Why might the images have been concealed?
The back panels show further religious scenes that include the Rinck family, intended to be visible when the triptych was closed. Covering these images would have prevented the identification of the Cologne patrons through the presence of their coat of arms. It also succeeded in obscuring possible information about the altarpiece's origins and history.
It is possible that the altarpiece's exporters in the 19th century might have deliberately concealed the scene to ease its transfer out of the country.
Restoration of the uncovered panels
Once the images had been revealed, the heads of the donors appeared to be the worst affected by previous damage and wear. The damaged areas were restored using thin glazes of pigment in a resin-wax medium, soluble in white spirit. No attempt was made to restore the section of the painting that had probably been burned by an altar candle. This is still visible today and can be seen on the back of the left wing, the Mass of St Gregory.
In Western and Northern Europe wood was the most widely used painting support until the emergence of woven canvasses in the 17th century. In early Renaissance panel painting the frame was an important part of the image, often an integral part of the whole construction. Frames, especially if elaborate, were expensive, sometimes costing almost half the amount of the altarpiece itself.
Due to the complexity and size of polyptychs, they were often divided into separate units and assembled on the altar once the painting and gilding was complete. In the 16th century, altarpieces were less likely to be free standing and were instead incorporated into the architecture of the church. The design and construction of the panels became more complex.
In the 17th century, wood panels continued to be widely used in Flanders and Holland for landscapes, portraits, genre scenes and devotional works. The Walker's triptych is most likely made from oak.
Constructing a panel painting
The choice of wood used to make a panel painting depended largely on its availability. In Germany artists used a wide range of timbers including oak, beech, chestnut, walnut, lime and cherry. In Italy, poplar wood was commonly used. It was important to choose woods with good properties of even grain, stability and durability. The method in which planks of wood were cut from the logs also affected their quality. Arrangements for the construction of an altarpiece were often made before the painting had even been commissioned.
Problems conserving panel paintings
The structure of panels can cause a variety of problems for the conservator, particularly if they have joins. In panels with one or more planks, it was important to construct them with the grain of the wood running in the same direction in each plank. If joins begin to fail, these often have to be re-aligned and re-glued by the conservator.
Larger altarpieces could be quite heavy once installed, and their weight can cause problems. Also, struts or battens attached to the reverse of panels can cause large stresses in the support, often resulting in splits. This will also lead to surface problems in the paint layers. The response of the panel to changing levels of moisture in the air (churches in northern Europe are often cold and damp) could result in splits or cracks across the surface and cause flaking and losses to the paint and ground. Some splits and cracks were found during the examination and conservation of the Walker's Altarpiece in 1962-63.
Surface preparation of a panel painting
A ground layer of gesso (calcium sulphate and animal glue) was usually applied to a panel to give a smooth painting surface. Sealing the wood surface in this way also helped to reduce the shrinking or swelling of the wood in response to moisture changes in the air. However, if only one side of the panel was coated with a ground layer, the uncovered reverse side was likely to lose or absorb moisture at a different rate, often causing the wood to warp.