Why old paintings look brown

Almost every oil painting made before the twentieth century was given a coat of natural resin varnish, and for good reasons: it saturates the colours, evens out the difference between matt and glossy passages, and protects the paint from dirt. What nobody could do anything about was the second half of the bargain. Natural resins yellow and grow cloudy, slowly and unevenly, and after a hundred years the picture is being seen through a filter the painter never chose.

The effect is not neutral across the spectrum. A yellowed varnish absorbs blue light most, so blues and cool greens go first: a sky turns green, a distant hill turns brown, and the cool half-tones in a face collapse into the warm ones. The painting does not merely get darker. It loses one end of its range while keeping the other, which is precisely the shift that makes so many pre-modern paintings look as though they were composed in amber.

Varnish can at least be removed. The pigments that departed cannot be brought back. The transparent reds that gave European painting its purples, pinks and warm flesh were lakes, and lakes fade — sometimes to nothing. Where a red glaze has gone, a violet robe reads as blue, a warm complexion reads waxy, a pink sky reads grey.

Van Gogh is the most thoroughly documented case because his materials are recent enough to be identified with confidence and his intentions are on paper in his letters. Canvases he built on a pink-and-green opposition now hang as white-and-green. The relationship he was actually working on is gone, and what remains is a composition that no longer performs the argument it was making.

Other pigments do the opposite and darken. Copper greens such as verdigris turn brown in the presence of sulphur; orpiment, a brilliant arsenic yellow, blackens on contact with lead white and copper pigments, which is why some sixteenth-century foliage now reads as soot. A single painting can be simultaneously fading in one passage and darkening in another.

The third mechanism is the least known and the least reversible. Oil paint becomes more transparent as it ages over centuries, and whatever lies underneath slowly comes up through it. On paintings built over dark grounds — Caravaggio's reddish-brown, the seventeenth century generally — the darkness that was meant to sit behind the image is now migrating into it. Shadow passages that once held a chair, a hand, an architectural edge have been eaten from below.

The same growing transparency is what makes pentimenti visible: an arm the painter moved and covered rises back into view like a bruise under the skin. It is a gift to art historians and a slow disaster for the picture, and it means the tonal structure of many famous paintings is no longer the structure their painters approved.

This is why picture cleaning generates public fights that appear, from outside, to be wildly out of proportion. When the Sistine ceiling was cleaned through the 1980s and 1990s and emerged in hard, acid pinks and greens, a substantial part of the objection was not technical but emotional: generations had learned Michelangelo as a sombre painter, and the sombreness turned out to belong to soot and glue.

The honest version of the debate is not "was the cleaning correct" but "which version of this painting have we decided is the real one" — the one that left the studio, the one that entered the canon in the nineteenth century, or the one that arrived in the gallery this morning. There is no neutral answer, because there is no untouched state to return to.

The last turn of the screw is that the accident became a style. By the nineteenth century, "old master tone" was something a painter could buy: a golden-brown key that made a new picture look venerable. Some painters got there with glazes; some used bitumen, an asphalt-based brown that never fully dries and has since destroyed the paintings it was used in, cracking them into wide, sunken fissures.

So the sequence runs: a painter uses the best materials available; time strips out one end of the palette and yellows the rest; a later generation admires the result and imitates it deliberately; and the imitation, made with worse materials, ages faster than the original. What we call the look of old painting is a composite of intention, decay and homage to decay — and the atlas that follows a single pigment across four centuries is, among other things, a record of what has quietly gone missing.