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.

Cone No.	Centigrade	Fahrenheit	Visual Color
022	600	1112	Dull Red
021	614	1137
020	635	1175
019	683	1261
018	717	1322
017	747	1376
016	792	1457
015	804	1479
014	838	1540
013	852	1565
012	884	1623	Cherry Red
011	894	1641
010	900	1652
09	923	1693
08	955	1751	Orange
07	984	1803
06	999	1830
05	1046	1914
04	1060	1940
031/2	1080	1976
03	1101	2014
02	1120	2048
01	1137	2079
1	1154	2109	Yellow
2	1162	2124
3	1168	2134
4	1186	2167
5	1196	2185
6	1222	2232
7	1240	2264
8	1263	2305
9	1280	2336
10	1305	2381	White
11	1315	2399
12	1326	2419
13	1346	2455

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