Crazing Clear Glaze

[Hailey]

I haven’t ventured into glaze chemistry so I don’t know what to do. A local supplier made me this glaze: https://glazy.org/recipes/143298 just to use as a clear liner for my soap dispensers and vases. It’s turned into a crackle glaze. Since they aren’t used for food, I don’t think it will be much of a problem and I figured I can fix it next year when I start doing my own glazes. With this crackle, is there anything non chemistry I can do to fix it? I’m planning to thin it out and try that. With a kiln I bought, I inherited some sodium silicate and soda ash that I don’t know how to use. Could either of those help? Is there any danger that the crackle glaze will fall off the piece? Like as shards of glass?

[Hulk]

Hi Hailey!

The craze pattern (how close together the lines) is medium to large, so, perhaps closer to a fit then if spaces between lines were very small.

For non chemical/recipe fix, likely that changing clay would be the only choice - a clay that matches up with the properties (COE - coefficient of expansion) of the glaze. Under firing, maybe the clay shrinks differently, however, not a fix in my opinion, for then the clay isn't as strong and water tight.

Changing that recipe to be lower expansion, my suggestion would be to start over. Small batches, once the problem is solved, then maybe reset the craze glaze with help from glaze calc software - or just use it on ware not for food.

I'm not seeing glaze fall off of crazed ware; I'd gone through a process of craze problems, which either get trashed, repurposed as planters, or used in our house. The cracks open up with wear and take a stain (time to retire it), but I'm not seeing glaze fall off yet on crazed ware. Maybe try some destructive testing to see how well it is stuck on there?

I didn't plug your glaze in (to glaze calc software) to see the numbers, etc., however, neph sye is high in sodium

Na2O (Sodium Oxide, Soda) (digitalfire.com)

Solving crazing, went from trying low-e recipes, then choosing a base to start with and adjusting it down - less sodium, more magnesium oxide, some lithium oxide, some zircopax - all which helped a lot - bigger craze patterns - however, for white stoneware, found two clays that fit with latest iteration of my low-e clear experiments, been in white stoneware for well over a year now, voila! (eh, got to get back to buff and red, and make yard ornaments from clay that the glaze doesn't fit).

 

[Hailey]

Unfortunately it’s crazing on all of my clay bodies. (Bmix, bmix with grog, San Clemente, speckled buff). I’m in a frustrating situation where I don’t have control over the firings, but when I can get my kiln set up, would it be helpful to let it cool down super slow? Also, sorry if this is a stupid question, but if the glaze is fit is “too small” for the clay, can I add ferro frit to help it? (Hopefully that doesn’t show my ignorance too much, just a similar clear recipe showed more of that than the one I used)

[Hailey]

4 hours ago, Hulk said:

Hi Hailey!

The craze pattern (how close together the lines) is medium to large, so, perhaps closer to a fit then if spaces between lines were very small.

For non chemical/recipe fix, likely that changing clay would be the only choice - a clay that matches up with the properties (COE - coefficient of expansion) of the glaze. Under firing, maybe the clay shrinks differently, however, not a fix in my opinion, for then the clay isn't as strong and water tight.

Changing that recipe to be lower expansion, my suggestion would be to start over. Small batches, once the problem is solved, then maybe reset the craze glaze with help from glaze calc software - or just use it on ware not for food.

I'm not seeing glaze fall off of crazed ware; I'd gone through a process of craze problems, which either get trashed, repurposed as planters, or used in our house. The cracks open up with wear and take a stain (time to retire it), but I'm not seeing glaze fall off yet on crazed ware. Maybe try some destructive testing to see how well it is stuck on there?

I didn't plug your glaze in (to glaze calc software) to see the numbers, etc., however, neph sye is high in sodium

Na2O (Sodium Oxide, Soda) (digitalfire.com)

Solving crazing, went from trying low-e recipes, then choosing a base to start with and adjusting it down - less sodium, more magnesium oxide, some lithium oxide, some zircopax - all which helped a lot - bigger craze patterns - however, for white stoneware, found two clays that fit with latest iteration of my low-e clear experiments, been in white stoneware for well over a year now, voila! (eh, got to get back to buff and red, and make yard ornaments from clay that the glaze doesn't fit).

 

So in finding a clay that it fits, is it just trial and error? Or can I look at the shrinkage rate of the clay? If so, do I want a higher or lower shrinkage rate than the clay that it’s  crazing on?

[Bill Kielb]

50 minutes ago, Hailey said:

Unfortunately it’s crazing on all of my clay bodies. (Bmix, bmix with grog, San Clemente, speckled buff). I’m in a frustrating situation where I don’t have control over the firings, but when I can get my kiln set up, would it be helpful to let it cool down super slow? Also, sorry if this is a stupid question, but if the glaze is fit is “too small” for the clay, can I add ferro frit to help it? (Hopefully that doesn’t show my ignorance too much, just a similar clear recipe showed more of that than the one I used)

45 minutes ago, Hailey said:

So in finding a clay that it fits, is it just trial and error? Or can I look at the shrinkage rate of the clay? If so, do I want a higher or lower shrinkage rate than the clay that it’s  crazing on?

 

The firings will not help. The clay and glaze fired coefficients of expansion must be a reasonable match otherwise this type of defect will occur. If the mismatch enough, the defect appears quickly. If they differ a small amount the defect can occur weeks, months, years later. No amount of slow cooling will fix this. If it’s gonna craze, it will because of the mismatch.

As far as matching the COE you can do it with glaze chemistry but Fritt is not necessarily a fixer. It’s more about reducing the high COE fluxes and increasing the lower expansion ones. This can’t really be calculated, one must test. Your glaze crazes because it shrinks faster than the clay so you are looking for a proven lower expansion recipe than you have.

The recipe on Glazy gets its boron from the Fritt. The recipe has what we know is enough boron to melt at cone 6 so adding more fritt is not necessarily a good idea.

Since this recipe doesn’t fit your clays, it’s probably much easier to find a better recipe. Recipes often don’t travel well so any recipe must be tested.

[Hulk]

3 hours ago, Hailey said:

So in finding a clay that it fits, is it just trial and error?

Hi Haley,

Your vendor may be able to help, or perhaps someone who uses the same clays.
When you show them - I'm getting this crazing on these clays - then ask, which clays will I have less crazing?

I'd tried a white stoneware from a new vendor a while back; my in progress low e glaze crazed, bad.
When I stopped by there again, I asked about one of their other white stoneware clays; the knowledgeable and helpful person affirmed that the other white stoneware is much less prone to crazing. Lucky me, same second stoneware is now my favorite clay.

Among the various clays I've tried, I've a good idea how they rank in terms of coefficient of expansion, yep, via testing.
That said, the one helpful vendor person helped me a lot, and they won my business.

[Mark C.]

Are these cone 6 clays? or cone 6 glazes on cone 10 clays?

[Callie Beller Diesel]

On 11/29/2021 at 3:13 PM, Hailey said:

in finding a clay that it fits, is it just trial and error? Or can I look at the shrinkage rate of the clay?

Tl;dr: yes we have to test everything, even when you’ve been at it a long time, and Tom’s suggestion to ask your supplier about combos that are already known to work are likely your best bet.

First we have to clarify a little terminology so there’s less confusion. If you go to your supplier and ask about the shrinkage rate of your clay, they’ll think you’re asking for dry and fired shrinkage rates. Those are numbers that are really easy to measure just by making a few test bars and measuring the difference in length. That number represents the difference in size from the mug you threw to the mug you wind up drinking out of.

COE is Coefficient Of (thermal) Expansion, sometimes also noted as CTE. Your supplier either doesn’t measure these numbers, or doesn't give them out anymore. I’ll get to that in a second. 

COE is the amount of expansion and contraction ceramic and glass materials undergo when they go through phase changes as they heat up and cool while in the kiln. Think like ice: it expands as it freezes and shrinks back down when it thaws. When the glaze and the clay do this at different rates, that’s when you get fit issues. That’s why you can’t fix crazing with slower firing ,or cooling, or applying glaze more thinly/thicker, or with any method other than changing the recipe or the clay you use it on. The incompatibilities are coded into the chemistry, and if they’re big ones, they show up right away. If they’re smaller differences, it can take some time to show up, but show up they will. If the differences are small, it can be as simple as adding some extra silica to the glaze. If the differences are large, you’re looking at changing materials, and that’s when things start to get interesting. 

Why doesn’t your supplier give out COE numbers? Because COE is something that is really difficult to measure IRL without expensive equipment. You can *kind of* use a calculated expansion rate based on measurements done on individual materials,  but it gets complicated, because not all materials expand in a linear fashion. There are enough materials that get used in both clay and glaze recipes that fall into the “doesn’t like being measured” category that a calculated COE winds up being of limited value. There isn’t a straightforward, easy to understand number that can be handed to a beginner, or even a lot of intermediate ceramic folks, and that causes more confusion than it solves. So suppliers that used to provide COE numbers for their clays have mostly stopped doing so.

Some glaze software will give you calculated COE numbers for glazes so you can work on a comparison basis, if you’re trying to adjust a specific recipe in relation to itself, and only if you’re keeping the materials the same. Like I said: only sort of helpful to try and put a number on it.

So yeah. We’re stuck doing a lot of testing when we first try to introduce a glaze to our repertoires. If you find a couple of base glazes that work with your clay and that look cool with each other, you can add colourants or other assorted modifiers to get different looks. I would suggest trying to find a glaze that is known to fit you clay body until you get to a point where you can play around with chemistry to try and understand it.

This glaze as a base wouldn’t be a bad place to start experimenting on when you’re ready to start that learning. It’s got a good silica: alumina ratio, and the flux balance is also in the ideal zone for food durability. It’s cheap, and the materials are mostly readily available. (I think Ferro has frit production back online now.)

[kswan]

I assume you're firing the B Mix 5, and not the one to Cone 10. I use B Mix 5, and I've been using Odyssey Clear on it with no crazing if you'd like to try that recipe in the future.  It is listed in Glazy (same exact recipe is named Sarah's Clear too but has nicer photos of it), and Gabriel Kline's book Amazing Glaze has it since he works at Odyssey center.  I haven't used it on any other clay than B Mix 5 though, since that's all I use. 

Odyssey Clear Cone 5-6

Nepheline syenite  30

Silica 30

Gerstley borate 20

Kaolin (EPK) 10

Wollastonite 10

Bentonite 2