Cleaning and Preservation of Coins - A Complete Guide, page 24
How To Straighten Bent Silver Coin Using Annealing Technique, Restore Impression
1. HOW TO UNBEND BENT COINS
The majority of cases of dug bent coins involve thin-sectioned silver or billon coins that were hammered or milled during Medieval and Late Medieval times. Such coins are not always of high silver content as they were made mostly from silver alloyed with various amounts of copper. This was done by medieval moneyers, and is still done by the coin mints today, for the purpose of not only creating and improving specific working properties of coinage - malleability (metal's ability to deform by hammering, rolling or pressing), density, hardness, and corrosion resistance, but also increasing the cost-effectiveness of the coin production.
For every category or group of coins, the silver content is reflected by Millesimal Fineness - a system of denoting the purity of silver alloys by parts per thousand of pure silver by mass in the alloy. For example, an alloy containing 62.5% silver is denoted as "625" or "625/1000" or ".625". The coin's millesimal fineness value should be taken into consideration before attempting any coin repair! Concerning the process of straightening a bent coin, the higher the millesimal fineness, the lesser a risk of destroying the coin during the process. For straightening bent coins, the millesimal fineness is an extremely important characteristic because this process involves ANNEALING.
Annealing a coin is the process of heating the coin to about 2/3 of its smelting point (see details below) and either allowing the coin to cool very slowly or quenching it in water. The coin annealing is used to restore the original structure or a regular pattern - lattice, of the coin metal, i.e. to allow the metal grains (crystallized molecules) to recrystalize, and the stresses built up in the coin are relaxed - any dislocations inside the lattice are effectively reset. Thus, annealing restores the original qualities of the coin metal. When the coin is bent, the grains undergo the process of work-hardening - they become smaller, and this causes Embrittlement - "precipitation of copper" within the lattice. Embrittlement can be described as separation of copper from its bond with silver in a silver alloy.
So if your bent coin's millesimal fineness is less than "203", you will most likely end up snapping the coin while straightening it because the copper's malleability is lower than the malleability of silver. Or the bent coin with low silver content will even crumble under a slight pressure after annealing! Annealing a badly embrittled coin might actually weaken it to a frangibility point. It would be best not to unbend such a coin. This is why it is important to know the coin metal's composition and millesimal fineness in order to make a right decision (whether or not to unbend the coin). All necessary information on coins in focus can be obtained from a detailed coin catalogue which you should have had by now.
Before attempting to treat the 17th century bent coin, it should be determined positively that a slight curvature on one side of the coin was not caused by the manufacturing process. During the process of milling coins, the blanks were passed between two dies mounted on rolls. When the rolls jammed for a second, the coins would come out slightly bent.
To straighten a bent silver coin successfully, you should follow these simple steps:
Step 1: Clean a bent coin thoroughly beforehand using an appropriate method which you can select from the "Cleaning Methods For Silver Coins" described on page 16. While being heated, all surface corrosion products such as sulphides, chlorides and oxides may not only cause terrible pitting on the coin's surface, but also fuse with the silver substratum at a coin's bend; thus, weakening it more.
Step 2: Make sure that you obtain and have the following items at hand:
• 1) a large plastic Rectangular Pan filled half way with distilled water;
• 2) a small stainless-steel Sauce Pan that is placed upside-down into the pan; the water level being half way below the saucepan's bottom (now top);
• 3) a gas or electric Oven or a smooth, flat and clean Steal Plate that can be heated on a gas burner;
• 4) a small propane gas Blow Torch (some hobby shops or a jewelry supply stores carry small blow torches);
• 5) a set of small Bent Nose Pliers with jaws "clothed" in rubber or chamois leather sleeves, or any non-metal tool to be used for actual straightening of the coin. Do not use bare metal tools for straightening the coin not to damage its surface and reduce its value. Use something softer than metal to cover working surfaces.
• 6) two pieces of thin sheet plastic (they can be cut out of a rectangular Philadelphia Cream Cheese container, or an old credit/debit card);
• 7) a round Dowel - a small rounded piece of hard wood (for final stages of straightening the coin using a rolling method);
• 8) a Block of smoothed wood to flatten the coin (the final stage of straightening).
Step 3: Put the coin in the oven and gradually heat it up to 350°F (177°C) slowly raising the temperature. This procedure will ensure that the coin will be able to handle the shock of heating by the blow torch next.
Step 4: As soon as the coin starts changing color, remove it from the oven and place it on the upside down sauce pan that is inside the plastic pan with water.
Step 5: Use the blow torch to heat the coin until it turns DULL PINK in color. This will indicate that a temperature of around 450°C-500°C (842°F-932°F) has been achieved, and the coin is now ready to undergo the second part of the annealing process. If the coin has been heated to a cherry red color, there is a risk of overheating it, and, instead of annealing the coin to achieve malleability, you may change the coin metal's properties for the worse. Wait until the coin cools down a little bit and...
Step 6: ...immediately quench the coin by shifting it off the sauce pan into the water. After the coin cools off, take it out and dry.
Step 7: Now the coin is ready for its FIRST straightening procedure. An illustrated example of unbending a crumpled coin using common plastic knives is given in my small Tutorial (excluding the coin annealing procedures):
Step 8: Do NOT try to straighten the coin after the first annealing! The best results are achieved if you unbend the coin 1 mm after each annealing. You can carry out several annealings without having adverse effects on the coin. And the less pressure you apply to straighten the coin, the better your chances of completing the task successfully. In case of crystalline silver, if the coin does not seem to be strong enough to be unbent more, let it cool down without quenching after heating it up and apply a little pressure with your finger to see if the coin is likely to snap. If it is, you should stop the process.
Step 9: Deliberately repeat the steps 5 to 7 as many times as necessary until the coin is almost unbent. The coin must be re-annealed after each unbending to reverse the work-hardening caused by applied pressure. When getting closer to a final stage of straightening, use a rolling technique with a round dowel on the coin placed on paper.
Step 10: Use the wooden block to completely flatten the coin. Heat up the coin for the last time but do NOT quench it in water! Allow the coin to cool down naturally. And... voilà! - you got a straight coin!
Now you can either tarnish the silver coin a little to give it a "naturally aged" look or just coat it with the microcrystalline wax for protection against affects of environment.
2. HOW TO RESTORE WORN-OUT COIN'S 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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