This question is often asked but rather few people are willing to read the great, 8 page answer: http://www.classicalcoins.com/flans1.html The is a lot of good information on the various pages of http://ptolemybronze.com/ but most people want short answers and many more have trouble accepting the truth of the matter. We all have trouble accepting the fact that the people who made these coins wanted the surfaces of the flans smoothed but were not bothered by the pits left in the center of the coins. This is quoted from page 3 of the 8. If you are not up to reading all 8, start with page 3 and go until you give out. http://www.classicalcoins.com/flans3.html