[Clayart] Chinese "raku"

Robert Harris robertgharris at gmail.com
Thu Jul 4 17:12:34 EDT 2019


Hi Rob,

What is important to note on the oil spots is that it has to be in
oxidation on the way up, or you don't get decomposition of the iron (to
produce O2) at about 2200F or so - hence using an electric kiln. The
production of Oxygen when the glaze is molten brings the iron particles to
the surface.

This gives pretty ordinary oil spots (particularly brown in black). What is
interesting is that you obviously get much brighter metallic oil spots if
you reduce on the way down - which is what they were doing in the video.

It looked to me like the elements are SiC Glo-bars which are pretty common
in industrial furnaces and don't have the same problems with reduction as
metal elements.

My assumption on the other video with the water is that there is "water
reduction" plus instant cooling to "fix" the colours of the oil spots.

Just off the top.of my head, I'd imagine that the blue oil spots are
totally reduced iron that are also undergoing a process similar to the
"blueing" of steel (there may be oil or something else mixed in with the
water).

Robert.





On Thu, Jul 4, 2019, 12:43 PM robert hackert <ndiaman at hotmail.com> wrote:

> Hi Robert:
> Pretty much what we saw in the first video. The kiln appears to be
> electric, but doing reduction, so heavy duty elements. Oxy probe, and
> pyrometer, so no hit and miss.
> It did look to be an iron glaze.
> Great video. Course they had 800 yrs., or more jump on everyone else.
>
> Rob Hackert
>
> Sent from my iPhone
>
> > On Jul 4, 2019, at 6:56 AM, Robert Harris <robertgharris at gmail.com>
> wrote:
> >
> > I have to admit, I thought the water was coloured because it had absorbed
> > stuff from the glaze (i.e. not stable very stable or so high in iron that
> > the oxide was coming off into the water) vs being that colour originally.
> >
> > I also thought that the point might be to "fix" the oil spots before they
> > re-oxidised, or something similar.
> >
> > On the topic of oil spots, I found this pretty interesting video a while
> > back, might be worth sharing too.
> >
> >
> https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DN6Knf7H_FwY&data=02%7C01%7C%7C5cadc5e14fa84cb065cc08d7006e3fb0%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636978345817797821&sdata=ZfGGogXIHrBXvsbfewcGT0h9hn9uQH5srSh%2BM%2BP1MT0%3D&reserved=0
> >
> >
> >
> >
> >
> >> On Wed, 3 Jul 2019 at 17:39, Douglas Fur <23drb50 at gmail.com> wrote:
> >>
> >> Some of the water used in this process appears colored. I wonder if they
> >> are using soluable metal salts to effect the glaze surface like fuming.
> >> Some have posited that the water is for reduction. Maybe its both the
> water
> >> reducing the salts so that the result is a metallic luster.
> >> Looks flash but I wonder how stable this surface would be. Would it
> survive
> >> a dishwasher, hand washing or hot acidic tea served in it?
> >> Duff
> >> Seola Creek
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