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Decomposition

In ceramic manufacture, knowing about the how and when materials decompose during firing is important in production troubleshooting and optimization

Details

Ceramic kilns are molecule-breaking machines. Mineral powders in a glaze, for example, can find themselves disassembled right down to the oxide level by the time a kiln reaches top temperature (the bonds holding oxides together are broken, weakest first, strongest later). Temperature, interactions and dissolving power of the increasingly fluid melt are the catalysts.

As clay bodies mature in a kiln they are partially broken down, some particles like quartz remain unchanged, others like feldspar melt and flow between the quartz and kaolin particle network. But the kaolin breaks down differently than the quartz and feldspar. For one thing, kaolin crystals are “hydrates”, they have water bonded into their chemical structure, ordinary drying won't remove it. But the heat generated by the kiln has the power to disconnect that water and enable it to escape as vapor. We call that escaping steam a "product of decomposition". All clays contain this type of water.

Glaze and porcelain materials tend to be refined and homogeneous, particles are generally smaller than 200 mesh and most have the same chemistry and mineralogy. But clay bodies containing minimally processed natural clays, by contrast, not only have much larger particle sizes but those particles can be a who's who of the mineral world! Many of the them are high in carbon and sulphur. Each type has a "thermal history" of when it breaks down and what gases it releases (e.g. oxygen, carbon dioxide, carbon monoxide, sulphur dioxide, metal fumes).

For clays that fire stable in color and maturity across a broad temperature range, products of decomposition may be limited to water and carbon, both of which escape early in the firing. But terra cotta clays, for example are different. The high and diverse population of gas-producing particles, many of which melt at lower temperatures, expels enough gases into overlying glazes to significantly impact quality and incidence of defects.

Carbon is integral in the crystal structure in some glaze materials (e.g. calcium carbonate, dolomite, barium carbonate). It takes higher temperatures (varying by material) to decompose and liberate CO2 and SO3 from the carbonates and sulphates. 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!

It is helpful to understand, as background information, what elements, oxides, compounds, solutions and mixtures are.

"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 ir is an oxide (there are about ten that are important in glazes). Ceramic materials are oxides bonded together. Theoretical soda feldspar, for example, is a compound of oxides: molecules of sodium (Na2O), alumina (Al2O3) and silica (SiO2) are bonded chemically to form a crystaline solid.

"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. A glaze 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 solid solution.

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.

Related Information

White spots and blisters in a high zircon glaze at cone 6

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.

Let me count the reasons this glossy white cone 6 glaze is pinholing

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.

The difference between these low fire transparents: Gerstley Borate vs. Ulexite

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.

Carbonate gassing can cause glaze blisters

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.

Calcium carbonate and glaze bubbles

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.

The perfect storm of high surface tension and high LOI: Blisters.

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.

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

Projects Temperatures
Glossary Glaze Chemistry
Glaze chemistry is the study of how the oxide chemistry of glazes relates to the way they fire. It accounts for color, surface, hardness, texturem, melting temperature, thermal expansion, etc.
Glossary Volatiles
Glossary Glass vs. Crystalline
In ceramics, understanding the difference between what a glass and crystal are provides the basis for understanding the physical presence of glazes and clay bodies.
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 Blisters
Blistering is a common surface defect that occurs with ceramic glazes. The problem emerges from the kiln and can occur erratically in production. And be difficult to solve.
Glossary Water Smoking
In ceramics, this is the period in the kiln firing where the final mechanical water is being removed. The temperature at which this can be done is higher than you might think.
Glossary Water in Ceramics
Water is the most important ceramic material, it is present every body, glaze or engobe and either the enabler or a participant in almost every ceramic process and phenomena.
Articles Where Do I Start?
Break your addiction to online recipes that don't work. Get control. Learn why glazes fire as they do. Why each material is used. Some chemistry. How to create perfect dipping and drying properties. Be empowered. Adjust recipes with issues rather than sta
URLs http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0366-69132008000300001
A study of the cmparative decompositions of MgCO3, CaCO3, SrCO3 and BaCO3
URLs https://digitalfire.com/4sight/temperatures/index.html
Milestone temperatures during firing
Tests Decomposition Temperature

By Tony Hansen


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