CaO (Calcium Oxide, Calcia)

Data

Co-efficient of Linear Expansion	0.148
Frit Softening Point	2910C (From The Oxide Handbook We also have a figure of 2572?)

Notes

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.

-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 (an incredible 2600C). It can be smelted with Al₂O₃ to produce refractory calcium aluminate cement (which can service to 2000C). But when soda and potash are added, CaO becomes a very active flux in both oxidation and reduction. 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).

-At higher temperatures, CaO contributed by wollastonite is more readily fusible than that contributed by whiting (calcium carbonate). Stain manufacturers recommend adding the molecular equivalent of calcium oxide using wollastonite, for better uniformity.

-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.

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. Float and container glass have 10% CaO.

-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.

Add 5% calcium carbonate to a tenmoku. What happens?

In the glaze on the left (90% Ravenscrag Slip and 10% iron oxide) the iron is saturating the melt crystallizing out during cooling. GR10-K1, on the right, is the same glaze but with 5% added calcium carbonate. This addition is enough to keep most of the iron in solution through cooling, so it contributes to the super-gloss deep tenmoku effect instead of precipitating out.

Comparing 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. 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 a 300F-to-ice water IWCT test). 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 this GLFL test for melt flow 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 high-CaOcone 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 more melt fluidity is needed, decrease the percentage of CaO in favor of a lower melting oxide, that will certainly help. There is a positive: For opaque glazes, this effect can actually enable the use of less opacifier.

Stull chart showing the SiO₂-Al₂O₃-(0.7CaO+0.3KNaO) system

Phase diagram of a SiO2:Al2O3:CaO:KNaO System

Phase diagram and stull chart showing the SiO₂-Al₂O₃-(0.7CaO+0.3KNaO) system. Courtesy of Matthew Katz, Alfred University

A glaze incompatible with chrome-tin stains (but great with inclusion stains)

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 (Mason specifies 10% minimum CaO, this almost has enough at 9.8%, but it also has 5% MgO and that is killing the color). Yet this same glaze produces a good red with #6021 encapsulated stain at only 5% (using 20% or more encapsulated stain is not unusual - so achieving this color with only 5% is amazing).

Two stains. 4 colors. Will the guilty oxide please step forward.

One stain produces clear in one glaze and red in another. Another stain blue and purple. Why?

These pairs are G2934 matte and G2916F glossy cone 6 base glaze recipes. The left pair has 5% maroon stain added, the right pair 5% purple stain. Why? The Mason Colorworks reference guide has the same precaution for both stains: the host glaze must be zincless and have 6.7-8.4% CaO (this is a little unclear, it is actually expressing a minimum, the more the CaO the better). The matte recipe calculates to 11% CaO, the glossy to 9%. So there is enough CaO. The problem is MgO (it is the mechanism of the matteness in the left two), it impedes the development of both colors (although not mentioned on the Mason docs).

Ceramic Oxide Periodic Table

All common traditional ceramic base glazes are made from only a dozen elements (plus oxygen). Materials decompose when glazes melt, sourcing these elements in oxide form. The kiln builds the glaze from these, it does not care what material sources what oxide (assuming, of course, that all materials do melt or dissolve completely into the melt to release those oxides). Each of these oxides contributes specific properties to the glass. So, you can look at a formula and make a good prediction of the properties of the fired glaze. And know what specific oxide to increase or decrease to move a property in a given direction (e.g. melting behavior, hardness, durability, thermal expansion, color, gloss, crystallization). And know about how they interact (affecting each other). This is powerful. And it is simpler than looking at glazes as recipes of hundreds of different materials (each sources multiple oxides so adjusting it affects multiple properties).

Links

Materials	Wollastonite
Materials	Calcium Carbonate In ceramics, calcium carbonate is primarily a source of CaO in raw stoneware and porcelain glazes.
Materials	Calcium Feldspar
Materials	Colemanite A natural source of boron that melts at a very low temperature.
Materials	Dolomite An inexpensive source of MgO and CaO for ceramic glazes, also a highly refractory material when fired in the absence of reactant fluxes.
Materials	Quicklime
Materials	Calcium Aluminate Cement
Glossary	Alkaline Earths Refers to a group of ceramic fluxing oxides that contribute similar properties to fired glazes. They contrast with the alkalis which are stronger fluxes.
Glossary	Oxide Interaction In ceramic glazes each oxide generally makes a specific contributions to the melting and freezing behavior of the glaze. However interactions are also important.
Glossary	Ceramic Oxide In glaze chemistry, the oxide is the basic unit of formulas and analyses. Knowledge of what materials supply an oxide and of how it affects the fired glass or glaze is a key to control.
Glossary	Limit Formula A way of establishing guideline for each oxide in the chemistry for different ceramic glaze types. Understanding the roles of each oxide and the limits of this approach are a key to effectively using these guidelines.
Glossary	Flux Fluxes are the reason we can fire clay bodies and glazes in common kilns, they make glazes melt and bodies vitrify at lower temperatures.
URLs	http://en.wikipedia.org/wiki/CaO Calcium Oxide on Wikipedia
Oxides	MgO - Magnesium Oxide, Magnesia
Minerals	Limestone Limestone forms by sedimentation, of coral and shells (biological limestone) or by the precipitation
Minerals	Dolomite Dolomite, as a rock, is called "dolomitic limestone". It is a carbonate, similar to limestone, but h
Media	Creating Rules for Calcium Carbonate - Wollastonite Substitution How to use Digitalfire Insight software to determine how much wollastonite to add and silica to remove to substitute for calcium carbonate in the glaze. Create substitution rules.

Mechanisms

Glaze Color	In oxidation, iron glazes calcia likes to form yellow crystalline compounds with the Fe2O3 producing a 'lime matte' even in an otherwise fluid glaze.
Glaze Color	Calcia likes to form yellow crystalline compounds with Fe2O3 producing a 'lime matte', especially in fluid glazes. Thus iron glossy brown or black glazes should be low in CaO.
Glaze Matteness	High molar amounts of calcia combined with adequate silica and preferably lower alumina will form a calcium silicate crystal matte (lime matte). The presence of zinc will increase the size of crystals.
Glaze Variegation	CaO can mottle glaze surfaces at high temperatures if significant amounts are present, it is a very strong flux and can contribute to phase separation.