Des & Jan Howard on tue 20 nov 01
Our base glaze for Cone 12 Red. is an emulsion type blue/white,
added copper gives a mauve/pink colour, (2 phase liquid gives the
blue, colloidal copper gives the red, combination gives the mauve,
I guess). The white & pink satin occur at the cooler end of the firing
range, as fine crystals I always thought, discussions on Clayart
indicate undissolved materials are more likely.
Obvious sugary crystals do develop as the temp goes up, then
the liquid/liquid stage is reached, there is only about a 1/2 cone
variation to this point. The liquid/liquid effect is good for at
least another 1/2 cone, then clears.
To get a darker blue for glaze on glaze deco we added 0.125%
cobalt carbonate to the mauve glaze, a bit too blue, set aside.
A softer glaze was needed for a cooler spot at the front of the kiln,
so, a Cone 10 copper red was added to the blue glaze 1:2,
blue was the result. It also worked as a blue deco in the hotter
parts of the kiln.
Because our unfired glazed pots are the same colour we add
food colouring powder to the glaze drums. The mauve glaze & the
blue glaze both had the same colour powder added to them
& to differentiate I painted "X" on the blue glazed pots with
a solution of brown food colour.
All of the blue pots came out of the firing with a
copper red "X" emblazoned on them!
Next firing. The blue deco was given a brush swipe of brown food colour.
Result, deep purple deco!
The cooler blue pots were given random sprays of brown colour.
Result, copper red blooms on the blue.
Next firing. The blue decorated pots were given a good spray
of the brown colour on their decorated inner surface.
Result, purple deco, in some cases on both sides of the pot
There are not, to my knowledge, any solubles in any of
the materials used, excluding the food colour.
On the non-decorated areas both the blue/white glaze & the mauve
glaze have the shadowy effect I associate with carbon trapping.
Thoughts anyone?
Des
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Des & Jan Howard
Lue Pottery
LUE NSW 2850
Australia
Ph/Fax 02 6373 6419
http://www.luepottery.hwy.com.au
Dannon Rhudy on tue 20 nov 01
Des said:
.... our unfired glazed pots are the same colour we add
>food colouring powder to the glaze drums.
...painted "X" on the blue glazed pots with
>a solution of brown food colour.
>....
>Next firing. The blue decorated pots were given a good spray
>of the brown colour on their decorated inner surface.
>Result, purple deco, in some cases on both sides of the pot
.....>Thoughts anyone?....
My first thought is that I'm glad I don't use food coloring
in my FOOD.
Second thought is that you've made some interesting observations -
no doubt you'll have folks wanting to know where to get the
same brand of colorings that you use. I've occasionally used
or had students use food colorings to tint look-alike glaze
slurries, and never noticed any effect at all on the fired
glazes. What does the ingredients list say is in your brown
powdered coloring?
regards
Dannon Rhudy
Des & Jan Howard on wed 21 nov 01
Dannon
I don't have access to a "this number = this additive/chemical"
reference book until my next town trip. The container is labelled
"BROWN POWDER COLOUR C.I. 20285, 42090 D/C 30%".
Des
Dannon Rhudy wrote:
> Second thought is that you've made some interesting observations -
> no doubt you'll have folks wanting to know where to get the
> same brand of colorings that you use. I've occasionally used
> or had students use food colorings to tint look-alike glaze
> slurries, and never noticed any effect at all on the fired
> glazes. What does the ingredients list say is in your brown
> powdered coloring?
Des & Jan Howard
Lue Pottery
LUE NSW 2850
Australia
Ph/Fax 02 6373 6419
http://www.luepottery.hwy.com.au
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