Rim Breaking

[madatmyrims]

Does anyone know why the rims of my pieces are doing this? I am a production potter and I make a large number of pieces and this has just started happening. It does it on multiple clay bodies but only every once in a while on dark glazes but usually always on our white glaze. We use the same base glaze for all and add colorants. Any help is appreciated. We fire to cone 6 oxidation- electric. https://www.dropbox.com/s/g0bgphvybm6bomc/Photo Nov 11%2C 12 45 19 PM.jpg?dl=0

[Bill Kielb]

Almost looks like it crawled but more importantly does that glaze fire to any gloss? Looks underfired in the picture. What colorant do you use for the white - zircopax? Post the recipe if known, that will likely help narrow down the issue.

[madatmyrims]

8 minutes ago, Bill Kielb said:

Almost looks like it crawled but more importantly does that glaze fire to any gloss? Looks underfired in the picture. What colorant do you use for the white - zircopax? Post the recipe if known, that will likely help narrow down the issue.

That’s what I was initially thinking, but I’ve been using this glaze for the last 4 years and it just recently started doing this. We buy all our materials from Laguna and I did notice a bag change in their Neph Sye. I will add the recipe. It is a matte glaze if fired to cone 6. 
 

The glaze is 

Neph Sye - 6360 

Dolomite-  2120

Ball Clay- 420

Bentonite- 250

Zircopax- 840

 

We use this on three different clay bodies, Speckled Buff, 55 Laguna, 65 Laguna, and I have noticed it do it on all of them a few times but it almost always does it on 65. It also does it a lot on reclaimed clay. 

[Min]

I agree with Bill, it looks underfired / unmelted. It's very unusual to see a glaze for functional ware at cone 6 that doesn't contain either boron or zinc. Glazes at cone 6 just don't want to fully melt without either of those. Since we are used to looking at recipes when they total 100 I redid yours, makes it easier to have a look at the proportions of materials, screenshot below. This has 70% nepheline syenite, which also sets off alarm bells for me. Expansion is really high because of all that sodium from the nepeline syenite, I would expect this glaze to craze over time as well as being unmelted. (given expansion figures are not accurate with matte glazes but the amount of sodium in the recipe indicates this) With all due respect I would start working on another recipe rather than trying to fix this one.

Welcome to the forum.

[madatmyrims]

Is there not something I can add to bring down the COE of the glaze? Maybe a Frit? The new clay I’m using is 4.8 COE when the clay I was using was 5.3 COE. Would that change cause this effect? I’ve been using this glaze for the last 5 years and haven’t ever had this issue until recently. It also works as the base glaze for other glazes I use and I don’t have issues with those. 

[madatmyrims]

And it doesn’t seem to craze- I’ve been using the same set for the last few years and there is no crazing on the set that I’m using. 

[madatmyrims]

Do you think I could decrease the amount of Neph Sye and add Frit 3134 in to help with the melt point? 

[neilestrick]

1 hour ago, madatmyrims said:

And it doesn’t seem to craze- I’ve been using the same set for the last few years and there is no crazing on the set that I’m using. 

It can be difficult to see crazing on a matte glaze. Try coloring a spot with a black sharpie then wiping the surface with alcohol. The sharpie will stay in the cracks if it's crazed. FYI, you won't be able to get the black out unless you refire it, so test on a pot you don't want or are willing to refire.

[Bill Kielb]

21 hours ago, madatmyrims said:

s there not something I can add to bring down the COE of the glaze? Maybe a Frit? The new clay I’m using is 4.8 COE when the clay I was using was 5.3 COE. Would that change cause this effect? I’ve been using this glaze for the last 5 years and haven’t ever had this issue until recently. It also works as the base glaze for other glazes I use and I don’t have issues with those. 

Some of the problems with this recipe:  it’s likely not fully melted so it’s expansion, durability etc…. Is  hard to predict with any confidence. It’s flux ratio is not good, For instance an R2O:RO in the rangę 0.2:0.8 to 0.3:0.7 is usually desirable for known durable glazes. It doesn’t mean glazes in this range are durable but it does imply the farther away one moves from this norm the less likely this will be durable. Yours is 0.4:0.6 which is significant. Glazes that are not fully melted often appear matte and can lack durability, since they are not mature. So for functional wares, most folks just avoid that situation because in general it more often than not ends up less durable.

As Min mentioned Your glaze does not contain boron nor zinc which is traditionally used to lower the melting point to cone 6. Approximately 0.15 boron or an approximate range of zinc, 0.2 -0.4 in a 0.7 RO base (known as a Bristol glaze). Anyway, nothing written in stone, but your recipe departs from these established norms so it is suspect. Fixing it, is a bit more complicated than just adding more stuff as the glaze designer wants to use best known practice to be as durable as practical.

When colored white, you have more Zircopaz than clay in your recipe. Zircopax is very refractory so that would add even more to the issue  with this glaze when colored white.

In other words there are  many issues with this recipe, I think most would say, find a new one rather than trying to make this into something.

BTW - fired COE is tricky and often hard to numerically work through so matching the COE is often a bit of trial and error observing trends rather than just manipulating numbers.

[Callie Beller Diesel]

I like the Stull chart overlay view that Glazy provides, because it gives a quick visual reference for what’s going on chemistry wise. They also provide an extended UMF calculation, which factors in certain colourants or opacifiers that are traditionally left out of the math. It’s not super clear in this image perhaps, but your glaze is in the middle of the semi-matte (due to underfiring) and crazed range, and would possibly be crazed even if fired to a hotter cone. This is clay body dependant, of course.

Stull doesn’t predict melting temperature, and was based off of a glaze with the aforementioned 0.3:0.7 flux ratio being fired to cone 11. Recent testing done has indicated that it does bear out at lower temperatures, however.

Here is a handy link to Glazy’s interpretation of Stull. If you don’t have a glaze chemistry software already purchased, their free version has a LOT of good functionality and tutorials.