Why Is That Painting Yellow? A Protective Layer That Ages Badly

Varnish performs two jobs that are worth separating. It protects the paint surface from dirt, from handling and from atmospheric pollutants, acting as a sacrificial layer that can be removed and replaced while the paint beneath is not touched. And it changes the appearance substantially, filling the microscopic roughness of the dried paint surface so that light passes into the paint rather than scattering off it, which deepens the darks, saturates the colours and unifies the surface gloss across areas that dried differently. That optical effect is the reason painters applied it deliberately rather than merely tolerating it, and it means an unvarnished painting looks different from what its maker intended.

Materials fall into groups with quite different ageing behaviour:

• •Natural resins including dammar and mastic dissolved in a solvent, traditional and prone to yellowing
• •Oil-resin varnishes, older and darker still, which are extremely difficult to remove safely
• •Synthetic resins developed from the mid twentieth century, which yellow far less and remain removable
• •Matt and gloss variants, produced by adding material that scatters light
• •Application by brush or by spray, which affects thickness and evenness
• •Isolating layers applied between paint and varnish in modern practice, which makes later removal safer

The degradation is chemical and well understood. Natural resins oxidise on exposure to air and to light, forming new compounds that absorb blue light preferentially, which makes the layer appear yellow and then brown, and the same process makes the resin harder, less soluble and more brittle over time. The change is gradual and continuous, so a painting darkens over decades without anybody noticing, and viewers come to regard the darkened state as how the work looks. Accumulated dirt and smoke deposits add to it, and a layer of old varnish traps that dirt within it. The brittleness matters as much as the colour, since a hardened varnish cracks and can pull at the paint beneath as it moves.

Modern practice surrounds the decision with evidence rather than relying on judgement alone. Examination under ultraviolet light distinguishes older varnish, which fluoresces, from later retouching, which does not, and reveals the extent of previous interventions. Infrared imaging shows underdrawing beneath the paint. X-rays show the structure, previous compositions and damage. Samples smaller than a pinhead are taken from existing losses and sectioned to show the layers in order, establishing exactly what sits above what. Solvent tests are made in inconspicuous areas and recorded. Cleaning proceeds in stages with the work examined constantly, and frequently stops short of complete removal, leaving a thin layer that preserves some of the accumulated appearance. Everything is documented so that a later conservator knows what was done.

Cleaning is contested and the disputes have been public and bitter. The case for removal is that the yellowed layer is not original, that it misrepresents the artist's colours substantially, and that leaving a brittle degraded layer in place risks the paint beneath. The case against is that solvents strong enough to dissolve an aged natural resin can also affect the uppermost paint, that artists sometimes applied thin coloured glazes over the paint which are easily mistaken for varnish and destroyed, and that judgement about where to stop is unavoidable and irreversible. Major cleanings of celebrated works have produced sustained public argument, with critics alleging over-cleaning and conservators defending the evidence. The modern response is extensive analysis beforehand and documented reversible materials afterwards.

Varnish protects the surface and, by filling the microscopic roughness, deepens the darks and saturates the colours, which is why painters applied it deliberately. Natural resins oxidise into compounds absorbing blue light, so the layer yellows and hardens over decades. Removal is irreversible and solvents that dissolve aged resin can also lift thin glazes, which is why major cleanings are argued about.