CaO (Calcium Oxide, Calcia)
Notes-Together with SrO, BaO, and MgO it is considered one of the Alkaline Earth group of oxides. It has a cubic crystal structure.
-Quicklime is pure calcia, but it reacts with water to produce calcium hydroxide or slaked lime. Calcium oxide, on the other hand, is an extremely stable compound.
-Calcia alone resists melting even at very high temperatures (around an incredible 2600C), but when soda and potash are added, it becomes very active in both oxidation and reduction. At higher temperatures, CaO contributed by wollastonite is more readily fusible than that contributed by whiting (calcium carbonate). This synergy between CaO and other fluxes and differences in the mechanism of its fluxing action generates some disagreement among experts regarding the nature of CaO (like MgO, it is not a 'stand-alone' flux).
-Calcium oxide is the principle flux in medium and high temperature glazes, beginning its action (within the glaze) around 1100C. It must be used with care in high-fire bodies because its active fluxing action can produce a body that is too volatile (melting if slightly overfired).
-Calcia usually hardens a glaze and makes it more scratch and acid resistant. This is especially so in alkaline and lead glazes. However the increased hardness does not necessarily mean greater tensile strength. Its thermal expansion is intermediate.
-CaO is not effective below cone 4 as a flux in glazes but in small amounts (less than 10%) it can dissolve in earthenware glaze melts especially with lead, soda, potash) to add hardness and resistance to leaching. In non-lead mixes it can also help reduce crazing. In larger amounts, it encourages the growth of crystals which can give decorative effects to glossy glazes and produce matteness (i.e. 30%).
-High CaO glazes tend to devitrify (crystallize). This occurs either because of the high melt fluidity imparted by CaO at higher temperatures or because of the readiness with which CaO forms crystals. Fastfire glazes can contain more CaO because the quick cooling does not give the crystallization a chance to occur.
-Calcia is a moderate flux in the cone 5-6 range, but a very active one at cone 10.
-High calcia glazes tend to have good (although sometimes unexpected) color responses. For example, in oxidation iron glazes calcia likes to form yellow crystalline compounds with the Fe2O3 producing a 'lime matte'. Without the calcia, glossy brown glazes are the norm.
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CaO is not found pure in nature but rather is contained in various abundant minerals (i.e. calcite, aragonite, limestone, marble) but vary greatly in their purity (impurities usually include magnesia, iron, alumina, silica, sulfur). Of these iron and sulfur are most troublesome (i.e. where clarity is important in glass). Lime minerals vary in the degree of crystallization and cohesion of the crystalline mass and the homogeneity of the matrix.
The term 'lime' encompasses several different minerals and manufactured products.
-The term 'Whiting' traditionally refers to calcium carbonate produced by the grinding of chalk from the cliffs of England, Belgium and France. However this title also refers to any ground calcium carbonate material (i.e. those processed from marble and calcite ores).
-Ground limestone and calcined limestone (burned lime) are used in the glass industry.
-Dolomite (magnesium carbonate) is a mineral which supplies some magnesia in addition to its CaO complement. It is preferred in many situations because it more readily fluxes and the magnesia imparts desirable properties.
-Wollastonite is a calcium silicate which is more expensive than other sources of calcium, but is used bodies, glaze, porcelains, enamels and frits for its many superior properties.
See Calcium Carbonate, Whiting.
The glaze on the left (90% Ravenscrag Slip and 10% iron oxide) is transformed into a highly fluid reduction tenmoku (right GR10-K1) with just 5% added calcium carbonate.
Compare two glazes having different mechanisms for their matteness
These are two cone 6 matte glazes (shown side by side in an account at Insight-live). G1214Z is high calcium and a high silica:alumina ratio (you can find more about it by googling 1214Z). It crystallizes during cooling to make the matte effect and the degree of matteness is adjustable by trimming the silica content (but notice how much it runs). The G2928C has high MgO and it produces the classic silky matte by micro-wrinkling the surface, its matteness is adjustable by trimming the calcined kaolin. CaO is a standard oxide that is in almost all glazes, 0.4 is not high for it. But you would never normally see more than 0.3 of MgO in a cone 6 glaze (if you do it will likely be unstable). The G2928C also has 5% tin, if that was not there it would be darker than the other one because Ravenscrag Slip has a little iron. This was made by recalculating the Moore's Matte recipe to use as much Ravenscrag Slip as possible yet keep the overall chemistry the same. This glaze actually has texture like a dolomite matte at cone 10R, it is great. And it has wonderful application properties. And it does not craze, on Plainsman M370 (it even survived and 300F to ice water plunge without cracking). This looks like it could be a great liner glaze.
CaO is a strong flux but it can cause crazing
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.
The difference between dolomite and calcium carbonate in a glaze
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.
Glaze chemistry works best when changing material amounts, not material types
Two glazes, same chemistry, different materials. The glaze on the left is sourcing CaO from wollastonite, the one on the right from calcium carbonate. Thus the oxide chemistry of the two is the same but the recipe of materials sourcing that chemistry is different. The difference in the melt flow you see here is an expression of how choosing different mineral sources to source an oxide can produce melting patterns that go outside what the chemistry suggests. The difference here is not extreme, but it can be much more. Glaze chemistry is relative, not absolute. It works best when you are changing material amounts, not material types. When you do introduce a very different mineral then you have a different system which has its own relative chemistry.
Boron blue in low fire transparent glazes
This high boron cone 04 glaze is generating calcium-borate crystals during cool down (called boron-blue). This is a common problem and a reason to control the boron levels in transparent glazes; use just enough to melt it well. If a more melt fluidity is needed, decrease the percentage of CaO. For opaque glazes, this effect can actually enable the use of less opacifier.
That middle sample is supposed to be red!
Left: a cone 6 matte glaze (G2934 with no colorant). Middle: 5% Mason 6006 chrome-tin red stain added. Right: 5% Mason 6021 encapsulated red stain added. Why is there absolutely no color in the center glaze? This host recipe does not have the needed chemistry to develop the #6006 chrome-tin color (it lacks enough CaO). Yet this same matte glaze intensifies the #6021 at only 5% (sometimes 20% or more encapsulated stain is needed to develop the color).
Two stains. 4 colors. Why? The chemistry of the base glazes.
We are looking at two pairs of samples, they demonstrate why knowing about glaze chemistry can be so important. Each pair shows the same stain on two different base glazes (G2934 cone 6 matte and PGF1 cone 6 glossy). Why does the maroon not develop in the left pair, why is the purple stain firing blue on the right? The Mason Colorworks color chart and reference guide specifies that the host glaze must be zincless and have 6.7-8.4% CaO (this is a little unclear, it actually is expressing a minimum, the more CaO the better). But the colorless one has 11% CaO, it should work (the maroon one has only 9% and it is working)! Likewise the purple color develops correctly in the 9% CaO but wrong in the 11% CaO base. Both stains have the same caution on the reference guide. What is going on? It is an undocumented issue: MgO. The 11% CaO base glaze is high in MgO (that is what makes it matte), that impedes the development of both colors. When you talk to tech support at Mason (or any stain company), they need to know the chemistry of your glaze to help, not the recipe.
Ceramic Oxide Periodic Table in SVG Format
The periodic table of common ceramic oxides in scalable vector format (SVG). Try scaling this thumbnail: It will be crystal-clear no matter how large you zoom it. All common pottery base glazes are made from only 11 elements (the grey boxes) plus oxygen. Materials decompose when glazes melt, sourcing these elements in oxide form; the kiln builds the glaze from these. The kiln does not care what material sources what oxide (unless the glaze is not melting completely). Each of these oxides contributes specific properties to the glass, so you can look at a formula and make a very good prediction of how it will fire. This is actually simpler than looking at glazes as recipes of hundreds of different materials.
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