Toasted look on the edge of glaze

[Liam V]

I am singularly obsessed with glazes that somehow cause clay to appear 'toasted' on its edges.

Could anyone tell me what ingredient in a glaze causes this or how I could emulate it?

Glazy recipes are much appreciated!

 

[Fred Sweet]

Liam-

Usually sodium or potassium, though other flux materials can also cause the halo. 

Regards,

Fred

[liambesaw]

Soda ash (sodium carbonate) does this for me.  

[Magnolia Mud Research]

 

1 hour ago, liambesaw said:

Soda ash (sodium carbonate) does this for me. 

so does baking powder,  baking soda, table salt, and trisodium phosphate -- always applied as a solution in water.   Try drawing on raw clay with a brush using a baking soda solution (or any of the above)  as ink.   

[Callie Beller Diesel]

If I had to guess, I’d say the fine line around the edge of the first image is some sort of sodium in the glaze just seeping into the clay. Some things will flash like that. The second image looks like a piece that was soda fired, but was in a dry spot in the kiln so it only caught a little flashing. 

[Liam V]

@Magnolia Mud Research Thanks for the tip, I love hearing about materials I can find in the home cupboard that make magic on my pots.

 

@Callie Beller Diesel I actually fired it at cone 10 oxidation in an electric kiln. It's a commercial glaze and turns that orange colour directly around where the glaze is applied. I'd like to mimic that flashing in other colours.

I'm curious to how you all learn about the indiviual effects of materials used in glazes, are there any books with such information or is this something you discover yourselves over time. I've been teaching myself everything from home and the books I've bought haven't really given me information like that.

https://glazy.org/recipes/28291

This glaze contains both sodium carbonate and potassium, is it likely to flash or does the presence of these compounds not always predicate flashing?

 

[Rae Reich]

@Liam V test test test - take notes

variables include clay body, glaze application, kiln, firing schedule and/or lab tech, propitiation of appropriate kiln gods

[Min]

4 hours ago, Liam V said:

This glaze contains both sodium carbonate and potassium, is it likely to flash or does the presence of these compounds not always predicate flashing?

I'm not seeing any flashing where the glaze line meets the claybody with that one. 

It could be a bit of a double edged sword getting glazes to flash in oxidation like yours did. It's likely there is a relatively high amount of alkali metals, my thought is sodium, in the glaze which is causing the flashing. Problem is that 2 of the 3 alkali metals potters use as fluxes also come with a high expansion therefore glazes tend to craze when heavily loaded with sodium and/or potassium. (lithium is the third alkali metal potters use as a flux, it is a low expansion flux but only moderate amounts of it are used as too much can cause shivering) I think that's why we don't tend to see a lot of flashing with oxidation fired glazes. If crazing isn't an issue for you look for recipes with high R20 levels, specifically sodium. I think you'll get more flashing with a low iron claybody rather than a high iron body but try your tests on all the claybodies you might be using.

[Callie Beller Diesel]

11 hours ago, Liam V said:

 

I'm curious to how you all learn about the indiviual effects of materials used in glazes, are there any books with such information or is this something you discover yourselves over time. I've been teaching myself everything from home and the books I've bought haven't really given me information like that.

 

 

I'm pretty sure we all wish it was all in one spot like a book, but I haven't found it yet. There are some good materials references out there, but they can't cover everything. Sharing information from knowledgeable folks in addition to the always annoying answer of test test test are the two big sources of info. The problem is that there's not really one single set of rules about how any singlematerial behaves. The outcome is always modified by the other ingredients in the recipe, how it's applied, the kind of clay it's applied to, etc. And let's not even start with the things that happen when you start messing with the firing cycle or varying how you heat your kiln. The answer is almost always "it depends."

 

On the topic of how the flashing happened: if the glaze has soda ash in it, which is very soluble in the bucket, if you wipe recently applied glaze off the surface of the pot, you can't remove the soda ash that is dissolved in the glaze water. It's been absorbed into the bisque, so it would stay behind and flux slightly. That could also explain some of the marking. It would also explain why there's a leading edge on the first pot. Like Min said, if you'd used it on a clay that wasn't prone to flashing like this, it might not be noticeable.

[Marcia Selsor]

I believe it is caused by soluble salts in fluxes as Fred pointed out. Solubles migrate on their own.

Marcia

[Liam V]

If I were to add 5% sodium carbonate to this:

https://glazy.org/recipes/22327

Would it work? Or would this require the removal of aluminium oxide, in an attempt to balance out the O3?

Or could I simply treat the sodium carbonate as a colorant without removing anything else frm the recipe?

 

[Callie Beller Diesel]

Soda ash is a flux, not a colourant. It can have an effect on colourants like copper, but it’s primary job is as a melter. It’s one of the materials that helps silica melt at the temperatures we work at, rather than the temperature it melts at when it’s by itself. Alabama Rain already contains a LOT of very active fluxes, including sodium from the Custer feldspar, and is prone to running as is.  If you add more flux and/or remove alumina, you will have a very broken glaze. When we’re speaking about sodium being the ingredient that’s getting you the flashing effect you’re looking for, it’s doing it as a side effect of its primary function. 

Is glazy your primary source of information? While it’s a great resource, it doesn’t come with a lot of information that helps you understand why everything is doing what it’s doing. You need some books. I recommend Clay and Glazes for the Potter by Daniel Rhodes, and there are other cone 6 specific references that others will chime in with I hope. I’d also like to recommend Digitalfire  as a materials reference and a source of excellent articles. https://digitalfire.com/4sight/education/index.html

Another recent favourite online resource for understanding how glazes behave in the bucket is Sue Macleod’s blog https://suemcleodceramics.com  She breaks the information down very nicely.

 

[liambesaw]

Mastering cone 6 glazes by Roy and hesselberth is a good one for cone 6.  There's a link to buy it for 25 dollars on their website masteringglazes.com

[Liam V]

I have developing glazes by Greg Daly and  The Handbook of Glaze Recipes by Linda Bloomfield;

I'll check out those recommendations as well,

thanks!