Treatment of The Triumph of the Winter Queen: Allegory of the Just, dated 1636, by Gerrit van Honthorst (Dutch, 1590–1656):
With cleaning complete, conservators can begin filling in all the losses of original ground and paint. The first step is the formulation of the fill material. A traditional gesso is made by slowing adding sifted chalk to rabbit skin glue until no more chalk can be absorbed. Dry pigments are then added to the mixture to approximate the light gray color of the original ground layer. Below are the materials used, with the finished gesso in the smallest bottle at left.
To prevent drying, the fresh gesso is kept warm on a cup warmer or in the hands of conservators as they work. A fine brush is used to apply the gesso into the losses. In many cases, multiple layers are necessary so that the gesso is built up high enough to match the thickness of the surrounding paint.
After the gesso hardens and dries, conservators use scalpels to make the fills level with the surrounding paint. To match the texture of the original paint, tiny dots and lines of gesso are brushed on and cracks are carved in the fills.
The following two details of the painting, from an area covering the leg of the youngest child, Gustavus Adolphus, to that of the lion, show the results of filling losses with toned gesso. The losses are unfilled at left; at right is the area after filling. In some cases, the gesso almost disappears as it is so close in tone to the surrounding paint.