Over the past few weeks I have been asked by several potter friends about the materials for Japanese shino glazes and their equivalents in the west. I haven't actually used a shino glaze for about fifteen years, but with some of the restaurant collaborations and requests from chefs, I may soon be revisiting the shino arena.
It is probably useful to firstly put shino into a frame of reference.
Once upon a time there was a tea master named Shino Soushin (1444-1523) who directed that the potters of Mino produce a white glaze. Their solution was to coat a low iron clay, either mogusa clay or gotomaki clay, with a local feldspar, occasionally mixed with some clay to make it more user friendly. The resulting glaze, wood fired of course, tended to be slightly grey in oxidation (Nezumi Shino meaning Mouse Shino) or pearly white with red flashing in reduction.
Generally Shino glaze is accredited to the Momoyama period (1568-1600), and, yes, some of the finest shino glazes come from that period, but it had its roots in the Muromachi period (1336-1573). Interesting to note the overlap.
So...what was the feldspar?!
The simple answer is Hiratsu Shino Choseki. Choseki is Feldspar, the Kanji "長石" means "long rock" referring to the rhombic crystalline structure of feldspar. This feldspar has a high Alumina to Silica ratio, combined with fairly high Sodium/Potassium content. It is in fact the Sodium and Alumina under reduction that give the distinctive soft pink and orange flashing of Shino. Which is why Nepheline Syenite lends itself so well to shino style glazes. Compare the analyses;
Hiratsu Shino Choseki ;
Ignition loss 1.59
Ignition loss 0.47
Many recipes have been based on this material, especially with its high sodium content. With the addition of up to 40% Clay (Ball clay, Kaolin, Porcelain, Terracotta or Fire clay) a huge range of variations can be achieved.
The representative Ball clays added to the feldspar in Japan were either Gairome;
Ignition loss 12.8
Or Kibushi Nendo;
Ignition loss 15.76
Other recipes have taken note of the high alumina to silica and high Alkaline flux and have included Soda ash into the mix. Others depend on fluxes other than Soda, the most significant being Lithium. In the west many Shino glazes use Spodumene as the base, which is basically;
Li2O. Al2O3. 4SiO2
This is as rare as hens teeth in Japan, and the best replacement is Petalite;
Li2O. Al2O3. 8SiO2
Of course the Silica present in the rest of the glaze needs to be reduced to compensate. This can sometimes be done by replacing Kaolin for Ball Clay, but it is important to test with the materials one has at hand, as there are huge regional variations. Alternatively an addition of Lithium Carbonate and Alumina will compensate, but you will need to do the math!
It is this high alumina, high alkaline flux that gives my pottery its distinctive peach blush, as I am using a porcelaineous, high alumina, low iron, clay in a wood firing, from which the natural salts vaporise from the wood, with the addition of a small amount of soda ash at the end of the firing. Not Shino, but based on the same principles.
There are books that purport to tell the secrets of Japanese wood firing, with lists of "American" shino recipes and no examples of any actual representative Japanese ceramics whatsoever. OK, I can live with that. As recipes they stay within the parameters of the original Hiratsu Shino style, and if you are satisfied with that, fine.
But that is NOT all, oh no that is not ALL!
Japanese shino glazes are not all made with Hiratsu Shino Choseki, and not all flashing is Soda/Alumina. One must not forget our old friend Iron, who so happily serves the potter with a huge gamut of glaze effects.
"And so", said the Cat in the Hat, "So, so, so, I will show you another good trick that I know!"
As my sempai Ken Matsuzaki demonstrates so well, any feldspar can make a Shino, given the presence of Iron either underneath the glaze or within range of the external glaze surface. It is the nature of Iron oxide that in a heavy reduction atmosphere, between the temperatures of 1090 and 1130, over an extended period of time, it will volatise and migrate to the surface of the glaze. This must be done before the glaze cinters, sealing it and preventing the Iron from seeping through the porous glaze matrix. By painting a decoration in Iron under the glaze, or by utilizing a small amount of Iron in or on the body beneath the glaze an Iron blush can be created on the opalescent white feldspathic glaze. A wash of Iron inside a saggar surrounding the work can give a similar effect.
Maintaining a heavy reduction during cooling is essential, but in the end the micro fine layer of Iron will re oxidize to give the distinctive Shino blush. Merely seeing Iron through a semi transparent glaze is not Shino. There is a lustre and warmth that springs from the natural process which makes shino special. Colours that resonate with sunrises and sunsets, that remind us of the soft warmth of human skin.
The Japanese traditional teaching system does not put the onus on the teacher to teach; It is the responsibility of the student to actively learn. Shimaoka sensei told me that there are no secrets. You just need to ask the right questions.