Crazing question

[Bam2015]

Morning All!

I know that crazing has been covered in a number of threads, but not sure the questions I am about to ask have been addressed. I have been using the same clear glaze with success for several kiln loads on both Laguna #15 cone 6 porcelain and Laguna #65 stoneware clay with success (no crazing), but this last load crazed, both the porcelain and stoneware. I did not mix the glaze, I purchased it from a pottery studio where I used to be a member, they mixed up a batch for me. 

I'm wondering if the following can help to reduce the risk of crazing:

1) Thin the glaze a little with water? I don't have any glaze making chemicals to add to fix this problem nor do I own a hydrometer, but I could buy one. 

2) Does glazing the inside of the pot and not glazing the outside reduce risk of glazing?

3) Does bisque temp/cone make a difference? 

I think I had one more thought, but it's left my brain. Thank you in advance.

To those forum members in Ian's path I hope you're ok.

I just edited my post because I first said that I don't want to mix my own clear glaze, but if that's the answer then why not? 

Betty

Edited September 30, 2022 by Bam2015

[Min]

Did you have cones in both the successful firings and the latest one with crazing? For the successful firings are you just eyeballing the glazes and looking for crazing or did you stress test them to check for delayed crazing?

1 - Nope, this will just delay the problem, it isn't a proper fix.

2 - Nope.

3 - Yes. That is why I asked if you had cones in the firings. Underfiring a glaze can cause it to craze.

[Dick White]

Further to Min's answers to your stated questions -

1. Don't buy a hydrometer. Contrary to popular wisdom, hydrometers are useless. Do a forum search for some of the other threads where we have discussed using a 10 or 100ml syringe and a digital scale to accurately measure specific gravity.

2. Not glazing the interior of a pot increases the risk that the pot itself will fail due to unequal tension/compression of only one side of the ceramic being glazed.

3. What Min said.

[Bam2015]

Min & Dick, thank you for your responses. Yes,Orton  witness cones on each shelf. The cones indicated proper firing.

Dick, I just measured the SG using the method you indicated. I read a number of articles against using a hydrometer. The SG  was a bit high, so I added water. I'll see if that helped. 

Oh well, what a bummer! Was hoping for an easy fix and maybe thinning it will help.

I guess I should I just start mixing my own glaze(s)? I just want a clear glaze, I use a lot of underglazes and mason stains.  

Just when you think you finally have things figure out.  

[Callie Beller Diesel]

If the glaze was working previously on all of these clay bodies, are any of them subject to the recent industry wide changes in talc supply? The new substitutes are playing havoc with current recipes and causing COE changes.

edited to add: many plastic, white firing clays at cone 6 use talc, and it’s a fairly common glaze ingredient that was up until recently used to control crazing. 

[Bam2015]

Callie, I pulled out some pieces that I made a few months ago, same clay bodies & same clear glaze. I now see crazing on those as well. I guess it just didn't happen initially.

I guess it's time for me to take another glazing workshop and start making my own glazes with testing, testing and more testing. 

Thank you for the input. 

[Min]

How bad is the crazing? Could you post a picture? Also, would the studio share the recipe with you? If it’s just slight crazing adding silica plus some kaolin can sometimes eliminate crazing if it isn’t too severe and there is room in the recipe for more of those two ingredients while maintaining a good melt.

[Bam2015]

Min, I am going to ask them for the recipe and if they provide it, I'll share. I doubt it's a recipe that they developed on their own.

Although I pulled out some pieces that I previously fired which crazed, I also have several pieces from when I first bought the glaze that did not craze. So maybe it just needed to be thinned? I'll do some testing now that I have thinned it a bit. 

[Min]

1 hour ago, Bam2015 said:

I also have several pieces from when I first bought the glaze that did not craze.

At the risk of repeating myself it is a good idea to stress test new glazes to see if you will get delayed crazing. I would strongly suggest doing this. Thinning the glaze is just going to delay the crazing.

A couple ways to do a stress test for crazing, the more gentle way is a boiling water to ice water test. More rigorous method is heat test tiles to 300F then plunge into ice water. 

 

[Bam2015]

Thank you Min. I will heed your advice and appreciate you taking the time to thoughtfully respond to my post.

I asked the studio if they would share their recipe and if they do, I'll post it, otherwise I will start from scratch with some of John Britt's recipes. 

Betty

[Pres]

@Bam2015 when you start your glaze search you may find M^6 Clear liner glaze of interest. I am using this with the addition of zircopax, tin and rutile for a pale buttery white. remove the rutile for a pure white.

 

best,

Pres

[Bam2015]

Thank you for the suggestion Pres. I will let you know if I try that recipe. Right now I am waiting for the recipe for the clear that is crazing. I'll post it on Mon when I get it. 

Betty

[Bam2015]

Hi All,

I said I would share the clear glaze recipe when I got it. Here it is from the studio who mixed it for me. 

EP Kaolin                 20.00

Ferro Frit 3134    20.00

G-200 Feldspar    20.00

Silica                            19.00

Wollastonite           15.00

Talc                                   6.00

It's also on this website. 

https://glazy.org/recipes/6798

Any thoughts on how to adjust this recipe to reduce crazing is appreciated. 

Betty

[Min]

How severe is the crazing you have? This recipe is probably just too high an expansion to remedy with a silica addition if you have more than just slight crazing. Closer the craze lines are together the more sever the crazing is. Silica and alumina levels are good as they are so probably couldn't add much more without affecting the melt. 

I had a quick look at the claybodies you are using, while expansion figures for claybodies are not very accurate they can get you in the ballpark as to which glazes might fit without crazing. Laguna posts the coefficient of expansion (COE) of the WC616 porcelain as 5.35 and the #65 stoneware as 4.8  So in theory you would be looking at glazes on the lower end of expansion (COE) figures. (the recipe you linked has an expansion of 6.8) Look for ones with a COE of around 6.2 or lower if you try mixing your own.

If you do start glaze testing have a look at the clear glaze recipes in Britt's cone 6 book. If you want to try a clear commercial glaze I would suggest trying Laguna's MS29, it's very clear and seems to fit low expansion clays quite well.

BTW G-200 Feldspar is no longer available.

[Bam2015]

Min, thank you for your thoughtful response, great info to have. I am going to start mixing & testing my own clears and have Britt's book, so I will start there. I am just starting to understand the chemistry behind this. Over the past year I have spent my time focusing on throwing skills. I see that I need to expand my knowledge and learn about glaze fits. 

Betty

[Kelly in AK]

Narrowing things down to a glaze that fits is a beautiful thing. After many years of making pots without concern for crazing (as long as the clay was vitreous-dead horse, won’t beat further), I’ve never appreciated a properly fitting glaze more.

It is absolutely delightful to flick the rim of a bowl several months after it’s fired and have it ring like a bell.

Worth the effort.