POST-EXCAVATION DECAY

Physical and chemical changes to finds do not stop once they are excavated but the agents of decay acting on the finds may change. The response of a find to the post-excavation environment is often linked to its condition following decay during burial. Understanding how the post-excavation environment differs from the burial environment and the impact that can have on finds is important for their preservation.

Agents of Decay Post-Excavation

In comparison to the burial environment, post-excavation environments generally:

Have higher oxygen levels.
Are drier, but still with a varying and often abundant supply of water in the form of water vapour.
Are warmer, but with a wider range of temperature fluctuations.
Have higher light levels.
Expose finds to greater risk from physical forces, for example, during handling.
Are richer in biological agents of decay, such as bacteria, fungal spores and insect pests.

This page explains the agents of decay acting in the post-excavation environment, their impact on archaeological materials and simple actions to minimise the risk to materials from these agents of decay. Table 4.1 indicates the main chemical agents of decay acting (oxygen and humidity) and Table 4.2 explains other agents (temperature, light, physical forces and pests).

Table 4.1 Agents of decay and their impact on archaeological materials post-excavation

Minimising Risk from Agents of Decay

Simple actions can minimise the risk to finds from the agencies of decay acting in the post-excavation environment. Many of these relate to careful handing, avoiding inappropriate cleaning during post-excavation examination of finds and packing and storing finds appropriately based on their material and condition. These actions are outlined for inorganic materials, metals and inorganic materials below.

Inorganic materials

Examine porous inorganic finds for the presence of soluble salts which may crystallise as humidity drops post-excavation (Figure 4.1). Consult a conservator if soluble salts are suspected.
Store large stone finds elevated on pallets to avoid water uptake from damp ground.
Protect any areas of pigment from light damage during storage. Store in an unlit space or cover with blackout material.
Ensure very careful handling of fragile inorganic finds and provide appropriate cushioning within packaging using foam or acid free tissue paper.
Package fragile ceramics ensuring that there can be no abrasion between sherds.

Damage to archaeological ceramic with spalling of surface caused by soluble salts (Cardiff University)

Figure 4.1 Damage to archaeological ceramic with spalling of surface caused by soluble salts (Image: Cardiff University)

Metals

Reduce corrosion risk to metal finds by drying immediately post-excavation and packaging with silica gel to desiccate the environment around the finds.
Monitor the relative humidity in boxes during storage to ensure it remains appropriate for the type of metal.
Remain vigilant for signs of active corrosion of metals post-excavation, particularly on iron and copper alloy finds (Figure 4.2).
Ensure careful handling of finds with laminating metal coatings and metal finds with fragile dissimilar materials such as inlays.
Provide appropriate support within packaging for finds of thin metal sheeting using foam or acid free tissue paper.

Wrought iron nail damaged by active corrosion

Copper alloy coin showing damage to surface from bronze disease

Figure 4.2 Post-excavation corrosion of metals caused by chloride salts. Iron - showing characteristic growth of orange akaganeite which has split the archaeological nail. Copper alloy - showing bright green, voluminous bronze disease which disfigures the surface of the coin (Images: Cardiff University)

Organic materials

If waterlogged, maintain waterlogging of finds during examination and storage before transferring to a conservator. Check water levels in bags and boxes regularly.
If possible, store waterlogged organics in the dark and at lower temperature to minimise biological activity.
Store painted wood and leather and dyed textiles in areas with reduced light levels or cover with blackout material.
Handle fragile embrittled textiles and laminating antler, ivory and horn with great care. Provide appropriate cushioning within packaging for these finds using foam or acid free tissue paper
Do not clean the surfaces of decayed organic materials, particularly when waterlogged. Running under clean water to remove silt can help prevent mould growth during storage. Handle carefully to avoid damage and compromise of any original tool marks
Maintain vigilance for insect pests in finds storage and processing areas and use blunder traps to monitor possible pest activity. Check regularly for mould growth (Figure 4.3). Store registered finds in airtight boxes.

Mould on organic material from high humidity storage

Figure 4.3 Mould growth on an organic artefact which had been stored at high humidity (Image: Cardiff University)

Composite objects

Identify the materials comprising the composite object, the agencies of decay acting on each and the suggested mitigations.
Consider any contraindications in the mitigations for different materials in the composite object. For example, the low humidity required to prevent corrosion of iron may cause cracking of wood which would ideally be stored in a mid-range humidity.
Seek the advice of a conservator in the case of conflicting requirements of materials.

Further Information

Cardiff University Guidelines for Storage of Archaeological Metals. Department of Conservation, Cardiff University
https://www.heritagepreservationguidance.co.uk/guidelines-for-storage

Cronyn, J. 1990. The Elements of Archaeological Conservation. Routledge, London, UK.

Department of Culture, Media and Sport (DCMS). 2005. Guidance for the Care of Human Remains in Museums. DCMS, London, UK.

Rimmer, M., Thickett, D., Watkinson, D. and Ganiaris, H. 2013. Guidelines for the Storage and Display of Archaeological Metalwork. English Heritage

https://orca.cardiff.ac.uk/id/eprint/61283

Selwyn, L. 2004. Metals and Corrosion: A handbook for the conservation professional. Canadian Conservation Institute.

Shashoua, Y. 2006. Inhibiting the Inevitable; Current Approaches to Slowing the Deterioration of Plastics. Macromol. Symposium 2006, 238 67-77.
https://doi.org/10.1002/masy.200650610

Shashoua, Y. 2008. Conservation of plastics; materials science, degradation and preservation. Butterworth-Heinmann, Oxford

Society of Museum Archaeologists (SMA). 2020. Standards and Guidance in the Care of Archaeological Collections and Materials Factsheets. (Editors G. Boyle and A. Rawden)

Thunberg, J. C., Watkinson, D., E. and Emmerson, N., J. 2021. Desiccated Microclimates for Heritage Metals: Creation and Management. Studies in Conservation, 66(3):127-153.