William Lucius on mon 20 aug 12
A BIO rim on a recently fired=3D2C large tenmoku bowl with carbon trap whi=
te=3D
glaze inside the bowl and above the outside faceting was supposed to be bl=
=3D
ack=3D2C so every time I see the blue rim it reminded me to ask someone why=
t=3D
hat happened. Perhaps in time I will be able to appreciate a blue rim .
As soon as I sent my post I wondered about the lithium as a possible culpri=
=3D
t. Sure enough=3D2C I rummaged through my rejects and found a small pig wi=
th=3D
a BIO belly band under a high zinc oxide (10.8%) matte white glaze with no=
=3D
lithium. It is acceptably black (except for on the ears=3D2C where it de=
fi=3D
nitely has a blue cast) under a muddy=3D2C grayish glaze that really does n=
ot=3D
perform well in reduction. So=3D2C in addition to this correlation=3D2C th=
e pr=3D
esence of lithium in the glaze suggests a chemical causation for the observ=
=3D
ed blue color. The blue ears led me to inspect my library of test tiles=3D2=
C =3D
which suggests that that there may be other variables at work. Each tile r=
=3D
ecords clay type (10 different native clays or combinations)=3D2C glaze # (=
on=3D
ly #11=3D2C Carbon Trap White is of interest here) and firing temperature a=
nd=3D
atmosphere (consistently Cone 5=3D2C Light Reduction). Each tile is paint=
ed=3D
with 5 different oxides prior to bisque firing (only BIO is of interest he=
=3D
re). When tabulated in a spreadsheet=3D2C BIO (n=3D3D13 tiles) comes out p=
redo=3D
minately blue (n=3D3D9)=3D2C occasionally black (n=3D3D3) and rarely brown =
(n=3D3D1=3D
). With the possible exception of the single brown color=3D2C clay type do=
es=3D
not seem to matter. Further inspection suggests that another operative va=
=3D
riable is glaze thickness - all nine blue occurrences are correlated with a=
=3D
thin=3D2C single dipped glaze. Two of the black occurrences correlate wit=
h =3D
an unctuous=3D2C double dipped glaze (the third black occurrence is on a se=
ve=3D
rely vitrified clay body). So=3D2C I need to see if I can consistently obta=
in=3D
a black underglaze design by double dipping the glaze=3D2C which I know fr=
om=3D
other tests with other white glazes tends to mute the otherwise bold desig=
=3D
ns. As a potter that would be sufficient=3D2C but the scientist in me stil=
l =3D
wonders how glaze thickness influences color expression. Offhand I would e=
=3D
xpect that a thick glaze somehow protects the underglaze from the kiln atmo=
=3D
sphere=3D2C probably effectively maintaining the iron in its reduction phas=
e =3D
at high temperature (BIO stays black after bisque oxidation). Of course=3D=
2C=3D
any suggestions about this idea would be welcome. =3D20
And just to complicate the argument=3D2C one of my grandson's pots with BIO=
u=3D
nder carbon trap came out a metallic bronze (almost gold)! He likes it giv=
=3D
en that he does not know that it was supposed to be black.
=3D20
Thank you both for your off-list replies
William A. Lucius=3D2C Ph.D.
Board President and Director
Institute for Archaeological Ceramic Research (IACR)
iacr@msn.com
http://leuppkilnconferenceorg.admin.melbourneitwebsites.com/
=3D20
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