These ancient ceramics potters were most likely semi-specialists who only dedicated a portion of their time to pottery production. Because no kilns have been discovered from this time period, it is assumed that vessels were fired over open flames. There is proof of local ceramic exchange across groups, but because the vessels are so similar, the transfer was most likely of the commodities held within the containers themselves. Radiometric dating studies show that pottery fragments were recovered in a cave as far as 12,700 BCE. Many people believe that Jomon pottery was manufactured much earlier than this date. However, because of uncertainty and many sources asserting specific dates based on various dating procedures, it is difficult to establish for certain how long Jomon pottery has been produced. According to some reports, archaeological finds date back to the 14th millennium BCE.