Petrography lower if wholly or partly vitreous. The specific gravity of the more common rocks range from about 2.5 to 3.2. Archaeologists use petrography to identify mineral components in pottery. This information ties the artifacts to geological areas where the raw materials for the pottery were obtained. In addition to clay, potters often used rock fragments, usually called "temper" or "aplastics", to modify the clay's properties. The geological information obtained from the pottery components provides insight into how potters selected and used local and non-local resources. Archaeologists are able to determine whether pottery found in a particular location was locally produced
What Is History? art. Though Carr conceded that historians cannot predict exact events in the future, he argued that historical generalizations can supply information useful to understanding both the present and the future. Carr argued that since scientists are not purely neutral observers, but have a reciprocal relationship with the objects under their study just like historians, that this supported identifying history with the sciences rather than the arts. Likewise, Carr contended that history like science has no moral judgements, which in his opinion supports the identification of history as a science. Carr was well known for his assertions in "What Is History?"