MELISSA BLOCK, host:

At the Massachusetts Museum of Contemporary Art, an acre of wall space is covered with drawings. They're the work of Sol LeWitt. Or more precisely, they're the work of an army of assistants and volunteers who executed the artist's ideas. LeWitt was one of the fathers of conceptual art, the movement dedicated to the notion that the concept was the thing. The actual finished piece was secondary. LeWitt died last year before the show was finished. But as Andrea Shea of member station WBUR reports, visitors will have plenty of time to see the show - 25 years in fact.

ANDREA SHEA: Thousands of lines, hundreds of huge circles, squares, and triangles in various stages of execution jump off the gallery walls and grab you.

Mr. JOCK REYNOLDS (Artist and Museum Director, Yale University School of Art): And all of a sudden, you're in a room where these different kinds of lines are literally just dancing together in the most eloquent and gorgeous visual fashion you can imagine.

SHEA: Jock Reynolds collaborated on this show, and he knew Sol LeWitt. He says the artist rarely painted the wall drawings himself, even when he was alive. LeWitt once said the idea becomes the machine that makes the art. In the late 1960s, it was a radical approach where the artist came up with a concept for others to produce. Sixty professional assistants, apprentices, and interns have come together for this retrospective. They're deployed here on three floors in this 27,000-square-foot former mill known here at MASS MoCA as Building No. 7.

Ms. JULIA WAGNER(ph) (Intern, Sol LeWitt: A Wall Drawing Retrospective, MASS MoCA): We're doing everything from pencil sharpening to masking walls to doing a lot of wall preparation.

SHEA: Julia Wagner is an intern in this modern guild system.

Ms. WAGNER: If anything surprised me, it was how much we, the interns, got an actual hand on what's on the wall.

SHEA: Their hands are drenched in balled-up rags and buckets of ink to slap geometric shapes with punchy color and texture. It's a process Anthony Sansotta knows well. He's co-directing the retrospective and worked as one of LeWitt's assistant for more than 20 years.

Mr. ANTHONY SANSOTTA (Co-Director, Sol LeWitt: A Wall Drawing Retrospective, MASS MoCA): Sol started doing his own wall drawings back in 1968. He soon realized that if he was going to be using the entire wall that he was going to need help.

SHEA: LeWitt himself compared this process to a musical performance, as he told the Smithsonian Institution in a rare 1974 interview.

(Soundbite of vintage recording)

Mr. SOL LeWITT (Artist): Every time you hear, you know, the same Bach piano or harpsichord thing it's the same. Whoever does it, leaves their mark on it. And in a way, it's good that the draftsman has a part, not just the artist doing it, but it's a collaboration.

SHEA: To make the collaboration work, Anthony Sansotta and LeWitt's other assistants followed detailed instructions.

Mr. SANSOTTA: Pretty much like an architect or a composer, we had either a set of working drawings or a set of notes.

SHEA: And while there is room for slight variation, those notes are more like algorithms or complex formulas.

Mr. REYNOLDS: A circle whose diameter is determined by the distance between two points, the first point is located where two lines would cross. If the first line...

SHEA: That's Jock Reynolds again. The director of the Yale University Art Gallery says his friend, Sol LeWitt, was content to remain in the background and was anything but a diva.

Mr. REYNOLDS: He was a fantastic human being. I mean, this was a guy who didn't want to spend a lot of time going to openings or playing the scene. He spent practically all of his time simply working, day in and day out, in his studio.

SHEA: Early in his career, LeWitt labored over basic lines, often thousands of them: vertical, horizontal, diagonal. He introduced primary colors and their combinations, then geometric and isometric shapes, inks, washes, arcs, grids, waves - many looking rather logical, some pretty trippy. Upstairs in what's called the Scribble Chamber, professional assistant Michael Benjamin shows the pencil work LeWitt was doing at the end of his life.

Mr. MICHAEL BENJAMIN (Professional Assistant, Sol LeWitt: A Wall Drawing Retrospective, MASS MoCA): What we have to do is scribble with pencils and scribble and keep scribbling until you get the gradation from black - very, very black - all the way to light. And it creates a three-dimensional feeling once it's done.

SHEA: So how long does it take?

Mr. BENJAMIN: There are days where the scribbling will render maybe two square feet. Right now, you're looking at nine foot by nine foot. It's a very obsessive process.

SHEA: But Benjamin, like everyone working on the MASS MoCA project, admires LeWitt's M.O.

Mr. BENJAMIN: I love the idea of art being accomplished by a group of people. It gives more respect to the art itself, and it kind of takes the focus away from Sol, too, because he's not actually there doing it.

SHEA: But in a way he is here, according to Jock Reynolds. Sol LeWitt died of cancer last year. And while the 78-year-old artist passed away before this retrospective began to go up, Reynolds says LeWitt didn't need to see the finished show. In fact, he says, the artist rarely visited any of his installations because he already saw the work complete in his mind.

Mr. REYNOLDS: He knew what this would look like. I feel very certain in knowing that he went, you know, went to the beyond knowing that this was going to happen, how it would look, and he knew it was going to get done, and he knew it would be here for a long time.

SHEA: Twenty-five years, a long time for any show, but especially for one spot-lighting Sol LeWitt because at the end of most exhibitions, his painstakingly executed wall drawings are painted over. But here at MASS MoCA, Sol LeWitt's legacy will span a generation, leaving future museum directors, curators, and artists to decide where LeWitt's work fits in 2033. For NPR News, I'm Andrea Shea. Transcript provided by NPR, Copyright NPR.