Cleaning and Preservation of Coins - A Complete Guide, page 24
Repairing Bent Coins & Restoring Coin's Worn Impression to Visibility
REPAIRING BENT COINS
Before attempting to treat a bent coin, it should be determined positively that a rolled impression was not involved in the manufacture.
This method was used in the 17th century especially. The blanks were passed between two dies mounted on rolls, which gave the coins a slight curvature on one side frequently.
The coins can be unbent by the following methods:
1) Bent coins cannot be straightened again by a vise, but can be hammered between two lead plates.
2) Another method involves hammering the bent coin between two beech wood blocks.
If a kink accompanies the curvature of the coin, it is practically impossible to repair the damage.
RESTORING WORN IMPRESSION TO VISIBILITY
To restore or see the worn former impressions of a coin, the following methods can be used:
1) The worn former impression of a coin can be seen or photographed in parallel light (lateral illumination).
2) The worn former impression can be seen or photographed at a certain instant during annealing of such a coin. For this purpose the coin is placed on a steel sheet over the flame of a gas stove or on a glowing hot plate.
The fact is that the metal crystals are more compressed in the region of the field than in the relief impression areas such as the inscription and figures. The coins come to incandescence at different times because of this.
3) This method also depends upon the difference in COMPACTION of the metal crystals. The coins involved are immersed in a bath of Nitric acid (HNO3), turned around and taken out with two wooden sticks after a few seconds. They are then washed in distilled water.
The nitric acid attacks the coin metal differently according to the density differences which are then revealed. If necessary, the process is repeated several times.
After the former impression is sufficiently perceptible, the coin is dipped into pure alcohol or spirit and then dried. This method may be used for silver, copper, bronze and nickel coins.
4) For gold coins, Aqua Regia must be used; it is to be applied with care.
Aqua Regia is prepared by mixing three parts of concentrated Hydrochloric acid (HCl) with one part concentrated Nitric acid (HNO3) followed by dilution to about 1:10.
NOTE: Aqua Regia is the only acid that will dissolve gold and platinum.
After immersion the gold coin is rinsed and then soaked for 30 minutes with frequent changes of water.
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