Archaeological Iron

Many objects come to us from museums and require conservation for repairs, in preparation for display, or to protect from further deterioration. Some objects however come from archaeologists and require conservation for research purposes.

My first solo venture was to conserve some archaeological iron which had been excavated from Ham Hill – the biggest Hill Fort in Britain. I had 17 iron objects in total, all covered in dirt and several layers of corrosion, obscuring their original shape; looking at them you wouldn't know that beneath all the dirt was an actual object!

The largest of my iron objects before treatment

The conservation requirement was to reveal the shape and enable the archaeologists to record accurately what they had found; after which they would be put into storage. So the conservation requirement was simple enough – clean.

In this context it required removal of the dirt and top layer of corrosion to reveal a more stable and aesthetically pleasing magnetite layer underneath. The original iron of the object is now vulnerable; if exposed it can react with moisture and oxygen in the environment and cause further corrosion and damage (you may come back to the piece in a few years to find it has completely disintegrated!)

Magnetite: A stable layer of corrosion; grey in colour, it shows the original shape of the iron underneath it, while protecting the iron from further corrosion.

X-Rays provided by my tutor allowed me to see the shape and amount of the original iron left underneath the corrosion, and gave me a guide for where to clean to.

And what did all this mean? This meant it was time to get to grips with the Air Abrasive!

The Air Abrasive machines use Aluminium Oxide powder to blast away the corrosion; you can vary the pressure and powder flow to control how powerful it is. The work is done under a glove box, with gloves, lab coat and dust mask. Here’s me air abrading!

Me using the Air Abrasive machine to clean one of my pieces of iron

Blast the corrosion off with a nozzle, seems simple right? It is harder than it seems! Removing the outer dirt was simple enough and gave me a strange sense of satisfaction, rather like power washing dirt off of a car and making it all clean and sparkly.

Revealing an even magnetite layer without exposing any of the iron underneath it however requires a lot of patience and concentration – particularly where the magnetite is thin. I was having difficulties, but a discussion with one of the tutors soon sorted me out.

Some of the pieces arrived broken and many were cracked and on the verge of falling apart. I did my research and decided to repair using Epoxy-Resin to restore the objects’ original shape, and consolidate using Paraloid B72 in Acetone to prevent further breakages.

Epoxy-Resin: I used a 5 minute epoxy which is a two part adhesive – this means that it dries 5 minutes after mixing together so no hanging around!

Paraloid B72 in Acetone: Paraloid B72 is an adhesive soluble in acetone. Being dissolved in Acetone makes it easy to work with and on drying the acetone evaporates, leaving the Paraloid in place. It is a favourite in the lab!

And here we have it, a finished article ...

The same iron object after treatment.

This is a Bill Hook - the cleaning revealed the sharp edge on the outside and you can see where it was once attached to the handle

Below is my favourite, but most challenging object: