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Lathe Machining of Bronze Coin Flans
 
Continued (p.8)

 Figure 19 illustrates how a cutting tool could be swung across a flan by a lever L rotating about a pivot P. The lever’s vertical position could be adjusted by a variety of methods including applying a constant vertical force pressing the cutting tool into the flan. It could also be adjusted by moving restraining collars along the pivot to hold the tool at a fixed height relative to the platen, or by raising or lowering the platen, which would be a very reasonable approach if we imagine that each side of the flan was faced on a different lathe set to the proper depth of cut. 


Fig. 19
Lever mounted cutting tool 

Figure 19 also shows some further details of this hypothetical lathe. The platen PL is mounted on the end of a rotating spindle S. The centering pin R is mounted in some kind of headstock that allows it to move vertically, to apply pressure to make the indentation in the flan. In the figure this motion is shown as being provided by a screw mechanism. The indenting force could equally well have been provided by a weight, lever or other simple mechanism. It should be noted that the centering pin, made from a hard tough material such as steel, did not necessarily have to rotate while the flan was being turned.  

A further step away from this hypothetical lathe allows visualization of the entire apparatus, as shown in Figure 20. Here the apparatus A includes the features previously discussed, mounted on a sturdy column C fixed to a base B. The spindle S is driven by any convenient source of rotary power, in this case a sheave driven by a belt (not shown), and rotates within a journal bearing J. The centering pin R is driven vertically by a screw mechanism in the headstock HS against the flan, by turning handle H


Fig. 20
Complete lathe apparatus 

We can be certain that the actual apparatus used would not have looked exactly like this, and that it doubtless used some different mechanisms. Nevertheless, this hypothetical apparatus could feasibly be constructed using methods and approaches well known to the ancients, and it would efficiently perform a flan facing process that would cause central indentations and concentric tool marks of the same type as those actually observed on the coins.

Conclusion

The machining of bronze flans began with the very large and heavy bronze coins that were issued by Ptolemy II. It is therefore reasonable to suspect that its development is related to technological problems that were discovered in the process of making these unprecedentedly large and heavy coins. Ptolemy I had truck bronze coins up to 31 mm in diameter whose flans were not machined, and the Roman Empire later struck sestertii in diameters up to 30 mm whose flans were not machined. There were also medallic Roman Provincial issues up to 40+ mm in diameter whose flans were not machined, at least not by any process that left a dimple or concentric marks. The machining process clearly was not necessary for striking large bronze coins, but that does not mean that it was not advantageous.

One might object that inventing a new technology of rotary cutting and a complex lathe machine to correct a simple casting quality problem is not reasonable. There is an assumption in this objection that is not justified: that the technology was new and that the machine that did this cutting was invented specifically for this purpose. Rather, it was most probably adapted from other related machines that were already being used to cut metal for other purposes. We must remember that the archaeological record is more than 99% incomplete with respect to ancient machines. There were not ever large numbers of these valuable devices, and they were very rarely lost or buried in such a way as to be preserved. They were much too valuable for the metal "scrap" and reusable parts in them.

It is also a misconception to think that the machine used was complex by the standards of the time. The machine that is illustrated here may be indeed more advanced in some ways than what was actually used (Robert Kokotailo has suggested that the rotating platen may actually have been a large stone flywheel, which seems quite reasonable), but it is not complex in the sense that it is more complicated or more difficult to make than other apparatus that was being built at the same time and in the same area, Hellenistic Egypt. In the early Ptolemaic period, Alexandria was the center of the most advanced technology in the Mediterranean world. The question of complexity really has to be judged in the context of what would have been needed to do the work with the necessary speed, quality and efficiency. The ancient Greeks, in fact, had wonderful technological capabilities which we are still learning about. One fascinating example of this was the Antikythera Mechanism, http://www.dreamscape.com/morgana/triton2.htm , an ancient analogue computer that in many ways was more advanced than the devices that later Europeans were able to make before the Industrial Revolution. It is thought to have been used for astrological purposes.

There were also elaborate mechanisms used in some ancient temples to instil a sense of awe in the visitor, such as for automatically opening doors, and many other amazing devices: http://www.tmth.edu.gr/en/expo/ancient_greek_technology.html presents some examples of these. More glimpses into ancient Greek technology can be found at http://www.greece.gr/ENVIRONMENT/ScienceAndTechnology/ancientyetsomodern.stm .

Finally, lathes were quite well known to the ancient Greeks (they were in fact used by the Egyptians before them) and were used to turn not only wooden rods such as chair legs, but also wooden bowls and plates, large wooden objects such as shields, stone objects as large as column drums, ceramic objects in the "green" or unfired state, and even to machine many axisymmetric metal objects, such as bells and goblets.

Short of the discovery of a well preserved Ptolemaic or Roman Provincial mint in which archaeologists can accurately reconstruct what was actually being done, which is not likely to happen any time soon, we have to base our interpretations on very incomplete information. What is needed now is more thought by numismatists and more data - more images of coins whose flans bear tooling marks, if possible images of coins with flan defects such as the Ptolemaic bronze cited here that give us hints about the nature of the tool used. Possibly fortune will really smile on the enterprise, and someone will make a connection between this monograph and some archaeological evidence. The reason for publishing this sort of speculation is, after all, to start people thinking about the subject. Then bits of information that others know or will come to know, which otherwise would not get any special attention, may be seen to add in some way to our knowledge of the subject. A few more pieces of the puzzle are fitted in, and our knowledge becomes a bit less incomplete. 


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