63 The Holy Cross retable Conservation strategy The conservation treatment had to deal with the severe lifting and flaking as well as the considerable losses in the gilding and the polychromy due to the hygroscopic response of the support. Surface cleaning was the obvious first step. Wood consolidation was necessary only in limited areas, the majority of the wooden structure was very well preserved and stable. The aim of the treatment was to stabilise the construction, polychromy, and gilding, and to enhance the aesthetic perception. In general, the approach follows the concept of minimal to moderate intervention as employed for the treatment of the opposing “Annunciation” retable. The deformation of the “Arma Christi” panels with extreme flaking of the paint layer posed a major challenge for treatment. There, big sheets of Japanese paper and strong glue had been applied in the course of an earlier conservation treatment. Shrinkage in the paper and warping of the wooden support caused further detachment of the paint layer. The conservation treatment started with consolidation of painted and gilded surfaces, hand in hand with surface cleaning. Several consolidants had to be used and specialised techniques for their application had to be developed depending on the problem at hand. The deteriorated (insect-eaten) wooden support of some parts of the altarpiece was consolidated as well.50 Wood conservation Only few damages by wood eating insects needed consolidation with Plex‐ igum PQ 611.51 First, a 5% solution of the resin was injected into holes and gaps to “wet” the wood. Then, a 20% solution was applied with a syringe. Bare wood with the need for consolidation was first treated with a 5% skin glue in order to saturate the wood surface and to prevent a discolouration by the application of the Plexigum resin. Paint layer consolidation A 10% solution of sturgeon glue was used to consolidate the blue paint layer (smalt), as the thick ground layer required high glueing power and staining had to be avoided. The consolidation of the friable gilding was executed with Plextol B 500 dissolved in acetone and water (Plextol B 500 : acetone : water = 2 : 2 : 1).
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