Week Twelve
It never ceases to amaze me that a ball of clay can miraculously turn into a cup in a few minutes.
I read an article on The Chemistry of Pottery, by Stephen Breuer in 2012. It answers a lot of questions I have had over the years about ceramics. For instance, why does clay shrink so much? Because it has a very high level of water content, apparently. Water is within the molecular structure of clay. He says that layers of clay sheets are separated by a layer of water molecules, bonded together by hydrogen. These bonds are weak enough to be manipulated by force, but strong enough to hold together once that force is removed. This allows clay to be formed into shapes that remain. The form will shrink 5 % from wet to bone dry condition. The platelets become closer together forming a stronger structure.
When the clay is slow fired, or bisque fired, more water is released. The object shrinks further.
"Once its temperature reaches around 500ºC, the changes in it have become irreversible. At this point the clay is very fragile and crumbly, but it can no longer be reconstituted into the original workable state. This stage is described as the driving off of the so-called chemically bound water...
The weak hydrogen bonds are replaced by stronger and shorter oxygen bridges." Beuer (2012)
The clay is first fired to 1000 degrees, after which it can then be glazed and fired to a higher temperature, with little further shrinkage. In an earthenware firing (1000 - 1150 degrees) the clay remains porous. Breuer explains that if you were to put an earthenware mug full of water in a microwave and boiled it, when you removed it from the oven you would find that the handle would be hot. This is because of the clay accepting moving water molecules during the boiling of the water. This does not happen in a stoneware firing (>1200 degrees). Of course, you have to use the correct clay in each type of firing. If you fired earthenware clay beyond 1200 degrees, it would literally melt.
Breuer goes on to say: "Most pots are glazed, ie they are covered by a thin coating of glass. This can be for aesthetic or for practical reasons, usually both. It is particularly important for pots holding food. The glaze usually has three main components: silicon dioxide to provide the main body,
aluminium oxide to enhance the viscosity of the glaze by cross-linking the silica networks and
fluxes, generally alkali or alkaline earth metal oxides, to lower the melting point of the mixture to the temperature of firing.2
In addition, it is common to include transition metal oxides to provide colour to the glaze." Breuer (2012)
This is an informative article and well worth a read for some fundamental chemistry knowledge of the ceramic process.
Breuer, S. (2012, July 1). The Chemistry of Pottery. Retrieved from https://eic.rsc.org/feature/the-chemistry-of-pottery/2020245.article
Clay is known to have a 'memory'. This means that if you made a thin slab and pulled and curled it from a surface while wet, chances are that when you fired the piece it would develop a curl. This memory is caused by the changing alignment of the clay platelets. A good demonstration of this can be found on a video by Fire Verse Ceramics.
Fire Verse Ceramics (2014, December 27). Pottery Lesson - Two Minute Tutorial: Clay Memory and Warping [Video File]. Retrieved from https://www.youtube.com/watch?v=4dUOw1IYAXk and also from http://www.fireverseceramics.com/claywork-tutorials.html
Wheel practice continues. I am starting to teach myself how to belly out the cylinders now. It's not easy and requires a gentle touch. It incorporates pushing out from the inside, collaring the neck and perhaps, bending the lip over to create a flair. I'm glad with my progress so far, although wish I could practice more.
I was lucky enough to listen in on a little instruction on kilns and bisque firing by Darren. I mostly already knew the information but it is always good to reinforce it. He explained that the purpose of the bisque firing was to extract the water content in the clay. If this is not done, the piece can explode in the kiln. He pointed out the various programs detailed in the notice under each kiln. He also commented on kiln wash, a mixture of kaolin and alumina hydrate. (There are many recipes for kiln, or batt, wash, some using silica. It is designed to protect the kiln shelves. It doesn't completely prevent glaze from sticking to the shelves, but it does make it easier to grind the excess glaze off the batt.
I recently discovered the breath-taking work of Japanese artist, Taketoshi Ito. He is known for his finely carved vessels and slip-worked bowls. He seems to be inspired by European gothic architecture and molded iron work.
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