Centering dots on Constantinian coins
 

       On the reverse of many Constantinian era coins, you may find a dot in the center. This is commonly referred to as a "centering dot". It is the consensus of most informed people that these dots were used in the manufacturing process. The authors of Late Roman Bronze Coinage believe these marks to be "merely operational". Some people occasionally think they have found a new variety, since the dot may or may not be mentioned in the reference they are using.

       The question is how or why is the dot on the reverse. There are no historical records of the minting process that tell of this dot and no ancient paintings (why would there be....it was pretty mundane work back then) showing engravers at work. Of course, much information has been lost through the ages. We can re-create a very plausible way these dots were created, though.

     The person that engraved the dies during Roman times had a difficult job. He needed to make a starting point. When working with a circle, the starting point is generally the middle. Today a compass is used to trace a circle. If you have ever used one, you will recall that one arm of the compass is very sharp and it makes a small hole as it spins. This small hole is also in the middle of the circle. The Romans could very well have used something very similar to aid in the engraving process. The rest of the design would have been laid out in the circle and then the engraving process started.

    Anything that was part of the coin design was carved out and the fields were left alone (when coins are struck, the metal flows into the carved out spaces). That is why the dot is found on some coins. If there was no design in the center field, the dot would be left. This is why there is never a dot on the obverse. The bust was engraved over the dot which completely obliterated the small dot. Not all coins have a dot even if the field is empty; this just means a celator took the extra time and filled the hole or smoothed the dot out or the hole was gradually filled in (like a clogged die) during striking process.
 

    This dot dilemma is more problematic for campgates. Sometimes the dot is a control mark and other times it seems to be merely a centering dot. This is another topic in itself; though, if in doubt, a good practice is to check the standard catalogue for this period-- Roman Imperial Coinage VII.