Japanese Shino recipes are regularly 90+% feldspar. The white color comes from the incomplete melting of the feldspar, and the red color is likely Alkali Flashing caused by the vaporization of sodium and potassium onto the clay body.
I used a stoneware known to flash red in reduction and applied several commercially available feldspars to test tiles. I applied Yellow Ochre as wash as Shino pots often have iron decorations.
All feldspars melted to have almost the same appearance. All feldspars melted to a glossy white. Even when very thin, the feldspar did not flash red. The surrounding bare clay did flash a bright orange though. Underfired feldspars (they begin to melt at cone 5 to 6) have a rough and prickly surface, completely unlike shino glazes. On the other hand, the well melted feldspars were too glossy. So it seems like modern shinos are not pure feldspars.
When looking at the chemical analyses of Japanese feldspars, they seem to have an unusually low amount of KNaO. I think these feldspars are contaminated with kaolin and quartz particles. They are partially dissolved by the albite (sodium feldspar) and orthoclase (potassium feldspar). Since the melting is incomplete, the glaze becomes slightly matte. The decrease in melt activity also reduces the glaze's ability to dissolve iron. Some iron contamination must be causing the flashing within the glaze.
Chemical weathering of feldspars converts albite and orthoclase into kaolinite and silica, so the idea is to "contaminate" the feldspar. For the red and cream coloring, the cause must be an iron bearing clay or maybe even pure iron oxide itself. There is precedent for this - Arakawa Toyozo reportedly coated the inside of saggars with high iron clay and Malcom Davis added Redart, a high iron clay to the glaze itself. Arakawa Akira adds 3% colorant to feldspar for flashing shino, but he doesn't specify what the colorant it is. Based on the color, it might be red iron oxide.
Since feldspars with varying chemistries and Silica-Alumina Ratios all melted to the same waxy semi-gloss, I think in the poor melt of the shino glaze, physics takes precedent over chemistry.
The orange where thin color in traditional shinos comes from the clay body, not the glaze. My Midfire Hiiro clay bodies reliably flash in both oxidation and reduction, so I'll be using that.
The first property of shino glazes is that they pinhole. Since feldspars soften very late in the firing, the pinholes cannot be from escaping gases. Furthermore, some pots have pinholes only on the outside.
My conclusion is that the pinholes are application pinholes. I can control where the pinholes occur by controlling how rough the bisque surface is. To make sure pinholes only occur on the outside, I wet trim the outside to reveal the grog. Then I stretch the pot from the inside to roughen the texture even more. The pits and cavities will form sites for pinholes.
The first step is to see how available feldspars melt when pure. The final surface and color will be partly dependent on the raw materials available.
Liner shino
Flashing shino
Crawling shino
Recipe: 100% Minspar +2% Sodium Bentonite (dirty) - Thick at 60%, no epsom salts needed for flocculation. Drips - Thinner at 70%, can be flocculated back up with epsom salts. Still drips somewhat.
More flocculation = less pinholing - Therefore want a glaze that has more bentonite, less epsom salts
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