About the restoration of the coins

 
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introduction

In addition to the digitization project in the State Museum of Prehistory in Halle (Saale), coins that had not yet been restored were processed from April 15, 2019 to February 29, 2020.
Most of these are found coins from antiquity to the 20th century, with the majority of the restored coins dating from the late Middle Ages and the early modern period.
Most of the alloys used at that time were represented: silver alloys, billon (copper-silver alloys with a low silver content) as well as bronze and copper coins.

State

While many silver coins were in good condition, the billon coins and coins made of copper alloys were partly very badly corroded. Some coins, mostly bracteates - were broken and had to be glued.

Restorative measures

The primary goal of the restoration work was to expose the original surface in order to make the legend of the coin legible again. In addition, the coins should be preserved to protect them from further corrosion and damage.
Chemical cleaning measures were primarily used for the silver and billon coins. Mechanical cleaning was only partially an option as an additional measure, as it can easily damage the relatively soft surface.
In contrast, bronze coins were mostly only cleaned mechanically. The stable green discolored patina, mostly consisting of copper carbonates, was retained for conservation and aesthetic reasons. The dangerous copper chlorides were removed as far as possible.

Chemical cleaning

The main agent used for cleaning the surface of copper compounds was EDTA as a 0.1 molar solution in water (37.224 g in 1000 ml). To accelerate the cleaning, the temperature was increased to 50 ° C if necessary. Usually 2-3 hours are enough to get good results.

If the corrosion layers could not be dissolved by EDTA, the chemicals given below were also used.

Formic acid (15%): to dissolve stubborn corrosion particles (often a mixture of copper carbonates , copper oxides and possibly copper and silver chlorides). It was also applied selectively in the form of a paste (meerschaum powder).

Sodium hydrogen sulfate solution (10%): to dissolve thick and hard copper oxide layers.

30% sodium thiosulphate or 15% ammonium thiosulphate solution: to remove the silver chlorides (horn silver) .

5% potassium peroxydisulfate solution: to remove copper deposited on the surface.

5% thiourea with 1% formic acid: partial removal of sulphide patina
 
The exposure time should always be kept as short as possible for all chemicals. Usually 30-60 minutes are enough.To observe the progress, the dissolved metal connections were repeatedly wiped off the surface with a soft brush.

The only dry cleaning applied to coins made from copper compounds was the 15% signette salt solution. The solution is slightly basic (8 pH) and only removes the salts of the copper and not the underlying oxides. This happens very slowly (12 to 24 hours) so that the process remains very controllable.

The Seignette salt (potassium sodium tartar) can also be used with silver coins, for example if only the least possible removal is intended.
 
Mechanical cleaning

The following tools were used for this cleaning: scalpels, triangular scrapers, fiberglass pens, burins as well as various brushes and rotating brushes. Soft abrasives such as zinc or calcite powder were also used for thin tempering patina (sulfide patina). The powder was rubbed onto the metal bumps with acetone and cotton swabs.

Bonding and consolidation

The bond was made with 30% Paraloid B 72 in ethyl acetate. If necessary, the coins were partially or completely covered on one side with fine Japanese paper (soaked with 20% Paraloid B 72).

Preservation

After chemical cleaning, the coins were cleaned in running water, placed in acetone for 24 hours and then dried on a hot plate.
They were then preserved with 10% Paraloid B 72 in ethyl acetate to protect them from further corrosion and incorrect handling.
The coins made of copper alloys also received a protective layer made of 10% Cosmoloid H 80.

The coins with a very strong and active copper chloride corrosion were previously treated with BTA (5% solution in ethanol for 2 hours at 60 ° C).