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Lathe Machining of Bronze Coin Flans

 Abstract

Central dimples in the flans of Ptolemaic and Roman Provincial bronze coins have long intrigued
 
numismatists. The features observed in these coins are discussed, and methods by which they may have been
created are proposed.

  Introduction

 The flans of many large ancient bronze coins bear central indentations or dimples that have long intrigued numismatists. An example of such central dimples is shown in Fig. 1. 


Fig. 1
Central Dimples in a Ptolemaic bronze
 

The central dimple on Ptolemaic bronze coins is discussed by Bouyon et al. [Systèmes et Technologie des Monnaies de Bronze, Moneta 19, Wettern 2000], which  provides invaluable technical detail including a metallurgical cross section. 

Similar central dimples are also found in the bronze civic coinage issued  by many cities in Moesia, Thrace and Asia Minor under the Roman Empire. This coinage (often referred to as Roman Provincial or Greek Imperial coinage) reached its peak between 150 and 250 a.d., after which the civic mints closed due to the effects of invasions and rampant inflation. 

It is not obvious from looking at these coins what the purpose of the central dimple was. Since many contemporary civic coins struck in Asia Minor did not have central dimples while others did, this feature clearly was not essential to the manufacture of bronze coins.

Dimples were created before striking 

A key issue in deciphering the reason for the existence of these dimples is determining whether the dimples were a part of the flan before it was struck, or whether dimples were the result of something that occurred after striking. The current preponderance of opinion is that the dimples were already  in the flan at the time it was struck, and were formed as a part of the flan preparation process. If the dimples were formed after striking, they would have had to be cut rather than impressed. The process discussed here follows and reinforces the conclusion that the dimples were part of the flan preparation. 

Casting of the Flans 

The flans for ancient bronze coins were nearly always cast. The tin bronze alloys used work-hardened rather rapidly and were not malleable enough to hammer into shape from cut strips. In a few cases such as  in the Parthian Kingdom, however, bronze coins of the smallest denominations were struck from flans crudely cut from rods or bars. 

The bronze alloys used had a high tin content and other alloying elements which made them quite fluid when molten. They were sometimes cast in individual molds, but were normally cast in molds containing  several cavities fed by runners.  These cavities were sometimes directly connected by the runners, so that the hot metal flowed from one cavity into the next until all were filled. Alternatively, cavities were grouped into a “tree” in which metal flowed from a central sprue into runners leading to the individual cavities.  These casting arrangements are illustrated in Figure 2. 


Fig. 2
Flan casting arrangements

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