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

Scanning Sulis Minerva

Bronze head of Sulis Minerva

This week at the Roman Baths, Dr Annemarie La Pensée, from Conservation Technologies at the National Museum of Liverpool, has been scanning the original bronze head of Sulis Minerva. We hope to make a 3D image from the scanning process to aid interpretation and possibly shed light on how she was made. Here’s a little bit more on the object and what we hope to learn from the scans…..

Sulis Minerva awaiting scanning

Vital Statistics:

Roman Name: Minerva

Position: A major deity and one of the 12 Olympian Gods

Jurisdiction: Roman Goddess of Wisdom

Mythical Family Tree/Relatives: Daughter of Jupiter and Juno

Depiction / Description / Symbol: the owl, the snake and the olive tree

Greek equivalent: Pallas Athena

Festival: Quinquatria - March 19 through to March 23.

Celtic influence: Linked to unreferenced local deity - Sulis

The gilt bronze head of the goddess Sulis Minerva is one of the best known objects from Roman Britain. Discovered in 1727, it was the first indication that the Roman site at Bath was not a typical settlement. Gilt bronze sculptures are rare finds from Roman Britain; only two other fragments are known.

The head has a fascinating story to tell that can be gleaned from the circumstances of discovery and its condition and from scientific examination.

First of all, we should note that the head is slightly larger than life size, suggesting that the original statue, of which it formed a part, was an imposing sight. For many of those who saw it, there may have been no previous encounter with an object of such awesome, golden brilliance.

Hidden in the hair line are several small holes which once held rivets that fixed her tall Corinthian helmet to her head.

Examination of the head has revealed that it has six layers of gilding. The first two use a technique known as fire gilding whilst the four latter layers are applied as gold leaf.

When looking closely at the head, we can see that it has a number of imperfections. There is corrosion which has affected it in parts where it lay in the ground for over a thousand years. There is also a strange rectangular cut beneath the chin. It is thought that this may have resulted from a flaw in the original casting process in which a bubble on the surface may have been cut out and filled with an inserted plate. When gilded over, it would not have been visible to a casual observer. This plate has subsequently fallen out as a result of corrosion whilst in the ground.

What more can scanning tell us?
In creating a 3D scan, we should be able to see these imperfections much more clearly. We also hope to better understand the casting process, in particular, where the molten metal would have been fed into the mould….

Sulis Minerva being scanned

Keep an eye on the BlogSpot for the latest news and results……

For an in-depth discussion on the head of Minerva, visit our webpage by following the link below:

http://www.romanbaths.co.uk/walkthrough/6_temple_courtyard/minervas_head,_detailed_look.aspx

Helen Harman - Collections Assistant

'Behind-the-Scenes' Tour - Digging a Little Deeper....

Birds Eye View of the Roman Baths

Are you interested in the history or archaeology of your local area? Have you ever wondered what secrets might be hidden beneath your feet? Well, here at the Roman Baths Museum, we may just be able to help you, as we hold archaeological material for the whole of the Bath and North East Somerset area. So what does that mean?

East Baths Bulk Archaeology

Whenever a new building development takes place, part of the process is to record any structures or objects that might be hidden below the surface and it’s these objects and records that form a large part of the Roman Baths Museum collection. Maps, plans, reports and objects from various sites and developments such as the Spa are stored here, as are the objects and records from historic work such as the digging of Victoria Gravel Pit and the excavation of Combe Down Villa. We don’t like to keep these things to ourselves and would invite you to come and visit our stores so we can share this information with you.

Roman bath stone burial chest-cist - Combe Down BATRM 1983.12.h.10 circa 200AD

So, if you fancy doing something a little bit different, then why not come along on a Behind-the-Scenes Tour? These Tours offer a glimpse into the stores of the Roman Baths Museum, and a chance to look ‘behind-the-scenes’ at the everyday workings of a busy and vibrant museum. See and handle objects in the reserve collections and find out why and how we care for them.

Forthcoming Tour Dates:
Thursday 26 January 2.30pm
Thursday 23 February 10.30am
Thursday 29 March 2.30pm
Thursday 26 April 10.00am

Numbers are strictly limited so advance booking is necessary on 01225 477779.

All children must be accompanied by an adult and have to be aged ten or over.

Normal admission charges apply. Bath and North East Somerset residents free with a Discovery Card.

For all the latest 'Behind-the-Scenes' Tour dates please follow this link

http://www.romanbaths.co.uk/whats_on/events/events_calendar/tunnel_tours_and_store_tours.aspx

Helen Harman - Collections Assistant