In oil painting, double bonds in the long carbon chains of the oil react with oxygen from the air as the artworks dry. This creates new chemical connections that stabilise the final, cured layers.

When conservators used X-rays to analyze Rembrandt’s 17th-century masterpiece “The Night Watch,” they discovered something unexpected under its surface: lead.

Apart from oil-based paint layers, some of the layers were made of natural resins, proteins, gums, and, in some cases, a resinous, varnish-like layer. Protein-based material can indicate the use ...