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copper reds, one more slice of pie

updated fri 15 aug 08


Wyndham Dennison on thu 14 aug 08

From Marian
> Adding tin to a copper
> glaze, as is common, probably enhances the reflection and refraction of
> light and thus the color we preceive. We are surely not going to form bronze
> by "smelting" tin and copper in our glazes.
From Jean
> hi ivor and hi marian,
> first of all im not a chemist like you two, but about de discussion on tin
> (that was a question of wyndham) i only can talk about my experiences
> and they tells me that tin changes the color of my recipe and it is like you
> say ivor, that the red becomes more orange, and i dislike it.
> so i have to continue without tin.
Like Jean said I'm no chemist as well but the 3 of us are exploring the
Elephant from our own perspective, which is as it should be. There is in
my POV one element not yet considered in the different equations, Carbon
or it maybe "BuckeyBalls, nano carbon" for all I know.
Lets also go back to the copper reds in China. One excerpt I read said
that heated bronze was treated with an acidic solution to produce
something like a patina scale that was ground into a powder and used in
the copper red glazes. That would seem to suggest an affinity for tin
and copper in a glaze melt. The Chinese also used saggers to fire there
ware, which would exclude the carbon from a reduction firing.
I have fired copper reds with heavy reduction 1500 deg f all the way til
cone 10 and have had muddy liver colored reds. I have backed off
reduction at about cone 6 and keep light reduction an my reds brightened
beautifully cooling rate is the same.
I don't know if some ingredients such as zinc and tin are needed since
most agree that zinc evaporates before cone 10, maybe zinc or tin create
a condition for copper to stay in the glaze and not evaporate away.
What we see and can analyze after the glaze is formed is not what is
there at certain points in the firing on the way up or down.
Here is another example, San Fransisco sour dough bread can not be make
in New York. New York Sour Dough will be made. Because of the yeast
strain in San Fransisco during the fermentation stage, you could not
test the finished bread to find the reason for the different breads.
I may try a sagger test to compare two pots glazed the same and fired in
and out of a sagger in the same firing side by side.
Each kiln fires differently and what I call lt or heavy reduction is
subjective to me and my quirks of firing as are to each of us.
I am glad that Jean has a method that produces beautiful red and we will
each learn something because of these methods. Thanks to all, no hard
feeling to any, just the opposite, Keep it coming :-D