Microexcavation
Alkham Bowl
The British Museum, London, UK
One of my favorite treatment projects from the British Museum was copper alloy fittings for an Iron Age wooden bowl (1990,0701.24). The bowl was excavated in 1989 from Alkham, Kent along with three copper alloy stave buckets. Despite having similar decorative techniques such as repoussé and a pinched rim, this object has no hoops and is possibly a turned wooden vessel.
What is left of the copper alloy fittings is mostly mineralized. During the original lifting, archaeologists heavily consolidated the fragments to the dirt with HMG, cellulose nitrate. As a result, consolidated dirt obscures the beautiful surface decoration, much of which is still intact.
My treatment goals were threefold: 1. to excavate pieces of the bowl from the dirt; 2. to make the fragments more easily handle-able and stable; and 3. to create a supportive mount, allowing the bowl to be visible while in storage and more easily handled by researchers.
See below for more details of my treatment process.
Before Treatment
My supervisor, Hayley Bullock, Senior Metals Conservator, had began treatment on the bowl, removing much of the bulk of the consolidated dirt. The consolidated dirt closest to the mineralized copper alloy had been left in place, awaiting further treatment.
Scanning Electron Microscopy
I found mineral-preserved wood underneath the copper, which was identified as ash (Fraxinus excelsior) by Dr Caroline Cartwright, British Museum.
SEM Image by Dr Caroline Cartwright, showing backscatter electron view of a radial longitudinal section of the wood
Treatment Process
All treatment was undertaken under the microscope at 1.0-2.0 magnification. To remove the consolidated dirt from the surface, I gently swabbed each piece with a combination of IMS (denatured alcohol) and acetone, then lightly brushed with a scalpel to remove (A). I removed unconsolidated dirt from the reverse of the pieces by softening it with 50:50 IMS and deionised water then gently scraping with a scalpel.
Throughout the removal process, some pieces crumbled and broke. I re-adhered them with 25-30% Paraloid B-72 w/v in acetone. I then reinforced the breaks with small tabs of Japanese Tissue Paper impregnated with Paraloid B-72 (B, before reactivating adhesive).
At the end of the treatment process, some dirt and pieces of flint remained on the objects (C). I left this in place due to the extreme fragility of the copper alloy; further excavation risked breakage. The pieces were housed in a bespoke mounting system inside a polyethylene box. I shaped cutouts into Plastazote (polyethylene) foam to cradle the fragile pieces and lined the cavities with Relic Wrap (polytetrafluoroethylene). I added small pieces of foam held in place with stainless steel pins to impart extra stability without putting pressure on the pieces (below).
A
B
C
