|Monthly Tech-Tip |
Quartz is the main crystalline mineral form of silica (SiO2). White or milky quartz is an opaque white, greasy-looking, sharply angular very hard rock. Natural deposits of relatively pure quartz are plentiful and grains of it are found in many other types of rocks. Pure quartz melts at 1713C, far beyond the range of an average potter's or industrial production kiln. It is the most abundant mineral on earth.
Quartz is a fascinating study in the importance of considering the mineralogy of materials (as opposed their chemistry). For example, quartz and fused silica glass have the same chemistry, 100% SiO2. But they have dramatically different physical properties (e.g. thermal expansion, hardness, specific gravity, melting temperature). To illustrate, consider diamond and graphite: they have the same chemistries. But they have radically different physical presences.
From a theoretical perspective, quartz is a mineral. However in ceramics it is also considered a material, often simply called "silica" (however original container bags often label it as "ground quartz"). Huge quantities of quartz powder are used in ceramic bodies and glazes. In bodies the quartz particles most often remain unchanged in the fired matrix, behaving as an embedded aggregate. They act as a "skeleton" in porcelain bodies. They raise the thermal expansion and make it easier to fit glazes. However in glazes, the quartz particles dissolve in the glass and become a part of its chemical structure (thus the original properties of the quartz are gone, giving way to the new physical presence of the glass). The term "silicate" refers to minerals or fired materials where SiO2 predominates in the inventory or molecules that make up its structure. Kaolin is a silicate. In kaolin crystals there are 2 molecules of SiO2 for every one of Al2O3. They combine chemically to form the familiar flat, stop-sign shape.
Closeup of in-situ quartz mineral at the MGK quarry site in India.
Cordierite In Quartz
Cassiterite W Quartz
Lazulite Siderite Quartz
These are glazed test bars of two fritted white clay bodies fired at cone 03. The difference: The one on the right contains 13% 200 mesh quartz, the one on the left substitutes that for 13% 200 mesh calcined alumina. Quartz has the highest thermal expansion of any traditional ceramic material, alumina has the lowest. As a result the alumina body does not "squeeze" the glaze (put it under some compression). The result is crazing. There is one other big difference: The silica body has 3% porosity at cone 03, the alumina one has 10%!
In ceramics, cristobalite is a form (polymorph) of silica. During firing quartz particles in porcelain can convert to cristobalite. This has implications on the thermal expansion of the fired matrix.
Raw ceramic materials are minerals or mixtures of minerals. By taking the characteristics of these into account technicians can rationalize the application of glaze chemistry.
Flameware is ceramic that can withstand sudden temperature changes without cracking. The low thermal expansion of true flameware makes craze-free glazes very difficult.
In ceramics, this refers to the sudden volume change in crystalline quartz particles experience as they pass up and down through 573C. Fired cracks are often related to this.
|Oxides||SiO2 - Silicon Dioxide, Silica|
|Materials||Newman Red Clay|
|Materials||Primas FA-200 Feldspar|
Quartz MSDS at ilo.org
Wikipedia quartz inversion
Quartz on WikiPedia
Quartz mineralogy data