If you make your own glaze recipes, you use ceramic raw materials with hot air… or “LOI” for short. Sometimes this LOI is removed from the raw material beforehand by “calcining” it. But what exactly is that and why do ceramists do it?
By hot air, I don’t mean (this time) all the crap that bloated types spew into the ether. I’ll probably talk about that another time, but this time I want to talk about “Loss On Ignition” or the abbreviation LOI. In other words, the evaporation of substances by intensely heating them: calcining.
Let me explain using the raw material kaolin and a kiln that we will fire at 750 0C (1382 F). Time for an experiment!
Ceramic raw materials: kaolin what is it?
Kaolin is a pure clay, an important component of porcelain, it is therefore also called “China Clay”. In the US, it is sold under the brand name EPK. But the kaolin I use is mined in the Czech Republic and is sold under the name “Kaolien” (Zettliz Ia) at Keramikos, among others.
The exact analysis varies by mine and may change (slightly) over time, but theoretically kaolin is the pure mineral kaolinite; with the chemical formula Al2 Si2O5(OH)4. For ceramists, it is usually written as 2SiO2. Al2 O3.2H2O.
So chemically, kaolin consists of 2 molecules SiO2 (called silica or quartz), 1 molecule of Al2O3 (aluminium oxide) and 2 molecules of H2O (water).
In glazes, this raw material is used to introduce Al2O3 into the glaze. You are forced to get the SiO2 (and if you need more in the glaze, enter it with the raw material quartz, which only contains SiO2).
But what happens to the water (H2O)?
Calcining: evaporation by heating
The chemically bound water “disappears” with the firing, between 450 – 600 0C (842-1112 F). Of course, the water does not disappear, but evaporates as water vapor from the kaolin (and the kiln). This heating of a raw material is called calcining. By expelling this chemically bound water, the raw material becomes lighter. We express this weight loss in a percentage “LOI”.
You can theoretically calculate that weight loss if you know the molar mass of the molecules in the raw material. In this case:
- Kaolin: 2SiO2. Al2O3.2H20=258.16 grams per mole (g/mol = molar mass)
- Without (chemically bound) water: 222.13 g/mol
- LOI=(258.16 – 222.13)/258.16 = 13.96%
Then it is another few million years of waiting for the water to recombine with the aluminum and quartz to kaolin…
Theory vs Practice: An Experiment
Okay so theoretically the kaolin will become 13.96% lighter if you heat it above 600 0C…. Well we’re going to try that!
In practice, you heat kaolin to approx. 750-800 0C to calcine it, then you know for sure that all chemically bound water has been removed. Heating at a higher temperature is also possible, but not too high because it is then more difficult to dissolve/melt in the glaze at a lower temperature than the temperature at which it was calcined. And you certainly want to fire lower than the melting temperature of the raw material, because then you don’t end up with a powder, but a melted lump.
To test my kaolin, I placed 100 grams (100.01 grams to be exact) in a sealed container. I then fired this to 748 0C (in accordance with PTCR ring). And then weighed again.
It turned out that the LOI of the kaolin I use is 10.6%, so 3.4 percentage points lower than according to theory. How is that possible again?
Unlike “theoretical kaolin”, “real kaolin” from the earth is not 100% pure. According to the supplier’s datasheet, the LOI is approx. 10% (analysis from 2010) to 12.8 (analysis 2021). In short, my test is confirmed by the supplier’s datasheets.
Why does LOI matter?
If you fire glaze raw materials, you are dealing with LOI. This can be water, as with kaolin, but also carbon (CO2), sulfur (SO2) etc. All substances that cannot withstand the high temperatures in the ceramic kiln. You don’t have to do anything for that, when firing they disappear completely by themselves… well if you ventilate your kiln space sufficiently…
But that’s not why it’s good to know how much weight your raw materials lose during firing. If you compose your own glazes, you calculate the UMF (Unity Molecular Formula or Seger formula) to predict the properties of the glaze. If you don’t take the LOI in account, your calculation will be inaccurate.
Don’t worry, there are many sites where the composition of the raw materials including LOI is listed, such as glazy.org.
The supplier of your glaze raw materials can also tell you what the composition is, including the LOI. You can then enter this in the software you use to calculate the UMF.
If you can’t find anything, you can always test it yourself! Because glaze theory is great, but it needs to be proven in practice.
(Oh yes, why do we want to calcine ceramic raw materials? That’s for a future blog)
References
- Debasish Sarkar, “Ceramic Processing Industrial Practices”, 2019 (p. 215-246)
- Salmang Scholze, “Keramik” 7 Auflage 2006 (p. 267-278)
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