Copper spot effect

[jerbis quilbin]

Hello! I'm new to the forum and only got my hands in clay about a year ago. So though I have been reading a bit about glaze formulation and chemistry I am really very much in the dark. Please forgive me for asking any basic questions!!

I am trying to recreate the effect in the two photos I attached. Both are chun glazes with copper. The one with big splotches was reduction fired chun with bits of copper wire sprinkled on the wet glaze, to my understanding. The other has copper carbonate and silicon carbide mixed in, and was oxidation fired.  I don't really get what's going on, why would it form green? That's carbonate, right, but I would think it would decompose. And why does it go from black, to green, to red/purple?

More importantly than those curiosity questions, I have some ideas: Will this work on any white glaze? I intend to try sprinkling copper on a white my studio already has and see what happens. I also thought I could maybe make a silicon carbide wash and apply that under glazes, to make trying SiC with various glazes easier.

How would you glaze wizards go about achieving this? Please let me know.

Here are the recipes for those images:

https://glazy.org/recipes/18035

https://glazy.org/recipes/19163

[liambesaw]

Silicon carbide is used for localized reduction (reduction effects inside of an oxidating atmosphere).  So copper in reduction (firing with an oxygen deficit) will range from dark red to pink, black or grey in some extreme cases.  But in oxidation copper is mostly green, but can show other colors in the blue green spectrum as well.

I believe the top one is the result of brushing copper carbonate onto a clear glaze and firing in reduction.  The second one looks like a mixture of oxidizing and reduced copper, which would be present in a glaze using chemical reduction (addition of silicon carbide).

By the way that Irvine chun is not what is giving that red and black hole effect, that is copper painted onto the surface of a bowl glazed with Irvine chun before it was fired.  If you mix that glaze it will just be a white glaze.

[jerbis quilbin]

1 hour ago, liambesaw said:

Silicon carbide is used for localized reduction (reduction effects inside of an oxidating atmosphere).  So copper in reduction (firing with an oxygen deficit) will range from dark red to pink, black or grey in some extreme cases.  But in oxidation copper is mostly green, but can show other colors in the blue green spectrum as well.

I believe the top one is the result of brushing copper carbonate onto a clear glaze and firing in reduction.  The second one looks like a mixture of oxidizing and reduced copper, which would be present in a glaze using chemical reduction (addition of silicon carbide).

By the way that Irvine chun is not what is giving that red and black hole effect, that is copper painted onto the surface of a bowl glazed with Irvine chun before it was fired.  If you mix that glaze it will just be a white glaze.

I see. Yes I think the poster said he sprinkled on small copper wire snippings. I am just wondering if there is a way to tell if this effect requires that chun glaze, or will work with any white glaze. I am also wondering if it would be feasible to make a silicon carbide wash and brush it on to reduce individual pots when you want?

[liambesaw]

Gotta experiment, copper won't do that on every glaze though, has to have enough calcium I think, probably works best with tin as well.

[C.Banks]

Some copper glazes work very well under magnesia mattes with zircopax. The spotting is similar to the 2nd photo but more distinguished.

I'm only a bumbling neophyte but as a side note - strontium carbonate will inhibit copper reds in reduction. If I remember correctly 6-7% works to create some interesting green effects in reduction and soda.

[GEP]

8 hours ago, jerbis quilbin said:

I see. Yes I think the poster said he sprinkled on small copper wire snippings. I am just wondering if there is a way to tell if this effect requires that chun glaze, or will work with any white glaze. I am also wondering if it would be feasible to make a silicon carbide wash and brush it on to reduce individual pots when you want?

Are you firing oxidation or reduction, or do you have access to both? The answer to the “will it work” question regarding your first photo starts with a reduction atmosphere. It sounds like you are asking if you can create reduction for this effect by using a silicon carbide wash, rather than a reduction kiln. Do I have that right? If so, the answer is a very common answer to many ceramics questions: there’s only one way to find out, try it! However, I suspect you won’t get that same effect. Silicon carbide applied to a pot creates tiny local pockets of reduction, and gives results like your second photo, not the first. I suspect the first pot needs to be completely surrounded by a reduction atmosphere, in order to create the fuming/spreading of red color. 

To answer another of your original questions, copper oxide looks black but it’s actually super concentrated green. Copper carbonate should look green as a raw material. Double check which one you have. Green is the natural color of copper in an oxidized state, so when fired in oxidation it remains green. When fired in reduction, the green is removed and the copper returns to a red color. Purple is the “half-reduced” state, mixture of green and red. 

[Marcia Selsor]

I have used very fine silicon carbide to achieve copper reds in ^10 oxidation without much spotting. If you want the copper spotting as on the first pot, you might try copper granules and fire the pot in a saggar for the glaze firing and include some charcoal briquets, Someone was discussing saggars in reduction on here in the past year or so.

Marcia