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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 With the advent of faster firing schedules in recent years whiting has been replaced by wollastonite and frits as a source of CaO in many applications (CaO oxide is advantageous in fast fire because it does not lower the melting point as much as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

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.

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.

GR10-B transparent glaze using 10% talc instead of 10% calcium carbonate. This version has a lower thermal expansion and is less likely to craze.

An example of how a test bar from 20% bentonite and 80% calcium carbonate can fume a glassy glaze onto nearby testbars at cone 10R. The glaze is thick enough to craze and is transparent glossy. Any ideas why this happens and why the whiting is completely

A cone 6 melt flow test to compare two calcium carbonates (they make up 27% of this glaze recipe that was designed to maximize their percentage). Notice the amount of bubbles (due to the high loss on ignition of the material). Different brand-names of the material obviously have slightly different chemistries so they exhibit different flow properties during firing.

The two cone 04 glazes on the right have the same chemistry but the center one sources it's CaO from 12% calcium carbonate and ulexite (the other from Gerstley Borate). The glaze on the far left? It is almost bubble free yet it has 27% calcium carbonate. Why? It is fired to cone 6. At lower temperatures carbonates and hydrates (in body and glaze) are more likely to form gas bubbles because that is where they are decomposing (into the oxides that stay around and build the glass and the ones that are escaping as a gas). By cone 6 the bubbles have had lots of time to clear.

Left: Worthington Clear cone 04 glaze (A) uses Gerstley Borate to supply the B2O3 and CaO. Right: A substitute using Ulexite and 12% calcium carbonate (B). The degree of melting is the same but the gassing of the calcium carbonate has disrupted the flow of B. Gerstley Borate gasses also, but does so at a stage in the firing that does not disrupt this recipe. However, as a glaze, B does not gel and produces a clearer glass. A further adjustment to source CaO from non-gassing wollastonite would likely improve it.

An example of how calcium carbonate can cause blistering as it decomposes during firing. This is a cone 6 Frit 3249 based transparent (G2867) with 15% CaO added (there is no blistering without the CaO).

These glaze cones are fired at cone 6 and have the same recipe: 20 Frit 3134, 21 EP Kaolin, 27 calcium carbonate, 32 silica. The difference: The one on the left uses dolomite instead of calcium carbonate. Notice how the MgO from the dolomite completely mattes the surface whereas the CaO from the calcium carbonate produces a brilliant gloss.

This chart compares the gassing behavior of 6 materials (5 of which are very common in ceramic glazes) as they are fired from 500-1700F. It is a reminder that some late gassers overlap early melters. The LOI (loss on ignition) of these materials can affect your glazes (e.g. bubbles, blisters, pinholes, crawling).


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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