Crazing versus firing temperature

[davidh4976]

Is it correct to say that if a glaze/clay combination does not craze at cone 5, then it should not craze at a higher cone firing?

If I normally fire to cone 6 and want to make sure that the new glaze/clay combo does not craze because of underfiring or a cool spot in the kiln, I think I can fire test pieces to cone 5 and then my normal cone 6 firings would be craze free. I use the alternating boiling/ice water baths for craze testing.

[Min]

2 hours ago, davidh4976 said:

Is it correct to say that if a glaze/clay combination does not craze at cone 5, then it should not craze at a higher cone firing?

 

Yes.

2 hours ago, davidh4976 said:

I use the alternating boiling/ice water baths for craze testing.

If you want a more extreme test then go from a 300F oven to ice water. Boiling to ice water has a temperature difference of approx 180F whereas oven to ice water is approx 268F  difference. 

I used to do a sequence of 300F/ice water then 310F/ice water then 315F/ice water but I think that once you have done 1 cycle of oven/ice water you have stressed the glaze to a point where further cycles aren't valid.

[Bill Kielb]

I think hard to generalize and no way to guarantee anything is craze free forever. I do think there is reasonable correlation though. Differences in the expansion rate of clay and glaze being the prime issue, followed by the long term reliance of the glaze to tolerate the slight inevitable difference. The fired COE is often a result of fully firing so a clay body or glaze not fired or fully melted may have a different COE than one that is. Just imagine a clay body or glaze under fired by two or thee cones, we would normally expect it to behave very differently than fully fired stuff because it is under fired. If both are under fired the same amount will their under developed  COE somehow be even between them?  While I think in the ranges you are working in, likely a decent indication, just firing hotter does not necessarily mean the glaze will have greater flexibility and could actually become harder and more prone to tensile failure.

I think your logic is reasonable, but believe it’s always best to test the the clay and glaze combination fired to cone.

[Min]

On 1/31/2024 at 9:17 AM, Bill Kielb said:

just firing hotter does not necessarily mean the glaze will have greater flexibility and could actually become harder and more prone to tensile failure.

 

As we all know both clay and glazes shrink as they are fired and then cooled. A cone 5 clay fired 1 cone hotter will probably shrink a small amount more than if fired to cone 5 or lower. An extreme example would be to compare the size of a mid or high fired piece of bisque to the same piece when fired to  fired mid or high fire. It shrinks.

We also know that both claybodies and glazes mature over a range, there isn't one magic temperature that either is mature. Given that crazing is most often a result of a claybody being too "large" for the glaze which results in the glaze "stretching", ie crazing, to the body as both cool (or X amount of time later in regards to delayed crazing or from moisture absorption). By firing a midrange body one cone hotter will likely shrink it ever so slightly from what it would be one cone cooler. It won't make the body larger therefore chance of crazing is reduced. Grossly overfiring a body will increase the porosity of the body but that's another issue.

As to if the glaze will have issues with firing 1 cone hotter, perhaps it will, and like all things ceramic it should be tested.

edit for clarification: referring to the physical claybody shrinkage as it relates to crazing.

@Bill Kielb, would you mind clarifying what you mean by the glaze and "tensile failure" in regards to the op's question?

Edited February 4 by Min
clarity

[Bill Kielb]

1 hour ago, Min said:

By firing a midrange body one cone hotter will likely shrink it ever so slightly from what it would be one cone cooler. It won't make the body larger therefore chance of crazing is reduced. Grossly overfiring a body will increase the porosity of the body but that's another issue.

As to if the glaze will have issues with firing 1 cone hotter, perhaps it will, and like all things ceramic it should be tested.

@Bill Kielb, would you mind clarifying what you mean by the glaze and "tensile failure" in regards to the op's question?

Crazing to me is a result of the differences in the rate of growth or shrinkage. Once fired clay and glaze  are the same size, so the clay has shrunk to what it will be and the rate of future expansion and contraction will be affected by its composition and its  physical (and thermal) properties such as density etc…. Firing the body to its rated cone will provide the best chance of a fully melted body with repeatable characteristics. So the COE will likely be different for a cone 6 body fired to cone 4 or even cone 5. Our glaze on the other hand if able, may tolerate the difference in speed of expansion / contraction  and not fail or craze as if pulled apart. As a glaze is fired higher will its COE change? Very likely yes. Will its physical characteristics change? Very likely yes. Glazes more resistant to wear as in surface hardness may fail more so in impact. 

I think the difference here for me, none of this is about the starting size and all about rates and whether the glaze can keep the clay body in a slight bit of compression without the glaze failing. So firing both to their rated cone likely gives the best repeatable representation of the match for both. Firing lower and assuming the next cone higher will improve the COE match between the clay and glaze is likely not really reliable in my view. So in the end, fire to rated cone, firing to lower cones is very likely not a good indicator for how these materials react when fired to rated cone.

Edited February 1 by Bill Kielb

[Min]

I think we all know about clay and glaze COE's. After firing hundreds of glazes on thousands of pots in multiple kilns and types of kilns (wood, gas, electric) I've yet to see a glaze that fits a claybody then craze when fired a cone higher. Many of the kilns I've fired, mostly pre the advent of controllers, often had a variance of a cone from top to bottom. Underfiring created issues with the glaze at times, overfiring resulting in crazing didn't.

Perhaps if you want to discuss theory of shrinkage, expansion, contraction, COE etc we could start a new thread.

[Bill Kielb]

2 hours ago, Min said:

Perhaps if you want to discuss theory of shrinkage, expansion, contraction, COE etc we could start a new thread.

I really do not - just responding originally to the OP and your specific question. I have fired a bit and never did I notice firing higher cures crazing. That’s just an observation though. The reason I believe what I believe is outlined above but it’s just an opinion with a basis. Start a new thread, maybe you have something and firing higher is another tool to help with crazing.

[Min]

18 minutes ago, Bill Kielb said:

maybe you have something and firing higher is another tool to help with crazing.

With all due respect Bill I didn't say that. (however underfiring a clay and glaze will contribute to crazing)

What I am trying to point out in the subtext is when someone asks a simple question we have the option of just answering the question or going into theory with much more detail and theory when oftentimes it really isn't necessary.  When we have a better understanding on a person's knowledge and experience it is easier to tailor a reply to fit the question, if not perhaps we can offer a solution or answer with an offer to go more in depth or into theory if that is warranted.

[Bill Kielb]

3 hours ago, Min said:

With all due respect Bill I didn't say that. (however underfiring a clay and glaze will contribute to crazing)

Yes of course simplicity is best. But I would suggest you re-read my original response directly addressing his post. Elaboration came as further requests were made. My first answer was intended to be reasonably brief but also a reasonable explanation to his question:  “Is it correct to say that if a glaze/clay combination does not craze at cone 5, then it should not craze at a higher cone firing?” And his direct follow of working at cone 6.

With due respect as well, I certainly apologize if I fell short of that goal in your view. Unfortunately my original answer was to the best of my ability an honest attempt to achieve that.

[Min]

4 hours ago, Bill Kielb said:

Yes of course simplicity is best.

Thank you. 

I believe I made my point. If you would like to debate this further then please feel free to PM me, any of the other mods or admin.

 

[C.Banks]

On 1/31/2024 at 6:13 AM, davidh4976 said:

Is it correct to say that if a glaze/clay combination does not craze at cone 5, then it should not craze at a higher cone firing?

This is an interesting question.

I've always understood glaze fit in terms of the big changes in silica. Maybe somone can help me underastand too. Once that last big in quartz happens at 573c there is no ther significant change in size until cooling through 250 - the last change in quartz that can put the 'squeeze' on.

In my limted understanding, firing one cone hotter does not involve any significant change in the size of silica.

I look forward to understanding this better though. I get tripped up easily but do enjoy trying to fit the pieces together in my brain.

Edited February 2 by C.Banks

[Hulk]

"Is it correct to say that if a glaze/clay combination does not craze at cone 5, then it should not craze at a higher cone firing?"

Good question.

imo, maybe!

"Maybe" on account of the crazing or not crazing may not matter so much if the clay isn't fully mature or is over fired.
For clays that take a higher cone without misbehaving, yes.

In my experience (limited, of course!), some clays have a wider firing range, while others are very much more sensitive to over firing.
Three clays, in particular - a red, buff, and a black clay - that I've tried matured at a solid cone 5 but misbehaved when fired into the top of cone 6 range.

"Fully mature" - where the clay is dense, strong, and has low absorbency*, err, as low as the particular clay can reasonably get, that is.
"Over fired" - where clay has begun to slump/melt, bloat, fizz/bubble...

This article This amazing difference 45 micron silica can make (digitalfire.com) got me thinking.
I wonder if my glaze fitting struggles were related to 200 mesh silica (which is what I have)?
If so, then, would more heat work help dissolve the silica?
If so, then, would more and/or more aggressive fluxes help dissolve the silica?
If so, would a drop & hold (after peak temp) help dissolve the silica?
I don't know.
The solution involved tweaking the formula to reduce COE, however, I'd adopted drop & hold at about the same time.

I'd like to have clay and glaze combinations that can take a half cone or more of "excess" heat work without problems.
The aforementioned sensitive clays, I've found other red and buff clays (haven't tried any other black clays yet).

Keep on with cone packs on each level, keep notes, and test (test, test)!

*article on maturity Maturity (digitalfire.com)
 

Edited February 2 by Hulk
punctuation

[High Bridge Pottery]

I would theorise that firing hotter probably lowers the expansion of the clay, possibly by dissolving more crystalline silica and could make a glaze more likely to craze.

I'm not sure it's a good idea to relate the permanent shrinkage firing clay with coefficient of thermal expansion. 

Edited February 2 by High Bridge Pottery

[Min]

6 hours ago, C.Banks said:

In my limted understanding, firing one cone hotter does not involve any significant change in the size of silica.

I had a look through Hamer's Pottery Dictionary of Materials and Techniques and under the Crazing topic. Snippet from there reads "Where a glaze is overfired and then crazes it is because the overfiring affected the body more than the glaze." also "The crystalline silica in the body becomes fused silica and in this form it has a much lower rate of contraction. The glaze still has the same rate of contraction and therefore in proportion it contracts more when overfired than when correctly fired."

Hamer doesn't quantify how much overfired. I haven't seen it with a single cone higher firing, maybe more prevalent with very tight firing range claybodies?  

[davidh4976]

20 hours ago, Hulk said:

I wonder if my glaze fitting struggles were related to 200 mesh silica (which is what I have)?

 

We had a situation about a year ago where we started to get crazing on glazes that had never crazed before. It took a lot of trial and error testing with known good batches of materials to determine the problem. It turns out that we had received a batch of 200 mesh silica by mistake instead of 325 mesh. Because there was no clear "mesh" marking on the bag, no one realized that we had received 200. The glaze mixers had unknowingly started using 200 instead of 325 in the glaze mixes. Switching back to 325 solved the problem.  Of note, the crazing happened only on cone 6. We did not get crazing on cone 10. My theory is that cone 6 was not hot enough to melt the 200 mesh completely, but cone 10 was able to completely melt all of the 200 mesh.

[C.Banks]

16 hours ago, Min said:

I had a look through Hamer's Pottery Dictionary of Materials and Techniques and under the Crazing topic. Snippet from there reads "Where a glaze is overfired and then crazes it is because the overfiring affected the body more than the glaze." also "The crystalline silica in the body becomes fused silica and in this form it has a much lower rate of contraction. The glaze still has the same rate of contraction and therefore in proportion it contracts more when overfired than when correctly fired."

Hamer doesn't quantify how much overfired. I haven't seen it with a single cone higher firing, maybe more prevalent with very tight firing range claybodies?  

aha

and from the dictionary too. I forget books are a thing apparently.

It didn't occur to me to consider overfiring.

This makes sense to me in terms of how much feldspar is present in glaze copmpared to clay. The way I understand it if there isn't sufficent feldspar present, in clay,  some critobalite gets left out and doesn't fit on cooling - like musical chairs.

 

Edited February 3 by C.Banks

[Hulk]

"Switching back to 325 solved the problem."
Thanks David!

I'm curious now, err, more curious, does drop & hold firing schedule help ameliorate the 200 mesh "non fully melting in" problems?
I'm still on the original silica supply I bought in 2018, and year afore last I bought more, a lot more.

 

[High Bridge Pottery]

They seem to agree with my theory that it's about crystalline silica and being high expansion in crystal form and low expansion when fused/melted/dissolved.

As a wise member once said "it depends"

Maybe your glaze is full of unmelted silica because you got 200 instead of 325 mesh and firing hotter lowers the expansion of your glaze, maybe your glaze is all amorphous and going hotter lowers the expansion of the clay crazing the glaze or a mixture of lots of chemistry melty and crystaly combined happens and it all evens out in the end and firing hotter makes no difference.