Conservation | Portable Antiquities Scheme blogs

Conservation Cleaning

To reveal important surface information (as described in the previous blog post), the coins have to be cleaned of surface dirt and corrosion products. This is done using a combination of two treatments; chemical and mechanical cleaning.

The photograph above demonstrates a typical example of the Frome coins once they have had an initial rinse in tap water and brush to remove surface dirt. The majority of coins in this photograph are covered in corrosion products rendering them illegible. These coins would first be treated chemically to reduce the majority of the corrosion products.

Chemical Cleaning

Two different chemical treatments are employed to clean the coins depending on the extent and type of corrosion present. After an initial visual assessment the conservator they will make a decision as to which chemical treatment is required. This is usually a treatment consisting of either a low percentage of formic acid, or an Alkaline solution, or both.

Chemical cleaning allows for large batches of coins to be cleaned at once, which is critical when cleaning such a large hoard. The conservator carefully monitors changes in the coins appearance and condition during chemical cleaning to ensure that the coins are not ‘over cleaned’. All coins which have been chemically cleaned are thoroughly rinsed with water to remove any traces of chemicals.

Coins in Alkaline Rochelle solution. This helps to reduce the level of blue/green copper carbonates.

Mechanical Cleaning

Mechanical cleaning is required when coins have small patches of corrosion obscuring important details. Coins with low levels of corrosion may not need chemical treatment, but will most likely require some mechanical cleaning. All coins which have been chemically cleaned require some form of mechanical cleaning.

Production line of Dremel rotary brushing.

Mechanical cleaning involves reducing corrosion products with a scalpel under magnification and brushing to remove surface corrosion product residue. The photo above shows a production line of conservators using natural fibre dental brushes, on rotary Dremels® . This allows for a greater quantity of coins to be brushed at one time, and is more efficient than brushing them by hand.

Following chemical cleaning corrosion and dirt residues remain on the coin. This coin has been brushed on one half to demonstrate the difference in legibility before and after.

Corrosion

This coin demonstrates the large red/purple crystals of cuprite on the lower half and green copper carbonate corrosion products on the upper half.

The amount of corrosion on the coins varied depending on where they were excavated from in the ceramic vessel. The diagram below shows a cross section of the vessel and the context layers as they were excavated. The annotations below describe the changes identified.

Image courtesy of Somerset Heritage Service Archaeology

Top- Layers 10-13- Coins are well preserved. Corrosion is minimal, dusty and easy to remove.

Middle- Layers 14- 17- The corrosion is very hard making the coins difficult and time consuming to clean. Coins display both green carbonates and cuprite corrosion.

Bottom- Layers 18-19- These coins respond to treatment more readily than the middle layers, however they are not as well preserved as those from the top layers.

The coins from the Frome Hoard were excavated from an agricultural site. Since its burial it is likely that the effects of a fluctuating water table have influenced how the coins have corroded. As the water level rose it caused the bottom of the pot to become more saturated than the top. The ceramic vessel in which the coins were contained would have caused any trapped water to drain slowly. The corrosion pattern noted allowed conservators to anticipate how difficult some coins may be to clean therefore influencing which treatments would be selected.

52,503 Coins: Where to start?

The Questions

Dave Crisp at the side of the trench

The quick thinking of metal detectorist David Crisp meant that the Frome hoard was not disturbed upon its initial discovery. This crucial action has allowed for a systematic study of the hoard beginning with its archaeological excavation. As the pot was excavated the ceramic sherds and coins were removed in layers. Each layer was carefully numbered and individually packaged. The British Museum received the hoard in around 60 bags, there being several bags from each layer of the pot. It was hoped that by excavating the hoard in layers it would reveal clues as to why and how the hoard was collected and deposited. These context layers formed a framework for organising the coins and retaining essential deposition information. Having reliable contextual information can help us answer questions like:

Were all the coins put in the pot at the same time? Because the latest coins, of Carausius, are in the middle of the pot this does seem to be the case.
Did the coins come from a variety of different individuals or sources? The answer seems to be yes because there are two distinct groups with coins of Carausius in them: a number of earlier Carausian coins were in the last group to be put in the hoard, at the top of the pot, while a large number of later Carausian coins were in a group in the middle of the pot.
How many different groups (smaller pots, leather bags etc) were emptied into the hoard? We hope that by analysing all the coins, by layer and bag once they have been catalogued, we will gain an insight into how many different groups of coins were in the hoard.

Forward towards some answers

The information stamped onto the coins can help answer these sorts of questions. The LEGEND is the text on a coin which names the emperor (Augustus and Caesar in Latin) ruling at the time of issue. Other titles include commander (imperator), pious (pius) and blessed (felix). Because the Frome hoard was buried towards the end of period in which there were several civil wars and many barbarian invasions, there are around 25 rulers represented over a 40 year period (AD 253 to around 290) The mintmarks on the reverse of the coin often appear in the FIELDS and the EXERGUE of the coin, giving us important information about where and when a coin was struck. In the illustration of a coin of Carausius (AD 286-93) below, the letters ‘B’ and ‘E’ are mint control letters in the field of the coin – we do not know what they stand for, but there was a large issue of coins bearing ‘B E’. The ‘MLXXI’ in the exergue stands for ‘Moneta Londiniensis’ (the mint of London), one part silver to twenty parts copper (i.e. 5% silver). The mint marks are often recorded in the format shown at the bottom right of the image. Carausius reigned from around AD 286 to 293, but we know that this coin was struck in the middle of his reign, around AD 290 and is one of the latest coins in the hoard. Carausius set up a breakaway empire in Britain and Pax (the personification of peace, as seen on this coin) was the most common reverse type used on his coinage.

A conserved coin of Carausius

After their initial rinse in 2010, the coins were further sorted into bags by emperor, still depending on the layer they were from. Funds were then raised to conserve the approximately 30,000 coins needing further conservation, from the total of 52,503 pieces in the hoard. The relative rarity of Carausius’ coins made them our first conservation priority. The next priority was the illegible coins, pieces with so much corrosion that it was not possible to identify the emperor. These made up roughly 15% of the hoard. Finally, most of the remaining coins were partially identifiable with discernible emperor types but essential information on the reverse remained obscured. Coins in this condition were third in priority for conservation cleaning and have been selected by the curators.

The coin pictured above has been conserved to a legible standard, that is, all textual information has been sufficiently revealed that our numismatist colleagues can identify them. Notice the portrait of Carausius on the obverse still has his eyes and nose covered by corrosion as these areas are not a research priority. The ‘IMP’ that precedes ‘CARAVSIVS’ remains hidden by the crystals of corrosion products. In this case, revealing the ‘IMP’ was not necessary as it is commonplace to find it preceding the name of the emperor and there is not much space for much else to fit there. However, there are instances where between the ‘IMP’ and the emperor’s name is a string of other letters that are shorthand for additional titles, such as ‘IMP C CARAVSIVS’ where the additional ‘C’ stands for ‘Caesar’. In other situations one finds ‘IMP C M CARAVSIVS’ where ‘C’ is again ‘Caesar’ and ‘M’ is for his given name ‘Mausaeus’. The presence of these additional letters implies that the coins were struck from different dies, which is important information for our investigations.

Carausius- (286-93): One of the five silver denarii from the Frome hoard after conservation, one of the finest specimens of its kind in existence

Staffordshire Hoard conservation opportunities

Birmingham Museums & Art Gallery and Stoke Potteries Museum are seeking expressions of interest for several Conservation opportunities associated with the amazing Staffordshire Hoard. These would suit experienced conservation professionals and students and allow the successful applicants to influence the future of these objects in a world-class museum setting. Competition will no doubt be very fierce for these posts. Below are the basic details for each role and a PDF of the brief and please note that dates of closing differ for the first post and the second two posts.

Hoard Conservation Advisory Panel

For conservation Professionals, Scientists, Archaeologists and related professionals who wish to to join the Hoard Conservation Advisory Panel. The deadline for applications is June 30th. Download information on the Advisory panel specification.

Conservation professional placements

For conservation professionals who wish to take advantage of a unique professional development opportunity through contributing to the conservation of the Staffordshire Hoard as part of a placement. The deadline for applications is July 30th. Download information on the Conservation professionals specification.

Student placements

Student placements to contribute to the conservation of the Staffordshire Hoard. The first deadline for applications is July 30th. Download information on the Student placement specification.

If you require any further information please email Deborah Cane