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Lathe Machining of Bronze Coin Flans
 
Continued (p.5)

 Tooling marks 

Evidence of a machining process on the flans prior to striking is provided by tooling marks which have survived the striking of some coins. These tool marks on the coin faces always appear to be concentric to the central dimples, even when the dimples themselves are noticeably off center.  These tool marks can be seen in incompletely struck areas on many coins when they are closely examined under magnification. There are also a few examples in which the tool marks are so prominent that they can be seen in photographic or digital images.

Ptolemaic Bronzes with Clear Tool Marks

The reverse of one Ptolemaic bronze coin, shown in Figure 11, has an interesting flan flaw (probably associated with the runner), which was shallow enough to be cut by the tool used to machine the flan yet deep enough to remain intact after striking.  


Fig. 11
Tooling marks on a Ptolemaic bronze coin 
(Hannes Mayer)

The tooling marks on this flan are clearly concentric to the dimple, indicating that during the machining process the dimple must have defined the axis of rotation between the flan and the cutting tool. 

The edge of this coin, shown in Figure 12, exhibits tool marks which appear to be localized to part of the edge, and may have been caused either by a tool which cut the flan rotationally, or by a coarse file used to smooth this area. Those who have experience in metal filing would recognize this diagonal orientation of the marks as characteristic of the filing stroke that a machinist would naturally use. Such filing could also have been performed on a flan that was rotating. It is interesting to note that the edge is biconical,  with a parting line that is much closer to the reverse side than to the obverse side. Clearly the mold was split at this parting line and it appears from the image that the runner must have been on the obverse side of the mold. 


Fig. 12
Edge of the coin in Fig. 11 

Later Ptolemaic bronzes are often carelessly struck, and either a poor strike or machining of an old coin to prepare it for restriking has provided the very clear example of concentric tool marks from the late Ptolemaic period shown in Figure 12a:


Fig. 12a
Concentric Tooling Marks
(Hannes Mayer)

From the above examples, it is not yet established whether the grooves in the coins are concentric rings (such as would be cut by irregularities in a fixed tool) or spirals (such as would be cut by a tool moving across a rotating flan). Because the pitch of the grooves is very small, as in a phonograph record, it is not easy to distinguish a continuous spiral from concentric circular grooves if the marks are only partially preserved.

More insight into the machining process is provided by some specimens in which the lathing operator evidently dragged the cutting tool rapidly out across the rotating flan after reaching its center. A very clear example of this which definitely shows the spiral nature of the lathing grooves is shown in Fig. 12b:


Fig. 12b
Concentric Tooling Marks
(Bart Lewis)

It would be reasonable to expect that it would take about 0.5 second to rapidly withdraw the tool across the flan, and since there are roughly 5 pitches to the spiral, this suggests that the flan was rotated at 600 RPM.

Another specimen in which the cutting tool apparently was not dragged out quite so rapidly is shown in Fig 12c:


Fig. 12c
Concentric Tooling Marks
(Robert Kokotailo)

Here the visible remnants of the grooves are highlighted in red, and extrapolations connecting them are shown in blue. It is again quite clear that the grooves form a spiral.

Roman Bronzes with Clear Tool Marks

On some Roman Provincial coins, tool marks may be found which are also concentric to the dimples. A bronze coin of Gordian III struck in Nikopolis ad Istrum in Moesia, shown in Figure 13, bears clear evidence of such tool marks in the incompletely struck areas.


Fig. 13
Concentric tool marks
 
(Flanumismatics)

Another bronze coin of Gordian III struck in Adrianople in Thrace, shown in Figure 14, bears a very prominent tooling mark on the reverse which is also concentric with the dimple. There is a similar but less prominent mark on the obverse of this coin.


Fig. 14
Concentric tool marks
(Moneta-L Posting)

These concentric tool marks suggest that the surfaces of the flans on these coins were smoothed or faced, in machinist’s terminology, by some kind of tool that rotated relative to the flan about the dimple, which must therefore have acted as a centering feature that held the flan in a fixed position relative to the cutting tool.

Tooling marks (and occasionally central dimples) are also found on some Roman Imperial bronzes, suggesting that these issues may have been struck in the Balkans or in Asia Minor. An example of this is shown in Fig. 14a:


Fig. 14a
Concentric Tooling Marks
(Robert Kokotailo)

Here the reverse of a sestertius of Antoninus Pius bears clear concentric tooling marks near its center. Robert Kokotailo reports that his microscopic examination of the grooves reveals that:

"After examining the coin under a binocular microscope, what I find is that the smooth areas in the rings are on the high points. The rougher areas between the smooth areas are the lower points. These low points have, in minor areas, some original incrustations, and the roughness is due to corrosion. What this demonstrates is that the rings are part of the original striking, because if they were lathed during cleaning, the corrosion and incrustations would have been removed where the lathe cut deepest (i.e. the low points)."

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