Investigating Lead Soaps in Oil Paintings with Chromatography

Old paintings and artworks are socially important as well as pieces of art — they are a window to a life in the past. With no film or photography in the sixteenth century to capture what life was like, we rely on paintings and the written word to provide a glimpse of history.

Conserving the past

It is important to preserve artworks for future generations — the job of art conservationists. They face many challenges, but museums can help and now the environment is carefully controlled around precious works — even controlling the amount of ozone in the air can affect a conservationist’s work as reported in: Conservation Secrets Revealed in Ancient Warship.

We can see the deterioration caused by too much light or a dusty environment — but what about when the deterioration is due to the painting itself? One example of this is “lead soap”, which has been identified and studied modern analytical techniques including chromatography.

Painting and Inclusions

An old oil painting is a complex mix of several elements. The paint can be an inorganic or organic pigment mixed with a binder to help spread and dry the pigment, and this is applied in several layers to a support which can be canvas or wood. Over this there is often a protective organic layer such as a varnish. All of these elements can affect the stability of a painting.

Any deterioration in one layer can trigger further deterioration due to chemical reactions taking place at the interfaces of the different layers. One of the reactions caused by deterioration made translucent white lumps — known as inclusions — that form under the surface of the paint and cause a mottled or grainy effect on the surface. It used to be thought that the mottled effect was a feature of the painting, perhaps used by the artist to give some texture to the work. But detailed analysis with modern analytical techniques has revealed the inclusions to be soap. How does soap get in an old master?

Soap Formation

Soaps are salts formed when fatty acids react with metal ions — but where do these compounds come from? The metal ions — in this case lead — come from the pigments used in the paints, with ‘red lead’ and ‘lead-tin yellow’ being examples of pigments containing heavy metals. The binders and protective coatings used in oil painting contained glycerides, which can undergo hydrolysis to fatty acids.

Analysis of the inclusions using gas chromatography-mass spectrometry has confirmed the presence of lead carboxylates — lead soaps —of stearic (C18) and palmitic (C16) fatty acid chains. Soap formation is not restricted to paintings, with copper and zinc soaps forming on the surface of old metallic objects treated with animal fats or oils.

The formation of inclusions in paintings is not fully understood yet. What triggers the process and how the reactions proceed are still unknown. But GC-MS is helping curators understand the formation of lead soaps and hopefully will allow conservationists to limit the damage — so future generations can enjoy Rembrandt’s work for years to come.