Kate Shakeshaft Murray on sun 8 nov 98
Hello all you good folks out there!
My sweetie has expressed a wish for a robin's egg blue celadon. I remember
reading a magazine article recently that said that trace amounts of
titanium in the kaolin clays readily available in the U.S. (kaolin present
in either the clay body or glaze recipe) will push celadon toward green or
gray-green rather than blue. We fire a light stomeware to cone 10-12 in
reduction, but can't afford to go Grolleg, and all the celadon tests we've
ever run have been green or greenish. Any suggestions or recipes?
Thank you! Kate in Florida, an off-and-on lurker
Jane Rekedal on mon 9 nov 98
Try adding 1/10 of 1% cobalt oxide to your favorite celadon for a pale blue.
Jane Rekedal
Tom Buck on thu 13 oct 05
Dear Ivor et al:
The Robin's Egg Blue is most likely an optical effect and perhaps
Michael Cardew provides an answer to your ??? about a clear glaze yielding
robin's egg blue in thick layers.
Cardew (Pioneer Pottery, 1st edition) writes on Pg140:
"Opacity in glazes is caused by suspended matter, whether gas (air
bubbles), liquid (emulsion glazes) or solids. A fine mist of of air
bubbles will only make a glaze semi-opaque. "Emulsion Whites" can be
obtained from certain glaze compositions which on melting separate into
two inmmiscible liquids. Each liquid would be transparent alone, but if
the globules of one are suspended as fine particles in the other, the
glaze will be a perfect opaque white. The firing conditions for emulsion
glazes must be carefully controlled because the whiteness depends on the
size range of the suspended globules."
and then Cardew provides two footnotes:
1. The limits for these compositions [emulsion whites], for earthenware
glazes containing borax frit, have been thoroughly investigated by Knisek,
B.C.S., 59 (1960), 339.
2. The size ranges [of suspended globules] are given as follows:
between 2 micrometres ["microns"] and 1 micrometre: [the colour of
the glaze] is Milky White;
between 1 micron and 0.1 micron: BLUE-WHITE emulsion;
between 0.1 micron and 0.05 micron: Grey semi-transparent;
below 0.05 micron: clear transparent.
B.C.S., 59 (1960), 374.
If a glaze contains some Boron-3 Oxide (eg borax, aka sodium borate) then
two different glasses are formed during the melt. Firstly, the B2O3 forms
a glass with flux oxides present (usually KNaO) -- this occurs at
relatively low temperatures (by Cone 010, 1648 oF, 900 oC).
Then at higher temperatures (above Cone 06, 1832 oF, 1000 oC) the
other glass (alumina/silica) forms and the two glasses may well be
immiscible (depends on many factors), and the B2O3 glass will disperse
in the Al2O3/SiO2 glass. And the Robin's Egg Blue colour is achieved where
the glaze coat is thick enough.
til later peace Tom.
Tom Buck ) -- primary address.
"alias" or secondary address.
tel: 905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street, Hamilton ON L8T 3E1 Canada
Ivor and Olive Lewis on fri 14 oct 05
Dear Tom Buck=20
Thank you for those insights.
Unfortunately a X 10 hand lens does not resolve things as small as 0.1 =
mu. But I agree with you that an optical effect seems most likely. Good =
Old Cardew.=20
Best regards,
Ivor.
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