking to sell the painting to the Metropolitan Museum for 30,000 pounds sterling and wanted Da Costa Greene to corroborate its identification as the work of Maestro Jorge Ingles, who was active in Spain around 1450.

She declared it a fake and because Ingles was Spanish, dubbed the artist the Spanish Forger. (The Metropolitan didn't buy).

William Voelkle has added about 100 works to the known examples of the forger's art in three years of research preparing for the Morgan show. Voelkle's work brings about 150 the paintings, manuscripts and single leaves from manuscripts attributed to the mysterious artist.

"I wouldn't be surprised if the oeuvre were doubled after this slow," Voelkle said in an interview. In fact, he hopes one result of the show will be that people will bring more possible forgeries to his attention. "The next one I see could provide a clue to the identity of the forger," said Voelkle, the associate curator of renaissance and medieval manuscripts at the Morgan library.

Most museums and collectors are reconciled to labelling their works by the forger as fakes, but in the first 30 years of the century there were battles over the origin of several Spanish Forger works.

When one expert challenged the authenticity of the Cincinnati Triptych in 1909, which was then attributed to Jean Fouquet, he was overcome by others who rose to the Triptych's defense. Today, Voelkle can point to a dozen mistakes the Spanish Forger made that make it almost inconceivable that the Triptych was done in medieval times.

The Spanish Forger didn't copy medieval works, but took elements from several different scenes and arranged them as he chose.

His chief compositional and thematic sources, Voelkle has determined, were five illustrated volumes on medieval and renaissance life and culture written by Paul Lacroix and published in Paris between 1869 and 1882.

These books not only provided the forger with material, but by their popularity also created a market for his works. A widely popular exhibit of French primitive painting in 1904 no doubt helped the forger's business.

In writing of that exhibit, at least one commentator warned of the fakes that were on the market.

The Morgan exhibit shows the source for each forgery and provides clear explanations of other techniques ranging from study of a painting's style to neutron autoradiography that Voelkle has used to determine that all the works in the exhibit are fakes.

Voelkle believes the forger was probably more than one person working from one Paris workshop. His mistakes in illustrating religious manuscripts show that his knowledge of liturgy was wanting, but that he could read some Latin.

One painting of chess players indicates the forger didn't understand chess. Beyond such scraps of information, the Spanish Forger remains a misnamed mystery.