1.2 These guidelines are aimed principally at excavators and finds processing staff who may require advice as to the best way of dealing with waterlogged archaeological leather when it occurs on site, and conservators and finds staff entrusted with its care after excavation and before it is deposited in a museum. Material from marine and submerged sites is not considered here, the ALG will produce guidelines for this in due course. More detailed advice on the care and investigation of waterlogged archaeological leather can be obtained from the contacts given in section 11.

1.3 It is expected that these guidelines may be revised in due course in the light of further work and any comments will be welcomed by the authors who can be contacted at the address given in section 11.

2 Project planning (MAP2 Phase 1)

2.1 It is recommended that a conservator should be a member of the core team of every project and will be on hand to give valuable advice at all stages of a project (MAP2, 4.6). Where the likelihood of leather recovery can be predicted it is good practice for the excavator to make initial contact with a leather artefact specialist so that procedures for its immediate post-excavation care can be agreed with the conservator and excavation team at the outset. As decisions on retention of material may have to be taken at an early stage in a project, contact with a museum professional from the designated museum should be made at an early stage (MAP2, 4.10).

3 Fieldwork (MAP2 Phase 2)

3.3 Material should not be discarded on site without reference to the relevant leather artefact specialist. It is important for specialists to see all the leather recovered from a site as the presence or absence of certain categories of leather is significant to the interpretation of the assemblage. Seemingly undiagnostic fragments may provide valuable evidence, those which do not can provide samples for experimental work at a later stage.

3.4 Following excavation the leather should be bagged by context only. Individual components from the same composite object should be kept together and isolated from apparently unassociated items from the same context. It should be indicated on the bag whether the leather has been grouped according to close association or general context and details of the context (e.g. well, pit etc) should be provided. It is preferable not to allocate finds numbers at this stage but where circumstances dictate that the leather should be individually numbered it must be remembered that the increased work this will generate in the laboratory during treatment will be reflected in the overall conservation costs (section 7.1).

3.5 While awaiting treatment waterlogged leather should be stored in two sealed polythene bags in air tight polythene containers. As much air as possible must be removed from the bags by gentle pressure before sealing. The leather should be kept in cool, dark conditions. Storing the containers in or lining them with light-excluding, black plastic refuse sacks will help prevent fungal growth as will keeping the leather in cold storage wherever possible. A domestic style refrigerator is suitable for this purpose, clearly labelled for the storage of wet organic finds only. The deep freezing of leather to enable long term wet storage is no longer recommended as freezing has been found to have a detrimental effect its cell structure.

3.6 The addition of fungicide to the storage water is not generally advisable and dark cold storage is preferable (section 3.5 ). Fungicide interferes with C14 dating results and may accumulate on the leather in concentrated deposits after freeze-drying. It must be remembered that the fungal growth may affect the analysis of dyes and tannins. Any decision on the use of fungicide must be made in conjunction with the conservator and leather artefact specialist who will be dealing with the material and the appropriate safety measures taken (COSHH 1994). If fungicide has been added precautions will have to be taken when handling the leather by both conservators and specialists as skin irritation may result. The material used and in what concentration must be clearly indicated on the packages; vinyl gloves should be worn while handling such leather and it may be necessary to prepare a risk assessment for this task which may require additional precautions (COSHH 1994).

4 After excavation

4.1 Thorough cleaning of the leather is essential as examination and conservation cannot proceed until the soil has been completely removed. Washing is often carried out by finds processing staff rather than conservators. The advantages of having the leather cleaned by professional conservation staff after excavation is completed must be weighed against the desirability of having information on dating and contamination/residuality of the waterlogged deposits available during the fieldwork stage when the leather is washed by on-site finds staff. The conservator and leather artefact specialist will be able to advise on the suitability of the leather for washing on site. With complex Roman and post-Medieval shoe constructions comprising multi-layered bottom units, it may be more appropriate for only the surface soil to be removed initially, leaving the soil between the individual layers present; this will serve to keep the components together until they can be deconstructed in the laboratory where the relationship of the individual components can be fully recorded.

4.2 The leather should be washed by hand in a gentle stream of running water. Thin disposable gloves should be worn, the soil being removed using small sponges and soft brushes where appropriate. Whoever performs this task must be informed of the necessity for careful and thorough examination during washing to preserve any colouration (dyes, pigmentation), decoration, preserved thread, moss stuffing, grass/hay, textile or fur linings, cork soles or wooden pegs in soles. The removal of hard soil deposits may be facilitated by a period of soaking but a conservator should be consulted should the deposits prove intractable.

4.3 After washing, the leather should be stored as described in section 3.5. It is advisable to place the leather in clean polythene bags rather than reusing the original packaging which may harbour fungal spores. Cold storage is strongly recommended.

4.4 Any x-radiography should be carried out prior to assessment. This will mainly involve Roman nailed shoe bottoms as this technique produces a clear record of any nailing patterns. It may also be informative to x-radiograph other leather artefacts with metal fittings.

5 Assessment (MAP2 Phase 3)

5.1 As soon as possible after excavation the washed leather should be scanned while wet by the conservator and leather artefact specialist, in consultation with the appropriate contextual information for the site, and assessed for its immediate and long term conservation requirements and potential for analysis (MAP2, 6.2 and 6.11).

5.2 The assessment will make recommendations on the selection of the type of treatment for the material. Leather with potential for residue analysis (looking for tannins, oils waxes, pigments etc) should not be freeze dried but kept wet, however it must be remembered that such leather cannot be stored in this way long term. In preference to dispersal leather can be air dried slowly in controlled conditions to permit long term storage without incurring more costly conservation treatments (section 7.3.5). The present state of knowledge of leather artefacts is such that the only categories of leather which should be considered for air-drying are scrap leather (i.e. fragments with all edges torn and which possess no diagnostic features) and waste from the manufacturing process.

5.3 Where appropriate in the assessment the leather artefact specialist in conjunction with the conservator will make recommendations as to the necessity for measuring and/or drawing waterlogged leather prior to treatment. Should it be thought necessary to have a record of significant artefacts before treatment the wet leather can be traced onto a waterproof film using a fine permanent marker pen; some conservation laboratories are able to undertake this as a matter of course. It is not always necessary to either measure or draw the entire leather assemblage prior to conservation treatment (section 7.2).

6 The need for conservation

6.1 Unconserved leather is potentially hazardous to health through fungal and bacterial infection, both of which can also cause damage to the leather itself. It is preferable to study leather which has been conserved and is dry rather than wet. In addition to these disadvantages wet leather is fragile and easily damaged by handling. The study and illustration of wet leather is messy and time consuming involving the unsealing of bags and blotting to remove excess water which can cause physical damage. The shiny surface of wet leather may also mask fine details such as stitching, surface decoration and grain pattern. After examination the leather has to be rewetted and the bags resealed. Wet leather is inconvenient and heavy both to store and to transport.

6.2 In the interests of health and to minimise damage to the material it is recommended that leather is conserved as soon as practicable following excavation. Whenever feasible the untreated leather should be conserved during the cooler months. Efficient project planning will ensure that the length of time leather is held in wet storage prior to treatment is kept to a minimum. The period of wet storage should not exceed two years duration.

7 Conservation treatments

7.1 The batch processing of leather (i.e. leather grouped together according to context rather than items which have been given individual small find numbers) has been found to be the most cost efficient method of conservation. If the material has been individually numbered this will extend the treatment time as individual pieces must be kept separate throughout the treatment.

7.2 Previously much time has been spent measuring leather before and after treatment to monitor the amount of shrinkage undergone during conservation. Recent work (Ibbs 1990) has suggested that unless the leather is measured to a very high degree of accuracy this has been of limited value. Research on leather shrinkage is best conducted through individual specific projects (section 10) and the routine measurement of all leather undergoing treatment is no longer recommended.

7.3 A number of leather conservation treatments have been used and some are listed below. The choice of treatment will depend on many factors including the condition of the leather, the facilities available and should ideally be discussed by the leather artefact specialist, the conservator and a representative from the museum which will receive the material.

7.3.1 Freeze drying
With the benefit of over a decade's experience the use of freeze drying to conserve waterlogged leather has been found to produce satisfactory results. this is carried out after pre-treatment using Polyethylene glycol (PEG) 400 or glycerol. Freeze-dried leather is dry, light and flexible with a good colour, showing surface detail extremely well. It is sufficiently durable and supple to allow repeated handling necessary during study and illustration. Its light weight makes it easy to transport and store. Recently some doubts have been expressed as to the long term effects of freeze-drying on leather as some examples have been seen to be excessively dry and friable; currently research is being undertaken on the topic (section 10).

7.3.2 Immersion in PEG 600
Controlled drying of leather after immersion in PEG 600 has also been used. The treatment is regarded as reversible as it is felt that the PEG can be removed if it is wished to apply other treatments at a later stage.

7.3.3 Solvent drying
The use of solvents in large quantities is hazardous as they are highly inflammable and suitable safety precautions must be taken (COSHH 1994). The solvents remove dyes and in the presence of dilute acid remove tannins. White spirit which is also used in the treatment may leave fumes which linger necessitating adequate ventilation when studying such material. Solvent treated leather can harden within a fairly short time requiring dressing of the leather, a time consuming process which darkens the surface and can obscure detail (section 7.3.6).

7.3.4 Wax treatments
The prolonged heating of leather necessary in wax treatment is known to cause structural damage by irreversible shrinkage of the collagen fibres. The high molecular weight PEG used also tends to darken the surface of the leather, obscures detail and also attracts dust.

7.3.5 Controlled air-drying
Wet leather can be allowed to dry out slowly under controlled conditions in order that it may be safely stored. This should be considered as a last resort if no other treatment options are open. However it may be useful for small samples from significant artefacts for future study of residues, dyes and similar materials. Air-dried leather is difficult to study and draw as it is rigid and easily damaged and needs to be rewetted before examination. It has been found that on re-wetting the leather is slightly less flexible than when excavated and that the dimensions are slightly reduced. The leather should be slowly air-dried under pressure to ensure wherever possible that originally flat objects remain flat. Air drying is not totally irreversible; techniques are currently being developed to allow air dried leather to be rewetted, treated with glycerol to increase flexibility and subsequently freeze dried (section 10).

7.3.6 Leather dressing
In most cases leather dressings should not be applied after freeze drying as they can obscure some fine detail, although this could be carried out after specialist examination. The correct application of dressing is a very time consuming process, small amounts being applied at a time to ensure that it is taken up by the leather and does not just remain on the surface. It must also be remembered that as well as darkening the surface leather dressings may encourage biological attack, oxidise over time to stiffen the material, cause dust accumulation and may impede future conservation treatments.

8 Post-treatment care

8.1 Packaging
After treatment care should be taken in the packaging of the leather. Groups of offcuts and flat pieces of leather can be stored in pierced polythene bags. Support to the latter can be given using polyethylene foam covered with or acid free card padded with acid free tissue paper. Three dimensional items should be stored in suitable sized acid free boxes supported by, but not wrapped with, acid free tissue paper.

8.2 Storage
Leather is susceptible to changes in Relative Humidity and temperature and the ideal conditions are 45-50% Relative Humidity (RH) with a temperature of 18° C (ICOM 1994). It should be stored in the dark within 45-55% RH and 18-22° C with as little fluctuation as possible. Acid free cardboard boxes and tissue paper used in the packaging will help buffer changes in conditions but ideally the leather should be removed to a suitable store as soon as possible.

8.3 Handling
Leather will have to be handled during the analysis phase by specialists and illustrators (MAP2 Phase 4) but this should be kept to a minimum. The use of disposable vinyl or cotton gloves will protect the leather from contamination from handling but care must also be taken to avoid physical damage.

8.4 Display
The leather artefact specialist must see material before it is reassembled for display. Details of construction, linings and seams may be obscured by this process and the specialist may also be able to advise on such reassembly. The conditions of the display should also be suitable for the leather (section 8.2).

9 Dispersal

Unlike other categories of material insufficient leather has been recovered from archaeological excavation on a national scale for the question of the advisability of total retention of all leather assemblages to be a serious problem at present. Given the spatial and financial constraints under which the recipient museums labour, however, the indiscriminate collection and retention of any material is now insupportable (SMA 1993). Ultimately, the criteria by which leather objects are selected for retention or dispersal lies with the archaeological curator of the recipient museum, in consultation with a specialist where necessary. However, decisions made during the assessment process as to the suitability for conservation will influence decisions made later (section 2.1). The questions this raises must be the subject for discussion in the near future. At present, leather not considered appropriate for conservation may be air-dried to allow for safe storage (sections 5.2 and 7.3.5) until a policy for retention and dispersal can be agreed.

10 Future research

10.1 Academic research topics
In order to further research into archaeological leather it is essential that those currently working in the field draw up a list of the most fruitful lines of academic enquiry. In this way it will be easier to gauge which assemblages are of local, regional, national or international significance and better target the resources available for their analysis. The ALG are currently engaged in compiling a list of research objectives for the guidance of those undertaking assessments and analysis of archaeological leather.

10.2 Conservation research
In order to eliminate costly duplication of research in conservation laboratories it will be necessary to instigate specific research programmes. The ALG are also in the process of compiling a list of conservation research topics. Projects at present underway include the freeze drying of air dried leather and a pre-treatment condition assessment to determine glycerol concentration to be used before freeze drying.

11 Further advice

Specialist advice can be obtained from the contacts named below.

Glynis Edwards
Ancient Monuments Laboratory
English Heritage
23 Savile Row
London
W1X 1AB
0171 973 3325

Esther Cameron
Secretary Archaeological Leather Group
Institute of Archaeology
36 Beaumont Street
Oxford
OX1 2PG
01865 278 253

12 References

Ibbs, B, 1990 Shrinkage Calculations, Conservation News No 41 March 1990

MAP2, 1991 Management of Archaeological Projects, English Heritage, London

SMA, 1993 Selection, Retention and Dispersal of Archaeological Collections, Society of Museum Archaeologists Guidelines

ICOM, 1994 Guidelines for costume, International Council of Museums, International Costume Committee

COSHH, 1994 The Control of Substances Hazardous to Health Regulations SI 1994 3246

Compiled by Glynis Edwards (G.Edwards@eng-h.gov.uk) and Quita Mould, Ancient Monuments Laboratory, with assistance from members of the Archaeological Leather Group, August 1995
Copyright © 1995 Historic Buildings & Monuments Commission for England.

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