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Flint is a microcrystalline form of quartz and has a completely different physical appearance. The crystals are so tiny that the mineral appears glassy and has no preferred way of breaking. The fine crystal structure owes to the quick cooling it experienced during formation. Quartz, on the other hand, cooled over long periods and thus formed a much coarser crystal structure. Flint is usually contaminated with a little limestone (thus the CaO content and the LOI). There can also be trace amounts of iron and MgO.
Flint was first formed as a metamorphic rock. While many sedimentary rocks contain significant proportions of flint that was weathered and transported, the material of interest in ceramics is what has remained as the pure mineral.
Flint pebbles are employed as grinding media in ball mills (because of their hardness and the fact that rounded pebbles of proper size and low impurity levels can be found in many areas). Historically it was employed as a source of quartz in bodies both because it was more convenient to find and grind into a fine powder (once calcined to 600C flint-stone can be easily crushed). Supposedly it produced bodies less prone to cracking through quartz inversion temperatures.
Today, however, quartz is universally used in bodies and glazes and modern machinery can grind it to any size needed. It is very likey that you have never actually seen real flint and will not be able to find the true material from any supplier. The terms flint, quartz and silica have come to be used interchangeably in ceramics and you will see them all employed in recipes; they are all the same thing. However, most correctly, the material used in ceramics is called simply "silica". "Quartz" refers to the macro-crystalline mineral we find in nature. The terms "pottery quartz" or "potter's sand" likewise refer to silica. The term "flint" surfaces most often when referring specifically to the tools ancient peoples made from the micro-crystalline mineral.
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Overview of quartz hazards in the ceramic industry and process
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