Everything flows, the philosopher knows, and yet museum directors of all countries wish that activist tomato juice would never flow over their precious paintings, nor should they liquefy themselves. In the case of some large formats by Pierre Soulages, the "painter of Outrenoir", who died in October last year and who only created panels with different depths of black texture, exactly the latter has now happened. The painter's preferred colour, a simple painter's black, has returned to its liquid state after about sixty years – i.e. after sufficient time to dry. From the depths of the pictures, drops of paint have recently come to the surface in several places, as if an oil spring were gently bubbling up in the background of the picture. The Soulages specialist Pauline Hélou de la Grandière even speaks of a "river of colour", which sometimes extends several centimetres across the canvas.

The conservators at the Centre National de la Recherche Scientifique in Paris, the equivalent of the Max Planck Society, are still at a loss as to what could be the cause of this liquefaction. All that is known so far is that all the pictures in question were painted between December 1959 and March 1960. According to the restorers, a prosaic, man-made, almost anthropocene reason could have been the extraordinarily strong sulphide pollution of the air in the lignite-fired capital during the cold winter at the time, so that the sulphur could have reacted chemically with the cheap black, which was always purchased from the same dealer at Soulages during this period.

More mystical contemporaries, on the other hand, may be more inclined to think of the miraculous liquefaction of the blood of Saint Gennaro in Naples, which recurs regularly and is interpreted as a positive sign – woe betide the city on the volcano if the red miracle substance in the viola no longer liquefies! Madonnas bleeding red paint and sweating icons of Christ have also been attested in Byzantine and Catholic art almost since the beginning. Substance sweating found its way into the world of popular images at the latest when the statue of a black Christ secreted blood and water in Madonna's epochal music video "Like a Prayer" – and even came to life afterwards. We don't know what will happen to Soulage's panels.

Until scientifically valid proof of the actual cause of liquidation, the beautiful sentence about his art by Soulage, who died at the biblical age of 102 and painted almost to the end, may claim validity: that the public cannot wander into the artificial worlds with his eyes in his paintings, as in those of other painters, but that they rather meet the viewer. And recently even literally flow towards it.