York Archaeological Trust Lecture on Conservation of Archaeological Finds

with thanks to Wessex Archaeology via Creative Commons

After another session of digging and planning to finish the day we started the next morning with a visit to the YAT’s conservation lab. We learned about the various procedures employed in administrating, conserving and researching finds material. In the first section of the lab we were shown the way in which dry finds were treated. Heavily corroded metal objects are x-rayed to reveal their shape underneath and to indicate what condition it is in. A report is thus typed up from these results with a concluding recommendation for either further research, storage or indicating that it is of no further use to the trust.

A paler and more flekky x-ray indicates an objected that is heavily corroded while a bright additional line will indicate the presence of another metal (such as white metal plating) an air braser machine can expose alloys and platings.

Consolidents are used in objects that are crumbling or falling apart, minimal chemicals are added to minimise any further corrosion or any conservational processes that cannot be reversed. Once the objects have been treated they are stored in air tight boxes with silica gel and good packaging (usually plastizote – an inert fabric to line boxes). Materials that recieve wet packing are bone and jet to prevent drying out and resultant brittleness.

Conservational Treatments for Pots

We were then shown several works in progress: the conservation of three Roman cremation urns. These were stuck together using a conservation-grade adhesive. The pieces of the pot that were found were stuck together and filler used to secure any points of pressure. The pieces of pot had experienced some distortion whilst laying in the ground that made sticking the pieces back together difficult. Soil pressure can casuse cracking and even the shape of the pot itself can be the root cause of ‘sprung cracking’ whereby the sheer ‘top heavy’ nature of the form causes cracking from underneath. Plaster of paris is used to fill cracks and subsequently painted a shade that is similar to the pot to subtly display it has been used as a means of conservation.

An example pot was shown to display the various pottery conservation techniques employed in the past including the use of clay meshing and textured conservation material which is painted to match the original pot material. Today, however, it is preferred that there is a distinction made between the original and new pieces of pot.

Wet conservation

We were then taken to the wet part of the lab that specialises in conservation of wood which involves a number of complex chemical steps to ensure further degredation of the wood does not occur.

When a wooden object enters the conservation lab for treatment it is analysed to determine whether or not its conservation is necessary. Wood is a material that is often thrown out in archaeology as it requires extra special treatment to conserve unlike that of metal or pot. The three categories a piece of wood may be allocated to are:

  1. Recommended for comparative or research purposes
  2. Needs restoration/conservation not covered in original excavation budget
  3. The artefact is of no further value

Every piece of wood experiences chemical or microbial damage during burial and when it experiences drying out it begins to split, distingrate and is therefore useless as a source for identifying tree rings which are essential for identifying the season in which it was cut down in or even the species of the tree.

Every piece of wood experiences chemical or microbial damage during burial and when it experiences drying out it begins to split, distingrate and is therefore useless as a source for identifying tree rings which are essential for identifying the season in which it was cut down in or even the species of the tree.

When water evaporates from the wood, strong hydrostatic forces draw the water out and this in turn pulls on weakened cell walls. Therefore several steps are taken to prevent this drying out process:

Consolidation: This is performed by using an inert conservation grade polymer known as Polyethylene glycol (or PEG for short). PEG is mixed with cold water is used to form a weak solution for which the wood being conserved is bathed in. The absorbancy rates of PEG vary from tree species to tree species and the greater degraded a piece of wood is the better it absorbs PEG. The time required for the PEG treatment varies from object to object but for the medieval stake we examined it takes about 8/9 months. Determining when an object is ready to be removed from the PEG solution and be freeze dried is when an object stops losing mass

After the object has been removed from the PEG solution it is then put into a vacuum freeze dryer which freezes the object down. The aim of a vacuum freeze dryer is to boil water without it going through the liquid state. A cyrol protector is added to the vacuum freeze dryer to prevent ice crystals from forming in the wood that would cause subsequent cracking, splitting and/or fracturing. Additionally, a pump is used to reduce pressure inside the vacuum freeze dryer to avoid a liquid state of water from forming. Once the freeze dryer reaches an ambient temperature, conservation of the object is now complete.

In the case of conserving wood objects that have been found in ship wrecks (sea water) they must first be soaked in tap water to leach out the salt present (minerals absorbed in the salt). The tap water is then replaced with distilled water (water with a highest water potential) to reduce salt level to an even further minimal level. The water is changed once a month until the object is deemed to be free of salt. The steps taken for PEG treatment are then undertaken.

Next up: Bulk finds packaging and an introduction to small finds!

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