Ruyao: What's in a glaze?
(Originally published June 16, 2019)
A beautiful sky blue glaze that is both shiny and cloudy gives ruyao its defining characteristic that is best described as “jade-like.” True ruyao or fanggu ruyao can be distinguished from cheap mass-produced factory “ruyao” by this characteristic, as well as by its composition and the steps in its manufacture.
The jade-like glaze of ruyao.
The allure of ruyao can be traced back all the way to the Song Dynasty. After a thousand years, the studios making fanggu ruyao have returned to the same small number of key ingredients that were used in original Song Dynasty ruyao. Feldspar and Silica SiO2, two of the most common minerals in nature, are the two most important ingredients in the recipe for ruyao and it is these two minerals that give ruyao its special jade-like quality. SiO2 is found naturally as quarts and as agate – a variety of quarts believed to be one of the original ingredients in Song Dynasty ruyao. These two minerals, along with at least 10 other ingredients, are mixed, applied and fired to produce true ruyao glaze.
In order to fire successfully, ruyao ware has a very thick layer of glaze applied over the surface. Thin-glazed ruyao is much less likely to survive firing. Thin-glazed ruyao is much more expensive as only a small percentage survive firing.
A prototype thin-glaze ruyao teapot by Lee Shanming.
One of the primary differences between true ruyao and imitations is the temperature used during firing. True ruyao is fired at a very high temperature and undergoes partial vitrification of the glaze (the glaze surface becomes impermeable and glass-like). However, underneath the surface the glaze appears cloudy and deep.
This is the result of only partial vitrification, and this is what gives ruyao its appearance. Known as “high temperature feldspar phase separation”. The secret lies in the use of both feldspar and SO2, and more specifically - in the different melting points for the two minerals.
Feldspar reaches its melting point in the kiln when ruyao is fired at temperatures between 1260-1350C. It vitrifies, becoming glass-like. However, SO2 melts at a higher temperature of 1700C. Since SO2 never reaches its melting point in the kiln, it never vitrifies. This gives ruyao its smooth and shiny surface and cloudy depth. Light traveling through the glaze hits the SO2 and scatters, giving it a jade-like luster and glow.
Finally, a reduced oxygen environment is an important part of the firing process. The sky blue or tianqing 天青 ruyao glaze recipe has less than 3% iron oxide. During reduction firing, the small percentage of iron oxide gives ruyao its light sky blue. Because ruyao requires a very high firing temperature, other ceramic pigments cannot be used, only natural minerals.
Sky Blue Ruyao Cup from Shan Kiln.
Imitation vs True Ruyao
To fire ruyao successfully, a list of factors has to be taken into account:
1) Formula of the glaze.
3) Formula/composition of the body.
4) Shaping of the body.
5) Glazing technique.
4) Atmosphere and temperature in the kiln.
5) Firing position.
6) Firing time.
7) Cooling conditions and time.
All of these factors influence the success rate of firing ruyao. A problem encountered in any of these factors may prevent a successful firing. The cost of ingredients; of trial and error to discover the correct recipe; of trial and error in technique before and after firing; of firing at the required temperature and environment; all of these factors increase the cost of manufacturing.
The manufacturers of cheap ruyao have chosen to forgo the expense by producing something quite different. Many of the cheap “ruyao” pieces seen on the market are matt-glazed imitations fired at a lower temperature and coloured with ceramic pigments. Their appearance lacks the depth and jade-like character of true ruyao. These are light blue coloured stoneware, not true ruyao porcelain.
Factory made "ruyao" stoneware fired at a lower temperature and using ceramic pigment.