jars are made of clay, which in unworked form occurs in beds of particles of a certain size formed from the weathering of rock. As different rocks are composed of different minerals, clay has also a certain range of compositions, all of which contain clay minerals and sand, which is weathered quartz. Mixed with water the particles of clay cohere in a plastic mass of loosely bonded grains. When fired, or baked in an oven, the grains indurate, or form chemical bonds between them, so that they can no longer slide over each other. Pottery is therefore constructed and shaped in the plastic phase and then placed in an oven of predetermined temperatures to cook for predetermined lengths of time. The ancients were aware of these factors and did vary temperature and time although not with today's precision. In the vocabulary of pottery, clay pots are considered earthenware ceramics and are typically labelled terracotta, etymologically "baked earth". In the last few decades of the 20th century a number of questions became current about the provenance of Mycenaean pottery excavated by the British Museum from Tell es-Sa'idiyeh in the Jordan Valley. The pottery was Mycenaean, but was it imported or local? As a result, the British Museum's Department of Scientific Research (now Conservation and Research) decided to run a series of scientific tests on stirrup jars as representative pottery to see what determinations might be made, such as the provenance of the clay from which they were manufactured. They would perform the same tests on a "control group" of pots of known provenance in the British Museum. Radiographic analysis The preliminary tests determined the construction of the stirrup jars by xeroradiography, which had been adapted to archaeological images from medical technology. It produced x-ray images on paper rather than film. Like x-rays of metal castings, these images were of the masses within the border surfaces, showing cracks and inclusions. All the pots turned out to be constructed in the same way, without consideration of time or place. First the body of the pot is constructed by one of a few methods: coils, slabs or the potter's wheel. Immediately after construction, the pot contains too much moisture to be fired, as its sudden loss would cause the pot to contract and crack. It is allowed to dry until shrinkage is complete and it reaches a state called in the trade leather-hard, a