Conserving the Hildesheim portable altar
This portable altar is made of a combination of materials including gilded copper, wood, ivory, vellum, rock crystal and Purbeck marble. Before being put on display in the Medieval Europe gallery at the British Museum, its condition was assessed and found to be unstable.
A protective coating applied in the past to protect the gilt-copper sheet on the front from tarnishing was deteriorating, and some of the nails holding the front and back sheets to the wooden core had become loose so that a gap had opened between them. One of the crystal windows seemed to be slightly out of position and there was a danger that if it moved any further it might rub against and damage the painted vellum behind it.
A team of British Museum specialists in the treatment of the specific materials worked to preserve the individual components and make the altar suitable for display.
It was decided to dismantle the front of the altar to gain access to the wooden core, and other areas. Repairs were then made to the wood and the nails which mounted the front and back sheets. The gilded copper front and sides were cleaned and the deteriorated protective coating was removed.
During cleaning of the ivory plaques traces of gold, silver and black pigment were revealed. The misaligned piece of crystal was repositioned and the pieces of vellum were supported with inserts of acid-free paper. A small piece of folded, unbleached linen was also inserted behind the paper to help support the loose crystal. This matched the arrangement under the other crystal, where an old piece of textile was still present. Finally the front was remounted with the nails.
The portable altar is now stable and in a condition suitable for display with all of the many components being appropriately treated to ensure their future preservation.