Limestone

Oxide	Analysis	Formula
CaO	52.60%	0.97
MgO	1.00%	0.03
Al2O3	1.00%	0.01
SiO2	1.00%	0.02
Fe2O3	0.50%	-
LOI	43.90%	n/a
Oxide Weight		58.29
Formula Weight		103.90

Notes

In ceramics, the value of calcium minerals relates to their readiness to form melt solutions with SiO₂ and these solidify as silicates on cooling (glasses and glazes).

While not intuitive (given the high LOI of limestone), in some countries (e.g. Thailand) limestone powder (about 10-20%) is added to clay bodies (they are fired 700-900C). Many benefits have been noted, including: Lower water content needed (and thus lower drying shrinkage), more dense fired matrix, elimination of black coring, increased strength at lower firing temperatures and lower clay percentages in the recipe.

The analysis provided here is hybrid (not theoretical or actual). We started with the theoretical calcium carbonate analysis of 56.1 CaO and 43.9 LOI. Then we added 1% impurities of MgO, Al₂O₃ and SiO₂ plus 0.5% Fe₂O₃ (all at the expense of CaO). These are amounts that would typically be found in commercial materials. Many are less pure than this (having a lower CaO content), some highly processed grades are more pure.

Related Information

What happens when a limestone clay mix is fired to cone 6?

A fired clay bar disintegrates on contact with water

The top bar is a mix of calcium carbonate and a middle temperature stoneware clay (equal parts). On removal from the kiln it appears and behaves like a normal stoneware clay body, hard and strong. However, pour water on it and something incredible happens: in a couple of minutes it disintegrates (as it rehydrates). And generates lots of heat as it does so.

Links

Materials	Dolomite An inexpensive source of MgO and CaO for ceramic glazes, also a highly refractory material when fired in the absence of reactant fluxes.
Materials	Calcium Carbonate In ceramics, calcium carbonate is primarily a source of CaO in raw stoneware and porcelain glazes.
Materials	Precipitated Lime
Materials	Oyster Shells
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.
Minerals	Limestone Limestone forms by sedimentation, of coral and shells (biological limestone) or by the precipitation
URLs	http://geology.com/rocks/limestone.shtml Limestone at Geology.com

By Tony Hansen
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