Da Blues (2)

There is something special about these Jun glazes, which puts them apart from all the others. This becomes obvious when you compare a Jun-glazed item with one where a pigment-glaze was used, say Cobalt.

                                 

Victoria & Albert Museum

This something special is called 'opalescence', like an opal. When you shine a light sideways at an opal, the stone itself appears blueish, but the light that shines through is yellow/orange!

We witness this phenomenon almost everyday in the blue of the sky. The light of the sun hits molecules in the air (not dust particles, but much smaller bits: the components of the air itself),
and gets scattered in all directions. The curious thing is that blue light becomes more scattered than other colours, which is why we see the whole sky as blue. Officially this is called Rayleigh scattering:

As you can gather from the first diagram, the shorter wavelength of blue light is the cause of this: The shorter the wavelength, the more that colour gets scattered.

This whole thing applies to a Jun glaze as well, but until recently the reasons were the subject of debate. Different glaze components were thought to be responsible, amongst others iron phosphate and lime phosphate. It took modern science, with scanning electron microscopes and 'optical coherence tomography' to reveal the true cause.
The composition of Jun glazes is such that during the cooling stage something happens called 'liquid-liquid phase separation' (are you still there?). While the glaze is cooling down, when it is still like thick syrup, tiny glass spherules form within the rest of the glaze. It's as if the glaze separates into two different types of glass.
With an electron microscope at a magnification of x 20,000, the glaze looks like caviar, but the droplets really are tiny, even smaller than the wavelength of blue light. And here we are back at the Rayleigh scattering in the sky, which only happens because likewise the air  molecules are very small.

A Jun glaze can have a range of colours, from violet to skyblue to moon-like white. The same principle still applies: the bigger the droplets, the whiter the colour. A whiteish Jun is a bit like milk, in which the droplets of fat are big enough to reflect white light. So the colour of a Jun glaze says something about the temperature to which is was fired.

There are various ways to mix a Jun glaze, whether high fire or cone 6 like mine. You can read all about it in Bailey's books mentioned below. I am still happy I found his recipe, and after years of experimenting it never ceases to amaze me.

Jun ware, Northern Song Dynasty

Michael Bailey: Glazes Cone 6 1240 C/2264 F, A & C Black London 2005.
Michael Bailey: Oriental Glazes, A & C Black London 2004.
Nigel Wood, Chinese Glazes, their origins, chemistry and recreation, A & C Black London 2011.

Da blues (1)

Looking for a new Cone 6 glaze I came across the chapter on Jun Glazes in Michael Bailey's wonderful book Glazes Cone 6. I tried his Jun Base 1 glaze, which gave me a very reliable, soft blue result on a buff stoneware clay (Creaton 359), and an even more subtle violet tint on a French stoneware clay (KPCL Limoges KF 100).

Also, on a clay with iron specks, the results were promising.

But the real possibilities of this base glaze became clear when I tested it (amongst many others) over a high temperature tenmoku. Depending on the thickness of the layers it gave me a bright sky blue, all the way to a violet-brown tint.

And what made it even better: the way it breaks over sharp edges and rims.

Later on I tried it over a commercial black glaze (Keramikos SG840). The blue changes to a midnight blue with black to grey rims.

Every book on glazes will tell you that these wonderful blues are not the result of a pigment in the glaze, like cobalt is a pigment, but "an optical effect, brought about by the scattering of light at the blue end of the spectrum" (Bailey).

In the next blog I'll try to sort out just what that means.