To all the new readers. This is a series that I have started on our (ceramists) relationship with heat. I am an artist, who works as a ceramic engineer and teach materials here At Alfred University. So I have a particular perspective on our relationship with heat. It has always seemed to me that artists do not have firm grip on what is going on when we fire a kiln, so this series is an attempt to explain things better. I hope you find it informative.
Where we left off was the development of heat and the kiln. The kiln was important because people discovered that the hotter the firing, the more durable the work. We take durability for granted in a world of plastic and mass production. But, back then, you had one plate and one cup. So It was important that they lasted.
Eventually the Europeans figured out how to work with stoneware and salt firing, creating kilns that could reach higher temperatures. Which was good, and durable, but it was not pretty. I know that some people will jump us and say that this is beautiful ware. I would argue that it is interesting, but the fact is, this was ware that was made to be purely functional. Wasting time on aesthetics wasted production time. Loss of production time, means reduced profits.
Around this time trading with Asia began, and everything changed. China, to be blunt, was awesome. China had discovered Porcelain, and it was good. Porcelain is pure, white, sensitive and pretty. All of that is important for objects to be considered "beautiful" by the upper class. I don't want to go too into depth about the value of whiteness and the desirability of that (Michael Jackson, anyone). To emphasize the importance of Whiteness, I present you a dramatic reading from Monty Python and the Holy Grail.
"How do you know he is a King?"
"He hasn't got shit all over him."
Simple as that. The world people occupied was inherently dirty. Whiteness represented a life that was superior to the the unwashed masses. But there was a catch, Europeans could not make this lovely, white, delicate, translucent material. Only the Chinese could. This was a problem. Europeans had to import Porcelain, and they did, in massive quantities. The called it white gold.
What the Chinese had figured out was two things. One, they had discovered Kaolin and chemistry. Kaolin, the pure white clay which is the basis for all clays, was found at Gaoling Mountain, outside Jingdezhen. The second is that they had invented kilns that were able to achieve the (at the time) incredible temperatures. The real secret to building heat is not so much the generation if heat, as much as it is the retention of the heat that you have already created. As I stated before, a kiln is not a heat building device, so much as a heat retention device.
Anyone who has every fired a modern barrel Kiln, you will know, there units are often stated to be "Cone 10" rated. Yest it is exceptionally difficult to achieve that temperature. it is not the faults of the elements. It is the fact that the kiln only has a couple of inches of refractory. So the heat that is built, just radiates out through the wall.
So, the Chinese had figured all this out. Chemistry and Heat. But why are these high temperatures necessary? For that we must return to the Bunsen Burner. As we discussed before, in Chemistry class we used the Bunsen Burner to induce a chemical reaction. The application of heat, applies energy to the system that induces the materials to interact and form new species.
So what does all this heat get us? It is pretty simple, we use heat to change Clay, Feldspar and Quartz into Mullite and Glass.
Clay systems are pretty simple. They seem complicated because we look at a lot of different materials (Clay, Feldspar, Quartz and Fillers) It gets even worse because we have lots of Brand names on top of that (Custer, G-200, EPK, Grolleg) the list goes on and on. It gets even worse. This morning I was talking with an Engineering student, and I had to explain to him that Quartz, Flint, and Silica were all the same thing. Ugh.
But I don't want to get you confused (I'll save that for later). And Chemistry is for another day. What I want to talk about is what happens in a firing. I will go into the subtleties of a firing in my next post, but here I want to talk about what happens in a high temperature firing. That is the formation of Mullite and Glass.
Think of Mullite and Glass, as Skeleton and Muscles. Mullite is the skeleton of clay and Glass is the muscles. They need each other to function, even though they are different things. One without the other is useless. Glass makes clay, hard and durable. Mullite allows for the glass to keep it's shape in the kiln.
Anyone who has seen glass blowing, knows that molten glass flows. If left to its own devices molten glass will puddle. That is why glass blowing is so difficult, you are trying to fight against the nature of the material to puddle. You are fighting gravity and chemistry.
Why Mullite is so awesome is that it helps the molten glass keep its shape.
Next time. What is going on in my kiln? (and why Quartz Inversion doesn't matter).