THE CATEGORY of "miscellaneous instruments" employed in jazz has so burgeoned in the last two decades that it now includes not only all the members of the brass, woodwind, string and percussion families but also a vast array of exotic instruments from Asia, Africa and other parts of the world.

Still, the harp remains a virtual stepchild of jazz, although its use dates from the 1930s, when Casper Reardon recorded on it with Jack Teagarden and Benny Goodman. Dorothy Ashby, whose history as a professional jazz harpist began in the early '50s, says that there are special reasons for the virtual absence of harpists from the ranks of jazz players.

Ashby will perform this Friday at 8 p.m. and Saturday at 9 p.m. in the Great Hall of the Folger Shakespeare Library, her first visit to the area in many years. Friday will find her in the company of saxophonist Orrington Hall and on Saturday she will be joined by saxophonist-flutist Frank Wess. Bassist Benny Nelson and drummer Steve Williams will be on hand both evenings.

"I think the harp is probably the most difficult instrument to improvise on," Ashby said from her home in Los Angeles.

"In creating things spontaneously you have to be thinking in several directions in that the harp doesn't have any way to make chromatics, and jazz is a rather chromatic kind of playing. The problem comes when you're trying to design chords that have accidentals in the changes and wanting one thing in your melodic line and being unable to get it because you're using particular intervals in your chordal structure, if you know what I mean. Like wanting middle C in your melody and needing C sharp in your chord."

Explaining why the harpist must think ahead, Ashby continued, "Your pedals have to be set before you get there. It's similar to having a piano minus the black keys. On the harp all that the black keys represent tonally are made with your foot pedals. Each pedal has three positions and you have seven pedals. The number of combinations possible goes into the several thousands."

Ashby had already received extensive instruction in harmony and melodic construction from her father, a self-taught jazz guitarist, when she switched from the piano to the harp in Detroit's Cass Technical High School, where she also played saxophone in the marching band with trumpeter Donald Byrd.

The legion of jazz musicians who came from that city in the '40s and '50s also included vibraphonist Milt Jackson, guitarist Kenny Burrell, saxophonist Pepper Adams and vocalist Sheila Jordan.

"All I was doing was transferring to the harp what I heard around me," Ashby recalled. "Nobody told me these things were not done on the harp."

She had been sitting in on piano with her father and his friends when they rehearsed, and when the opportunity arose to train on one of the five school-owned harps, she "just tried to do jazz on the harp."

Among other obstacles in the way of the harp becoming established in jazz expression is the public attitude toward the instrument. Ashby points out that people "equate the harp with classical music. In the old days, when I first started playing, the audiences I was trying to reach were not interested in harp, period, classically or otherwise, and they were certainly not interested in seeing a black woman play the harp. I think I had to pave my own way."

Finally, she explains, there is the dearth of jazz material written for the harp. "Writers often do not experience playing with or writing for harp and often no efforts are made toward making the harp a solo voice. I guess in that area I can claim to be a pioneer."

In addition to club appearances over the years with her trio, Ashby has many compositions to her credit and keeps busy with studio work. The sounds of her harp can be heard on the albums and singles of hundreds of performers, including Stevie Wonder, Aretha Franklin, the Manhattans, Freddie Hubbard and Nancy Wilson.

As for Ashby's booking at the Folger, the library's director, O.B. Hardison, concedes that it will be the institution's "first high-profile jazz event" but he dismisses the notion that it is a change in policy.

While admitting that the major energy of the Folger "concentrates on the Renaissance," he says, "we've had practically anything you want to name."

As examples of the far-flung cultural outreach of the institution, he cites the presentation in recent years of "Dionysus Wants You," a soft-rock version of Euripides' "Bacchae," readings by area poets, a contemporary opera and D.C. composer Patrick Kavanaugh's composition for woodwinds and pool table. The last-named production was "the only concert we ever had in the Folger that was covered by Sports Illustrated," Hardison says.