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How best to straighten a bent hammered coin
A recent discussion on how best to straighten a bent hammered coin prompted me to provide a basic explanation of the relevant metallurgical processes and techniques. My background in sculpture, jewellery and their restoration will assist me to provide this information as the fundamental techniques are similar, However, I am not a coin restorer by profession. In fact I have very little experience with coins, although I have on other archaeological artefacts. I therefore hope that this essay might be viewed as a foundation on which to build, editing and adding to it as new tips and techniques come to light and as such welcome any comments and input from the reader.
Metals are naturally occurring chemical elements. These elements are usually found in combination with other non-metallic element, such as minerals oxides, sulphides, chlorides and so on. It is generally necessary through a process of refining to extract the pure metal component from the non-metallic dross. It is possible to separate these components into nearly a pure state because at high temperature the atoms of individual metals will link together and fuse in a liquid state. This process is called smelting. The atoms of metal cluster together to form molecules which as the liquid cools and returns to solid will create crystals. These crystals are referred to as grains that will order themselves into a regular pattern called a lattice. The structure of this lattice will largely determine the physical properties of that metal, its density, malleability, conductivity and so on.
WORK-HARDENING AND ANNEALLING
The structure can however be changed, the size of the grains can become smaller or larger which in turn will change the metals physical properties. For example as the grains become smaller the metal will become less malleable, harder to bend or brittle. The grains can in fact become so small that they may not be able to link together to form the lattice. The metals integrity is lost and it cracks. Similarly if the grains become too large this will lead to it becoming weak.
The usual way grains become small is through being worked, being bent, hammered and rolled. This process is referred to as work-hardening
The original structure can however be restored if the metal is heated to about 2/3 its smelting point and either allowed to cool very slowly or quenched in water. Precisely what method will depend on the metal used. This will allow the grains to re crystallise restoring the integrity of the lattice, which will restore the original qualities of the metal. This process of heating and quenching is called annealing. More about that later.
Work hardening is not however the only process that can alter the physical property of a metal. Metals are very rarely used in their pure state, more commonly two or more pure metals being fused together to make a compound called an alloy. This may be done to change the metal’s colour but more probably to create specific working properties such as hardness, density or corrosion resistance. Most alloys are not however permanently stable and will eventually break down. For example when silver is alloyed with copper. Pure silver is relatively soft and would be unsuitable for coins unless it was alloyed with a small amount of copper. The resulting metal is much harder. Copper might also be alloyed to reduce the quantity of silver for financial reasons as it is much cheaper. The separation of copper from its bond with silver in a silver alloy is called the precipitation of copper and leads to a condition called embrittlement. Some coin collectors will often refer to this as crystallization.
To understand how embrittlement occurs, imagine the structure of the alloys lattice as a dry stone wall. Imagine the rocks in the wall as representing the grains of metal. Our metal being an alloy will have grains made of fused copper and silver. So imagine the rocks made of copper and silver, all mixed together. Over a long time the copper begins to move to the surface of each rock. Eventually all the copper moves and we are left with rocks composed of silver that are very heavily holed where the copper has vacated. All the gaps between the rocks of our dry stone wall are now filled up with copper. The surface of each rock could be described as its boundary and that’s why copper is said to have precipitated out to the grain boundaries. Try now to imagine the dry stonewall as our metal lattice. It's a real mess. There are grains of silver with loads of holes in them and a layer of copper around those grains. Very little is actually fused together. The metal's integrity is completely lost, and naturally its original properties. The metal will in fact be very brittle and in the most extreme case could even become friable or crumbly. Some early Saxon coins suffer from this condition. This process is irreversible.
It is with the two processes of work hardening and embrittlement that we can now begin to consider a bent hammered coin
A BENT COIN
When a coin is bent the process of work hardening takes place as described earlier and the grains around the bend will be made smaller. The malleability will also be decreased and possibly the lattice may have lost its integrity and cracked. In order to straighten the coin we will have to make it do even more work so we need to regain some malleability through annealing. As described earlier the process of annealing is going to allow for the small worked grains to re crystallise forming larger grains through the application of heat. To anneal silver it needs to be quenched in water. Most metals have different temperatures that must be reached for re crystallization to be complete. I specifically say complete because this is a process and re crystallization might start at a much lower temperature but only be complete at a higher temperature. For example, pure silver will begin to anneal at 80C but only be complete at 500C. A silver alloy might need a higher temperature to begin to anneal.
Below is a table of the beginning and complete annealing temperatures of some metals.
METAL BEGINS COMPLETE
Pure silver 80°C 500°C
7.5% copper 230°C 600°C
20% copper 300°C 700°C
Coin silver can have up to 25% copper, its annealing temperatures judged from the % and temperatures above.
Pure gold 80°C 200°C
22 carat 290°C 600°C
18 carat 300°C 600°C
Copper 275°C 400°C
But back to our coin.
One might assume that it makes sense to completely anneal it, but what’s that process called embrittlement?. What if our coin is suffering from that condition?. Annealing won't cure it; in fact heating a badly embrittled coin might actually make it substantially weaker. I recently saw the remains of an early Saxon coin that just crumbled before the restorer’s eyes moments after heating. Unfortunately the factors which need to be considered don't end there. Applying heat might have an effect on surface corrosion products.
The heat may burn them out leaving the surface horribly pitted, but there are far more insidious results that might occur. The sulphides, chlorides and oxides that often make up those surface corrosion products might fuse with the silver substrate. This fusing will create new compounds the properties of which may be very difficult to predict. If our coin has cracks caused by work hardening, the silver holding together might be very thin and if fused with the corrosion products will usually give a very brittle property. We may think we've annealed it when in fact we've made it much weaker. It would be disastrous to then try to manipulate or work it. We might however be able to avoid this if we clean the coin very thoroughly beforehand, removing all the corrosion products. There is plenty of good advice on how best to do this in the standard coin-cleaning guides.
Another factor that needs to be considered is what effect heating the coin quickly might have. An already weakened coin might simply not be able to handle the shock. It might be important to consider how we apply the heat; it might be preferable to apply the heat very slowly. For example the coin could be heated in the oven slowly raising its temperature before removing it and further raising the temperature through the direct application of a flame.
Once a decision to heat has been made, we need to know to what temperature and how to achieve that temperature. Our guide should be the most gentle application of heat to achieve the lowest temperature that is necessary. Remember that annealing is a process and we might be able to get away with not having to achieve a complete anneal. This will reduce the risks of those other factors coming into play.
Lets explore this further using some possible scenarios:
We have a slightly bent coin that upon magnification does not have any cracks around the bend. It isn't a very old coin, there doesn't appear to be any crystallization on it and it was made with a high proportion of silver. We consider the table of temperature and decide that a temperature of around 300C might just produce enough annealing to straighten this coin. 300C can be achieved in the oven or a chip pan. We heat the coin and immediately quench it in water. The coin can then be placed between two pieces of leather-covered wood and a gentle pressure applied. We feel the coin slowly flatten and stop when satisfied that the jobs complete. Any oil should however be washed away if chip pan heating was the method adopted.
As we applied the gentle pressure on our coin we felt it wasn't giving way easily. STOP. We need more of an anneal, this time applying more heat. A gentle method of achieving this is to place a smooth, flat and clean steal plate on the gas cooker. Stainless steal would be best. Place the coin on it. The heat will transfer though the plate and gently heat our coin. We however do need a guide as to the temperatures being obtained. As silver heats, it appears to change colour. It starts off whitish and will slowly move towards a dull pink and continue towards a cherry red. Our visual perception of this event can be determined by the ambient light conditions. For example in bright sunlight the gentle glowing dull pink might in fact be a much higher temperature than if that colour was observed in darker conditions. One person’s idea of a colour might be different from another so it will help to have a visual guide. The dull pink is the same colour as an earthworm. It represents a temperature of around 450°C-500°C and is the next step to which we want to take our annealing. As soon as the coin matches our earthworm colour we pick it up and quench. Another and perhaps a more accurate method involves making a small mark on the coin with a black felt tip pen. Use one of those big ones used to write on packages. When the coin is then heated, the ink will burn away at about 500°C -550°C. So using one of these techniques bring the coin to the desired temperature and quench, then try to apply pressure to flatten the coin.
Annealing coins to the temperatures above will in most cases be sufficient. Heating to temperatures above this should be regarded as extreme and not standard practice. Once the temperature reached starts to resemble a cherry red colour it is very easy to overheat and instead of annealing our metal to achieve malleability, we begin to give the metal a different character. There is also a disproportional increase in the risks of the other factors coming into play.
This time our coin is badly misshapen and we cannot heat it evenly through our metal plate method or reach a high enough heat with our other methods. The direct application of heat with a flame will be needed. The gas oven ring will do but a propane gas torch will give far more control. Small torches can be obtained from some hobby shops or a jewellery supply merchants. A pair of long nose pliers can be used to hold the coin in the flame but great care must be taken not to mark or damage the coin. Once a coin has been annealed, pliers with a chamois leather wrapping can be used to manipulate the coin into the desired shape. A small rounded piece of hard wood can be used to push out any dents. If you have trouble holding the coin a small wooden jewellery ring clamp might help to hold the coin firmly in place.
A badly misshapen coin might need a lot of work. You should consider carefully beforehand exactly how you are going to manipulate it; a sequence of very deliberate steps is far better than just getting stuck in. Each step might need the coin to be re-annealed to reverse any work hardening caused by your work. A particular task might require several anneals to complete. Be very patient and take as long as you need. A coin does not need to be completely restored in one session. The appearance of a coin might also just benefit from being partially restored, one having considered certain damage too risky to attempt work on.
Copyright 2004 Clive Hallam
To prevent any confusion I would like to draw attention to the use of some words. Brittleness is a property that may be caused by Work-hardening but is also used in relation to the completely separate process called Embrittlement. Same property, different cause. Similarly the term Crystallization as used by coin collectors should not be confused with the re crystallization that takes place in annealing. I have also deliberately not referred to the process of Hardening but only to Work-hardening. The process of Hardening is a separate process. I did not consider that it had any relevance to this essay and that its inclusion would only lead to confusion.