Beneath the Varnish: Justus Sustermans’ Portrait of Cosimo III de’ Medici as a Child

Halfway through her fellowship at The Phoebus Foundation, Brazilian paintings conservator Aline Assumpção from São Paulo is restoring Portrait of Cosimo III de’ Medici as a Child, a delicate work by Flemish master Justus Sustermans (1597–1681). Dating from 1649, when Cosimo was just eight, the portrait reflects Sustermans’ role as court painter to the Medici family and his ability to fuse Italian and Flemish style. Here, he captured the young Cosimo in a tender play of light and shadow.

Conservator working on the child portrait of Cosimo III de’ Medici by Justus Sustermans (1649), during a detailed conservation treatment in The Phoebus Foundation’s studio.

The painting presented a heavily oxidised varnish that had significantly darkened the composition and obscured the nuances and contrasts of colour. Retouchings and additions bore witness to earlier restoration campaigns. The work had also been reinforced with a lining canvas, which altered its original dimensions through the addition of six centimeters of linen along the lower edge. As a result, the proportions changed slightly and the composition was extended. This may also have prompted the reinforcement. Impressions from previous stretcher marks reveal that the painting was once stretched on a smaller frame.

Front and reverse of the child portrait of Cosimo III de’ Medici by Justus Sustermans (1649), prior to conservation, showing yellowed varnish on the front and a lined canvas with a distorted stretcher on the reverse.

What is curious about this glue-based lining is that it consists of four piece of linen sewn together, forming a kind of patchwork structure. The reason for this approach remains unknown, although it is possible that a canvas of the desired size was not available at the time. Since the support showed evident deformations, a structural intervention was necessary.

The lining consists of a fine and tightly woven linen, in contrast to the original canvas, which has a much coarser weave. This structural tension between the two, together with the visible seams and the inadequate attachment to the stretcher, appears to have caused further distortions.

Detail of seams in the lined canvas on the reverse of a painting, visible prior to conservation, showing traces of earlier reinforcements and ageing of the linen. — Seams in the lining canvas

Detail of the edge of a painting, showing the difference in weave structure between the original canvas and the lined canvas. — The difference in weave structure between the original canvas and the lining

Cleaning the aged varnish layers and earlier retouching required great delicacy. The original canvas was loosely woven, and the paint layer revealed a fine craquelure and a porous structure. As a result, solvents could easily penetrate the canvas and cause the material to stiffen. To prevent this, a controlled and superficial cleaning was carried out, dissolving the varnish only at the surface without allowing it to pass through the canvas.

For the cleaning, PVA-Borax gels enriched with agar were used, forming a subtle double-network system. This allowed for a highly controlled and superficial action of the solvent, softening the varnish layer gently without penetrating the canvas. In combination with Evolon, a fine microfibre textile that helps regulate the solvent’s activity, the yellowed varnish could be removed gradually. The result is a clearer image, revealing the decorative details in the sitter’s clothing and the delicate subtleties of the artist’s brushwork.

Detail of the child portrait of Cosimo III de’ Medici during conservation, showing the use of a PVA–borax–agar gel for the controlled removal of varnish from the painting surface. — PVA-Borax/agar gel during varnish removal

Sequence of images showing a PVA–borax–agar gel gradually changing colour during conservation as dissolved varnish is absorbed into the gel matrix. — The gel gradually absorbing the dissolved varnish into its matrix

Comparison of the child portrait of Cosimo III de’ Medici before and after varnish removal, with yellowed varnish on the left and restored colour, detail, and clarity on the right. — Before and after varnish removal

During the preparation of the treatment, we had the opportunity to consult with Simon Bobak, London-based paintings conservator specialising in structural conservation. During his visit, we discussed several ongoing cases in the studio and the specific challenges each work presents.

For the treatment of the Portrait of Cosimo as a Child, it was decided to address the problem as locally as possible, with minimal intervention as the guiding principle. The upper edge of the canvas was found to be secured by only two nails. Through the careful use of weights and gentle humidification, the planar distortions were gradually reduced and the surface stabilised. The upper edge was then released from the stretcher and strengthened with a narrow strip-lining. The deformations have since improved, and this precise local intervention may well suffice to restore the painting’s structural integrity and visual coherence.

Comparison of the child portrait of Cosimo III de’ Medici before and during a local treatment to reduce deformations, with the upper image showing the condition prior to intervention and the lower image showing the painting during treatment under raking light. — Before and during localised treatment for removal of deformations (raking light)

The restoration is still in progress. After cleaning, a protective varnish will be applied to isolate the pain layer before the next stages of treatment, such as the the filling of losses, the pictorial reintegration, and the final varnish that will complete the process.

Stay tuned on our website and newsletter for the next chapter of this restoration!