AD. Frit was a significant ingredient. A recipe for "fritware" dating to c. 1300 AD written by Abu’l Qasim reports that the ratio of quartz to "frit-glass" to white clay is 10:1:1. This type of pottery has also been referred to as "stonepaste" and "faience" among other names. A ninth-century corpus of "proto-stonepaste" from Baghdad has "relict glass fragments" in its fabric. The glass is alkali-lime-lead-silica and, when the paste was fired or cooled, wollastonite and diopside crystals formed within the glass fragments. The lack of "inclusions of crushed pottery" suggests these fragments did not come from a glaze. The reason for their addition would have been to release alkali into the matrix on firing, which would "accelerate vitrification at a relatively low firing temperature, and thus increase the hardness and density of the [ceramic] body". Whether these "relict glass fragments" are actually "frit" in the more ancient sense remains to be seen. Iznik pottery was produced in Ottoman Turkey as early as the 15th century AD. It consists of a body, slip, and glaze, where the body and glaze are "quartz-frit". The "frits" in both cases "are unusual in that they contain lead oxide as well as soda"; the lead oxide would help reduce the thermal expansion coefficient of the ceramic. Microscopic analysis reveals that the material that has been labeled "frit" is "interstitial glass" which serves to connect the quartz particles. Tite argues that this glass was added as frit and that the interstitial glass formed on firing. Frit was also a significant component in some early European porcelains. Famous manufacturers of the 18th century included Sèvres in France, and at Chelsea, Derby, Bow, Worcester and Longton Hall in England. Frit porcelain is produced at Belleek, County Fermanagh, Northern Ireland. This factory, established in 1857, produces ware that is characterised by its thinness, slightly iridescent surface and that the body is formulated with a significant proportion of frit. A small manufacturing cluster of fritware exists around Jaipur, Rajasthan in India, where it is known as 'Blue Pottery' due its most popular glaze. The technique may have arrived in India with the Mughals, with production in Jaipur dating to at least as early as the 17th century. Modern frit Frits are indispensable constituents of most industrial ceramic glazes which mature at temperatures below 1150 °C. Frits are typically intermediates in the production of raw glass, as opposed to pigments