dig_labworks.pdf

Preservation Techniques

Glass

Glass is normally quite fragile when excavated so is first cleaned with swabs

under a microscope by a conservator. Its condition is then assessed. Normally,

glass beads will be dried by passing them through containers of an organic

solvent, such as acetone, which will dehydrate the glass. This work is done in a

fume cupboard for reasons of safety.

Treating Glass Beads

Finally, the beads are put into a solution of a synthetic resin, which is soluble in

the same solvent. The resin, specially selected to have physical and chemical

properties suitable for glass, acts as a consolidant and lends strength to the

internal structure. This protects the surface of the glass as the solvent dries.

Careful packaging and labelling is then required to keep the glass beads

physically safe and to stop them from getting mixed up or mislaid

Preservation Techniques

Wooden Bowls

When wood is excavated it is soft and fragile. Wooden bowls, therefore, require

very careful handling whilst still wet. Bowls and bowl fragments are treated by

immersion in a solution of a synthetic wax-like water-soluble polymer called ‘poly-

ethylene glycol’. After a few weeks all the wood cells that make up the bowl are

filled with this solution.

The bowl is then placed in a freeze-drying unit, and frozen to about -20°C. All the

air is removed from the chamber of the freeze-drying unit with a vacuum pump to

create a near vacuum, under which conditions the ice in the wood sublimes away

as water vapour. This is a fast and safe way to dry fragile organic materials which

otherwise tend to warp and shrivel because of the surface tension exerted by

liquid water.

Once dry, the surface of the wood is lightly cleaned again with brushes and the

loose fragments of bowl are fixed back in place with a special adhesive.

Piecing a Wooden Bowl Back Together

Preservation Techniques

Iron Objects

When iron objects are found they are generally covered in a mixture of dirt and

corrosion, so they often appear as shapeless lumps of ‘rust’, actually a mixture of

several corrosion products. These are formed by complex chemical reactions, the

iron slowly reacting with a range of components included in the archaeological

deposit — a process called ‘oxidation’. The item is therefore first X-rayed, so that

its true shape can be seen, and other details such as method of manufacture,

composition and condition become more clearly visible.

If selected for further recording, the object may be cleaned using hand tools

under a low-power binocular microscope. This is delicate and time-consuming

work and it is often only necessary to clean selected areas of the object, to show

its cross-section shape, for example.

Much of the work of removing

corrosion is done using ‘air-abrasive’

equipment. The air-abrasive is a

miniature shot-blaster that uses very

fine aluminium oxide powder. A fine

stream of the powder is shot with

compressed air through a tiny nozzle

attached to a hand-piece at the area

of the object to be cleaned, and the

corrosion is gradually abraded away

to reveal the original surface.

Cleaning takes place in a glove-box

to contain the spent powder and

corrosion, and a microscope is

normally also used so that the

conservator can monitor progress in

detail. One false move and you can

make a hole in the object quite

easily! Although time-consuming,

air-abrasion is a very effective way

to reveal fine detail on delicate

corroded surfaces. Many of the iron

objects on display in JORVIK were

cleaned in this way.

Using Air Abrasion Equipment

Once the investigative cleaning work is completed, the object can be recorded for

publication by drawing and photography. But the conservation work is not

completed as, in the above-ground environment, the object is still liable to react

with oxygen and moisture in the atmosphere, causing it to flake, split and finally

break up altogether. Although there are chemical ways of stopping this

happening, the treatments are complicated and messy, and can cause more

harm than good. So, at York Archaeological Trust we prefer to prevent corrosion

by storing the iron in very dry conditions, using airtight boxes and silica gel, which

will absorb all the moisture from the air in the box. This type of technique is

known as ‘passive’ or ‘preventive’ conservation.

An Iron Anvil from Coppergate

Preservation Techniques

Shoes

When excavated, the leather that constitutes shoes is a very fragile material, so

great care must be taken in the conservation process. The upper surfaces are

first cleaned with water and soft brushes, removing any deposits, so that the shoe

can be recorded ‘as found’ before being dismantled. The parts are then

individually washed and recorded again, and placed between plastic boards to

keep them flat.

A Squashed Viking Shoe

Those few leather shoes which are complete enough to be pre-selected for

display will be put through a chemical reagent that will remove some of the black

discolouration from the leather to improve its appearance. The leather is then

immersed in a solution of water-soluble synthetic ‘oil’ called glycerol, which helps

to plump the leather out and keep it flexible after drying.

Finally, the leather is placed in the freeze-

drier, frozen to about -20°C before the

chamber is evacuated and the ice sublimed

away as water vapour until the leather is dry.

This is a fast and safe way to dry fragile

organic materials which otherwise tend to

warp and shrivel because of the surface

tension exerted by liquid water.

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