In ceramic manufacture, knowing about the how and when materials decompose during firing is important in production troubleshooting and optimization
Decomposition is the breaking of inter-molecule bonds during melting in the kiln. To understand it we need to understand elements, oxides, compounds, solutions and mixtures (from the chemistry jargon point-of-view).
"Elements" are one kind of atom which cannot be broken down any further (except by nuclear reactions). Atoms are a nucleus with varying numbers of electrons in orbit. Oxygen, silicon, carbon, etc. are among the 100+ elements.
"Compounds" are atoms of more than one element bound together chemically. How? Atoms share electrons or electro-statically attract others having opposite charges. An "oxide" is a compound containing oxygen. A molecule of SiO2 is one atom of silicon and two of oxygen, thus is it an oxide (there are about ten that are important in glazes). Oxides are bonded together really well.
"Solutions" are mixtures of molecules in a liquid. In salt water, for example, the NaCl molecules move independently. But as the water evaporates the mobility of the water enables them to bond in a preferred orientation: crystalline.
Theoretical soda feldspar is a compound of oxides: molecules of sodium (Na2O), alumina (Al2O3) and silica (SiO2) are bonded chemically to form a crystaline solid.
A bucket of glaze made of feldspar, kaolin and silica powders is a "mixture", the particles are not chemically bonded and they are not dissolved. Rather, they float freely in "suspension" in the water.
Now, if we dry a glaze and subject it to increasing heat in a kiln, the bonds holding oxides together (in their compounds) will be broken (weakest first, strongest later). An increasingly liquid "melt" will form as temperature rises. This melt is a "solution", particles are dissolving into it.
Since glaze melts are much more viscous than water solutions, the opportunities for orientation of particles into crystals during cooling of the melt in the kiln are dramatically less. So they freeze as a "glass". A glass is a compound, but a rather novel one: a solid solution.
The above is the theoretical process. In practice a variety of other things are also occurring as the kiln fires.
Remaining water is driven off during early stages of firing. It is not decomposition because no molecular bonds are broken. However many materials are compounds that have water molecules bonded into their chemical structure (e.g. clays). These are called "hydrates" and they can account for up to 12% or better of the weight. Relatively low temperatures (e.g. red heat) are sufficient to break these water molecules away (sometimes called "water smoking"). The product of decomposition: steam.
There is another class of 'volatiles' or gases that are driven off: Carbon and sulphur. Carbon is integral in the crystal structure in some materials (e.g. calcium carbonate, dolomite, barium carbonate). Clays contain sulphates and organic carbon. Red heat is sufficient to burn away the organic carbon as CO2 but it takes higher temperatures (varying by material) to decompose and liberate CO2 and SO3 from the carbonates and sulphates (e.g. calcium carbonate loses 45% it is weight).
Some materials experience decompositions that disqualify them for use in glazes. For example, hydrated lime is a good source of CaO, but 25% of its weight is converted to water at 500C. To say this would be an inconvenient event in a firing would be an understatement! Other materials that do generate significant gases during decomposition may not be an issue because the host glaze and firing method can tolerate this. Soluble materials are no-nos in glazes, they concentrate on the surface was water evaporates.
This is also a common problem at low fire on earthenware clay (but can also appear on a buff stonewares). Those white spots you see on the beetle also cover the entire glaze surface (although not visible). They are sites of gas escaping (from particles decomposing in the body). The spots likely percolate during soaking at top temperate. Some of them, notably on the almost vertical inner walls of this bowl, having not smoothed over during cool down. What can you do? Use the highest possible bisque temperature, even cone 02 (make the glaze thixotropic so it will hang on to the denser body, see the link below about this). Adjust the glaze chemistry to melt later after gassing has finished (more zinc, less boron). Apply a thinner glaze layer (more thixotropy and lower specific gravity will enable a more even coverage with less thickness). Instead of soaking at temperature, drop 100 degrees and soak there instead (gassing is much less and the increasing viscosity of the melt overcomes the surface tension). Use a body not having any large particles that decompose (and gas) on firing. Use cones to verify the temperature your electronic controller reports.
First, the layer is very thick. Second, the body was only bisque fired to cone 06 and it is a raw brown burning stoneware with lots of coarser particles that generate gases as they are heated. Third, the glaze contains zircopax, it stiffens the melt and makes it less able to heal disruptions in the surface. Fourth, the glaze is high in B2O3, so it starts melting early (around 1450F) and seals the surface so the gases must bubble up through. Fifth, the firing was soaked at the end rather than dropping the temperature a little first (e.g. 100F) and soaking there instead.
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 a carbonate can cause blistering. Carbonates produce gases during decomposition. This glaze (G2415B) contains 10% lithium carbonate, which likely pushes the initial melting temperature down toward the most active decomposition temperatures.
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.
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.
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