Rouen Cathedral

This composition is painted extensively in gouache and some transparent watercolour washes applied to blue paper. For the sky, reddish ultramarine washes and very thin applications of gouache combine with the underlying blue paper to depict a very bright day. The cathedral is largely painted in small, thickly applied localised brush-strokes of gouache that are placed so that the blue paper left visible functions like blue brush-strokes. The building on the left was created in a similar manner, but with transparent washes of brown earth pigments instead of gouache. Both materials can be seen in the crowd in the foreground, as well as a range of others: ultramarine, Mars red (a manufactured earth colour, brighter than the natural ones), yellow ochre and emerald green. X-radiography of the sheet confirmed the presence of lead white in all the gouache, as well as emerald green. Both these materials are very opaque to X-rays, while the other possible materials available to Turner in these colours are not. Turner used emerald green extensively in his oil paintings, and was the earliest adopter of this pigment in watercolour. Later it would be used extensively by the Pre-Raphaelites in both media.

X-radiography is good way of monitoring the survival of the lead white-based gouache, which Turner used with very little gum water to bind it together. Here, it is applied so thickly that it is in danger of flaking off, since the paper support flexes whenever the work is handled. Tiny cracks could be detected by this means, and treated locally by paper conservators to ensure that they are fixed in place.