Deep cracks in the glaze

[Cenknst]

Hello everyone, hope you're doing well.

I noticed some cracks that I believe were not there right after the glaze firing - but probably appeared later (though I can only be %90 sure about that now, I normally notice such defects instantly)

So weeks later after the firing, my cracks happen to be very deep but not like web-like cracks all over. You can see in the photos that there are only two lines on one item and more on the pinky one. 

Another fact is that these can only be seen if you examine under various angles of light. 

The blue item is clear glaze coated on underglazed earthenware, and the pinky one is a dry glaze applied on earthenware. Fired at 1040degrees celsius. I also doubt whether the clear glaze was also dry glaze - but cannot remember. 

I could either blame the thickness while applying the glazes or the fact that they are dry glazes and I'm not handy enough to use them..

I'd appreciate your comments- I could only upload one photo, will try the other one again.

[Cenknst]

Here is the other  cracked sample.

[Cenknst]

Another one with clear glaze over underglaze.

[Roberta12]

It appears to me to be crazing.  I have had it happen weeks later as well.  It usually means your clay and glaze are not a good fit. ( I discovered I was putting cone 5/6 glaze over a cone 10 clay body)    Do you know the cone rating of your clay and glaze??  Could you explain "dry glaze"  I wasn't sure what that meant.  

Roberta

[Cenknst]

Hi Roberta, I mean the glaze we buy as ready powder and mix with water. 

I always pick them among the ones sold as 'low fire' and I use them on earthenware, so their temperatures actually fit but I heard that misfit may still happen. But when that happens I thought crazing covers all over  and doesn't stay local like these. But I'm not an expert and open to suggestions

[glazenerd]

Cen:

when crazing occurs after the firing: the two primary reasons are 1. Cristobalite inversion that occurs around 450F which causes sudden expansion. The crazing pattern is usually anticular- single crack line with smaller cracks extending from it. 2. Expansion caused by absorption of atmospheric moisture (humidity) or absorption from fluids (coffe/tea) that creates just enough added stress to cause widely spaced crazing. I would recommend doing an absorption test on an Unglazed test tile of the clay used in your photos. The clay body is stoneware from what I can see.

Nerd

[Min]

Delayed crazing can happen days, weeks or months after removing a pot from the kiln. Over time I would expect further crazing on those pieces, the long lines joined together by finer horizontal ones. Just because the clay and glaze mature at the same temperature / cone doesn't mean the glaze will fit the claybody.  Need to test each claybody with each glaze you are using. Since you are using earthenware it's important the glaze doesn't craze as the claybody will still be porous and by having a crazed glaze it will allow more moisture to be absorbed by the porous body and exacerbate the crazing. If you have some ink rub it onto the pots then rinse it off, craze lines will be easier to see.

[Cenknst]

Glazenerd,

thanks for the reply. It's earthenware / a plain white local clay. These are vases waiting on the shelves, therefore they had no touch with liquid. 

As there are more than one type of glazes involved(clear and that matte pink),  and the glazes are by Carl Jager (not personally formulated), I believe there's a technical mistake I'm doing.

[Cenknst]

Thanks Min. I also think these cracks will be worse, they're decorative vases - not mugs or serveware luckily. But still they look defective so I can only keep them to myself..

The results were always better with the ready liquid glazes - I guess I have to move on with them rather than these powdered glazes, looks risky.

[neilestrick]

How big a batch are you making at a time? If you're not making the entire batch of dry material, are you mixing the big batch of dry before taking any out? Do you run the wet glaze through a sieve when you mix it? How are you applying the glaze?

[Cenknst]

18 minutes ago, neilestrick said:

How big a batch are you making at a time? If you're not making the entire batch of dry material, are you mixing the big batch of dry before taking any out? Do you run the wet glaze through a sieve when you mix it? How are you applying the glaze?

I don't do big batches, I even do one per a certain glaze sometimes. Only with clear one I glazed like 5-6 pieces. I applied some of these by dipping and some with brush (I later learned that this is not the correct way for dry ones but it used to work)

and this just in, I had glazed a batch of ornaments in another firing - with totally different pack of earthenware, and this time with ready liquid clear glaze (local turkish brand that people are commonly satisfied with and use for food-safe products as they dont include hazardous chemicals) and now I noticed the same type of cracks... I brushed the glaze on these, but not quite like brushing but more like spreading the glaze as one coat (as they were small pieces)

I really can't see why.

[liambesaw]

It's a glaze fit issue. Not something you can adjust with water or mixing differently.  It's an incompatibility between the rate of thermal expansion between the glaze and the clay.  Contact the glaze manufacturer and ask which specific clay that these glaze will fit.

[Cenknst]

16 minutes ago, liambesaw said:

It's a glaze fit issue. Not something you can adjust with water or mixing differently.  It's an incompatibility between the rate of thermal expansion between the glaze and the clay.  Contact the glaze manufacturer and ask which specific clay that these glaze will fit.

We're talking about at least 2 different earthenware clays ( the same type but bought at different times) and 4 different glazes - liquid and dry... From the cases I've seen from such misfits the glaze was crazing severely. These are not like them.

[liambesaw]

18 minutes ago, Cenknst said:

We're talking about at least 2 different earthenware clays ( the same type but bought at different times) and 4 different glazes - liquid and dry... From the cases I've seen from such misfits the glaze was crazing severely. These are not like them.

But the cause of crazing is always the same.  White earthenware is generally a talc body, which are low expansion.  A low expansion body with a "normal" expansion glaze will end either with delayed or immediate crazing in most situations.

[PeterH]

24 minutes ago, liambesaw said:

But the cause of crazing is always the same.  White earthenware is generally a talc body, which are low expansion.  A low expansion body with a "normal" expansion glaze will end either with delayed or immediate crazing in most situations.

No disagreement, but from the 3rd image posted the cracks seem parallel rather than in the usual hierarchical structure.

I'm not sure, but is there an unglazed section of the body (near the thumb) with cracks in it? Detail of 3rd posted image...

[Min]

@PeterH, in Japan there are names for the different patterns of crazing. I believe the fine crazing in your black and white example would be along the lines of a "fish roe" pattern whereas the wider spaced (less severe) crazing can be of the "crab claw" pattern, over time the long vertical cracks can fill in with finer horizontal cracks giving the crab claw effect. Greater the misfit between clay and glaze the finer the crazing pattern therefore widely spaced crazing patterns have a better chance of having the glaze altered to remove crazing than glazes with a fine craze pattern.

[Cenknst]

I came up with another point today after making eliminations of cause of crazing, could it be because the bisque  was fired as low as cone 09-010 and later fired to cone 05 with the glaze? If the bisque was cone 05 too maybe this wouldn't happen as it would not condense any further?

[Min]

5 hours ago, Cenknst said:

...could it be because the bisque  was fired as low as cone 09-010 and later fired to cone 05 with the glaze?

No. You could have glazed the raw clay with no bisque firing at all and the glaze shouldn't craze if the glaze fits the clay you are using. If the glaze is underfired that can cause crazing. As already mentioned above, earthenware and porous claybodies can absorb moisture (atmospheric and from use of pots), when this happens the moisture can cause and exacerbate crazing. 

After reaching peak temperature the glaze and clay both will start cooling and as they do so if the glaze wants to contract more than the clay you will get crazing. Crazing can be visible as you are unloading the kiln or in the days, weeks or months later. An oversimplified way of looking at this is to think of glaze fit as a pair of pants going on a person, if I wear a size 8 and try and put on my daughters size 2 pants they aren't going to fit, they will tear (craze). Jeans (glaze) is too small to fit. Opposite of this is called shivering, if my daughter puts on my jeans they are going to be too big, they won't fit her and will fall off, in ceramics terms this is called shivering. 

 

[Hulk]

shivering

[Cenknst]

22 hours ago, Min said:

No. You could have glazed the raw clay with no bisque firing at all and the glaze shouldn't craze if the glaze fits the clay you are using. If the glaze is underfired that can cause crazing. As already mentioned above, earthenware and porous claybodies can absorb moisture (atmospheric and from use of pots), when this happens the moisture can cause and exacerbate crazing. 

After reaching peak temperature the glaze and clay both will start cooling and as they do so if the glaze wants to contract more than the clay you will get crazing. Crazing can be visible as you are unloading the kiln or in the days, weeks or months later. An oversimplified way of looking at this is to think of glaze fit as a pair of pants going on a person, if I wear a size 8 and try and put on my daughters size 2 pants they aren't going to fit, they will tear (craze). Jeans (glaze) is too small to fit. Opposite of this is called shivering, if my daughter puts on my jeans they are going to be too big, they won't fit her and will fall off, in ceramics terms this is called shivering. 

 

Thank you for your time and patience

Still, I'm not convinced how these commonly used clays and glazes do not fit, as these are frequently bought ones on the ceramic market, and the cracks emerged on the works across different bags of clay, at least 3 different brands of ready clear glaze and some dry glazes. 

As far as I'm concerned, not all glazes are compatible for applying on greenware btw, except for the specialties like Mayco Stroke and Coat. I mean you may try and succeed with some others, but not all of them.

I've just read about the points on this site and I'm going to try eliminating these; though it looks like covering every possible thing that could happen in ceramics:

http://www.countryloveceramics.com/articles/crazing-delayed-potter-bisque-ceramics-causes/ 

 

[Hulk]

Hi Cenkst!

In good light, with magnification, are you seeing any other/more cracks?

In your "clear glaze with underglaze" pic, looks like a fine network of lil' crazes there, which don't jump out like the big lines.

A few cracks should indicate that the fit is close, whilst a fine network would indicate a bad/worse fit.

I've bought two made up glaze powders - a clear and a blue; the clear crazes on several of the clays from the same vendor! I'm well down the mixing glazes myself road...

Here's another article on crazin'

https://digitalfire.com/glossary/crazing

[Min]

59 minutes ago, Cenknst said:

As far as I'm concerned, not all glazes are compatible for applying on greenware btw, except for the specialties like Mayco Stroke and Coat. I mean you may try and succeed with some others, but not all of them.

Agreed, I wasn't trying to imply otherwise, sorry if there was ambiguity there. What is the maturity temperature/cone of the clay you are using? If it's higher than the ^05 you are glazing to then a higher glaze or bisque firing would be worth trying. If the recommended cone for the glaze is ^05 but for the clay it's ^04 or hotter then you have a couple choices to try; first see if your glaze can take a hotter firing (use a scrap test pot)or secondly try bisque firing a test piece to the recommended cone for the clay maturity then see if it's still porous enough to accept the glaze well.

[ronfire]

I had the same issues when I switched my cone 6 clear glaze. Have a look at the attached thread  , I only had this on my large sinks and was fine on smaller items.

 

 

[liambesaw]

@ronfireyour problem was the clay breaking, his problem is crazing 

[Bill Kielb]

 

9 hours ago, Cenknst said:

I've just read about the points on this site and I'm going to try eliminating these; though it looks like covering every possible thing that could happen in ceramics:

As far as I'm concerned, not all glazes are compatible for applying on greenware btw, except for the specialties like Mayco Stroke and Coat. I mean you may try and succeed with some others, but not all of them.

The clear over white appears very thick, maybe just the picture but has what appear to be pinholes as well so I am not sure this is a great combination to begin with. I mention this because differences in the COE will be affected by thick applications of glaze, so maybe this goes in your list of trials. BTW, stroke and coat is a lowfire glaze basically that survives to cone 10 I believe. So a nice glaze but not a super mystery actually there are several out there.

Glazes that heal well, that is to say their fired surface tension allows them to recombine and heal generally perform better in once fire situations than those that don’t. Testing is always necessary as well as a once fire schedule that resembles the speed of a bisque fire. The operative word - generally.