Alternate Names: Carbonate of Lime, Whiting, Aragonite, Calcite, CaCO3
|If this formula is not unified correctly please contact us.|
|DENS - Density (Specific Gravity)||2.80|
|HMOH - Hardness (Moh)||3.01|
|GSPT - Frit Softening Point||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 With the advent of faster firing schedules in recent years whiting has been replaced by wollastonite and frits as a source of CaO in many applications (CaO oxide is advantageous in fast fire because it does not lower the melting point as much as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).
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. With lots of heat.
Examples of calcium carbonate (top) and dolomite (both mixed with 25% bentonite to make them plastic enough to make a test bars). They are fired to cone 9. Both bars are porous and refractory, even powdery. However, put either of these in a mix with other ceramic minerals and they interact strongly to become fluxes.
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 10% (test by Kat Valenzuela). Adding a flux only reduces the SiO2 and Al2O3, this pushes the thermal expansion upwards. 5% is actually sufficient. An alternative would be to use wollastonite, it supplies SiO2 also.
Because this glaze employs 10% dolomite instead of 10% calcium carbonate it has a lower thermal expansion and is less likely to craze. While the dolomite is contributing MgO, which normally mattes glazes, there is not enough to do it here.
It fumes a glassy glaze onto nearby test bars at cone 10R. This fumed glaze layer on the other bars is thick enough to craze and is transparent and glossy. Any ideas why this happens? Please let me know.
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 Ferro Frit 3249 based transparent (G2867) with 15% CaO added (there is no blistering without the CaO). Calcium carbonate has a very high loss on ignition (LOI) and for this glaze, the gases of its decomposition are coming out at the wrong time. While there likely exists a firing schedule that takes this into account and could mature it to a perfect surface, the glaze is high in MgO, it has a high surface tension. That is likely enabling bubbles to form and hold better.
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). Notice that talc is not finished until after 1650F (many glazes have already begin melting by then).
This is a cone 10 glossy glaze. It should be crystal clear and smooth. But it contains strontium carbonate, talc and calcium carbonate. They produce gases as they decompose, if that gas needs to come out at the wrong time it turns the glaze into a Swiss cheeze of micro bubbles. One solution is to use non-gassing sources of MgO, SrO and CaO. Or, better, do a study to isolate which of these three materials is the problem and it might be possible to adjust the firing to accommodate it. Or, an adjustment could be make to the chemistry of the glaze such that the melting happened later and more vigorously (rather than earlier and more slowly). The latter is actually the likely cause, this glaze contains a small amount of boron frit. Boron melts very early so the glaze is likely already fluid while gases that normally escape before other cone 10 glazes even get started melting are being trapped by this one.
Out Bound Links
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In Bound Links
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