A close look to better understand Vermeer's painting technique
This type of investigation is a collaboration between the conservation, curatorial and scientific departments at the National Gallery and provides information about the materials Vermeer used and how he combined them in different ways. The research can also indicate when materials have changed over time, altering the appearance of the paintings.
The only blue pigment used by Vermeer in both National Gallery paintings is natural ultramarine. This pigment, extracted from finely ground lapis lazuli which came from Afghanistan, was extremely expensive and many artists only used it for bright blue areas so they could derive maximum effect from its intense colour.
We can actually see the pigment mixture by taking a tiny sample of paint from the skirt and mounting it in polyester resin to produce a paint cross-section.
This sample is very small, so must be examined under a microscope. The scale bar on the left hand side is 20 microns, or about a quarter of the diameter of a human hair, so it really is a very small sample!
Using a microscope, we can see that the intense blue pigment particles of ultramarine are mixed with the particles of green earth (and a few tiny particles of red earth) in the same paint layer.
The colour and shape of pigment particles can be quite characteristic, but their identification is usually confirmed using another piece of equipment; a scanning electron microscope fitted with a special detector which can tell us exactly what chemical elements are present in each particle within the sample.
The exact mechanism of this degradation process is complex and not fully understood, though there is some indication that ultramarine may interact in some way with an oil binder.
The deterioration is often most prevalent in areas where the blue was used alone or with only a small amount of added white pigment.
As these dark areas become lighter, the modelling (balance of light and shade) in the painting can be affected, becoming less obvious.
These protrusions can be seen in a Young Woman Standing at a Virginal, for example, in the painting-within-the-painting.
Vermeer has used thick dashes and dots of bright yellow to render the play of light on its gilt frame, but the beige spots visible in the brownish-yellow ochre base colour are small protrusions of disrupted paint and are not deliberate.
Analysis has shown that these have formed as a result of chemical reactions between the oil binder and lead-containing pigments such as lead-tin yellow or red lead and are therefore an unintended consequence of the materials used by Vermeer which would not have been present when he was painting.
In fact, similar protrusions can be observed on many paintings by a variety of artists.
If you would like to learn more about Vermeer and his materials or would like to find out more about the work of the scientific department, please visit the National Gallery website, where all of the topics mentioned here are explored in greater depth.