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Calcium Carbonate

Formula: CaCO3
Alternate Names: Carbonate of Lime, Whiting, Aragonite, Calcite, CaCO3

OxideAnalysisFormula
CaO56.10%1.000
CO243.90
Oxide Weight56.10
Formula Weight100.00
Enter the formula and formula weight directly into the Insight MDT dialog (since it records materials as formulas).
Enter the analysis into an Insight recipe and enter the LOI using Override Calculated LOI (in the Calc menu). It will calculate the formula.
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 With the advent of faster firing schedules in recent years whiting has been replaced by wollastonite and frits as a source of CaO in many applications (CaO oxide is advantageous in fast fire because it does not lower the melting point as much as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 a tutorial video at digitalfire.com). However, remember that CaO is not an active melter below about cone 8, so particle size can make a big difference in its willingness to enter the glaze melt. A 325 mesh material could create a glossy glaze whereas a 200 mesh could create a silky matte, exclusively because of the difference in particle size. A 325 mesh material may have a mean particle size of only 10 microns (or even less), whereas a 200 mesh grade might be two or three times that (yet both powders feel the same).

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


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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.

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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.

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2,5,10,15% calcium carbonate added to Ravenscrag Slip on a buff stoneware fired at cone 10R. It gets progressively glossier toward 15%, crazing starts at cone 10%. By Kat Valenzuela.

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A cone 6 melt flow test to compare two calcium carbonates (they make up 27% of this glaze recipe). Notice the amount of bubbles (due to the high loss on ignition of the material). Also, different brand-names of the material have slightly different chemistries.

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Close-up of how calcium carbonate can produce glaze bubbles.The two cone 04 glazes on the right have the same chemistry but the left one sources the CaO from 12% calcium carbonate and ulexite, the other from Gerstley Borate. The glaze on the left is almost bubble free, yet it has 27% calcium carbonate (in a different recipe at cone 6). Why? Carbonates are more likely to form gas bubbles because body and glaze carbonates and hydrates decompose fairly low. By cone 6 this effect is normally gone (except in glazes that begin melting very early, high boron for example).

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Worthington Clear cone 04 glaze left (uses Gerstley Borate to supply the B2O3 and CaO) and a substitute using Ulexite and 12% calcium carbonate) right. This melt flow test demonstrates that the degree of melting is the same but the gassing of the calcium carbonate has disrupted the flow (of the one on the right). However, as a glaze, the 2931A does not gel and produces a clearer glass. A further adjustment to source CaO from non-gassing wollastonite is needed.

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By Tony Hansen

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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