Through thick and thin! Stabilizing the Plaster on Minirdis’ Coffin

Conservator works on an Egyptian coffin.

Conservator Morgan Nau stabilizes plaster fills with adhesive injected with a syringe. Photo credit: JP Brown. Object number 111517.

After cleaning, the first part of Minirdis’ burial equipment that we treated was his coffin.  The coffin was constructed of wood panels joined with wood dowels. A layer of an orange colored plaster like material had been applied over the wood to fill gaps between the wood panels and provide a smooth surface. On top of the plaster layer, the coffin had been painted black with red and yellow decoration.

a3627_111517_axialsectionshowingwoodgrainanddowels.jpg

CT cross section of the coffin (with mummy inside). Biscuit dowels can be seen at the top and bottom joining the planks. The bright white areas on the interior and exterior sides of the wood is the plaster-like fill material. The grain of the wood is visible on all of the planks and it can be seen that the sides are from the same piece of wood that has been split. Rendering by JP Brown. Object number 111517.

In our condition assessment we found many areas that needed to be addressed on the coffin before it could go on tour, both structurally and visually, and now that it was clean we could really see what was going on.

Structurally, we found that the wood panels had separated. This led to gaps between the panels – in some areas the gaps were so big that one of the long sides of the coffin bottom was falling off! The plaster had become crumbly and was easily dislodged by the gentlest touch.  Every time the coffin was moved, small piles of plaster crumbs were left behind. The plaster needed to be stabilized before any other work could be done.

overall_and_detail_crumbling_plaster.jpg

Interior of the bottom half of the coffin, showing the continued loss to the plaster fill material and paint layer. Photo credit: Morgan Nau. Object number 111517.

Fixing the plaster was much trickier than we expected. The plaster had swollen and crumbled, and it no longer had any firm bond to itself or to the wood planks. Our treatment had to address all of the plaster, from the exposed surface down to where it joined with the wood several centimeters below the surface, and everything in between. It took a lot of experimentation to find a solution to this problem.

In the end we found that the key was varying the dilution of the adhesive, starting with a relatively thick outer layer.  A more viscous form of the adhesive was applied on the exposed surfaces of the plaster. Once this adhesive set the plaster had a firm outer crust. With the crust in place it was possible to insert a fine syringe needle through the crust and into the body of the plaster. A thinner solution of the adhesive could then be injected; the lower viscosity of the injected material allowed the adhesive to penetrate deeply and secure the plaster to the wood and to itself.

When the adhesive set the plaster was significantly more stable and the object could be handled and moved without risk of further damage. As a precaution, however, a barrier layer of tissue paper was adhered on top of the plaster. This way if any of the plaster did become dislodged it would still remain in place until it could be readhered by a conservator.

plaster_barrier_ba_large.jpg

Top image – Plaster fill on interior of top side of coffin top, before treatment. Bottom image – Plaster fill after consolidation with adhesive and application of supportive tissue barrier. Photo credit: Morgan Nau. Object number 111517.