coin, silver

 

origin: Midlaren (province of Drenthe, The Netherlands)

date: 822-840 AD

 

 

 

This coin has been found during the excavation of a settlement in Midlaren (Drenthe, the Netherlands). The excavation of the site was carried out by the Groningen Institute of Archaeology (GIA), in cooperation with ARC1. The occupation of the settlement covers the period between 200 BC - 1100 AD, after which it was abandoned.

Figures 1 and 2 show the object shortly after excavation. Although the coin marks can be detected on either side under a layer of dirt and corrosion, a precise dating was not possible. This gave cause for cleaning of the coin.

Microscopical investigation confirmed our presumption that this coin is a silver based alloy. The visible corrosion products  consist of silver chloride (AgCl) and silver bromide (AgBr).
silver chloride can be recognised as a relatively soft, wax like matter, in colour grey to white.The official name of this mineral is Cerargirite or horn silver,  aiming on the possibility to cut this metarial with a knife, comparable to cutting a piece of bone or antler. Silver bromide or Bromirite is hard and opaque brown,  though it may show pinkish stains due to the presence of Cuprite (Cu2O), or it may have darkened due to the presence of copper/silver sulfides. When severe corrosion is present, the combination of silver chloride and silver bromide is detectable as a thick, sometimes swollen crust with incluisons of sand and dirt. The coin presented here posesses all of the characteristics described. Possibly, only a small amount of the original silver remains2.

Silver is a rather soft metal, due to which it is not uncommon to find corrosion products that are harder compared to the underlying metal.  Some silver objects develop a thin layer of secondary silver or irregularly deposited silver chloride between the original metal and the hard corrosion products. In these cases the corrosion crust can be chipped of by means of a scalpel. It involves a great deal of skill to avoid damaging the metal2. The coin presented here does not s suchowh a layer. Therefore the coin will be cleaned by means of electrochemical treatment and this is only possible when the object posesses a metal core. The treatment must be carried out in a series of brief and monitored sessions.

 

treatment
An x-ray image - before treatment - indicated the presence of sufficient silver to perform the suggested approach. On the basis of methods published3 the cleaning has been carried out using a  5% solution of formic acid (HCO2H) in destilled water. The coin has been attached to a stainless steel rod, functioning as the cathode. Stainless steel anodes at either side had a total surface of 10 cm2. The treatment has been carried out with 8.2V/0,4A. In the beginning each session lasted 15 seconds. After e few sessions the indication of silver amount still being present was confirmed. The treatment sessions were extended to 1 minute each. In between sessions the object has been placed briefly in an ultrasonic bath with ethanol. The entire electrochemical treatment time took 34 minutes. 


passive conservation
After treatment the object must be stored under conditioned circumstances. Sulphur in the surrounding air may give the surface a dark or dull appearance. Storage in an air tight box or container prevails here. Sulphur may also be present in the package materials or the exhibition show case. Both should be checked for this possible threat. 
It is not possible to garuantee the removal of all of the chloride contamination. For this reason the coin should be kept under dry atmosphere in the air tight box by means of silicagel2.

Figures 3 and 4 show the coin after treatment. On the front it reads: XPISTIANA RELIGIO, 'Christian faith'. In the centre is depicted a temple. The back shows a cross with four dots and the text:  LUDOVICCUS IMP, 'Louis emperor'. Louis the Pius (768-840) has been one of the Carolingian emperors. He reigned between 814-840 AD.

After treatment the coin can be dated on the basis of coin marks: 822-840 AD. Coins with these marks have been minted throughout the entire Carolingian empire, usually without a specific mint location4.

 

1 ARC: Archaeological Research & Consultancy, www.arcbv.nl

 

literatuur:

2 Cronyn, J.M.; The Elements of Archaeological Conservation; London 1990; pp 230-235.
 
3 Stambolov, T.; The Corrosion and Conservation of Metallic Antiquities and Works of Art; Amsterdam, 1985; pp 186-198.
 
4 Grierson, P. en Blackburn, M.; Medieval European Coinage, 1 The Early Middle Ages (5th - 10th centuries); Cambridge, 1986; p 530 en plaat 36