Much that was veiled in the past is now clearer. -- Piet Mondrian, 1941.

DID MONDRIAN deceive us, or have we deceived ourselves?

At the mention of his name, a sharply focused image clicks into the mind: Strong black lines on white, no colors but the primaries, no angles but the right one, no subject but the grid.

That image is a lie.

"Mondrian: Drawings and Watercolors," at the Baltimore Museum of Art, beautifully demolishes that familiar crisp cartoon. This superb exhibition restores to the Dutch master the gray veil that reveals the spirit of his art.

Mondrians, in memory, have a clarity that blinds. Here, at last, we look upon the works of art themselves and see that each is misted. The mystery within outweights the mathematics. The Mondrian we meet here is less concerned with clarity than with nuance. He is never a mere formalist. He borrowed less from Euclid than from the theosophy of Rudolf Steiner and Helena Petrovna Blavatsky. Those who saw his whitewashed studios in Holland and in France, in London and New York, were reminded of a monk's cell, yet throughout his long career, the catalogue informs us, this "Calvinistically strict bachelor" spent his evenings dancing the fox trot and the tango, the Charleston and the Shimmy, with "scientific fervor." He liked jazz more than Bach.

Mondrian the draftsman is never a mere formalist. His art is full of moonlight, of dunes and fogs and flowers. Each certainty he shows us is born of contradiction. In almost every picture here he reconciles opposites, positive and negative, void and volume, light and dark, vertical and horizontal, assurance and the lack of it, landscaprk, eertical and horizontal, assurance and the lack of it, landscape and abstraction. This is supremely honest art.

There are 132 Mondrians on view in the Baltimore exhibit. Organized in Germany by Ulrike Gauss, the show, in slightly different form, was first seen in Stuttgart and later in The Hague. It is worth a trip to Baltimore for this, its only U.S. showing.

How little we know Mondrian. In ways both large and small, this display surprises. The Dutch landscapes of his early years, with their thatched roofs and flat fields, have the vigor of van. Gogh's; his shadowed northern nightscapes with their silhouetted mills, their moons and leafless trees, are as haunted and disturbing as the dreams of Edvard Munch. And Mondrian's selfportraits, although far from numerous, have a dark, hypnotic power that holds the viewer's eyes.

One leaves the Mondrian exhibit with two compelling memories. The first is of his pilgrimage, his almost saint-like ardour and unwavering integrity. The second is of color.

The textbooks still insist that the master, early on, purged his work of reference and of every color save black and white, of course, and yellow, red, and blue. That is not what we see. The color of his show is the color of the fog, of the North Sea mist and the North Sea. His whites aren't white, his blacks aren't black, the primaries don't rule the pictures on display. From the start until the end of his long career (he was born in 1872 and died in Manhattan in 1944) the hues that ruled his drawings wre endlessly inflected, subtle shades of gray.

That he took much from the Cubists has often been remarked. (He also gleaned much from Matisse, a debt that, though suggested by the flatness of his color, is even more apparent in the drawing of his striking "Portrait of a Woman," 1909-'10.)

His father and his uncle were both skillful artists, and young Mondrian spent years copying old masters and sketching plaster casts. But from the very start, old conventions chafed him. He fought for the essential, for the glowing and unaging truth that the Theosophists had shown him behind the shifting veil of the material world.

Mondrian believed, and lets us believe, too, that in his art he might distill the essence of all art that had come before him. But he was far too honet to banish from his vision all traces of the world.

His linearities, no doubt, owe much to the shattered planes he saw in 1910 in French Cubist paintings, but the Cubist all were city folk, and Mondrian, in contrast, even when he lived in towns, and danced his serious fox trots, was something of a hermit. The Cubists dawdled in cafes and filled their close-packed pictures with shards of ads and ashtrays. Mondrian drew his vision from the land.

In this exhibition (it was beautifully installed by Baltimore's Brenda Richardson) a full wall is devoted to the drawings that he made of Dutch windmills under moon-bright clouds. Another long wall is devoted to his increasingly abstract studies of fruit trees. A third presents his dunescapes, a fourth wall is devoted to the drawings that he made of the church at Domberg. His flowers are here, too.

No sudden breaks are seen. Mondrian evolved. The colored paper tapes of his last New York abstractions are prefigured in the paper patchings of his barge studies and fieldscapes of the 1890s. The darkness flecked with silver in his charcoal compositions of the 1940s is the same seen in his tree studies of 1906.

Despite the textbook's claims, and those he made himself, the master never wholly banished nature from his art. His every horizontal summons to memory the flatness of a Holland rescued from the sea. In his every vertical one feels the aspiration that draws plants to the light, that led man to pay homage to the spire and the cross. Even in the charcoal grays of his latest grids, including those he left unfinished at his death, one feels the throbbing presence of that least mechanical of powers, that which leads to growth.

Many later artists, some of whom were masters, would sense the presence of the holy in grids and colored fields. Mondrian, however, saw within the rigorous rhythms of his art more than the ineffable. He has too long been seen as an austere minimalist. The Baltimore exhibit shows how much of the world, of its fields, seas and skies, of its cities and its sands, and even of its saintliness, he poured into his grand and inclusive art. The USF&G Insurance Co. helped pay for the show. It closes Sept. 20.