Clear Glaze Crazing

[Julie_R]

Hi,

Ive been using a Duncan Pure Brilliance clear brush on glaze on white & terracotta earthen ware clays. 

I have tried it over slips, underglazes & directly onto the clay body (2 to 3 coats as recommended on 05/04 bisque)

I am constantly getting crazing on the surface immediately when kiln is opened & also a few days later.

I fire in a gas Port-O-Kiln which is ceramic fibre lined.

I have tried.....ramping kiln slower, holding target temp longer, firing down to 700 before shutting off kiln. None of this seems to help.

Im very careful not to open kiln when still warm.

The reason I chose this glaze was because the info said it was craze resistatnt,  was brushable & had a firing temp of 06.

Could it be that the glaze just doesnt fit my clay or is it just that my kiln cools too quickly when i shut it off?

My gut feeling is that the glaze doesnt handle that rapid cool as happens with these types of kilns.

If thats the case, can I add anything to the glaze to help it resist thermal shock? ( i have about 5 litres of the stuff)

Alternatively, can anyone suggest where i might find a clear glaze recipe that would work on earthenware clay fired in this type of gas kiln?

Thanking you in advance.

[Bill Kielb]

1 hour ago, Julie_R said:

Could it be that the glaze just doesnt fit my clay or is it just that my kiln cools too quickly when i shut it off?

My gut feeling is that the glaze doesnt handle that rapid cool as happens with these types of kilns.

 

Glazes almost always craze because they don’t fit the clay. Especially delayed crazing. I would say most likely that glaze does not fit the clay.

[Kelly in AK]

I agree with @Bill Kielb, glaze fit is notoriously hard at low fire temps. It’s easy to imagine quick cooling is the culprit, but it’s unlikely that’s the cause. Industrial producers have astonishingly fast firing cycles, literally three or four hours from green to glaze fired, no crazing. It’s not deep magic, it’s because the glaze fits (they do have some tricks, but they’re mostly centered around avoiding other defects).

I’m happy to confirm that most of the things you’ve tried will have no bearing on your problem, none. The one thing you mentioned that might is trying a variety of clays. The glaze is shrinking a lot more than the clay. 

The other avenue to explore is temperature. In the range you’re firing at glaze fit can change radically within a cone, even half a cone. I say that from experience because I fire my work to cone 03, low fire. The clay is unique, and the glaze is formulated to fit, but a cone lower and it’s going to craze, a cone too high and the pots will dunt. This narrow range is true for many red earthenwares and clays designed to vitrify at low fire temperatures. Also, I should note that one of my kilns is fiber board and cools very quickly. It’s recently retired, but I fired dozens of glaze loads in it. Shut off at 10:00 pm and cool by morning. No crazing. That’s another reason I’m confident quick cooling is not the root of your problem.

I suggest you approach the two variables of clay body and glaze firing temperature (Some people fire their bisque quite high and do a lower glaze firing, I can’t speak to that with experience so won’t suggest it. It may well work.). Your cone 06 glaze might do fine at cone 04 and actually fit. I regularly fire commercial cone 06 glazes to 04 and 03 without problems.

There is the avenue of glaze formulation, but that’s an entirely different can of worms.

[Julie_R]

Thanks for your valuable replies.  At least you've set me straight on what the problem isn't.

I will do more testing at higher temps before I get the can opener out! 

[davidh4976]

It all depends on the fit between your glaze and your clay.  So, someone can recommend a glaze that works for their clay, but it may not work for your clay.

If mixing your own glaze is not desirable, I suggest trying various other commercial clear glazes and just doing some testing.

An effective way to test for crazing/fit is to take a test piece after it is glazed fired and alternately plunge it into boiling water and then ice water. Do this for 3-to-5 minutes in each bath and repeat the process 3 times. Dry the piece and then rub some India ink or dry erase marker on the piece and wipe off the excess. The ink will stay in the cracks making them easier to see, but you should also  closely inspect, using a magnifying glass to see if there is any crazing.  This test will accelerate any crazing that might otherwise only show up later (it's called delayed crazing).

If you want to tackle this by mixing your own glaze, you should become familiar with the Stull chart and how recipes land within it's different areas predicting crazing, etc. You can find some good tutorials online for that. Then, look at some recipes and where they fall on the chart. If you use glazy.org and input a recipe there, the site will plot the recipe on a Stull chart for you.

 

[Hulk]

+ Kelly's suggestion for other clay(s), particularly if your vendor(s) can point to a clay they know crazes less than the one(s) you're using.
In my craze journey, that's what happened! I'd scrapped several recipes, started over, re-formulated - getting closer, larger pattern - then went with a different white stoneware, problem solved.

[High Bridge Pottery]

13 hours ago, Kelly in AK said:

The other avenue to explore is temperature. In the range you’re firing at glaze fit can change radically within a cone, even half a cone. I say that from experience because I fire my work to cone 03, low fire. The clay is unique, and the glaze is formulated to fit, but a cone lower and it’s going to craze, a cone too high and the pots will dunt.

They had the same problem at the old place I used to work. They were selling premade bisque forms and duncans PB but if the manufacturer fired the bisque hotter than usual customers would start complaining about glazes shivering/popping off.  I would try firing your bisque a cone hotter to see if that stops the crazing.

[Julie_R]

Thanks again everyone for sharing your knowledge. Uuurgh.... so much to learn. Definitely not a generic "one size fits all" kind of thing with glazes & clay bodies is it?   Everything I've learned so far has been by spending hours on the internet, researching, watching videos (clayflix,  yay), forums like this one, trial & (a lot of) error & testing, testing, testing. Any classes or workshops are only available in the major cities so not an option for me. Sometimes I feel like a one legged woman in an arse kicking competition . But I love the process & I love ceramics & I wont give up!

[Bill Kielb]

3 hours ago, Julie_R said:

Any classes or workshops are only available in the major cities so not an option for me.

You may want to take a look at sites like: https://suemcleodceramics.com/. Lots of free stuff to read as well as online work shops. More free and paid resources https://digitalfire.com/glossary/tony+hansen, https://glazy.org/kilnschedules, https://ceramicmaterialsworkshop.com/

[Callie Beller Diesel]

On 10/27/2023 at 1:29 AM, Julie_R said:

Definitely not a generic "one size fits all" kind of thing with glazes & clay bodies is it? 

To the dismay of everyone who has ever worked in clay in the history of ever, sadly no.