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Cryolite

Alternate Names: Cryloite, Sodium Fluoaluminate, Kryolith

Oxide Analysis Formula
Na2O 44.31% 1.00
Al2O3 24.25% 0.33
F 54.31% 4.00
Oxide Weight 171.92
Formula Weight 171.92

Notes

A fluoride of aluminum and sodium associated with granite. It is a valuable source of insoluble sodium used in enameling and sometimes in frits and glazes. This material is a very active melter.

Because cryolite lacks oxygen it is useful in creating artificial reduction glazes for electric firing.

Since this material is looking for oxygen to satisfy vacancies resulting from the gassing away of fluorine, it will take it from the kiln atmosphere or from neighboring molecules. The net effect of this take up is that cryolite has a lower than expected LOI.

The reaction is likely 2 Na3AlF6 producing 3 Na2O, 1 Al2O3 and 12 F. 2 Na3AlF6 has a weight of 419.8, 3 Na2O. Al2O3 has a weight of 287.8 and 12 F is 228. In a theoretical reaction 419.8 grams goes in and 515.8 comes out (assuming F stays in the elemental state and oxygen is absorbed from elsewhere as needed). However various factors can play to effect a partial gassing of fluorine (an incomplete supply of oxygen to fill all vacancies and the fact the fluorine can form silicates by supplanting oxygen and thereby releasing it as a gas). The complexity of the situation can be demonstrated by mixing it half-and-half with kaolin, weighing, firing to cone 6 and weighing again. In one such test we did the cryolite lost 13.9% of its weight. Considering that the kaolin component loses 13% there still should have been a net gain. The mixture was an incredibly active melter (it needs to be fired in a deep crucible), this suggests that F was gassing.

Related Information

Links

Typecodes Flux Source
Materials that source Na2O, K2O, Li2O, CaO, MgO and other fluxes but are not feldspars or frits. Remember that materials can be flux sources but also perform many other roles. For example, talc is a flux in high temperature glazes, but a matting agent in low temperatures ones. It can also be a flux, a filler and an expansion increaser in bodies.
Typecodes Generic Material
Generic materials are those with no brand name. Normally they are theoretical, the chemistry portrays what a specimen would be if it had no contamination. Generic materials are helpful in educational situations where students need to study material theory (later they graduate to dealing with real world materials). They are also helpful where the chemistry of an actual material is not known. Often the accuracy of calculations is sufficient using generic materials.
URLs http://mineral.galleries.com/minerals/halides/cryolite/cryolite.htm
Cryolite at Mineral.galleries.com
URLs http://webmineral.com/data/Cryolite.shtml
Cryolite at WebMineral.com
URLs http://www.mindat.org/min-1161.html
Cryolite at MinDat.org
URLs http://en.wikipedia.org/wiki/Cryolite
Wikipedia Cryolite
Materials Sodium Fluosilicate
Oxides F - Fluorine
Hazards Cryolite and Ceramics
Hazards Fluorine Gas

Data

Hardness (Moh)2.5-3
Frit Softening Point1832C D
Density (Specific Gravity)2.95

By Tony Hansen


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