She was already respected for her large and colorful abstract paintings; then Helen Frankenthaler tried printmaking, too.

She was reluctant at the outset, bothered -- she has said -- by the apparent anachronism of a modern artist working with rocks and stones. Eventually, friends persuaded her to try her hand anyway. That was 20 years ago, and she's been making prints whole-heartedly ever since.

In the interim, Frankenthaler's lithographs, woodcuts, intaglios, silkscreens and pochoirs have added a formidable contemporary dimension to the art of printmaking. About a hundred of them are on exhibit at the Phillips Collection in the first full-scale retrospective of the artist's work on paper.

"Helen Frankenthaler: Graphic Works From 1961-1979" is a vast treasure of expressionist colors and shapes. The gem in the collection is "Essence Mulberry," an intricate, low-key seven-color woodcut that's hard to walk away from. It's hung alongside several trial and working proofs, one with Frankenthaler's hand-written instructions to the artists and technicians who help produce the prints: "Like vividness where mulberry meets ochre. Ochre must remain. Don't allow mulberry to hit paper edge."

Several other groupings include trial runs: the four-color intaglio "Earth Slice" is shown with plate proofs and a number of "experimental" impressions as well; the simple "White Portal," a four-color lithograph, hangs with its proofs and with the related "Silent Curtain."

The proofs afford a valuable glimpse at the artist's instincts and techniques, and comparing them to their final prints is fascinating.

The single drawing in the show, "K.T.'s Mulberry," was done with real mulberry juice and green crayon and served as inspiration for the "Mulberry" group. A wallful of works-in-progress and some posters designed for previous showings round out this excellent exhibit.