Let’s make an incense burner (Kouro) 香炉 Part 2.5 ‘Test work’

I’ve made 2 small projects with clay so far, more for testing purposes than anything else. Both turned out ‘ok’ to some degree. A shitty tea cup that looks like it was made by a 10 year old (literally, I’ve seen racks of shit 10 year olds make and theirs was probably better), and a flower vase that kind of fell apart once it was dried. Talking with Susan who works at the Wildflower Art Centre in northwest Calgary has helped patch up a lot of things about clay that I didn’t understand before.

What I made was supposed to be a dry flower vase. It was produced using a method called ‘coiling’ where clay is rolled between your hands into long strings, and coiled vertically to produce the shape. I used cutting rings on flat hammered clay instead as I felt it was more consistent. The layers are pinched together and then rubbed at the creases to blend the layers together. The coil method produces a pretty interesting result, although you should take extra precautions to ensure that each layer is stuck together well. If not, the piece may fall apart once it has reached its dry, pre-firing stage. I don’t think this method is what I will use for the daruma, but we’ll see.

One thing I feel I need to do in the future if I attempt the coil method again is to cut grooves or teeth in the layers so that they fit together better. I certainly was not expecting the base to fall off when it dried, but without any indicator of what is the 12:00 point on the base, I have to turn it like a jerk until it seems to be correct. Unacceptable. 

The clay I made is very likely low fire earthenware, it was fired at cone5 and turned out to be terracotta red due to a high iron content. It’s still quite absorbent at this stage, which is a sign that it may be under-fired. The consistency was very good though. Susan will do some testing for me at higher temperatures to see if the clay vitrifies. Once I know it’s vitrification point, I can more accurately produce slips and glazes for the decoration.

Kaolin alone does not produce a suitable slip for my clay. As great as it looked in the leather-hard stage, once the body was dry the kaolin was just flaking off. After the firing even more so, because the vitrification point of kaolin clay is much higher than that of the body, firing the piece doesn’t would not affect the glaze.

This is something I’ve learned about the firing process which is crucial. All clays/slips/glazes have melting points (vitrification), where they will essentially turn to magma. Earthenware melts at the lowest temperature, Stoneware and Porcelain at a much higher one. I’ve seen the term low fired earthenware, high fired stoneware before, but I thought it was just a style, not a law. Knowing this I can move forward in planning the materials for the design. 

The white glaze or slip that I produce must have an equal or lower vitrification point than the clay body. So I’ll have to remove the iron from the clay (if that’s possible, I wonder if I can put a magnet into the slip to suck out the iron…?), or use another material to get a white result. It will have to stay earthenware I guess. I don’t really know how to proceed until I hear back about the firing temperatures.

http://www.ceramicstoday.com/cones.htm