The next time you visit London, take a few moments to go the National Gallery and locate this small jewel of a painting. It’s as exquisite as the finest Fabergé egg, and as minute in detail. No one knows who painted it, although it’s most probable that it was made in France towards the end of the fourteenth century. In this article, I’ll consider how the Wilton Diptych was created.

This painting was a luxury object intended from the outset for the personal devotions of a monarch, or someone of close rank and stature. Its interior shows on the left, King Richard II (its most probable owner) kneeling as he is presented by the three saints, Saint John the Baptist (carrying the Lamb of God), Saint Edward the Confessor (holding the ring he gave to Saint John the Evangelist), and Saint Edmund (holding an arrow from his martyrdom). On the right is the Virgin Mary holding the Christ Child with a throng of eleven angels, one of whom bears the standard of the Cross of Saint John.

It was painted on two small panels of oak wood using egg tempera, in a workshop which was clearly very experienced at making such works. Each panel is made of one wider board and a narrower strip. The two parts of a panel were joined by a craftsman using simple butt joints and were glued together with such care that the joins are almost invisible. They started off about 2.5 cm (1 inch) thick, and were then carved down to form an integral frame with a recessed painting surface.

The two panels are hinged together using gilded iron fittings, so that the completed diptych could be folded shut for portability.

To prepare the panels for painting, the bare wood was first covered with a thin layer of parchment, then over that a single layer of gesso was applied. This was composed, as was traditional, of natural chalk and animal-derived glue. The gesso extended over the frame mouldings to prepare them for gilding.

As the origin of the diptych remains a mystery, there are no surviving drawings or studies, but the paintings would undoubtedly have undergone careful development, possibly even requiring the approval of the patron. When the artist was ready, they then made a detailed outline on the gesso surface using any combination of black chalk, metalpoint, and brushed paint or ink. Although the final painting has many small changes relative to this underdrawing, the overall designs were followed quite closely.

Much of the surface of the panels was then to be gilded. Those areas were first marked out with incisions into the gesso ground, then covered with a thin layer of red bole (clay), containing animal-derived glue. The gold leaf was then applied with dilute glue in water, and after a couple of hours the gold leaf was burnished into place. These gilded areas were then patterned using a range of different punches. The resulting effect is of a jewelled surface, with intricate reflected patterns from different sections of the gilding.

Some details used a different method of gilding, known as mordant gilding, in which a binding medium is applied to give low relief, and the gold leaf applied onto that without burnishing. The optical properties of unburnished and burnished gold generate additional surface effects.

The first paint to be applied was an undermodelling layer of dull green earth (with lead white) as the base for all flesh, and probably some tonal modelling such as charcoal black for dark areas. All paint used appears to have been egg tempera, freshly prepared each day using the yolk of freshly-laid chicken eggs, pigment and a little water.

Michelangelo’s abandoned ‘Manchester Madonna’ shows this process well, with different sections ranging from the terre verte underlayer and bare gesso through to fully-modelled flesh.

Paint was applied using fine brushes and a slow build-up of hatched strokes. Egg tempera becomes near-dry in a matter of seconds, and there is little or no opportunity for correction or reworking, particularly when working in such fine detail. As thin layers were built up, glazes were applied to enrich the colours and optical properties. Unfortunately some of these thin glaze layers have worn away over the last six hundred years.

Details of jewels and similar objects such as the white hart brooches were raised using thicker areas of lead white, to give the impression of enamelling. Coupled with mordant gilding, they mimic the three-dimensional form of jewels and act as point reflectors of light, sparkling as if they really were gems in the paint layer. Although it has been suggested that these details may have been added using oil paint, no evidence of that has yet been found.

The finest strokes of paint seen here are less than 0.5 mm across.

Yet we don’t even know who painted this, or where.

Reference

Dillian Gordon, Ashok Roy, Rachel Billinge, Caroline M Barron, Martin Wyld (2015) The Wilton Diptych, Yale UP for The National Gallery. ISBN 978 1 857 09583 8