424 (12)

397

Conservation

Instrumental analysis

More-accurate analysis of metal composition, corrosion and decorative coatings was carried out where necessary by outside analytical laborator-ies, usually the Ancient Monuments Laboratory (HBMC), using various instrumental technicjues -atomie absorption (AA), x-ray fluoresence spec-troscopy (XRF), and x-ray diffraction.

Analysis of non-metallic components Visual examination using stereo and polarising microscopy, microchemical lests, and measure-ment of physical properties, eg specific gravity, enabled preliminary idcntification to be madę of non-metallic components such as gem-setting media and gem-stones. Outside expert analysis was often reąuired to confirm results.

Treatmenl

Soil, concretions and corrosion were removed where appropriate using mechanical aids - scal-pels, needles, wooden cocktail sticks, soft- and glass-bristle brushes, power pen and airbrasive eąuipment. The progress of remoyal was closely controlled by viewing the work through a stereo binocular microscope. Occasionally corrosion was removed by Chemical methods or by eleclrolytic reduction. Where necessary melals were stabil-ised to inhibit further corrosion by well-established tcclmiąues, and then coated with an appropriate lacąuer for protection. A detailed record is kept of the condition, investigation and treatment of each object. The x-radiographs also form part of the archival record of the objeets.

Condition

The appearance and condition of metal objeets from the anaerobic enyironment of these water-logged sites was both surprizing and sometimes deceptive.

Copper alloys

Objeets ranged in colour from gold- to copper-coloured and were bright, shiny and apparently uncorroded, though closer inspection revealed etched surfaces. The degree of survival of sur-face detail is such that even metallographical analysis is possible by observation of the surface (current research, D. Goodburn-Brown).

Many objeets showed a characteristic duli gold-coloured ‘bubbly’ layer (colour pl 4C, lower part of object), sometimes thin and tenacious, morę often a thicker crust of up to several millimetres, loosely attached to the underlying layer. Patches of iridescent colours were noted occasionally on this crust. The underlying layer was either the metal surface itself, or a black corrosion layer on the metal. The black layer, which also appeared on some objeets without the gold crust, ranged in colour from brown-black to shiny blue-black and was soft and powdery, brittle or hard and tena-cous (colour pl 4C, top part of object). The crust generally obscured shape and surface detail, though it sometimes faithfully mimicked surface contours. Beneath these layers the metal ranged in condition from sound to very thin and fragile. Its surface was again often bright and shiny though etched, fissured and pitted. The formation and composition of these sulphides has been studied (Duncan and Ganiaris 1987).

This duli-gold coloured layer was sometimes so smooth and thin that it was at first mistaken for gilding or some other artificially produced surface coating. Its presence on lead-tin objeets caused some initial misidcntifications (see colour pis 1E & F, and 6B & E).

Although microscopic examination and microchemical tests resolved most problems, some instrumental analysis was necessary. For exam-ple, the bright, silver-coloured shiny but ton (no. 1384, colour pl 7B) was initially thought to be either an intrusive modern object or of silver. Microchemical tests suggested that it was com-posed of tin. Later XRF and AA established that despile its colour, it is a high-tin coppcr-alloy. A dull-gold coloured mirror case (no. 1718, colour pl 12C) was first identified visually as copper alloy: XRF analysis at the Ancient Monuments Laboratory subseąuently showed it to be gilded silver.

Iron

The condition of the iron items was very varied. Some objeets were very well preserved, with an even blue/black-coloured surface patina. Others were very corroded, with large concretions obscuring their shape. Most objeets fell between these extremes. They had patchy concretions which sometimes disfigured fine, black-patinated