Vandalised Rothko painting back on display at Tate Modern after 18-month repair job

A Mark Rothko painting, which had been seriously vandalised with graffiti, has been placed back on public display at Tate Modern after a remarkable 18-month restoration operation which has removed virtually any visible trace of the attack.

Wlodzimierz Umaniec defaced the 1958 painting Black On Maroon and was jailed for two years for the crime in December 2012.

The vandal, who co-founded the artistic movement "yellowism", stepped over a barrier in the gallery and daubed his name and the words "12 a potential piece of yellowism" before fleeing.

Rothko donated Black On Maroon, one of his acclaimed Seagram murals, to the Tate in 1970. Sir Nicholas Serota, Tate director, said he feared that the “sickening” act of vandalism may have permanently scarred the work, which the artist produced from a complex combination of oils, resin, eggs and glues. Umaniec had used a quick-drying indelible black ink to cause maximum damage.

But the Tate’s “emergency response unit”, sent in within an hour of the vandalism, has succeeded in restoring the work after a painstaking conservation process, employing a special solvent made with the assistance of the Dow Chemical Company.

“The damage has been removed, and what you see is what Rothko painted,” Mr Serota said as he placed the 1958 work by the abstract painter, valued between £5-9 million, back on display, after the £200,000-repair job.

Security had been reviewed in the wake of the incident – there are CCTV cameras in the murals room and a knee-high barrier around the works - but the director did not want to turn the gallery into "Fort Knox or a prison".

“This is a gallery, not a prison. We want people to enjoy the paintings in the way in which Rothko meant them to be enjoyed,” he said.

Rachel Barker, paintings conservator at Tate, said the restoration process was so slow that on some days she was only able to remove less than an inch of ink at a time.

The indelible graffiti ink had penetrated several layers, in some cases soaking through to the back of the canvas.

The Tate had to find a chemical solvent which could remove the ink while limiting damage to the original paint.

A team spent nine months analysing microscopic samples of layers on the painting and whittling down hundreds of potential solvents until they found the right one - a blend of benzyl alcohol and ethyl lactate - before even attempting to restore the work.

Special test canvases were used to assess solvents and cleaning methods, while the Rothko family also donated a canvas for testing - one which the artist had primed with maroon paint at the time of the Seagram commission in the 1950s.

A further nine months was spent working on Black On Maroon, removing the majority of the surface ink before reversible conservation-grade materials were used to restore the painting's surface.

Ms Barker said: “I came to see these murals as a child. To play a part in caring for them is an extraordinary privilege, but it's my job as well. Yes, I was nervous. But I had a job to do.”

Umaniec, a Polish artist, acknowledged that his act of vandalism had failed. In a statement, he said: “I apologise to [the] British people for what I did. I suppose I wanted to change the art world but of course I did it in a very, very wrong way.

“I spent almost a year and a half in prison and the British people have paid huge restoration costs, so it definitely wasn't worth dong it, and I'm sure the restoration team has done a wonderful job and I encourage everyone to see the restored picture.”

Sir Nicholas said of the vandal: “I'm really sorry that he felt the necessity to do what he did and from time to time people make mad gestures of this kind. But art endures and these paintings will continue to be here for the next 50 or 100 years, I hope.”

The family of Mark Rothko praised the “thoroughness and dedication” of the Tate’s conservation team.