Crazing (food safety in glaze)

#pottery #crazing #foodsafe
Hello potters, how are you ? today we're going to talk about one facet of food safety when it comes to your ceramic artwork and functional wear. were taking about crazing in pottery and how it affects the food safety of your glaze, some of the misinformation behind it, and how to prevent it. near the end of this video I will give you a reassurance that it's not as bad as it seems along with a explanation of what causes crazing and how to fix or prevent it.
it took me quite some time to understand what crazing or craze ware is and how it related to the strength and safety of my work for my customers. although it's a far easier explanation than it seems from a chemistry level. it mostly has to do with thermal expansion and how that works together with your clay body and glaze.
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[@AL-og4sp]

Matching the COE for the clay and glaze is THE core issue. You can alter either the clay or the glaze to make the two match. Having several clay bodies on-hand to fit a glazes seems like more trouble than adjusting the glazes to fit a single clay body. Just the cleaning to the extruder, canvas, pug machine etc. to switch between clays bodies would drive me nuts. But, 'to each their own'. Adding silica is definitely one method of adjusting the COE of a glaze, as silica is a low expansion component of glaze. Every glaze has silica, and many glazes will function with additional silica added. In addition to lowering the COE silica can increase the gloss (bad for matte/stain glazes) and will also raise the temperature needed to melt the glaze. Do a line test of the glaze, a 2%, 4%, 6%, 8%, 10%. silica to the glaze, see what happens. Crazing can happen for a long time after the firing. Quickly heating and cooling the test pieces will accelerate the process. Tony Hansens website is a great source if you wish to look into calculating glazes, which is what is needed to have control of the glaze chemistry.

[@EarthNationCeramics]

I love this comment because it answers to me the questions I could not address myself although I do have a question of my own. If I want no crazing is the job to the coe # to match my clay ? 1-1 ratio?

[@AL-og4sp]

​@@EarthNationCeramics I have not found a glaze clay software that outputs numbers that match up exactly with the clay COE from the manufacturers data. My work around is to use a base glaze that I know does not craze on a given body & I use that glaze's COE as the number I am trying to calculate subsequent glazes at. For example: Laguna's cone five bee mix is listed at a COE of 5.74, my well traveled clear base is calculated at COE of 6.45. The glaze does not craze & hasn't on pottery that I've used for many years. The calculations are more of a theoretical ballpark figure than perfectly accurate numbers due to the chemical complexity of glaze materials and the nominal numbers imputed for the glaze materials. PS thanks for your approachable way of discussing pottery, it hit's better than many the stuffy pottery videos on KZbin.

[@EarthNationCeramics]

@@AL-og4sp I think that's my channel's main approach. It's to take things that seem complex and whittle them down. Although I do know there are definitely experts in the field who are displeased with me giving a very" tip of the iceberg" explanation of something that is seemingly complex I also don't want ppl to be too intimidated by a very fixable thing like this. As for the c.o.e. yeah I agree with you. In the video I had noted that I don't know the numbers that would be the correct ratio for the coe or the glaze vs the clay to say "if these numbers match up it will not craze" so I usually take a base glaze and match it up as well. But I will say generally speaking I assume most of my viewers don't know how to make their own glaze or even read a still chart so instead of going to the Gambit of testing which requires you to make your own glaze in the first place I generally just tell them to change their clay.

[@shuvalassaf]

@@EarthNationCeramics yes, and it's relatively easy task to match COE, and you can use glaze calculator to calculate COE, and simulate changes in glaze...

[@naplin]

@@shuvalassaf Wrong. Your glaze calculator cannot calculate the COE ACCURATELY (thats the key here) before you mix and fire a batch. The COE in the glaze in going to depend heavily on how much of the silica in the glaze actually melted.

[@dangersparoxysm]

You always post just what I need right when I need it!

[@EarthNationCeramics]

If you have time please click the link sources down below. Filled with info I couldn't fit in the video.

[@jenntepper7588]

Are you starting a series on glaze flaws? Because that would be amazing!! I’ve been really struggling with pinholes and craters using commercial glazes and would love some troubleshooting tips

[@EarthNationCeramics]

I'm tryin but it takes alot of reading so I don't mess up or give misinformation. We might do shrinkage next.

[@erincinkant4357]

This was SO helpful, thank you!! I'd love to hear your thoughts on pinholes/pits one day!

[@msb4511]

As always .... full of helpful advice and entertaining. Thanks!

[@marisastocker1566]

Thanks allot for this video, I just had this question after my last kiln opening. Now I know for sure to change my clay.

[@martapatino7346]

I love your channel. Greetings forma Colombia. I am a beginner and all you teach is really important. Thanks

[@Webpixum]

Thanks for all the info by the way.

[@juliestrater439]

Excellent video. I had to stop the video before the end, and look to see if my glazes listed the COE...LOL! Drives me crazy when I see potters selling functional crazed pots at art shows, too.

[@glenntroyer]

Great informative video!

[@kdcthelioness]

*looks at pot I've been using for food* *pot has crazy crazing* *looks back to this video* I gotta get rid of that thing, don't I. Thanks for the info! I've been doing pottery for quite a while and never knew this, so I'm very glad to have learned!

[@darrenjackson5]

IT'S A 600 DEGREE BOX OF FIRE!!! 🤣🤣

[@danielastanova6538]

Thank you for this video. So usefull for me!

[@riccardopoetz7100]

Another easy way to fix this crazing issue is to work on your glaze COE to try matching it with your clay body. there are two types of crazing, first one looks like the one shown at 12:11 and is due to the COE of the glaze being higher than the one of the clay body, wich means that it is going, while cooling down, to retract faster. This one can be fixed by dereasing the percentage of alkali or increasing the percentage of silica. Alkali have a high COE and silica quite a low one, ther fore those modifications help decrasing de glaze COE. (btw, alkali and silica are not the only materials that can help, there are tons of them, each having others effects, for exemple, needing higher tempeerature, ading color the the glaze, making it more ore less liquid while firing etc...) For the other crazing, it is basically the opposite. The craks are formed on circles and they are not perpendicular to the clay body. Those are du to the COE of the glaze being to low, andf the clay shrinking to fast while cooling (not the same shrinking as while drying or firing, this shrinking is due to the COE) To fix it, do the opposite than before and try removing some silica or adding materials that increase the COE of the glaze! I hope my english isnt so bad that you guys and girls can't read me and understand my explenations!

[@kylerlunberg9694]

the statement that "Crazing is not food safe" should be made on a case by case basis, and the decision may vary for certain scenarios. A lot of atmospheric (wood, soda, salt) work has crazing and is completely food safe. If the clay is fully vitrified the crazing is not a concern typically. Some celadons and shinos in cone 10 craze often and are also food safe. In my personal experience the only time I've had food safety issues with crazing is in cone 6 and low fire work. I have done quite a few durability tests and haven't found that durability is compromised. I would guess the butter leaked into the actual clay body because it was not vitrified. But I will take a look at your links and see if there is something I am missing.

[@kandaceanneservello7323]

You are the best!

[@JennaDanelle]

So helpful, thank you!

[@wenfernandez9733]

will you make a series on how to properly use/apply glazes and how to use a kiln? I'm very hesitant about purchasing one because I don't want to lit my new studio on fire😩 Your videos are EXTREMELY helpful! Thanks so much

[@jourdanhamme3426]

This is an awesome video. Pottery science is cool :)

[@raymondbrowncentennialhs2501]

Old potter's tales.....figuring out what is real and what is an old potter tales feels like have of my learning.

[@kelseylakia9578]

Omg!!! 😂 Enjoyed the first 12 min of the video, but hilarious to find the solution to be change your claybody. 🙈 I couldn't stick around to find out if you want into COE. 😶

[@dinaeltorkycr6235]

Great video i use egyptian earthware that really do that crazing thing and really pissed me unlike the Italian clay that has no crazing , but i thought what if i raised temp bisq firing of earthware and then added talc in trans glaze would that work ?? Or just replace the clay body ?

[@saltydogfarm]

top stuff!

[@lasrubieras]

Hey Donte! Thanks for the info. Much appreciated. Here's my question: you talked about the coe for the claybody, what about the glaze's? I couldn't find that info,say, for Mayco's Celadon Bloom. Is there a special place in the internet heart of hearts where that information lives?

[@bom417]

Hi there, I have two questions. The info on my clay says the following: Coefficient of thermal expansion (CTE): 20 °C - 400 °C 63 20° C - 500 °C 65 20 °C - 600 °C 77 CTE * 10-7/K So what is the thermal expansion number? How do I find out the thermal expansion of my glaze?

[@knowingthenetherlands662]

You are one craze-y person!

[@EarthNationCeramics]

You on time out

[@micheledickey4066]

I was really hoping you were going to tell us how to fix an already crazed pot! Ugh. I had a very full kiln load of my first cone 6 glazed pottery and it overrides to about a cone 8!

[@Plantfoodandshelter]

I recently over glazed a crazed pot with a low-fire clear fired at 05 and it came out great! All crazed areas filled in!

[@user-vf4jb5zf6n]

Thank youuuuuuu thank youuuuuu. THANK YOUUUUUUUUUU

[@AnthonyEynard]

I have crazing with light flux over winterwood on bmix 5. Light flux doesn’t craze over any of my other glazes on the same clay body. Can you provide some insight when the issue has to do with glaze combos and not clay body?

[@Aaron.101]

Thanks

[@AR96289]

Question ~ I have found crazing in only a couple of my pots and only after they have run in the dishwasher a few times like a week or two after it came out of the kiln. It doesn’t happen to all my pots though, is it just a fluke?

[@patsmith4940]

A second potential medical hazard with crazed ceramics is that if the clay contains any toxic metals, such as lead, the toxic metal can leach out into your food, according to the MD I once worked for. Good sanitary practices do not resolve this problem, and the leaching can be sufficiently slow over the years that the affected individual may not make the connection.

[@effiebriest1278]

If you open your kiln too early it just makes the crazing visible very "early". Otherwise it would get visible over time, cause the more often things expand and contract, cracks will widen even on a microscopic level. And some claybodies are not suitable for every kind of use, like using a pasta plate for baking. Changing your claybody is an excellent idea, so easy, but still not the one potters seem to go to, including me. I was tought to kind of try and change the galze - thats close to impossible for me. It takes a lot of knowlege, costly materials and time. So thanks for making my life more simple.😘

[@jourdinreinhart6455]

When I asked laguna what I needed to do about their forest green crazing, they specifically told me to use it on stoneware and not porcelain. So since then, it's definitely change the clay.

[@EarthNationCeramics]

Yup. That's my general answer. Even if you're someone like me who knows how to calculate thermal expansion in your glazes when you make them it's still more work to test that glaze on multiple clay bodies than it is to just buy another clay. Multiple tests and firing versus just buying another bag. I could change the chemistry by hand but.... It's just easier for most people to buy a different clay.

[@wiwiic]

Hai! I had this whole batch that was glazed and after the last fire whenever its wet it creates what looks like inside humidity spots. is this considered crazing? I've been researching and i can't find anything about it :(

[@jadesines5343]

I used low fire clay & used low fire Mayco Lapis Lagoon-no grazing until days later….could you give me some advice? Why didn’t it craze right away when they came out?

[@dylanpottery]

Do you have a way to find COE of glaze?

[@jameswills2205]

Several of my first peices crazed and my teacher said it was likely because the glaze was applied too thickly. All the peices that crazed were the same glaze (a clear earthenware gloss) on a terracotta clay body I don't know if any other students work crazed but all the glazes were provided by the teacher

[@LindaKing-lf8nk]

OK got some new plates from Christmas. And when I went to wash them four of the plates had this dark gray discoloration after being put in water. That lightened after the plate dried. And three of the plates had crazing in them. And these are new plates. I would assume all the ones with crazing should go back because they’re not food safe but the ones that had the gray discoloration I would assume the glaze was Porous and should not use them??

[@rebeccaszeto5708]

Is there a way to coat an already glazed.fired non safe ceramic to "seal in the toxins" to use it for food?

[@karri8998]

Will it craze right away or will you end up getting it back from customers later on?

[@JoeMama-sx4qz]

I made a tea pot but it has crazing going on, can i still use it for tea? I do want to mention i will be sterilizing it once a week with bleach if used consistantly

[@patsmith4940]

Are "scuff marks" from knives and other utensils the same as crazing, and do "scuff marks" create potential medical hazards? Is there a way to glaze the pottery in such a way that "scuff marks" will not occur when the pottery is used to eat from?

[@DempseydeLange]

Sometimes I find a pot only crazes a week or two later before I have even used it. Usually only Transparent glaze seems to craze. ???

[@fang2111]

Could you do pinholes too? Haha.

[@robertogodoy4882]

Mayco makes a food safe cone 6 Matte crackle glaze, I was pretty much wondering how their crackle glaze is different from crazing? Hard to say if your pieces are crazing if the glaze is suppose to crackle? And how could it be good safe with cracks?!

[@EarthNationCeramics]

I honestly don't know that because from my point of view if it's actual crazing it would not be food safe. Although their company might have figured out a recipe that mimics the aesthetic of cracks in a glaze without it being in the glaze matrix or down to the clay body

[@daniellecaron3025]

Purchase my cone 6 porcelain from a manufacturer/store here in Canada that also sells a transparent glaze that, as it says on the label "is especially made for their cone 6 porcelain". I ruined many-many pieces of hand decorated pieces before realizing that their glaze COE didn't fit their clay. The crazing happened after a few weeks of use. And on top of that, they do not tell you their clay's COE. The least info the better. I had to figure it out with time and $$$$. I have yet to find out a glaze recipe that would do well (without spodumene). Other glaze manufacturers have been more generous in their explanations, but commercial glaze is costly. Upsetting.

[@sphaerashaney]

🙏

[@Zoomfreaky]

New to the whole pottery business. Thank you for sharing this. I had a thought. Is you have crazing and you refire your piece with a clear glaze that matches the clay body ?expension? rate...is it then possible to close/seal the crazing and make your piece food safe again? You will have to fire 3 times instead of 2. But maybe if you like the effect? Is that a possibility? Thanks again...and I now will go and binge watch your other video's...oh no. Down the rabbit hole I go...:)

[@firedupceramica5464]

Actually, I've seen my crazing go away on plates that have shown crazing after I re-fired them. I don't know the logic there, but it has worked for me a couple of times.

[@Zoomfreaky]

@@firedupceramica5464 Thank you...:) It is worth a try to refire.

[@nickohare7187]

What about glazes specifically made for crazing that you stain after glazing with india ink? Are they food safe?

[@EarthNationCeramics]

Technically no. Like raku glazes called "crackle glazes" it's still crazing.

[@jayantasharma128]

Why my stoneware glazed craze after fire

[@WhaleTank]

I always seem to see crazing in low fire mugs are they are used for tea or coffee. Is that just because the coffee or tea colors the crazes?

[@shuvalassaf]

Low fire glaze is more prone to crazing due to the lower percentage of silica to flux ratio, and its hard to successfully making them food safe. Discoloration does make the cracks more visible.

[@EarthNationCeramics]

As a side note especially with colored glazes and clay it's more difficult to see the crazing. So a lot of people assume that using a different glaze works simply because they do not see the craze but realistically if you took a microscope it's still crazed.

[@shelleydoxtdator753]

I make my own glaze also but I have this one glaze that peels away from my pot. I dip my pieces and a few minutes later it peels away from the piece. Do you know why? Hopefully 🙏😕

[@EarthNationCeramics]

To thick. Add little water and shake well .

[@shelleydoxtdator753]

@@EarthNationCeramics I’ve done that but I will try again, thanks 😊

[@WhaleTank]

Are crackle glazes different than crazing?

[@____Ann____]

No, just a different word.

[@scarletwatson5108]

Write a book……PLEASE! I promise if you write a trilogy about pottery I won’t be the only one who will buy it. Self publish it! Make as much as you can off your teaching. You are good! And I hate to say it, yes I’m an old lady but it wouldn’t hurt to make a DVD because there are some studios out there including mine that I work at that have old TVs with DVD players attached to them and nothing to watch but Disney movies. Nothing against Disney but I want my students to be immersed at all times hearing and seeing things Pottery and because as a non-profit we still don’t have a smart TV that is what many of us need you to do;)

[@Webpixum]

Is there a difference with crackle glaze and crazing?

[@EarthNationCeramics]

Theoretically the differences that crazing weekends the pottery and touches the clay body itself as for a crackle glaze is just the glaze itself. I've had this question before and I would have to ask the company but to my knowledge that is the difference.

[@scarletwatson5108]

The companies don’t consider the crackle glazed safe as pretty as they are.

[@SydneyTravellersGuide]

megaman!!

[@deannak1841]

Wouldn't it be easier n cheaper to change your glaze?

[@EarthNationCeramics]

Well, not really. Glaze (assuming your working with the same cone every time) doesn't usually change expansion rate all that drastically, unlike clay. Perhaps from cone 06-6, yes, but most of us stay one cone. In order for that to work you would have to be the one making your own glaze and even then you would have to be using drastically different glazes possibly at different cones which is extremely uncommon in one kiln. Tldr: changing your clay body will change the thermal expansion drastically as for changing your glaze usually does not within a certain cone or subtype of glaze. Especially that of commercial glaze which usually have the same base with different colorants so it's honestly just the same glaze over and over again with different pigments so the thermal expansion won't change.

[@JohnSmith-fw2et]

I disagree, as a porcelain worker the supply house has one porcelain rated for cone 6. There is no other porcelain to choose from. What would be better is for the supply houses to offer solutions to crazing, they have the equipment and materials to solve the issue of crazing.

[@EarthNationCeramics]

There are other solutions but I find telling students to use a different clay body is the easiest fix. I would love to hear about the solutions you talk about though.

[@cmonjose]

this guy can't stop repeating himself, video could have been 4 mins

[@Angeliqueceramique]

You don’t have to watch

[@EarthNationCeramics]

Side note, repeating information also helps knowledge stick.

[@cmonjose]

@@EarthNationCeramicsok good point

[@igotbit9454]

You’re so sarcastic it’s annoying

[@EarthNationCeramics]

Who?