Quartz stone at room temperature is a stable mineral, insoluble in water and sulfuric acid, hydrochloric acid, nitric acid. Hardness 7, specific gravity of about 2.65, melting point 1713 ℃. Glossy glassy, sometimes fat-like. Texture is purely colorless; with impurities are red, yellow, blue, black, brown, purple, green; transparent to opaque, brittle Fracture was shell-like or staggered. Crystal was hexagonal columnar, column with stripes. There is a difference between left and right crystals. Twin crystals are common. Quartz is a non-plastic material, and its clay in the high temperature generated mullite crystal to give porcelain higher mechanical strength and chemical stability, Quartz and can increase the translucent body, is the preparation of white glaze good raw materials.
First, the role of quartz in porcelain billet
(1) quartz raw materials are barren raw materials, it has a ceramic body in the normal temperature heating has an important impact. Before sintering, the quartz in the blank material on the plasticity of the plastic from the role of conditioning, and can reduce the shrinkage in drying, Quartz shorten the drying time and prevent the deformation of the body (including deformation and collapse).
(2) quartz in the firing process, Quartz the emergence of crystal transformation, volume expansion, can offset the partial shrinkage of the body. As the quartz crystal transition will cause changes in volume, so in the heating and cooling process should be carefully controlled cooling and heating rate, to avoid cracking products.
(3) When firing, the heating expansion of quartz properly counteracts the sintering of the billet, and when the glass phase is present in a large amount, the quartz becomes the skeleton of the ceramic, which prevents the body from bending during firing And other defects.
(4) high temperature chemical affinity is very strong, Quartz with a variety of other oxide compounds, high temperature liquid viscosity is very high, increase the green body binding capacity.
(5) to increase the mechanical strength and transparency of the body.
Second, the role of quartz in the enamel
(1) Quartz in the glaze, the more fusible CaO, Na2O, K2O, MgO melt to produce transparent quartz glass, so that glazed bright, improve the glaze gloss.
(2) quartz in the glaze in the thermal expansion coefficient is small, good heat resistance.
(3) quartz softening temperature is high, high temperature liquid viscosity.
(4) quartz can increase the mechanical strength of glazed, improve the glaze hardness, so that porcelain wear, and chemical stability Ye Hao, not affected by the erosion of acid.
(5) can make the glaze transparent light.
In the production of glazed tiles, the amount of quartz in the green body is generally 10-20%, in a firing glazed tile body in the amount of less, usually 10-15%, in the amount of glaze frit 20-40%. White billet for quartz purity requirements are higher, while the colored body can be used less pure quartz. In a burnt wall tiles of the body is often not directly added to the quartz, but by the introduction of clay with clay.
Third, the crystal in the firing process of crystal transformation
(1) low-temperature rapid transformation of quartz crystal: the conversion temperature is low, the crystal table at the same time transformation, conversion speed, reversible reversible direction, before and after the transformation of the body volume changes little, the green body caused by cracking process cracking Big:
A, β - quartz reversible into α - quartz, the conversion temperature is 573 ℃, the volume expansion is 0.82%;
B, γ-scale quartz reversible into β-squamous quartz, the conversion temperature is 117 ℃, the volume expansion is 0.20%;
C, β-scale quartz reversible into α-scale quartz, the conversion temperature is 163 ℃, the volume expansion of 0. 20%;
D, β-fused silica is reversibly converted to α-cristobalite with a conversion temperature of 180-270 ° C and a volume expansion of 2.80%
(2) quartz crystal high temperature slow conversion: high conversion temperature, from the surface to the internal transformation, Quartz the transformation process is slow. The transformation is accompanied by a large volume change, due to slow conversion and phase buffer effect, so the green body caused by the cracking process of little impact:
A, α-quartz reversible into α-scale quartz, the conversion temperature is 870 ℃, the volume expansion of 16%;
B, α-scale quartz reversible into α-cristobalite, the conversion temperature is 1470 ℃, the volume expansion is 4.7%;
C, α-quartz reversible into molten quartz, the conversion temperature is 1713 ℃, the volume expansion of 0.1%;