A Boss worth Battling for!

By Carola Del Mese, MA Conservation Studies student specialising in Metals

Explore Categories

Fig 1 : Left – Iron shield boss from Worthing Museum in need of conservation. Right – Example of a conserved shield boss from the Netherlands.
Fig 1 : Left – Iron shield boss from Worthing Museum in need of conservation. Right – Example of a conserved shield boss from the Netherlands.

Shield Wall!

If you are a fan of the Vikings you will have heard of a shield wall and a shield maiden, but what is a shield boss? A shield boss is the central part of a shield, and this one has definitely been in the wars. (Fig. 1, left) It belongs to Worthing Museum and came from a Saxon warrior graveyard in Sussex. It is around 1500 years old, and although it now looks completely unrecognisable, it was once similar to the image in Fig 1, right. As we can see, it’s completely corroded and cannot be restored, so what can be done when an object is not what it originally was?

When is a shield boss not a shield boss?

Worthing Museum has several complete shield bosses, so this one won’t go on display. The object’s new function will be for research - it is now a very interesting historical document! Its features can tell us a number of things:

  • The shape - it fits into a recognised category (Dickinson and Heinrich 1992) and will add to historical knowledge of the Saxon people it belonged to, showing how far they travelled, and maybe even where they came from.
  • It will add to the history of the Sussex area.
  • There may be ‘mineralised organic’ remains of wood or leather attached to the back which could reveal more about the shield itself. (‘Mineralised organics’ are fragile remains of organic matter preserved within the corrosion).
  • There could be some remaining metal and features such as rivets - analysis could be carried out by future researchers.

Taking all this into account, I drew up a battle plan to preserve the boss.

The Battle Plan

The first step was to x-ray the object, which confirmed that it was mainly held together by corrosion and soil. (Fig 2) The x-ray also showed there was still some fragile metal remaining around the ‘walls’, suggesting that a corrosion inhibitor should be included in our plan.

The second step was to take detailed photographs – you can never take too many! If any of the tiny fragments collapse, the photos will help to reconstruct the jigsaw puzzle. (Fig 3)

Fig 2 : X-ray - plan view of the shield boss.
Fig 2 : X-ray - plan view of the shield boss.
Fig 3: Detailed photograph showing placement of the fragments
Fig 3: Detailed photograph showing placement of the fragments

The Strategy

  1. Investigate how much of the object is still metallic by using a weak magnet. This indicates where to apply the corrosion inhibitor.
  2. Gently explore how much of the outer layer consists of loose fragments, and how much is still attached to the underlying layer. If some fragments are resting on others, I will need to consolidate (strengthen) the underlying layer first, before attaching the loose fragments on top. (Fig 4)
  3. Take note of any mineralised organics attached to the surface.
  4. What to do with the soil? Iron corrosion will grow to encompass its immediate environment. In this case, the soil and corrosion have fused, and removing it would cause serious damage to the boss. In this case, the soil will be left in place until the loose fragments are consolidated, then it can be reconsidered.
  5. The remaining iron may still corrode, so tannic acid can be used as a corrosion inhibitor. It will darken the surface, however the colour change is acceptable. (Figs 5 and 6)
  6. Begin to consolidate the loose fragments from the centre. Use a higher concentration for broken and loose fragments. For cracks, apply the lower concentration solution by dripping small amounts from a brush and allowing it to travel along the cracks by capillary action.
  7. If there are mineralised organics, what is the safest option? They could be left in position or removed to safe storage.
  8. When the outer layer is stable, it can be turned over and gently rested in a Tyvek™ cushion (lint and acid-free ‘fabric’) loosely filled with polystyrene balls for support. Then the inside can be assessed and the next steps planned.

Fig 4 : Replacing a tiny fragment using a magnet
Fig 4 : Replacing a tiny fragment using a magnet
Fig 5 : Tannic acid tests of varying strength solutions
Fig 5 : Tannic acid tests of varying strength solutions
Fig 6 : Brushing  the tannic acid solution on to the remaining metal
Fig 6 : Brushing the tannic acid solution on to the remaining metal

Let’s regroup!

This is a good time to evaluate the progress of the treatment and consider any further actions. If the shield boss is now stable, it should be stored in a low relative humidity environment and checked regularly for signs of corrosion. This will be a challenging treatment, requiring a steady hand and patient approach, and it may take some time to complete.

If you are interested in archaeological conservation see the ‘Further Reading’ section below.

References:

Dickinson T, Heinrich H. (1992), ‘II: Typology of Metal Shield Fittings’. Archaeologia, 110, pp 430 doi:10.1017/S0261340900028137

Further Reading:

Laird, J. Viggers, M. (2014) ‘Object biographies: An Anglo Saxon shield boss from Magdalen Bridge, Oxford.’ https://britisharchaeology.ashmus.ox.ac.uk/east-oxford/ob-shield-boss.html Accessed 5/12/21

Caple, C. (2021) ‘The Challenges of Archaeological Conservation’, in Caple C, Garlick V. (eds) Studies in Archaeological Conservation. London: Routledge

Figures – All images of the shield boss were taken with the permission of Worthing Museum.

Fig 1 left – Author’s own

Fig 1 right - https://geheugen.delpher.nl/nl/geheugen/view?coll=ngvn&identifier=RMO01%3A008541  Accessed 6/12/21

Fig 2 – X-ray. 3mA, 85 seconds, 90kV.

Figs 3-7 – Author’s own.