Brooching questions: conserving a silver brooch from the Galloway Hoard

After spending over 1,000 years in the soils of Galloway, it’s no wonder objects from the Galloway Hoard needed some serious work before going on display. Bethan Bryan talks through the complex process of conserving a silver brooch, from mending “mini-volcanoes” of copper erosion to using porcupine quills and algae-derived gels.

It has been a delight and privilege to conserve one of the brooches from the Galloway Hoard. My background is in early medieval archaeology, and so it was a dream come true to be offered the chance to work on such a beautiful and historically important object. The work involved detailed scientific examination and analysis of the corrosion and materials of the brooch, and then conservation of the brooch itself.

A circular brooch densely decorated with interlaced vines and four enigmatic beasts, one in each of the brooch's four quadrants. The brooch is tinted green with age.
The brooch before conservation work began.

The Galloway Hoard, discovered in September 2014 by a metal detectorist, contains over 100 objects including silver jewellery and large quantities of silver ingots. Many of the hoard’s objects, including several silver brooches, were contained within a silver gilt vessel buried at the beginning of the 10th century. Clearly, silver makes up a considerable part of the Galloway Hoard’s story.

Top-down view into a barrel-shaped vessel. Inside the vessel are a tangle of objects including a heavily oxidised circular brooch, clumps of textile, and other metallic objects.
The brooches within the copper vessel in which they were discovered. Image via AOC Archaeology.

The object I worked on was a silver and copper-alloy composite disc brooch. It is the smaller of a pair of very similar brooches within the hoard, which overall contained seven Anglo-Saxon disc brooches of a kind not found before in Scotland. A similar brooch was found in the Pentney Hoard from Norfolk, England.

Side-by-side look at the front of the circular brooch before conservation laid on a white surface next to a ruler, and a closeup of two of the beasts on the brooch facing each other. They resemble dogs or wolves.
The larger brooch of the pair from the Galloway Hoard and a close-up of two of its beast-heads.
Two images side-by-side showing the front (left) and back (right) of a different circular brooch. This one has a flowing interlace design with gold colouring, and its back is rusted and flat with a simple, broad pin.
Front and back of a similar Anglo-Saxon brooch from the Pentney Hoard. Image via the British Museum.

The front of the brooch is a disc of silver with pierced or/cut open-work vine decoration and a central, equal-armed cross. The gilded front surface of the copper-alloy back of the disc can be seen through the cut. The front of the brooch is decorated with four open-mouthed beasts’ heads, each adorned with niello inserts. Niello is a black metallic alloy of sulphur, usually alloyed with silver or copper. It is used to fill designs that have been cut into the silver surface. There are also five silver, spherical bosses. The silver and copper-alloy discs are riveted together with silver pins. On the back of the brooch is a spring-hinged pin and catch plate also riveted on to the copper-alloy disc.

Side-by-side images of a closeup of a beast head and metal studs on the main circular brooch, and the back of the brooch showing the pin with a thick layer of green oxidised material on it.
Left: One of the beast-heads of the front of the brooch with the spherical silver bosses and two arms of the central cross (before conservation). Right: The back of the brooch showing the clasp for the pin and the rivets holding the back plate to the silver front disc (before conservation).

The decorated front has been cut by hand. Imperfections can allow us to feel a human connection with the maker; tool cut marks are visible in places on the silver, and the vines are irregular in shape and size. The four beasts’ heads have also been decorated individually, each bearing its own unique pattern of niello-inlay.

A very wide view in microscopic detail of the interlace design on the brooch. Four red circles highlight areas where there are maker's marks or small imperfections in the metalwork.
On these photomicrographs we can see the cut marks left by the sharp metal tools used to cut out the vine-scroll decoration (before conservation).

Before any conservation work could begin, the brooch was fully assessed, recorded and photographed. This included taking X-rays and photomicrographs to record the surface in detail, small samples of the corrosion products, and analysis under the Scanning Electron Microscope.

First-person view of a desk in a busy lab environment. A computer monitor shows an extreme closeup of one of the beast heads on the brooch. Objects on the desk include a microscope, notebook, and various brushes.
The recording of the brooch before conservation included taking photomicrographs of the surface with a digital microscope. The software used enabled us to stack the photos, allowing us to obtain images with a much larger depth of field than would be possible in a single photo.

Once the brooch was fully recorded, it was time to conserve it. The back of the brooch was heavily corroded around the edges. Cleaning tests were performed on samples of copper alloys and silver by Dr Mary Davis, the lead conservator for the Galloway Hoard project, to assess their potential effect on the brooch.

  • The back of the brooch before conservation, showing the copper-alloy back plate and spring-hinged bin and clasp, and the heavy copper corrosion around the edges.

It was decided from these tests that the best way to remove the corrosion from the copper-alloy back-plate of the brooch was to do so manually with a scalpel, under a microscope. This way the corrosion removal could be carefully controlled, and no chemicals would potentially change the underlying surface condition and colour of the copper-alloy.

A woman with blonde hair, wearing a dark green plaid shirt and purple gloves, sits upright at a desk while examining the brooch through a large microscope. She holds a metal utensil and is cleaning the brooch very carefully.
Bethan Bryan removing the copper corrosion from the back-plate under a microscope.

The copper-alloy back-plate was very thin and delicate in many places, especially around the edges. Some areas had to be consolidated with Paraloid B72 to give them enough strength to prevent loss of material during the conservation process. Paraloid B72 is an acrylic polymer and is used a lot in conservation as both an adhesive and a consolidant. In some places it was discovered that the back-plate had corroded through entirely, and so removal of the corrosion revealed small holes. The edges of these holes were also consolidated with Paraloid B72.

Collage of four images, each showing the circular brooch with beasts heads partway through conservation. It is now much shinier than before, with the gold-coloured metal shining through.
The gold surface of the copper-alloy back plate was revealed slowly by removing the corrosion spots with a porcupine quill.

There was much less corrosion on the front of the brooch, and the silver was in a better and more stable condition than the copper-alloy of the back plate. The corrosion on the gilt surface of the front of the back-plate was removed with a porcupine quill tip, under a microscope. The porcupine quill was strong enough to manually remove the copper corrosion spots, but soft enough not to scratch the soft and thin layer of gold.

Closeup of the bottom half of the circular brooch, highlighting corrosion along the edges of the metal interlace decoration.
Most of the friable corrosion could be removed from the surface of the silver disc with a scalpel, leaving the tougher corrosion to be softened with a gel before removal.

The softer areas of green copper corrosion on the front silver disc of the brooch were also removed manually with a scalpel, under a microscope. The tougher corrosion spots on the silver were first softened with an agarose gel, and then removed with the scalpel.

Agarose gel is made from agar and is non-toxic and eco-friendly. Agar is derived from certain types of algae and is a type of carbohydrate (for example, starch or cellulose) called a polysaccharide. It is soluble in hot water, can be used to form a gel and is stable in both alkaline and acidic conditions. The gel acts as a molecular sponge and solubilises impurities, drawing them away from the surface, and then holds the impurities within the gel matrix. The gel I used included in the mix alkaline Rochelle solution (a mix of sodium hydroxide, potassium sodium tartrate and water). This solution has been used at the British Museum for the past 50 years for cleaning debased silver coins (silver which has a large copper content).

Side-by-side views through a microscope of the brooch. On the left you see it in full with a translucent gel applied to it. On the right is an extreme closeup of one of the metal studs partly covered in the gel.
Applying the agarose gel to one of the silver bosses to soften the copper corrosion spots. The agarose gel was applied to soften copper corrosion spots on the silver disc.

In some parts, you can see where the copper-alloy back-plate has completely corroded, even creating small holes through the gold layer on the front where the corrosion products have forced their way through. Many copper corrosion products take up more room than just pure copper metal or the copper-alloys used in metalwork; they are more voluminous. The result of this is what I like to call ‘mini-volcanoes’ of copper corrosion bursting through the gold layer.

Extreme microscopic closeup of a triangular section of copper framed by silver lines. Tiny green dots of corrosion break through the copper in several places.
An area on the front gilded copper-alloy plate where the copper has completed corroded away and corrosion products have forced their way through the surface of the gold.

Once the corrosion products were removed from both the silver disc and the gilt surfaces, I consolidated the gold front of the copper-alloy back-plate this time with Paraloid B48N. This is stronger than Paraloid B72. I used this to provide extra strength to the gold on the corroded copper-alloy disc and the front of the disc itself. Doing this, would hopefully allow for further removal of corrosion from the back of the brooch.

The circular brooch rests atop four rectangular pads under a bright light. A metal tool is being used to delicately remove corrosion from its copper and green surface.
Consolidating the back of the brooch.

Once the gilt front of the copper-alloy disc had been consolidated, more of the voluminous copper corrosion could be removed safely from the back to reveal the original surfaces. This was a time-consuming and delicate job with a scalpel and dental tools, as the copper-alloy disc was still very fragile.

The almost fully conserved circular brooch viewed head-on against a white surface. All traces of green corrosion are gone, emphasising the silver interlace pattern and gold-coloured base.
The silver front of the brooch with the conservation process almost complete.

Once the corrosion had been removed to an acceptable level and the conservation process was complete on the brooch, I consolidated the whole of the brooch with a protective layer of B48N. This acts as a consolidant, lacquer and moisture barrier to protect the brooch. The brooch was now ready for display.

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