The sculptor Anthony Caro (left) and his longtime assistant and studio director, Patrick Cunningham, in 2012 working on 'Wandering,' a piece from Caro's 'Park Avenue' series. John Hammond/Courtesy of Barford Sculptures Limited

I've been trying to sort out my memories of Anthony Caro, a nearly impossible task, given the four decades that I was fortunate enough to enjoy his friendship and to spend time with him in his studio in London's Camden Town district. Time there was always thrilling—seeing new work in progress or seeing the final version of sculptures last encountered in intermediate stages.

While Tony was invariably pleased when you were excited and enthusiastic about particular works, he was far more interested in the problematic, recalcitrant ones. And he was always eager to respond to suggestions from fresh eyes—respond immediately, that is, by working on the sculpture. Patience was not Tony's strong suit. (When he said he'd pick me up to go to the studio at 10, I knew I had to be ready at 9:30.) Nor was he interested in imagining alternatives. He always wanted to see what would happen right now, in the actual materials and at real scale, even though those materials were large pieces of industrial steel or cast iron that were difficult to manipulate. "Let's try that," he'd say, and one of his young helpers would be summoned to take something off a sculpture or hold an additional element in place for consideration.

Especially after everyone's favorite toy, the miniature fork lift, became part of the studio equipment, large pieces of steel would be dangled in midair against an evolving work, to test where they might take their places. "No. I don't like that at all. Take it away," Tony would say. He always reacted quickly, trusting his unmediated intuition about what he was doing. The most encouraging thing you'd hear was "It's getting clearer." But reaching that state of perfect clarity—a process Tony called "taking away everything that isn't necessary to the sculpture"—could take a very long time. Hence the sculptures with dates stretching over years.

One of the most remarkable moments I experienced in the studio was something probably commonplace for Tony. We were walking from the big studio to the smaller space, near the office, past the big pile of steel that occupied one of the parking spots in the courtyard. Tony suddenly stopped, jabbed at a distinctively shaped piece of metal that I half-recalled having seen many times before, and shouted to his longtime assistant: "Pat! Pat! I know what I want to do with this." Tony went by that pile several times every day, usually, it seemed, without concentrating on it. Yet he was clearly taking in whatever he saw and thinking constantly about sculpture. (The piece in question found its way into an evolving work later that day.)

Time spent in the studio was obviously pure joy and amazingly energizing for Tony. A few years ago, at dinner, the talk turned to the effects of time. At one point Tony, who was then in his mid-80s, said, "When I'm in the studio, I always feel fine." He always seemed tireless and wholly focused there. For the visitor, though, time in the studio was hard work. Your eye and your ability to articulate your responses were constantly being tested. But I learned more about how to look at sculpture and how to think about it in Tony's studio, over the years, than almost anywhere. I owe him a lot.

Tony always surprised me. When "The Last Judgement"—his complex, multipart meditation on the disasters and upheavals of the 20th century—was installed in the Antichi Granai in Venice, as part of the 1999 Biennale, I kept going back day after day. I knew that I'd seen the various components many times before, as the individual sections of the work evolved in the studio, but I still felt that each encounter with these strange, brooding tableaux was a new discovery.

I had the same response to the monumental, athletically poised "Park Avenue" series, which I first met about 2010 when they were trestlelike structures, part of a proposal for a huge public installation in New York. I'd seen the trestles translated into quarter-scale steel models for the vast project, and seen them pulled out into the courtyard, to be photographed. That day, you could see Tony's impatience at having to deal with something at one remove, not full size. His desire to turn the quarter-scale steel trestles into "real" sculptures, unconstrained by the practical necessities of enlarging them or of engineering, was palpable. And he did just that over the next two years, using the trestles as provocative stimuli and utterly transforming them into one of his most potent series, with a remarkable combination of refinement and strength, delicacy and fierceness. The exhibition of these generously scaled works at Gagosian Gallery in London last summer was a triumphant penultimate act of a long career. (A last series of steel and plexiglass sculptures will be shown in London in the fall.)

My memories of Tony form a kaleidescope of images in a surprising number of locations: the Tate Gallery; the Yorkshire Sculpture Park; the Museum of Modern Art; the Metropolitan Museum of Art; the ruined patio of a former orphanage in Barcelona, with Tony making sculptures out of derelict balcony railings; and more. But of all of them, it's those long, rigorous, stimulating days in the Camden Town studios that I will miss the most. We will always have the sculptures. But I'm having a lot of trouble getting used to the world without the passionate, vital force of nature who made them.

Ms. Wilkin is a critic and independent curator. This article is adapted from remarks delivered at a memorial to Anthony Caro, who died last October, held at the Metropolitan Museum of Art on March 28.