What happens to clay when it goes into the kiln, to harden and then to be glazed?
“It comes from the Earth. I think we get our clay from India,” said senior Alex Warner, who explained clay as being “sand-mud.” When his piece goes into the kiln he says the high temperature would make the molecules “realign to be straight.”
Senior Dreaven Clark explained the firing method as “kind of a teamwork thing, like the glaze is hardening along with the clay and the two different molecular structures are combining with each other.”
Create your own (more scientific) hypothesis:
Hypothesize what will happen to the mass of the clay when it is fired for the first time in the kiln.
Hypothesize how the glaze will affect and interact with the clay when they are fired together.
Be sure to identify the dependant and independant variables in your hypothesis. Think about changes in states of matter, temperature, kinetic energy, and conservation of mass.
So what does firing a mug actually have to do with hydrogen bonds and molecular structures?
Adrian Wade and Mark Chaplin teach pottery and chemistry, respectively. Their subjects, though quite different, relate directly in many ways. Adrian explains to her students the process of bisquing a piece (firing the clay to harden it) as the particles aligning like a zipper, combing the particles through each other so they are all going the same way. The clay is fired in the bisque kiln at 1819 °F for 12 hours to completely dry out and harden the piece.
From the perspective of chemistry, Mark explains that the minerals in clay are part of a hydroxide bond (an oxygen and hydrogen structure). This bond is very weak, which makes the clay moldable and easy to manipulate while it is wet.
On a basic level, when clay is fired at a high enough temperature, the water from the clay (H2O) is released into the air, causing the clay particles to shrink and create a tighter covalent bond with the remaining oxygen. This makes the clay harden.
Next the clay is glazed and fired to finish. The glaze is a very important part of the process, says Adrian, if a piece is not glazed it is not vitrified, meaning without the glaze the bowl you are making would not hold water and could poison you if you tried to eat out of it. The temperature of the kiln for glazing is much hotter, at 2165 °F.
Our glaze at school comes in a powder form to which we add water. The glaze’s key ingredients are flux, (which lowers the melting point of the glaze), metal for color, and silica that reacts with the flux to help the glaze harden.
Pottery students know that glaze is a glass and therefore is made of sand. When the glaze reaches the high temperature of the kiln it melts and turns into glass.
It is important that the glaze cool quickly in order come out smooth and not make crystals. The flux helps the glaze to not crystallize as it cools by lowering the melting point of the glaze; this way the kiln doesn’t have to heat up as much and it takes less time for it to cool down. At this stage, the kiln goes for approximately 4 to 7 hours. When it reaches peak temperature, the kiln turns off and the pieces must cool for hours to harden.
The metals in the glaze also react with the heat to create the brilliant colors that coat the pieces displayed in the art shows at our school and the ceramics we use every day in our kitchens.
Discussion Questions
- Water boils at 212°F. Why do you think the kiln has to be close to 9 times hotter in order to drive the water out of the clay?
- As the glaze goes from a solid (powder form) to a liquid (when coating the piece) to a solid (hardened on the piece) what is going on at the molecular level in the glaze?
