Northern Black Polished Ware is subject of enquiry in this paper. This is an important pottery tradition of the Ganga Valley. This ware has a very high lustrous finish and shows variations in colour. There are many problems related to its origin, distribution and technology of manufacturing. In this paper an attempt has been made, using X-ray diffraction techniques, to address the problems related to its firing technology. It is being proposed that NBPW production was distributed over a wider area of the Ganga Valley and it was a specialized craft in ancient India. It has been found that due to special technique of firing the clay minerals present on the outer surface of pottery underwent mineral transformation and produced a lustrous glossy slip.