HELP! Green oribe glaze was underfired and came out black - Can I refire to correct temp??

[Elke]

Hello glaze makers!

I have a very niche issue and thought someone here maybe able to help me.

 

I was commissioned to make a series of 8 large bespoke ceramic pitchers in a specific colour. They are to be presented as awards at a commemorative event on Dec 10.

My customer selected a soft green finish from one of my existing pieces that was glazed in a traditional oribe glaze and fired in oxidation in a wood kiln ( in a group fire in Gunderoo NSW)

I have a small Port-o-Kiln and assumed I could replicate the green colour by using the exact glaze (same original batch) and firing to cone 9-10 in oxidation in gas.

When I opened the kiln this morning all were BLACK!!

 

I am not sure why. Its either because I didn't go high enough - I found out the group Gunderoo wood fire went to cone 12-13.

Or because it Port-O-Kilns cool quickly and when you look up oribe on Wikipedia there is a section that says Black Oribe is achieved with a rapid cool down.

Either way I have to urgently refire existing pieces to cone 12-13 or remake all agin which is a ton of work.

Does anyone have any understanding of the chemistry behind this?

Could I refire to higher temp and the glaze continue to mature and achieve the green colour?

Any advice or insight is greatly appreciated.

 

 

RECIPE

Potash Feldspar 60

Wood Ash 20

Whiting 10

Ball clay (clay ceram ) 10

Bentonite 1 or 2%

Copper carb 7.5

 

 

[Hulk]

Hi Elke,

Welcome to the Forum!
Good questions.
I don't have answers, just wanted to say Hi and bump your thread.

Check back for updates!

[PeterH]

7 hours ago, Elke said:

Could I refire to higher temp and the glaze continue to mature and achieve the green colour?

I've no experience in this area, but your problems might be to do with allowing time for crystal growth rather than reaching "maturity".

Just a standard reference to slow cooling in an electric kiln, to emphasise sometimes changes in appearance are controlled more by the cooling cycle than by the peak temperature. Which may require down-firing in a kiln that cools too rapidly for that glaze/effect.

Super Cool! Slow Cooling in an Electric Kiln
https://ceramicartsnetwork.org/daily/article/Super-Cool-Slow-Cooling-in-an-Electric-Kiln
 

[Babs]

I rhink you inadvertently did a reduction firing. Easy to do in a small gas kiln.

Do you have time to refire one as a test making sure you have a  fully oxidised atmosphere? With that amount of copper I could get black pots without trying in my gas kiln if not careful re atmosphere.

[Min]

Liner glaze in the pitchers?

[neilestrick]

Lots of possibilities here, but it all comes down to you can't expect a glaze to do the same thing in a totally different kiln. First, wood kilns don't just fire in oxidation. They cycle between oxidation and reduction every time the kiln is stoked. Second, you've got a different atmosphere in the wood kiln due to ash and vapor from the wood. Third, the temperature was different, but maybe not depending on where it was placed in the kiln. Fourth, the cooling cycle was different. Different clay body? At this point I would hold off on refiring your pitchers and instead run a couple of test firings with other pieces at a higher temp and with a slower cooling cycle and see if you get the desired results. Once you've figured out what's needed, then refire the pitchers and hope they do okay with refiring. In meantime I would also start making a new batch of pitchers and offer your client a different color that you know will come out as intended. FYI, many of us, including me, have been in a similar situation at some point.

[Min]

One more issue that could come up is how the already fired pots take to a second firing. Both the clay and the glaze getting that much more heatwork could very well have an effect on both.

[Elke]

Thank you thank you everyone!!

After more research I do think it was partly a temperature issue, copper needs high temps to burn out and for the glaze to move and run. I alos used my gas Port-o-kiln which is desigend to rapid cool unlike the wood kiln I had used before.

This batch of glaze had been tested in a kiln run off gas and also waste vegetable oil (a very experimental sustainable kiln), in oxidation , and it came out perfectly transparent, shiny and green.

I definitely did not go into reduction, I have fired this glaze in reduction to cone 10-11 and it goes a marvellous shiny spotty glossy red.

The matt metallic black was a surprise.

MY QUESTION (for those how know a bit about glaze chemistry and heatwork) 

Will the metallic black burn off if I refire to cone 12-13 and cool slowly?

I am thin on time, so I want to crowd source some advice to make the smartest move.

 

I have attached screengrab of the original wood-fired  green pot and my misfires black pitchers. Both were lined with Tenmoku. 

THANKS!

[neilestrick]

12 minutes ago, Elke said:

This batch of glaze had been tested in a kiln run off gas and also waste vegetable oil (a very experimental sustainable kiln), in oxidation , and it came out perfectly transparent, shiny and green.

Knowing this, firing hotter should allow the glaze to take the copper into the melt and go green.  I don't know how the cooling will affect it, but I expect if it went glossy green it should stay that way with a fast cooling. The question will be whether or not it will respond the same in a refire as it does in a first firing. Nothing to do but test it!

[Babs]

Noticing there are tinges of green, glaze thinner? Near base edge of glaze and glaze flaw in handle. SG and glaze application of glaze same as in the testing in the alternative kiln? 

Re refiring the clay body, have you done this with the clay you are using previously?

If you post what claybody you are using, someone may have experience with this.

[Min]

Good spotting the green at the bottom glaze edge  Babs. Does suggest a thickness issue could be the cause.

[Chilly]

On 11/28/2023 at 1:07 AM, Elke said:

The matt metallic black was a surprise.

It is lovely, I'm sure you will find an alternate customer.