Uncovering Art’s Mysteries with Chromatography

Eggs, glue and vegetable oils are three of the things that art conservationists find in paintings — and these are not objects painted by Rembrandt or Van Gogh (although his Sunflowers might yield a small drop of oil). They are compounds that the artist used in the manufacture of his paint — specifically they bind the pigment together to produce the finished paint. Modern analytical techniques — including chromatography — help conservationists and other researchers to understand how a painting was made, what materials were used and what restoration it has undergone previously. This knowledge can allow them to repair and protect valuable paintings — it can also help to detect fake paintings. Let’s take a look at how chromatography can help peel back the layers on art.

Anatomy of a Painting

Old paintings generally consist of three layers — canvas, paint and varnish — and to make sure a painting can be preserved, the conservationist needs to know the make-up of each of them. Chromatography is an ideal tool to aid researchers in their identification of the various components used in each of the layers.

Varnishes are applied to paintings for two main purposes: as a protective coating or to add an effect to the finished painting — although not all artists like to add varnish as discussed in this article on Van Gogh. An outer clear varnish can stop atmospheric pollutants and chemicals causing deterioration to the surface layers of a painting through chemical reactions. The types of varnish changed through the years and a restorer cannot rely on the age of the painting to determine the varnish type as the painting may have been cleaned or restored previously — so chemical analysis is necessary.

Gas chromatography coupled with mass spectrometry (GC-MS) is often used to analyse varnishes and outer coatings on paintings.

Binding the Pigments

Paint is made of two main ingredients — pigment for the colour, and binder to hold the pigment together and allow the paint to spread and dry. There are many different binders used, and they are generally organic in nature: this is where the egg yolk, glue and oil come in — they have all been used as binders in works of art.

Pigments are the components that give colour to the paint — and are generally mineral or organic in origin. Natural organic pigments include the colours obtained from plants and animals; whilst alchemy gave the artists artificial inorganic pigments including metal based pigments that gave rise to the problem known as ‘lead soap’.

Separating the Layers

Chromatography is the science of separation, and both HPLC and GC have a role to play in analysing art. Alongside GC-MS, the technique of pyrolysis GC-MS is used to identify the components in varnishes and binders. Pyrolysis is a way of thermally decomposing the larger sample molecules using a hot filament to produce smaller volatile molecules that can be analysed using GC-MS.

HPLC is used by art conservationists to analyse the organic molecules from naturally produced pigments that are non-volatile.