COMPLEX FINDS

Certain types of find present an immediate first aid challenge due to their complexity arising from their size, condition and fragmentary nature. Understanding and planning for these challenges permits more effective and timely first aid actions to be carried out.

Challenges and first aid actions relating to complex objects are exemplified on this page with reference to:

Large waterlogged wooden finds
Mosaics
Wall paintings

Large Waterlogged Wood Finds

Despite the wide usage of wood throughout human history, its survival on UK archaeological sites is relatively rare. It is usually only expected in waterlogged conditions, when charred, or when preserved by proximity to corroding metal artefacts. Of these, waterlogging is the most common and challenging of the preservation mechanisms to deal with on site and following excavation.

Challenges of large waterlogged wood finds

The challenges to first aid of large waterlogged finds are detailed in Table 8.1:

Challenges of first aid for large waterlogged finds

Table 8.1 Challenges of first aid for large waterlogged wood finds

Excavation and recording of large waterlogged wood finds

Careful planning for complex waterlogged wood finds is required. Agree a preservation strategy which is informed by input from a conservator and wood technology specialist. This will shape the approach taken to excavation and recording, including selection and retention, sampling, labelling and preparations for any in situ preservation, including short and longer-term reburial.

Protect the waterlogged wood while it is in situ during excavation and recording by preventing drying out of the timbers, which causes irreversible shrinkage and warping.

Keep timber wet whilst being worked on:
- Use a hand spray for smaller timbers.
- Use a pressurised hand spray, watering can or hosepipe with sprinkler for larger timbers, avoiding strong jets of water.
Do not rewet a timber that has dried accidentally.
If timbers are in situ for several days during excavation and recording:
- Cover with wet polyether foam or capillary matting and black polyethylene sheeting.
- Water the polyether foam or capillary matting frequently, using a leaky pipe garden hose system for larger areas.
Do not allow waterlogged wood to freeze. This will disrupt the degraded cellular structure.

When excavating the finds:

Consult a conservator for advice.

Remove adhering mud carefully with sponges, soft haired brushes and blunt plastic or wooden tools using plenty of water. Do not use trowels which could damage the surface and confuse original tool marks.
Take care while excavating the outer degraded layers, including preserved bark which may need additional protection. Stainless steel pins and surgical bandage can be used to prevent loss of bark and other associated fragments.

When recording complex timber structures and articulated waterlogged wood finds:

Record timber in situ using photography and drawings.
Note the condition of the wood and record any associated materials.
Record any carpentry marks or other marks acquired during the use-life of the object.
Label all timbers to preserve information on location, orientation and association between timbers, whether they are to be lifted or preserved in situ.
For a complex find, devise a labelling system which allows later reassembly.
- Cask staves: number each stave individually and attach a label on its upper, inside face. Number sequentially from a given point to preserve the orientation of each stave and allow exact repositioning during future reconstruction.
- Boats: consult a nautical archaeologist for advice on appropriate labelling.
Include the labelling system in the site Data Management Plan and annotate drawings and photographs with relevant labels. Ensure that the labelling system is interpretable by a reader with no knowledge of the find or site.

Lifting large waterlogged wood finds

Consult a conservator and wood technology specialist for bespoke lifting advice for a large waterlogged wood find. In an emergency, it may be possible to lift finds with the following preparation:

Ensure timbers have been excavated, surveyed, recorded and labelled.
Complete the excavation of the archaeology around the timbers or protect it from damage during the lifting process.
Gather supplies and equipment for supporting timbers and maintaining their waterlogged condition during lifting and transport.
Undertake a risk assessment of the lift procedure.
Ensure personnel for the lift of heavy timbers are trained in manual handling, have read and understood the risk assessment and have rehearsed the lift procedure.
Ensure that operators are trained in the use of any lifting tackle and have the necessary personal protective equipment (PPE) as identified by the risk assessment.
Arrange for immediate transport of timbers to the conservation facility.
Prepare interim wet storage on site for lifted waterlogged timbers if transport is delayed.
It may be necessary to cut especially large timbers to permit lifting.

The materials required for lifting large, waterlogged wood finds are given in Table 8.2:

Common materials required for lifting large waterlogged finds

Table 8.2 Common materials required for lifting large, waterlogged wood finds

Whether or not a timber is sufficiently robust to support its own weight during lifting will determine the procedure necessary. Those that can support their own weight can be lifted directly into storage containers with water or onto pallets with wet polyether foam. Timbers that cannot support their own weight will need additional support during the lift.

To lift large waterlogged wood finds with extra support:

Excavate around the find using wooden or plastic tools, sponges and soft brushes.
Undercut the find to loosen it from the underlying soil matrix.
Slide a support underneath the find and lift.
Transfer the find to the prepared container or pallet. If the find is particularly fragile, use a pallet with three sides which allows the find to be slid on rather than lifted.
Cover the find with wet polyether foam.
Secure the find and foam into the container or onto the pallet with plastic cord, strapping or ties (Figure 8.1). Add extra padding under straps or ties to prevent damage to soft outer wood surfaces.
Cover the whole in heavy duty polyethylene sheeting or place in lay-flat polyethylene tubing and tie off or heat seal ends to retain moisture.
Label the find inside and outside the packaging, including orientation to context data.

A large waterlogged wood find secured onto a pallet

Figure 8.1 A lifted large waterlogged wood find secured onto a pallet

Support pallets for large waterlogged timbers should:

Be made of water and rot-resistant materials.
- Marine plywood or softwood battens pressure-treated with wood preservative are suitable.
- Do not use ordinary plywood and composite woods such as MDF. They are not waterproof and will disintegrate.
Not be constructed of blue dyed wood as the dye can run.
Be designed to support a timber from site through conservation treatment.
Have raised sides to prevent a timber sliding off.
Have runners beneath to aid moving by pallet truck, ideally (Figure 8.1).
Have battens on only three sides for weak timbers.
Be lined with wet polyether foam (open celled) as padding and to keep the timber wet.

Mosaics

Mosaics are most commonly found on Romano-British sites, although renewed popularity from the mid-18th century onwards means that they are encountered in more modern buildings, especially as door thresholds and patios. Mosaics may be high-status and highly decorative or simpler and more crude in design. Their quality and construction will vary depending on their status, but mosaics were usually constructed using tesserae (small tiles) of locally available stone (e.g., black limestone, white chalk, green and yellow sandstone), ceramic (e.g., red terracotta) and, occasionally, coloured glass.

A generalised mosaic structure is shown in Figure 8.2. A mosaic may contain some or all of the various bedding layers shown.

Figure 8.2 Generalised cross section of a Roman mosaic

Challenges of mosaics

The challenges of first aid for mosaics are given in Table 8.3:

Table 8.3 Challenges of first aid for mosaics

Excavation and recording of mosaics

Whether a mosaic is to be left in situ or lifted, recommended first aid actions are as follows:

Establish elevated or cushioned planking walkways to avoid walking on the surface of the mosaic.
Trowel carefully down to just above the surface of the mosaic.
Remove the last few millimetres of soil with wooden spatulas.
Brush loose soil away with soft, long-bristled brushes and plastic dustpan and brushes.
Wash the surfaces of fixed tesserae by sponging sparingly with clean water. Use high density sponges with a large capacity for holding water.
If advised by a specialist:
- Where the original position is known, replace loose tesserae using modelling clay which may be removed using water later. Record the location of these replaced tesserae.
- Protect exposed edges of the mosaic with modelling clay or plaster of Paris applied sparingly with a cling film release layer between it and the mosaic.
Where deposits allow, dig a gully around the mosaic to facilitate rainwater drainage.
Keep mosaics constantly damp and avoid wetting and drying cycles. Protect from direct sunlight and cover with damp polyether foam and layers of black polyethylene sheeting when not being worked on.
Be vigilant for the appearance of white efflorescence on tesserae surfaces which may be due to soluble salts.

Recording of a mosaic and its condition should be thorough and ongoing before, during and after cleaning. Recording method will include photography, photogrammetry, drawing, 1:1 tracings and written descriptions. Scales, datum lines and coordinates are essential, especially on undulating mosaics.

Lifting mosaics

Lifting of mosaics is a specialist process and should be undertaken only by trained and experienced conservators and personnel. Lifting a large mosaic normally involves facing it up with a synthetic textile adhered using a reversible adhesive. The mosaic is then divided into sections along lines in its decorative scheme or the edges of damaged areas, undercut using a long-bladed tool and lifted onto thin, rigid boards inserted under each section. Occasionally, a technique involving rolling the mosaic face down onto a wooden drum may be used. Both approaches are specialist processes and must only be undertaken by trained and experienced conservators and personnel.

In an emergency, and having taken advice from a conservator, small sections of a mosaic may be lifted using the encapsulation technique. Be aware that where the mortar bedding of the mosaic is in good condition, this can create difficulties when undercutting.

In situ preservation of mosaics

Leaving a mosaic in situ may offer a more cost-effective and practical approach to its preservation than lifting it. Designing an in situ preservation strategy will require input from an conservator with specialism in ancient mosaics and reburial. Even where open display in situ is the aim, short or medium-term reburial of the mosaic is likely to be necessary.

Short or medium-term reburial

To rebury a mosaic for up to a year (e.g., between field seasons):

Record the mosaic fully, keeping damp throughout.
Cover the mosaic with geotextile (e.g., Terram), a synthetic, porous, degradation resistant textile which allows water and water vapour to pass through.
Lay at least 150mm of clean, washed sand (e.g., Buckland or equivalent) on top of the geotextile to act as a marker for future excavation. Use a white or pale sand that does not contain iron (do not use yellow or builders’ sand which can stain the mosaic).
Backfill with at least 50cm of soil to protect the mosaic from frost and physical damage.
Mark the area (e.g., with stakes) to denote the extent of the mosaic if this does not create a security issue.

Long-term reburial

If a mosaic is to be preserved permanently by in situ preservation, a strategy must be agreed in consultation with a specialist conservator and other stakeholders, including the landowner. Expect significant financial input to be necessary. Further information to support decision-making can be obtained from specialist organisations, such as the Association for Study and Preservation of Roman Mosaics (ASPROM).

Wall Paintings

From the Roman period onwards in the UK, lime-based plaster was applied as a finishing layer on walls of public and domestic buildings. Interior walls and ceilings were often then decorated with coloured friezes and dados. Naturally occurring pigments, such as haematite (red), yellow ochre, green earth and carbon black, were used commonly and more expensive pigments, such as blue frit and cinnabar (red), occasionally. Wall painting techniques varied by period, as demonstrated when considering Roman and medieval wall paintings.

The generalised structure of Roman or medieval wall paintings is given in Figure 8.3:

Figure 8.2 Generalised cross section of a wall painting (Roman or medieval). Fresco would be expected on Roman sites with tempera more common from the medieval period

Challenges of wall paintings

The challenges of first aid for wall paintings are detailed in Table 8.4:

Table 8.3 Challenges of first aid for wall paintings

Excavation and recording of wall paintings

Where possible, record the relative positions of every fragment of a wall painting to facilitate understanding and possible reconstruction of the decorative scheme. In practice, this may not be feasible where fragments are very small and are recovered from redeposited layers of mixed building debris.

Retain white fragments of plaster in addition to those with pigment on. The number and volume of fragments and variations in thickness and fabric may provide information about their original context. Retain fragments of backing plaster which may bear impressions of the original wall structure (e.g., stone, tile, wattle and daub, or rammed earth).

During careful excavation and recording of a wall painting (whether fallen or attached to a standing wall), maintain its water content as discovered. If discovered damp:

Lightly mist the wall painting with clean water using a hand-held plant spray.
- Do not use excessive volumes of water or a strong jet of spray which may loosen or dissolve pigments and plaster.
- Do not saturate the wall painting with water to achieve a good photograph.
Cover the wall painting with black polyethylene or tarpaulin to reduce the action of microorganisms and prevent drying.
- Ensure covering material does not rub against painted surfaces.
- Protecting painted surfaces with damp polyether foam prior to covering can reduce the risk of abrasion damage and reduces frost damage.
Erect a temporary shelter for shade from direct sunlight and protection from wind and rain if necessary.

The condition of the wall painting will dictate appropriate cleaning procedures which should only be carried out by a conservator or experienced personnel. Avoid cleaning any painted surfaces.

Recording of a wall painting will likely involve photography (include a colour scale), 1:1 tracing, drawing and written descriptions and should be ongoing throughout the excavation process. Ensure that the following are recorded:

Position and orientation of wall painting fragments relative to associated building structures. This will assist with determining original locations, including whether it was on a wall or ceiling.
Surface cracking and flaking and any separation from the plaster support to inform planning for lifting and recovery. Very light tapping of the painted surface with a knuckle will locate any voids behind the plaster that could initiate further damage.

Lifting wall paintings

Whenever possible, wall paintings should be retained in situ on their support to avoid loss of context. Lifting a wall painting is an action of last resort and must be undertaken only following extensive discussion at management and policy level and on the advice of a specialist.

In emergencies, areas of collapsed plaster may be lifted following the methods outlined for lifting fragile finds with the following general guidelines:

Consult a conservator for advice on suitable personnel, supplies and techniques.
Assess the condition of the plaster accurately and plan thoroughly for the lift.
Be aware that there may be more plaster beneath that which is visible, especially if it originated on a ceiling.
Small fragments which can support their own weight can be excavated and lifted with spatulas and trowels and packed as a fragile, damp registered find.
Lift medium-sized fragments by carefully excavating earth from beneath and gently easing a plywood support board underneath. Cushion with polyethylene or polyether foam and tape to the board if necessary. Enclose in a box for support and pad the box interior.
Larger pieces of wall painting are likely to be heavy and may require customised pallets and boxes for lifting and transport.
Record details of packing procedures on boxes and pallets.

In situ preservation of wall paintings

Wall paintings may be preserved in situ either on open display or buried. Decisions on the preservation strategy must result from discussion between specialists and stakeholders.

Where a cleaned wall painting is to be preserved in situ and has porous or decayed plaster and pigments which are water soluble or flaking, consolidation may be required. This is a specialist and irreversible process which is likely to change the appearance of the painted surface, usually causing darkening. Consolidation should be avoided if possible and only carried out by a conservator or experienced personnel.

Further Information

British Standards Institute. 2016. Conservation of cultural heritage. Guidelines for the management of waterlogged wood on archaeological terrestrial sites BS EN 16873:2016

Guichen, G. de and Nardi, R. 2008. Mosaics Conservation– Fifty Years of Modern Practice. Getty Conservation Institute, Los Angeles, CA.

Historic England.

2006. Guidelines on the X-radiography of archaeological metalwork.

2008. Guidelines for the Curation of Waterlogged Macroscopic Plant and Invertebrate Remains.

2018. Waterlogged Organic Artefacts: Guidelines on their Recovery, Analysis and Conservation.

2018. Guidelines on the Recording, Sampling, Conservation and Curation of Waterlogged Wood.

2020. Care and Conservation of Historic Architectural Tiles.

Jensen, P. and Gregory, D. J. 2006. Selected Physical Parameters to Characterize the State of Preservation of Waterlogged Archaeological Wood: a Practical Guide for their Determination. Journal of Archaeological Science 33(4):551-559.

Squires, K., Errickson, D. and Márquez-Grant, N. (Editors). 2019. Ethical Challenges of Working with Human Remains. Springer, London, UK.