Conservation Consternation

There are often times when I wonder how some archaeological finds have managed to hold together for thousands of years. I find this thought particularly alarming when handling metal objects – they are so easily damaged by long periods of time in damp or waterlogged conditions.

A Saxon Spearhead from Batheaston Bypass, still in one piece and stable.

Even metal objects found in good condition are threatened by corrosion, and all objects sometimes require specific care and treatment by experts. For example, the pewter vessels and other artefacts from the King’s Bath corridor are soon to be redisplayed, and have been sent out in groups for conservation. The objects were conserved when they were found, but over time, the state of their treatment and attitudes towards care has changed. The pewter objects in particular looked dull and were coated in a waxy substance that seemed only to attract dust. When the items were returned from conservation, the difference was amazing.

Pewter patera after conservation

It turns out that the majority of the work done was to remove the waxy coating and degrease the surface of each object. The coating was replaced with a fine layer of microcrystalline wax to provide protection, and buffed to shine. In a few cases, fragments were reattached and broken areas were filled. However, it seems that when considering conservation, less is without doubt more – the purpose is to protect an artefact without changing its composition or encouraging degradation.

Learning about different types of metals, surface treatments and the process of corrosion is very useful, if only to highlight how much could go wrong with metal objects! A major problem is corrosion, caused when a metal object combines or reacts with other elements to form an undesirable compound, such as tarnish or rust. Corrosion is different from patination, which is generally more benign and will slow and stop without intervention – this occurrence can be seen on copper clad roofs as a blue green compound known as verdigris.

Statue of Liberty – the blue green colour is a result of natural patination

Corrosion generally occurs preferentially, which means that it will affect a reactive metal in preference to a more noble metal. This is the primary reason for galvanising iron with a coating of zinc, which is preferentially eroded to protect the main iron sheet. The process of corrosion can only happen with the presence of moisture, so all metal is stored at a very low humidity. The corrosion of iron, even if contaminated, will not take place if the relative humidity of the surrounding atmosphere is 20% or less. For archaeological iron, such as the spearhead, a relative humidity of 18% or less would be safest, preventing any possible further deterioration.

If you’re looking to find out more about the scientific side to archaeology, National Science and Engineering Week at the Roman Baths, 9th-18th March 2012, is definitely a date for your diary!



Useful links:

http://cci-icc.gc.ca/crc/articles/metals/corrosion-eng.pdf  

http://www.collectionslink.org.uk/media/com_form2content/documents/c1/a130/f6/003024.pdf