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Calcium Carbonate

Formula: CaCO3

Chemistry %

CaO56.10
 

Volatiles %

CO243.90
DENS - Density (Specific Gravity) 2.80
HMOH - Hardness (Moh) 3.01
MLPT - Melting Point (MP) 825C D

Whiting has traditionally been a source of CaO in raw glazes and glass (however whitings also typically contain some dolomite as a contaminant). Whiting is generally inexpensive and there is a large calcium carbonate industry worldwide for non-ceramic uses of this mineral. Well known deposits are the chalk cliffs of England, France and Belgium. Marble and calcite ores are abundant in many places.

Inexpensive non ceramic grades of whiting tend to lack the quality and consistency needed for use in glazes (especially for industrial use). Also whiting produces a very large volume of gases while decomposing, it loses more than 40% by weight. While these gases should be gone well before 1100C (and therefore should not disturb the glaze melt), in low or fast fire they can contribute to imperfections and faults in the glaze surface. With the advent of faster firing schedules in recent years whiting has been replaced by wollastonite and frits as a source of CaO in many applications (CaO oxide is advantageous in fast fire because it does not lower the melting point as much as the alkalies). Since LOI is a good indicator of variation in chemistry it may be practical to do an LOI test on shipments by firing a specimen of powder in a thin bisqued bowl to confirm the consistency of shipments.

There are many alternate no-LOI sources of CaO (e.g. wollastonite, frits) and incorporating one of them to source the CaO instead is a classic application of ceramic chemistry calculations (it is dealt with in the lessons section of the INSIGHT software manual).

In low-fire bodies, calcium carbonate is sometimes added in small amounts as a filler to reduce fired shrinkage and act as a whitener. It is also common to see 5% whiting included in porous earthenware body recipes to prevent moisture expansion (which causes glazes to craze).


Mechanisms

Out Bound Links

In Bound Links


Pictures



The top bar is a mix of calcium carbonate and clay fired to cone 6. The bottom is a couple of minutes after water was poured onto it.


Example of calcium carbonate (top) and dolomite (both mixed with 25% bentonite). They are fired to cone 9. Both bars are porous and refractory, even powdery.


XML for Import into INSIGHT

<?xml version="1.0" encoding="UTF-8"?>
<material name="Calcium Carbonate" descrip="" searchkey="Carbonate of Lime, Whiting, Aragonite, Calcite, CaCO3" loi="0.00" casnumber="471-34-1">
<oxides>
<oxide symbol="CaO" name="Calcium Oxide, Calcia" status="U" percent="56.100" tolerance=""/>
</oxides>
<volatiles>
<volatile symbol="CO2" name="Carbon Dioxide" percent="43.900" tolerance=""/>
</volatiles>
</material>


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