Oxblood Glaze, How To Have A Green Where The Glazed Is Not Reduced

[marcello]

Hello,

I am testing some oxblood glazes on tea bowls. I am trying to have the red where the bowls are reduced and green where they are oxidized, just like the one i am posting here. In my versions, the areas are not reduced are always transparent/colourless. Do you think it is  a problem of chemistry or a firing problem?

Looking at the color clay of this bowl, It seems it is fired with a light reduction. Do you agree? If I use an iron-bearing clay, it comes much darker after reduction.

 

many thanks

 

Marcello

 

 

 

[Callie Beller Diesel]

This bowl looks like the iron from the stoneware is colouring the clear part of the glaze, where the copper has burned out. That's why it's turning that greenish shade. More iron from a darker clay=more green.

[marcello]

in my tests using a iron bearing clay does not produce a green... but a black. the clay also becomes much darker.... Maybe the reduction should be lighter?

 

 

[Marcia Selsor]

I have gotten green and red in ^6 reduction using Panama Red from John Britt's article in CM . Will post a photo. 

Here are some recipes from his article.

 

 

GLAZE RECIPES  Cone 6 Reduction

from John Britt

 

 

JEFF’S  RED 1 Cone 6 reduction

41.90 Potash Feldspar

26.20 Silica 

8.40 Whiting 

8.70 Dolomite

13.00 Ferro Frit 3134 

1.70 Zinc 

4.40 Barium Carbonate  

 

0.50 Copper Carbonate 

 2.60 Tin Oxide

1.00 Bentonite      

 

PANAMA RED Cone 6 Reduction   Can go green on Porcelain where oxidized.

45.50 Potash Feldspar                 

16.30 Silica  

2.70 Whiting

2.70 Kaolin                      

8.00 Dolomite                       

11.00 Gerstley Borate                 

10.00 Ferro Ferro 3110                      

4.30 Strontium Carbonate             

2.70 Zinc Oxide                      

 

2.70 Tin Oxide                       

1.80 Copper Carbonate   

 

            

Cone 6 Copper Red Cone 6 Reduction

43.70 Ferro 3110                      

21.00 Silica

15.10 Kaolin                      

 1.00 Whiting                         

 4.20 Dolomite                       

 9.40 Gerstley Borate                 

 1.90 Zinc Oxide                      

 

2.00 Tin Oxide                       

2.00 Copper Carbonate   

 

Selsor Copper Red Cone 6 reduction

56.25 Nepheline Syenite             

12.50 Gerstley Borate                 

10.41 Whiting                         

20.83 Silica                         

 

1.5 Tin Oxide                       

0.8 Copper Carbonate                

 

from John Britts  alteration in CM Oct. 2009 or 10 some glazes had errors and were corrected in Nov.issue.

Selsor Oribe Cone 6 reduction  This is a green glaze that John developed from the above red.

56.25 Nepheline Syenite             

12.50 Gerstley Borate                 

10.41 Whiting                         

20.83 Silica                         

 

1.0  - 5.00 Copper Carbonate     

[marcello]

Thank you Marcia!

 

can I please see also the foot of  your nice work?

Any additional info about the firing schedule would help. I will certainly try, even if I fire at cone 10. 

Certainly with a lower temperature the iron bearing clay should come out lighter.

I have tested some britt's oxblood at cone 10- Some of them are really good (I am posting an example) but they come colorless where copper hasn't turned red.

 

I am thinking also what would happen adding a little iron in the glaze, How knows if it would effect the reduction? If copper burns away, maybe a little iron would keep the glaze green. It would be a celadon maybe?

 

 

[neilestrick]

That is not an area of oxidation. The area where it is not red has most likely had the copper burned out by contact with the flame in the kiln. It is going green because of the iron in the stoneware clay. Technically, the glaze there is clear, not green. Put your pots near the edge of the shelf where they are likely to contact the flame and you'll be able to get this effect.

[Callie Beller Diesel]

If you're looking for copper reds, many an undergraduate student is advised to start with a celadon base glaze and add copper. Celadons were traditionally used on stoneware clay (think Koryo dynasty in Korea), not red clays. A tiny amount of iron from the clay body leaching into the glaze is the source of the green. If you start adding it to the glaze itself, it becomes too intense very rapidly. In my experience, the iron will overpower the copper. That's not to say you won't get an appealing colour of some kind, it just won't be the effect you're looking for here.

[TJR]

If you're looking for copper reds, many an undergraduate student is advised to start with a celadon base glaze and add copper. Celadons were traditionally used on stoneware clay (think Koryo dynasty in Korea), not red clays. A tiny amount of iron from the clay body leaching into the glaze is the source of the green. If you start adding it to the glaze itself, it becomes too intense very rapidly. In my experience, the iron will overpower the copper. That's not to say you won't get an appealing colour of some kind, it just won't be the effect you're looking for here.

Diesel;

Let me get this straight. Do you begin with a Celadon glaze that already has 2% iron in it, or do you get the iron from the clay body and the copper is added to a plain clear glaze to get the copper red?

TJR.

[marcello]

thank you, a lot of informations for my tests!

 

About the clay, what is your opinion on how to get a yellow clay after a reduction firing? I get this color only in oxidation, and i get a dark brown in reduction....

 

thanks

Marcello

[bciskepottery]

Consult with the place you buy your clay from, or the manufacturer's web site . . . they often have samples of clay bodies fired in both reduction and oxidation.  The look in reduction will be determined by the composition of your clay body and how the materials change as they go through that oxygen-starved firing segment called reduction. 

[Marcia Selsor]

Thank you Marcia!

 

can I please see also the foot of  your nice work?

Any additional info about the firing schedule would help. I will certainly try, even if I fire at cone 10. 

Certainly with a lower temperature the iron bearing clay should come out lighter.

I have tested some britt's oxblood at cone 10- Some of them are really good (I am posting an example) but they come colorless where copper hasn't turned red.

 

I am thinking also what would happen adding a little iron in the glaze, How knows if it would effect the reduction? If copper burns away, maybe a little iron would keep the glaze green. It would be a celadon maybe?

 

11219511_10206549485807305_3784303503918761558_n.jpg

 

I apolgize for not being able to show you a photo of the foot. It is white porcelain. I am in the midst of packing for a 6 week residency and I leave at 5 am Wed. morning. It was fired at UT Brownsville when I was teaching there. I fired it in a large Olsen updraft. And Neil is correct, the green area was blasted by one of 14 burners from the along the edge of the shelf. 

 

Marcia

[Callie Beller Diesel]

 

If you're looking for copper reds, many an undergraduate student is advised to start with a celadon base glaze and add copper. Celadons were traditionally used on stoneware clay (think Koryo dynasty in Korea), not red clays. A tiny amount of iron from the clay body leaching into the glaze is the source of the green. If you start adding it to the glaze itself, it becomes too intense very rapidly. In my experience, the iron will overpower the copper. That's not to say you won't get an appealing colour of some kind, it just won't be the effect you're looking for here.

 

Diesel;

Let me get this straight. Do you begin with a Celadon glaze that already has 2% iron in it, or do you get the iron from the clay body and the copper is added to a plain clear glaze to get the copper red?

TJR.

e

 

You take the celadon, remove the iron from the recipe and swap in some copper. Start in the neighbourhood of 2-4%. That's as specific as any of my instructors ever got. It was up to the student to figure out how much copper to add. Given the variances in recipes, this was a good thing, because they'll all respond a bit differently, and you really do have to test under your own conditions with firing, local clay bodies, etc. They are all sensitive.

 

Celadon type glazes *tend* to borrow iron from the clay below in an aesthetically pleasing fashion, if it's stoneware or a ball clay based porcelain. Thus the greenish blue colour, under ideal conditions. I did a big 10 page paper on it once. I'd go full on nerd here, but this isn't my thread.

[marcello]

It would be interesting to read your 10 page paper

Thanks

Marcello

[Callie Beller Diesel]

It was on celadon, not copper reds, but if you like, I'll see if I still have the file.

[neilestrick]

A celadon base will get you a get start to a copper red, but reds generally need a little more melt than celadons. It's a fine line between runny enough and burning out the copper. You'll often find that copper reds have very low clay content in the recipe, which make suspension and application trickier. Copper reds also need something to be the nucleus for the copper crystal, which is why they use tin oxide. Historically iron was used, but it tends to darken or muddy the glaze more than tin. Most copper reds that I have seen actually use very small amounts of copper, as low as 0.25%, but generally not more than 1.5%.

[Tim T]

A couple of points:

- the copper red is just from a very thin layer in the middle of the glaze. If the glaze layer is too thin, it doesn't form, and you get a clear glaze.

- If you are doing a "proper" copper red rather than a lower firing look-alike then you are at a temperature where copper is vaporising from the glaze, and this may be another cause of a copper deficit giving a clear glaze

If you want more technical details, Tichane's book is probably the best I've come across - I forget the title - it is out of print but readily available 2nd hand.

 

A friend of mine accidentally got a wonderful red with green flashes. He has gas bottles for his kiln with a manual changeover. He forgot to check the gas left in the bottle before firing, and it ran out close to top temperature. Luckily he was there and caught it, and quickly switched over to the other bottle. The pieces came out copper red with green flashes where oxygen had entered the kiln whilst there was no flame to burn it out. I've been trying to get him to try and repeat this, as the pieces were really lovely.

[marcello]

I have gotten green and red in ^6 reduction using Panama Red from John Britt's article in CM . Will post a photo. 

Here are some recipes from his article.

Hello Marcia,

can you please give some details about the firing? I am ready to test the panama red glaze, but it would be helpful to know if it was a strong or light reduction firing, when you started the reduction, for how long, and so on.

Many thanks

Marcello

[Brad325]

Marcello,

      I am fairly new to all this, yet I don't think it has to do with changing your glaze formula.  The clay body is the same throughout thet pot, right?  Therefore,  my belief is  it has to be that atmosphere within the kiln.   I have more questions then answers.  The solid red around the pot  leads me to believe you got reduction and for some reason the green  patch or clear spot of glaze didn't get the same amount of  reduction.  Why?   What was it near?   Was it open there?  Where was it on the kiln shelf? Where was it inside the kiln?   Do you know better locations in your kiln that give you complete even reduction all around a pot? Are you keeping good notes about your firing?  It might take you a while, but maybe you have already figured out a solution.  Best wishes,  Brad

[neilestrick]

42 minutes ago, Brad325 said:

Marcello,

      I am fairly new to all this, yet I don't think it has to do with changing your glaze formula.  The clay body is the same throughout thet pot, right?  Therefore,  my belief is  it has to be that atmosphere within the kiln.   I have more questions then answers.  The solid red around the pot  leads me to believe you got reduction and for some reason the green  patch or clear spot of glaze didn't get the same amount of  reduction.  Why?   What was it near?   Was it open there?  Where was it on the kiln shelf? Where was it inside the kiln?   Do you know better locations in your kiln that give you complete even reduction all around a pot? Are you keeping good notes about your firing?  It might take you a while, but maybe you have already figured out a solution.  Best wishes,  Brad

The last post was from 2015, so if you want to hear from Marcello you might want to sent him a direct message.

[Marcia Selsor]

I haven't fired ^6 reduction for a while.My friend finished his kiln a few blocks from my home and asked me to help fire it. Here are some of my Selsor Copper Red results. these are from 2 firings in the last month. The darker reds are 1/2 cone hotter.