New version of this page available at digitalfire.com: Click here to go now

Silica

Quartz, Flint

Chemistry

Chemistry available on new site, see link above

Links to Other Materials

Hazards

Miscellaneous

Notes

The term 'silica' can be misleading. It is important to understand the difference between 'silica mineral', 'silicates', and 'silica glass'. Quartz is the best example of a natural mineral that is almost pure silicon dioxide (it is the most abundant mineral on planet earth). Other ceramic minerals like feldspar and clay contain some 'free silica' (accessory quartz). However these also usually contain 'silicates', that is, SiO2 chemically combined with other oxides to form crystalline minerals. Other silica-containing rocks and minerals are andalusite, barite, beach sand, bentonite, calcite, diatomaceous earth, kaolin, limestone, mica, pyrophyllite, talc, tripoli, rutile, wollastonite, zeolite, zirconium sand, vermiculite, granite, and sandstone. Silica is also available as a silicate glass (in frits).

Pure silica minerals (like quartz) have high melting points. In ceramic bodies and glazes other oxides are added to complement it, they form silicates with it or occupy the network between particles of quartz. In the latter case silica is considered a 'filler' (e.g. porcelain clay bodies). It is interesting that some special purpose (and expensive) clay bodies replace the silica filler with calcined alumina, this greatly increases body strength and reduces thermal expansion.

Individual particles of quartz have a high thermal expansion (and associated contraction) and significantly change their volume as they pass up and down through 'inversion' temperature points during firing. This can cause a form of body cracking called 'dunting' where the silica does not get dissolved in the feldspar glass melt. The cracking occurs as microcracks radiate out from each microscopic particle of quartz and propagate into larger cracks. High quartz bodies are usually unsuitable for ovenware and ware that must tolerate sudden temperature changes. However this behavior is advantageous to glaze fit since it puts the 'squeeze' on the glaze to prevent crazing. At the same time silica in glazes tends to dissolve and form low expansion silicates that reduce glaze expansion and also prevent crazing. In both cases, silica powder of small grain size is advantageous.

High temperature bodies tend to have up to 30% silica whereas low fire ones have much less or none (because of its refractory nature). However in recent years many companies substitute kyanite, pyrophyllite or similar minerals for part of the quartz to minimize thermal expansion (see article in Studio Potter vol 28 #1 by Peter Sohngen). Apparently very fine grades of silica aid in cristobalite formation in stoneware bodies (cristobalite is a form of silica that goes through it's inversion at about 200C).

High temperature glazes can have 40% or more silica at times, if enough flux is available to react and form silicates.

Understanding that quartz mineral and silica glass have vastly different physical properties is often the beginnings of understanding the relationship between the mineralogy of the materials we use and their chemistry. Fused silica, for example, is one of the lowest thermal expansion materials available (0.2% at 2000F). Some industries, for example, use fused silica slabs weighing more than a ton as valves in large pipes where temperatures are not only high but suddenly change, yet these slabs do not crack. Quartz, on the other hand, is one of the least thermal expansion tolerant minerals (1.5% at 2000F). Yet both have the same SiO2 chemistry.

(Richard Willis)

The dioxide form, SiO2, of silicon, apportioned to virtually 100% in quartz, flint, cristobalite, obsidian, silex, and refined sand; and to around 50% in the great number and variety of aluminum silicates. Insoluble in water. Melts around 1700ºC. Can be calcinated up to 1250ºC without any alteration in aspect or behavior — any color changes will be due to impurities in the sample not to the molecule SiO2. A feldspar, by
contrast, will change color and congeal at 1250º.
Silica, especially as quartz, is the principal responsible for low-temperature “crackles” — having over-passed its correct proportion: just that too much for a glaze’s fluid and adhesive needs and just that too little to prevent fusion. By contrast, an over-abundance of silica at high temperatures (over 1100º) will retard and even prevent crackling.
The proportion of silica to alumina (Al2O3) is the major influence over a glaze’s matte to gloss range: the more the alumina and the less the silica, the more the matte; and the more the silica and the less the alumina, the more the gloss. This relation can also affect a glaze’s opaque to transparency range, but other ingredients (iron, tin, magnesium, etc.) can override the relation between silica and alumina and become the principals
in producing opaque. Also, “colorants” (cobalt, etc.) produce a degree of opaqueness even when the over-all aspect is of a transparent color. see silicates and silicon below for the most commonly used forms of silica

Data

Linked Articles

Authors

Pictures

XML

Available on new site, see link above


Much more information with complete interlinking to many related
databases can be found by logging into the www.ceramicmaterials.info database

Copyright 2003 http://digitalfire.com, All Rights Reserved
Please support http://ceramicmaterials.info to improve this library
instrial.gif (4460 bytes)

INSIGHT is ceramic chemistry
calculation software that runs on
Windows, Mac and Linux and talks
to this web site.