Treatment of The Triumph of the Winter Queen: Allegory of the Just, dated 1636, by Gerrit van Honthorst (Dutch, 1590–1656):
Cross Sections and Scientific Analysis:
During varnish removal, it became apparent that the painting had been restored several times in the past. Varnish removal from the figures proceeded quickly, revealing the skillfully applied and well preserved paint layers below. However the background was trickier – it had been cleaned less and so had more varnish layers present. Furthermore some areas seemed to be extensively covered with several layers of old repaint, making it difficult to evaluate the condition of the original paint underneath. Finally, varnish removal revealed that the appearance of a few of the colors, the blue in particular, had altered over time.
In order to help clarify some of these issues, cross sections are taken from several areas of the painting.
Using an ophthalmic knife and microscope, conservators remove very small chips of paint, about the size of a pinhead.
Each paint sample is embedded in liquid resin that dries and hardens upon curing. Below is the liquid resin in a mold (left) and a resin block with an embedded cross section (right).
After polishing the resin, conservators view the layers present in the cross section under a scientific microscope at 100 to 1000 times magnification. Cross sections appear as tiny “core” samples through the painting, with the varnish layer(s) at the top, then paint layers in the middle and the ground layers at the bottom.
In the below cross section from the painting’s brown background (200x magnification), two distinct layers of overpaint are visible, with varnish between the layers. At the bottom is a double ground layer (reddish brown with a thin layer of gray above).
The varnish layers are clear in the following detail of the top layers of the same cross section (500x magnification), viewed under ultraviolet light.
The background of the painting has been repainted several times over the past 300 years, resulting in a patchy, uneven appearance overall. The sample taken from the background points to at least two repainting campaigns, and is helping guide the removal of the majority of the overpaint.
Cross sections can also be viewed and analyzed in a scanning electron microscope, with which elemental analysis can be done on individual pigment particles, often identifying the pigments used by the artist.
To investigate why the main figure’s blue dress is flat and brown in some areas (mainly in the dark shadows) and not in others, conservators take a sample from a bright blue portion of the dress. The cross section below (200x magnification) shows a very thin layer of natural ultramarine blue over a thick layer of smalt mixed with lead white.
Ultramarine was a very expensive pigment in the seventeenth century, while smalt was quite cheap, so painting the bulk of the blue dress with smalt, then thinly glazing with ultramarine was an economical strategy. Smalt, a colored glass made with cobalt, would have been bright, deep blue when first applied. Unfortunately it discolors over time, and this is why the shadows of the dress have turned brown. Pure smalt was used in those areas, with no ultramarine on top.
This type of analysis gives conservators invaluable information both about how the painting was originally made and what has happened to it over time.