Clear Glaze Changing The Color Of Glaze/clay Body Below?

[SydneyGee]

I wish I had the recipes for you guys, but unfortunately all I have are photos. I have tried our basic clear glaze on top of about a dozen pots with different glazes/engobes, and every one came out with a weird reaction with the glazes and clay body beneath.

 

We are firing to cone 6 reduction, and the clear glaze causes a strange blue to appear on the glaze and clay body. For example, the photo is a recycled body clay high in iron, with iron filings mixed in, and a black engobe brushed on top. The clear turns everything blue! Sometimes it's more blue than other times, but it with out fail, will cause a blue to appear wherever the clear glaze is applied.

 

The two marbled tumblers are the recycled clay with a black engobe, and the last one is a red clay (Laguna's Calico clay) with just clear over a white porcelain slip on the inside. The inside crazed badly.

 

Any ideas why the clear glaze creates a blue reaction on the pots? I have just about given up using it, I would rather just use a white satin and have a predictable solid color than this weird mess.

[preeta]

what creates the black in the black engobe?

 

is there some cobalt carbonate? becausae i see the blue only where the engobe is. if its mason stain then the same thing happens to me. 

 

my conjecture (which i have not been able to test well yet in reduction is) - a lot of the stuff disappears under clear glaze in oxidation. i dont use iron, rutile or copper under clear as just a tad bit is left. but under our clear cobalt does very well. 

 

yikes. i dont know about the 3rd one. 

[Callie Beller Diesel]

Aha!

You have inadvertenly discovered the mechanism that creates floating blue!

 

Iron underneath the glaze (iron filings, black engobe, red clay body, etc) + too much boron and not enough silica and alumina (hello crazing,)= boron clouding.

 

Your clear is not a balanced glaze.

 

https://digitalfire.com/4sight/glossary/glossary_transparent_glazes.html

 

Some commentary on clear glazes over iron from digitalfire. Note the 5th picture down.

[SydneyGee]

Thank you two!

 

@Preeta: Probably iron and manganese. I glanced at the recipe once, but could not tell you in what amount they make up the engobe.

 

This is one thing I hate about being in a school studio, I have no control over the quality, ingredients, or outcome of my work. I have been experimenting extensively to find out what works, often with success, but also many times with failure... The clear glaze is one that has never worked well for me. I have found that I would rather just leave bare clay (if that is my intention) or to use a more stable and consistent glaze.

 

Why don't we fix these problems in the glaze?? That is beyond me...

[Callie Beller Diesel]

If you surf around that website, you'll find areas where Tony worked on clear glazes that are more stable than that one. Is there a reason you can't mix your own?

[SydneyGee]

I guess because they are worried if the glaze is not stable it will ruin the kiln and others works. If I take the glaze design class I can make my own glaze, but mostly it might be a problem of funding and taking the "what if" chance.

[neilestrick]

Looks to me like it's picking up some cobalt from the engobe.

[Babs]

Neil said it, the black of hte engobe prob. is from Mang. iron and cobalt.  and that is causing a cobalt blue to emerge.

That is a seriously crazed glaze though. Does it perform on other clays without the crazing?

[SydneyGee]

It crazes on everything. I have never seen it effectively coat a piece clear, even porcelain. It has ruined otherwise nice forms by crazing and tinting the other clay bodies (including turning porcelain a slight blue). My guess is it is an imperfection in the recipe, or cross contamination with other glazes near by.

 

It is unfortunate they do not take the time to correct such a simple recipe. A few of their glazes are like this, they just never quite work properly.

[SydneyGee]

I talked to my professor and the engobe has cobalt in it. Apparently I missed the jar labeled "No Cobalt" fwoops.

[High Bridge Pottery]

I have seen clear glazes in oxidation tend towards blue in reduction firings on porcelain. Not sure what the cause of it is but reduction brings out the blues.

 

Here is a quote from digital fire in the blue section, any iron will seem to send the glazes blue in reduction. I think that is giving the blue hue throughout the glaze with cobalt sending the glaze brighter blue.

 

 

Oxides - Fe2O3  

In reduction glazes Fe2O3 tends to fire bluish or turquoise to apple green with high soda (boric oxide may enhance). 0.5% iron with K2O may give delicate blue to blue green.

 

They also say "Iron glazes will move toward blue if alumina is low."

 

Bit more reading and I found this on another website.

"The cup (fig 4) with iron glazes was fired under reducing conditions. The glaze on the inside contains 0.5% iron(III) oxide and the glaze on the outside 10%; the reducing power of the CO was enough to convert the iron in the inner glaze into Fe(II), but the concentration of iron in the outer glaze was just too high."