numbers of pots have long been known, and they suggest very large loads of 25,000–30,000 vessels. Though not all the kilns at this, or other, manufacturing sites were so large, the excavation of the grand four (big kiln) at La Graufesenque, which was in use in the late 1st and early 2nd century, confirms the scale of the industry. It is a rectangular stone-built structure measuring 11.3 m. by 6.8 m. externally, with an original height estimated at 7 metres. With up to nine 'storeys' within (dismantled after each firing), formed of tile floors and vertical columns in the form of clay pipes or tubes, which also served to conduct the heat, it has been estimated that it was capable of firing 30,000–40,000 vessels at a time, at a temperature of around 1000 °C. A 2005 work has shown that the slip is a matrix of mainly silicon and aluminium oxides, within which are suspended sub-microscopic crystals of haematite and corundum. The matrix itself does not contain any metallic ions, the haematite is substituted in aluminium and titanium while the corundum is substituted in iron. The two crystal populations are homogenously dispersed within the matrix. The colour of haematite depends on the crystal size. Large crystals of this mineral are black but as the size decreases to sub-micron the colour shifts to red. The fraction of aluminium has a similar effect. It was formerly thought that the difference between 'red' and 'black' samian was due to the presence (black) or absence (red) of reducing gases from the kiln and that the construction of the kiln was so arranged as to prevent the reducing gases from the fuel from coming into contact with the pottery. The presence of iron oxides in the clay/slip was thought to be reflected in the colour according to the oxidation state of the iron (Fe[III] for the red and Fe[II] for the black, the latter produced by the reducing gases coming into contact with the pottery during firing). It now appears as a result of this recent work that this is not the case and that the colour of the glossy slip is in fact due to no more than the crystal size of the minerals dispersed within the matrix glass. Forerunners Arretine ware, in spite of its very distinctive appearance, was an integral part of the wider picture of fine ceramic tablewares in the Graeco-Roman