Calcinated flint

When it consists of only silica and water, flint is basically colorless. Impurities within the stone, however, render colored varieties. Thus, much flint is tan, beige, or jet black with a brown homy appearance (the latter color is caused mainly by iron impurities), but there are also gray, pink, and even red varieties. If dark flint is heated to temperatures above 450°C, the outer layer changes to white. Flint that underwent such a heating process is known as burned or calcinated flint, a misnomer probably used because the white surface appears to have changed into lime (Luedtke 1992 Sieveking and Bart 1986). Burned flint is easier to work than is the natural mineral, a property... 

Fireclays can be dividea into plastic clays and hard flint clays they may also be classified as to alumina content. Firebricks are usually made of a blended mixture of flint clays and plastic clays which is formed, after mixing with water, to the requirea shape. Some or all of the flint clay may be replaced by highly burned or calcined clay, called... 

Chert Cristobalite A mir rocryslalline form of silica. An impure form of flint used in abrasives. A crystalline form of free silica, extremely hard and inert chemically very resistant to heal. Quartz in refractory bricks and amorphous silica in rlialomaceous earth are altered to cristobalite when exposed to high temperatures (calcined). Cristobalite is extensively used in precision casting by the hot wax process, dental laboratory work, and certain speciality ceramics. 

Refractories for the glass industry are made, as a rule, from a mixture of calcined clay, consisting partly of crushed old pot material, carefully cleaned, and plastic bond clay. The proportion of calcined (grog) to plastic clay usually is between 1 1 to 3 2. In some glass plants no clay but the Grossalmerode material and pot shell was used in others the plastic clay from Missouri was introduced, and, again, in some cases a smaller amount of raw flint clay was added. Some of these mixtures are as follows ... 

When flint is calcined, the combined water is driven off about 400°C, causing the large aggregates to lose their strength and become crumbly finally, at 1100°C, the quartz changes to cristo-balite, this conversion being promoted by the presence of calcium oxide, an impurity. 

Coade Stone. A vitreous ware, used for architectural ornament, made in London by Mrs Coade from 1771 until her death in 1796 manufacture finally discontinued in about 1840. The body consisted of a kaolinitic clay, finely ground quartz and flint, and a flux (possibly ground glass). Coated Dolomite Grain. Also known as unfired semi-stable dolomite refractory, this comprises calcined dolomite bonded with tar or oil. 

Flint. Nodular chalcedonic silica from the chalk deposits of W Europe and elsewhere. These flint pebbles are calcined and ground for use in earthenware and tile bodies. In USA the term flint is often applied to other finely ground high-silica rocks used in whiteware manufacture. See also... 

Shaft Kiln. A vertical kiln charged at the top and discharged at the bottom. If solid fuel is used it is fed in with the charge, but shaft kilns can also be fired with gas or oil by burners placed nearer to the bottom of the shaft. Such kilns are used for the calcination of flint, dolomite, fireclay, etc. 

Refractories - In North America certain foimdry refi actories contain flint clay to give a dense, strong product able to withstand much higher temperatures than ordinary clay-based refractories. In Europe calcined ball clay (chamotte) is used in place of flint clay. China clay is used in cordierite-based kiln furniture and insulating firebrick. 

The multitude of refractory aggregates (for example, calcined clay, flint clay, andalusite, mulhte, bauxite, fused white or brown alumina, sintered alumina, etc.) and the different grain sizes of these materials—in addition to the big variety of bonding agents, additives, and mixing liquids—enable endless combination possibihties in development work on unshaped refractory products. Therefore, a qualitative standardization is not really possible as it is the case for refractory bricks. As a result of the ENV 1402-1 standard, which now provides a more precise definition of the products, there will probably be adjustments on the market in the long run. If also considering economic aspects, suitable products will obviously be developed on the basis of all the available constituents. 

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