Have you ever wondered what would happen if you ground up the entire earth’s crust, mixed it thoroughly, and then mixed it with water? Then you get… a ceramic glaze!
But how do you actually prove that? Of course, I could fire the mixture in a kiln at about 1280 °C to see what the result is. But how do you get a completely ground earth’s crust?
Fortunately, there is another way to show that a glaze based on the earth’s crust produces a shiny, deep dark brown ceramic glaze. You just have to do the math…
What does the Earth’s crust actually consist of?
The earth’s crust roughly consists of three main types of rocks:
Igneous rocks
Created by cooling and solidification of magma or lava.
Sedimentary rocks
Created by deposition and hardening of material that has broken down from other rocks (erosion).
Metamorphic rocks
Formed from existing rocks (igneous or sedimentary) that have changed under high pressure and temperature.
But it doesn’t stop there…
Minerals: the building blocks of rocks
These rocks consist of numerous minerals: naturally occurring, inorganic crystalline substances with a specific chemical composition and structure. Think of quartz, feldspar, mica, pyroxene, amphibole, and so on.
According to the International Mineralogical Association (IMA), there are about 5,900 to 6,000 recognized minerals worldwide.
From minerals to rocks
Rocks consist of one or more minerals. Not every mineral forms a rock; some are rare or unstable. Others together form well-known rocks such as granite, dolomite or Cornish stone.
Depending on the classification system, between 2,000 and 2,500 rock types are distinguished worldwide.
Chemistry makes it clearer
The study of the earth’s crust is complex. Geology (the science of rocks and minerals) is divided into:
- Mineralogy – the study of minerals
- Petrology – the study of rocks
And even then, they are both gigantic research areas, where you don’t have an overview in an hour of googling. But fortunately we can also look at it more simply: chemistry.
Atoms: the real basis
Although the earth’s crust consists of thousands of types of rocks, they are all made up of atoms – the building blocks of the universe. And they are clearer: just over 100 in total.
For the ceramist, only about 12 are really relevant:
- Alkali metals & alkaline earth metals: Li, Na, K, Mg, Ca, Sr, Ba
- Other oxides: Zn, Pb, Al, Si, B
And for color, there are a few more:
- Coloring oxides: Sn, Fe, Cu, Co, Mn, Ti, Cr, Zr
The earth’s crust in 8 oxides
Chemically, the earth’s crust consists mainly of the following oxides (source: Wikipedia):
- SiO₂ (silicon dioxide) – 62%
- Al₂O₃ (Alumina) – 16%
- Fe₂O₃ (iron oxide) – 6%
- CaO (calcium oxide) – 5%
- Na₂O (sodium oxide) – 4%
- MgO (magnesium oxide) – 4%
- K₂O (potassium oxide) – 3%
And let these be exactly the oxides that potters have been using to make glazes for thousands of years…
From analysis to UMF
The chemical composition of glazes is not expressed in mass, but in moles (number of particles). Therefore, we need to convert the weight percentages to mole percentages, and then to a UMF (Unity Molecular Formula, also known as Seger formula).
This can easily be done via Glazy.org.
From UMF to glaze recipe
Based on the UMF of the Earth’s crust, I have composed a glaze recipe that (approximately 😄) matches the chemical composition of the Earth’s crust.
- Feldspar (sodium) – 27,8%
- Sillica – 19,0%
- Feldspar (potash) – 15,6%
- Kaolin – 15,1%
- Dolomite – 14,9%
- Iron oxide (red) – 5,9%
- Magnesium carbonate – 1,7%
See this glaze on Glazy: Earth Crust Glaze Recipe
From Earth’s Crust to Glaze Dust
Just fire it at around 1250–1280 °C (cone 8–10) and you’ll have a beautiful glaze, straight from Mother Nature!
I love using this glaze on my work – and luckily, it’ll be a while before I’ve truly fired through the entire Earth’s crust. 😉
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