The Theory Of Trail Formation


 Simply put, trail dies are a die anomaly, similar to but not like a doubled die. This puts them into a class called “die varieties” since they will be found on every coin struck with that die from the beginning of its use to the end.

To understand why these anomalies are a variety, we must explore how they are created. Since they are associated to a die fault the most logical place for trails to occur is when the die is hubbed and this is where we will focus our attention to their creation.

First, I must explain that the theory explaining trail development is based on our common knowledge of what happens during a hubbing process. Communications with the U. S. Mint concerning these anomalies has accomplished very little in adding to the information already known. In 2003 and also in 2007, this subject was broached and it has been established that the MINT does know about these anomalies and has tried to recreate the same affect that produced these die faults. However, not being able to do so, they terminated that study and are reluctant to reopen that study to determine the cause of trails.

One other point that I would like to bring forth, to avoid confusion, is the term wavy steps. Extensive study of these dies has shown that wavy steps are in fact a form of trails. So, keeping that in mind, when I do talk of trailswavy steps are included in that descriptive word.

Now to the hubbing of a die (this includes both master and working). When a blank, conical die is placed in the hubbing press a huge amount of pressure is applied to it. This effectively transfers the image that is on the hub (either master hub or working hub), through compression, to the die. Without atypical conditions occurring horizontal movement between the hub and the die is not produced and the result is a normal image being transferred to the die. However, at times faults do occur and the results lead to an imperfect image transfer.

We are all familiar with the explanation of how a doubled die occurs when dealing with multiple hubbing. It is a difference between the first image made with a hub to the second (or other more) image made with either the same hub or a different hub. With the use of the single squeeze hubbing system, things did change a bit resulting in a different way a doubled die could occur. Since there is no multiple hubbing in the single squeeze hubbing (there have been some reported instances), just how does a doubled die occur?

Both situations of a single squeeze doubled die occur when the die is not in the correct position in relationship to the hub. In the first instance, the hub makes contact with the die and seeing that the position is incorrect, the hub is backed off the die and the die re-positioned. This initial contact or “first kiss” produces a small portion of the image, generally located in or around the center of the die. The second application of the hub to the die produces a second image that is offset from the first incomplete image resulting in a doubled die. This is very similar to multiple hubbing and the way a doubled die was produced using that system.

The other situation occurs again with the die not being in the correct position in relationship to the hub. The difference in this situation is that the hubbing continues and a positional correction is not made after the “first kiss”. At some point in time during that hubbing, the pressure applied corrects the position of the die to the hub resulting in a doubled die or an offset from the first partial image to the second complete image on the die. It is important to understand how a doubled die occurs in the single squeeze process, especially in the second situation for that involves horizontal movement, between the hub and the die, during the hubbing procedure.

Now, lets us look a typical trail die. If we take into account the area affected, it is normally over 10% of the die face. Also, there is no trail die that affects 100% of the die either; there is always some portion of the die that is left unmarked by trails. If we try to apply the first scenario of doubling to this situation, it just would not fit. All the doubled dies produced by re-positioning the dies after the “first kiss” are always towards the center of the die. However, trails are not confined to that area, so that situation cannot be applied to the formation of trails.

The second doubled die situation, with its horizontal movement of the die in relation to the hub can be a key to understanding what is going on with the formation of trails. While both of these anomaly types (doubled dies and trails) are produced by die movement, one can see the obvious difference in their appearance. The doubled die duplicates a design element, either partially or in whole. Trails do not duplicate a design element but extend a portion of that design. The difference is simple and this is what separates the two from being similar.

With the information provided above, we can establish that trails are not formed at the beginning of hubbing. We can also commit ourselves into believing that trails are not formed in the latter half of the hubbing process; the hub has sunk into the die face too far to allow horizontal movement between itself and the die. That leaves us with a time period that is not between the “first kiss” of the hub to the die and the point where the hub is sunk too far into the die to create horizontal movement. Is there another time period possible that is more conducive to producing the affect that we are seeing in the anomaly called trails? There is and it happens at the very end of the hubbing, when the pressure to the hub is relieved.

Let’s examine what happens at the immediate end of the hubbing procedure. At the end of that cycle, the hub has sunk into the die at a pre-described depth and the full design has been transferred. Once the pressure applied to the hub has been removed, the only weight on the finished die is the hub and its attached collar to the hub. What would happen if the die did move horizontally against the hub?

 Before answering that question, we should explore what could make the die have that horizontal movement. The first cause is thermal energy. With the compression of the steel in the die the atomic layers are rubbed against each other. There is a force that opposes this movement and it is called friction. When kinetic energy occurs between the atomic layers, friction converts that energy into thermal energy or heat. This fact has been confirmed by the MINT in its statement that the die is very warm to the touch after the hubbing process has been completed. With this thermal energy present, you have what is called thermal expansion.

Thermal expansion is the expansion or contraction of a physical object without the loss or addition of weight in that object. One only has to look at a mercury thermometer to understand this principle; as the temperature increase, the mercury in the glass tube expands, filling more of the glass tube. If the temperature decreases, the mercury contracts in the glass tube and indicate the temperature drop. The steel in the die acts much the same way as it does with mercury; however, the degree of expansion in the steel is six times less than that of the mercury for an equal temperature change in both substances.

While contraction and expansion does occur due to heat it is not the only factor to be considered when dealing with thermal energy. Under ideal conditions, contraction and expansion will be uniform throughout the object affected. In other words, if we were to hang a steel ball in an oven and apply a temperature of 100 degrees Celsius, the steel ball would expand equal on all areas of its surface. Reduce that temperature and the steel ball will contract accordingly.  However, we are not dealing with an object that has an equally applied heat to all areas; what we have is a steel rod that has all of the thermal energy generated in its top portion. Even then, if the steel rod was of a uniform consistency, the expansion rate would be equal an all horizontal planes and thus the expansion would be equal in each given plane in accordance with the temperature felt. But the steel rod is not of equal consistencies, with some areas having impurities, or unequal thermal conductivity which will result in the thermal energy not being able to expand or contract equally on a given plane. This is called thermal warping and this action can produce the horizontal movement that we have been looking for in the die.

Besides thermal warping, there is another force that must be added to the equation that will produce movement in the horizontal plane and that is called elasticity. Elasticity is the ability of an atomic structure to “remember” its original shape and return to it after it has been altered through compression or expansion. If we were to squeeze a square shaped sponge into a tight ball, after the applied pressure was released, it would return to its original square shape. This type of object has what is called a high elasticity. As solid objects become denser in their atomic or molecular structure, they tend to become less elastic and move towards being more brittle. So, even though metal has a dense atomic structure, it does have some elasticity or “snap back” qualities when pressure is applied to it. Conversely, if too much pressure is applied to the metal, the atomic layers tend to roll over each other (creating thermal energy through friction) and the elasticity is lost as the atomic structure moves towards a crystalline or brittle state.

So, we now have two forces that can cause movement in the die with thermal warping being the greater of these two forces.  One other factor that must be understood is the ability of the hub to leave marks (trails) on the die face. Yes, they are both made from the same carbon steel, but there is a marked difference in their respective hardness to each other. The die blank has been annealed, which softens the metal (makes it less brittle) by using a heating process design to produce that desired effect. The hub, on the other hand, has been hardened by a process called tempering. This tempering enhances the hubs ability to impress the design it has into the blank die. It also gives the hub the ability to score or mar the die due to this hardening.

We now have a possible cause for motion and a tool (the hub) that can add extra features to the die face. All it takes now is an understanding of what may happen to create these trail lines. At the end of the hubbing cycle, the hub is fully impressed into the die face with a large amount of hydraulic pressure applied to the hub and its collar. When that hydraulic pressure is relieved, other forces begin to take effect. There will be a point when the hydraulic pressure will decrease enough that the force of thermal expansion will start. This thermal expansion will be greatest towards the center point of the hub and the die for that area will have been subjected longest to the pressure from hubbing and has the least amount of area for heat dissipation. If there are no faults in the die to cause horizontal movement, the hub will leave the edge of the die first and work inwards due to this thermal expansion. The last area that the hub will have contact with the die is the exact center of the two devices.

We also must remember that the hub is slight concave, which will cause the die to be slightly convex. The reason behind this is to facilitate the flow of metal on the planchet when struck by the convex die. We should also remember that the hub and its collar are stationary in the horizontal plane, only having the ability to move in the vertical plane.

Now, let us suppose that there is some fault in the die that will cause it to have motion in the horizontal plane.  At some point in time, when the pressure from the hubbing process has decreased enough to allow that movement, the die will move across the hub face. This movement will cause the high points on the hub (either master or working) to dig into the die’s surface. This will create lines on that surface, which will taper off as the pressure decreases from the hub to the die. There can be a situation where the pressure is not relieved enough and the die will actually move in two different directions. This dual movement is generally reciprocal and is caused by the die trying to retain its former shape. This action can also take place in the working hub, when the image is transferred from the master die. In this case, the high points are the areas formed by the field and the incused designed elements. When the working hub “drags” across the master die, the same high points will mar the surface of the working hub. When this happens, the image transfer from these trails will appear as incused lines on the coin.

While we may never understand the full complexities of how trail dies are formed, this is a beginning to understanding why they do happen. As new information is gathered, it will be analyzed and fit into the bigger picture of just how trails come into being.

Thank you,

BJ Neff