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glazing - cone 6 terra cotta clay

updated wed 3 dec 03


Stephani Stephenson on tue 2 dec 03

Karin wrote
I just got through with a glaze test firing of some Cone 6 terracotta
I used undergllazes on some shallow bowls with a clear glaze over it.
The pieces are an awful gray-brown ......... My question is - at what
Cone does one get a decent terracotta color

Hi Karin
Was the clay itself, i.e the unglazed clay, an awful gray brown? or
just the glazed part ?
It can be harder than hell to get a good clear glaze over red terra
cotta, while maintaining the beautiful red.
or even to get get a good terra cotta red glaze by itself.
Iron is so reactive, that is why I love it, but it can drive you nuts
It seems a cruel joke that the insistent red in red clay can become so
fugitive when under a glaze.

The iron which so faithfully blooms a lovely brick red on its own, in
the clay, turns all shades of gray , dull mud,etc
under a glaze. Did you have any period of reduction in the firing,
especially around cone 010 to 06? This will gray.
Did you heat and cool slowly ? This allows adequate oxygen in the
atmosphere , and into the iron molecules,
which will promote iron reds. Finally the make up of the glaze affects
the color, though I haven't found the magic bullet here.
Are you using a commercial glaze? I have tested different frit based
glazes over clay bodies, though at lower temp. At lower temps , popular
boron frits such as 3134 tend to draw out green tones from the iron in
the clay. 3195 seems pretty neutral, others bring out yellow tones.
I have been experimenting with spodumene in glazes on terra cotta. it
seems to help promote development of reds from the iron in the clay. a
bit of tin oxide is useful in this area, but can adversely affect other
colors, primarily chrome greens.
I would also like to hear what others have learned in this area.

It is possible there are some red terra cottas which hold up to cone
5, though this is at the upper end of the firing range for them.
Aardvark clay has a clay called Dixon red, very beautiful, though it
too starts to move to brown at this temp, (though it is a rich brown).
Also ,caution, this clay has a HIGH grog content! (Snail would
probably like this clay!!!!) it is very very chunky stuff.
Most of the terrra cotta redsshow good color at cone 04-3 or so, but
again, check the unglazed part of your pot to see if it is just the
glaze, or the clay as well, at this temp. good luck!

Stephani Stephenson