iandol on tue 5 dec 00
Shino Colours.
Seeing the abundantly rich colour and reflective texture of examples of =
Shino work made by Josh Gold in Ceramics Technical No11 has been a =
revelation. I now appreciate why this style of surface creates a lot of =
interest. So simple, a white glaze on a seemingly indifferent clay is =
strongly reduced to create such beauty.
Looking over a recent porcelain test tile bearing recipes based on a =
high soda felspar, the freshness of Josh Golds=92s work still at the =
forefront of my mind, I noticed a pale orange halo around some of the =
melts. The anomalous nature of this colour became caught my attention =
because the glaze samples associated with the colour were well reduced =
and exhibited what seemed to be a typical blue green celadon hue. I =
started to wonder how it could be possible for iron to behave as though =
it has been both oxidised and reduced in areas that are immediately =
adjacent.
So, how are the orange colours, which develop where a Shino glaze is =
thin, explained? There seems to be a contradiction. Books about glazing =
tell us that iron gives yellow, orange, black and brown in oxidation and =
green blues in reduction. So, who can explain orange in reduction? If =
the colour is not from Iron, just what is going on?
Ivor Lewis. Redhill, South Australia.
Hank Murrow on tue 5 dec 00
>Ivor Lewis wrote in part;
>Looking over a recent porcelain test tile bearing recipes based on a high
>soda felspar I noticed a >pale orange halo around some of the melts. The
>anomalous nature of this colour became caught my >attention because the
>glaze samples associated with the colour were well reduced and exhibited
>what >seemed to be a typical blue green celadon hue. I started to wonder
>how it could be possible for >iron to behave as though it has been both
>oxidised and reduced in areas that are immediately >adjacent. So, how are
>the orange colours, which develop where a Shino glaze is thin, explained?
>>There seems to be a contradiction. So, who can explain orange in
>reduction?
Dear Ivor;
The orange color is from iron which has been reduced, congregated in a
micro-crystaline network at the surface of the glaze, and oxidized for some
time after the reduction. Sometimes i fire in reduction to C/2 and for ten
hours after that in oxidation to C/10, folowed by a slow cool. Other times
I fire in reduction to C/10, cool to C/3, then relight the kiln, hovering
there in oxidation for several hours,then cooling normally. If you break a
piece of orange shino open and examine the shard, you will discover that
the orange layer is so thin you can't measure its thickness without the aid
of a microscope with a reticulated eyepiece, as the orange layer is only
20-40 microns thick, and the glaze is white underneath the orange! Where
your glaze is showing a celadon color, the iron did not crystallize out,
remaining in solution in its reduced form to yield a celadon. If you want
the whole piece to go orange, try cooling much slower to encourage the
micro-crystalline growth. It also helps to get tiny amounts of chlorine or
fluorine into the recipe to loosen the iron from the clay and carry it to
the surface where it stays behind to re-oxidize, while the Cl or F
outgasses.
I hope you have success with these suggestions, Hank in Eugene
Craig Martell on tue 5 dec 00
The ever inquisitive Ivor Lewis asked:
>Shino Colours.
>So, how are the orange colours, which develop where a Shino glaze is thin,
>explained? There seems to be a contradiction. Books about glazing tell us
>that iron gives yellow, orange, black and brown in oxidation and green
>blues in reduction. So, who can explain orange in reduction? If the colour
>is not from Iron, just what is going on?
Hello Ivor:
I've done 3, 35 glaze biaxial grids with shino glazes. These are Ian
Currie volumetric biaxials by the way. These were original flux blends
based past work of others.
Here are some of my findings. I think I'm reading the tiles fairly well
but others may think differently. If iron is taken into solution in a
glaze that is fired in reduction you will most likely see greens and blues
when the iron percentage isn't too high, say between .5 and 2 percent. As
the percentage increases we see olive celadons, honey colors, and finally
some sort of dark tenmoku or something similar. When there is a plethora
of iron, some or almost all of the iron is taken into solution but there is
an excess that will precipitate and form crystals. Depending on the
structure of the glaze, we will see iron reds and tenmokus with crystals
suspended in the glassy matix.
With Shinos, we are adjusting the glaze to not allow much iron at all to go
into solution. In a shino glaze, we are using about the same amount or
less of iron that we find in celadons. There may be even less or almost no
iron in the glaze and the fire color or orange is formed by the glaze
picking up iron from the claybody. This accounts for orange where the
glaze is thinner. You can see the result of iron transfer into the glaze
at the interface. I use porcelain so I don't have any help from the
clay. I use iron bearing stone in my shinos. It comes out to a less than
2% addition. So to keep this small amount of iron from going into solution
we must keep a shino as low in calcium as possible, push the alumina way
above the seger formula suggested limit. The suggested upper limit for
alumina in glazes fired to cone 10 is .5 moles. Most shino glazes have
about .9 moles of alumina. As the alumina goes higher, into the 1.0 to 1.5
mole range we see red shinos develop. As the alumina and silica increase,
the color goes more to hot orange and finally ligher oranges into
white. The abundance of silica here is almost like a nuka glaze.
I might say that in the shino biaxials, I did not add any sort of kaolin or
ball clay to the glazes for alumina. I used 325 mesh alumina oxide so I
could see the affects of alumina and silica in different blends. As most
know, clays add both alumina and silica and I wanted to isolate the affects
of the two. As you would expect, where the alumina and silica were in
lower propotion with the fluxes, I saw glazes were celadon due to the iron
being allowed to go into solution. I concluded that the alumina, in higher
proportion was critical to the development of fire color as in reds and
oranges.
That's the view from here, Craig Martell in Oregon
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