Transparent glaze crazing over cone 6 porcelain

[Danielle Caron]

Hello all,

I have been using the Tucker's Bright White porcelain (cone 6) for several months now, nice to throw etc. Love it, but when it comes to glazes, that's another matter.

I love to paint using underglazes covered with transparent glaze, but so far, after many dozens of trials, using 2 different commercial transparent glazes and trying 5 other recipes suggested for use on porcelain (on digitalfire, etc.), I inevitably end up with crazing. Either straight out from the kiln or after a few uses.  It must be a question of thermal expansion, but when I get to that topic, it seems I could spend months or years searching for the solution. A little overwhelming for a beginner like me.

So far, I have fired at cone 6, the "slow glaze" option on the Bartlett controller of my new ConeArt kiln and before that at a community studio which, I believe, was firing at a similar standard schedule (crazing also there).

Does anyone has an idea of how to fix it ? My pieces are standard thickness (beakers, plates, bowls).  Thanks in advance.

[Roberta12]

It's tricky to find a clear/transparent glaze that will match your porcelain.  But certainly not impossible. There are people on this forum who will be able to analyze the clear you have been using and suggest some changes for you.  I have one clear that I have used for years on most of my clay.  It is from the Mastering Cone 6 book.  But when I tried a couple of different clays, it crazed!  A LOT!!  I know from experience you have to find one that will work for your clay (I use Laguna #16 for porcelain)   Let us see what the experts say!  I will get the recipe from MC6 and send it to you.  But it may not work for your clay.

Roberta

[Min]

@Danielle Caron, have you tried contacting Tuckers and asking what they recommend? I believe Ron Roy is a clay consultant there so I would be surprised if they didn't have a glaze recommendation. Also, post the recipes, or the names of them, here so we can see what you've tried so far. One other thing, if it's a cone 6 porcelain you are firing all the way to cone 6 and not underfiring?

Welcome to the forum 

[Danielle Caron]

Thank you  Min .

Yes, I have sent an e-mail to Tuckers and I will try calling them on Monday.

Here are the glazes I tried so far :
Spectrum 1100 transparent , G2926B (DigitalFire - bubbles that translate into pale blue dots), G2934Y from DigitalFire (changes the underglaze colors), G3806C from DigitalFire, WCAC  Clear Celadon (don't know the CTE eitheir) from Glazy, Easy E Clear from John Britt, High Calcium semi-matte from John Britt, PSH True Colors clear glaze, the one that produces the best underglaze colors, but so far not clear at all if it crazes, my first trials were not documented enough, but if I can avoid a commercial expensive glaze, I would.

From what I understand so far, it would seem that the porcelain has a much higher CTE than all those glazes?  Or lower?This is what I get from all the readings I did since my first post earlier. But CTEs are rarely indicated with a recipe.... and it seems that clay suppliers are not often willing to talk about the CTE of their products..

 

[neilestrick]

I would try the Digitalfire G1214W. I've also attached another recipe that works very well on Standard 365 cone 6 porcelain, and may be close for your clay body.

 

G1214W.pdf Neil's Perfect Clear.pdf

[Danielle Caron]

@neilestrick
Thanks!
Going to try them right away.  

D

[Min]

8 minutes ago, Danielle Caron said:

From what I understand so far, it would seem that the porcelain has a much higher CTE than all those glazes?  Or lower?This is what I get from all the readings I did since my first post earlier. But CTEs are rarely indicated with a recipe.... and it seems that clay suppliers are not often willing to talk about the CTE of their products..

There are different points of view on posted CTE rates for both clay and glazes. The CTE of a claybody is not a static number, where on the firing curve it is measured can alter the number. This is why some clay manufacturers don't include it. With glazes it also gets a bit convoluted. CTE works best to compare  base recipes with the same ingredients. Add or remove one of the ingredients in the base and a different CTE might fit the body (or not). Where it is really useful is to give you a ballpark idea of if you are looking at a relatively high or low CTE glaze and then how to adjust it. You can rule out a lot of glaze recipes just by looking at them. If they are high in say nepheline syenite, custer or soda feldspar or frit 3110 they are going to be high expansion glazes and likely to craze on a low expansion body. If you are seeing a fair bit of spodumene or frit 3249 then chances are it's a lower expansion glaze.

There can be a huge difference in porcelains in regards to what fits it. For example Laguna's Frost Porcelain has a really high CTE of 6.99 (this is going to be +/- a bit depending on firing but gives an idea of being a high CTE). On the other hand some porcelains, like the one you are using from Tuckers and those from Plainsman, have much lower expansion. 

Which underglazes are you using that are not working? Some stains (which will be what is used in the underglazes) need specific requirements to work. Examples would be any green underglazes that contain a chrome stain (most do) need to have a zinc free glaze. Chrome tin pink/red/purple stains need to have a high calcium glaze and so forth. 

G2926B crazed on my porcelain too, G1215U from digitalfire might work, I have a version of it without the 3249 frit if you want to try it. With the frit will melt slightly better but the one without works well too. There is room to lower to expansion further if necessary. For a satin matte if you use digitalfires ^6 magnesium matte and blend it with whichever clear you land up using it will make a matte that you can fine tune the amount of gloss with.

 

[Danielle Caron]

@ Min. Yes, the dark green stains get on the grey side. I didn't realize the size of the challenge when I started working with porcelain and underglaze.

I will definitely try the G1215U, and would like to try your version without 3249 frit as well, however I am not sure that I am experienced enough to adjust all the other quantities to fit 100%.  Will do tests with df magnesium matte and blend it with a clear. That is interesting as well.

D

[Min]

This is my version of ^6 G1215U with a lower expansion and 0 frits. It's going to be thick in the bucket from all the epk plus gerstley borate. Just try a 100 grams of it and see if it crazes. You can always use the original recipe with the frit 3249 but it's expensive. Gerstley Borate can be swapped out with a frit if you do find this fits so it's not as thick a glaze in the bucket. I'ld just try a small test batch of this to see if it fits first.

[liambesaw]

Spodumene and talc, nice low expansion minerals

[Danielle Caron]

@Min.

Thanks again for your great generosity and your quick and detailed replies. This is so great! Will try it all and post good results.

Danielle

 

[Roberta12]

2 hours ago, Min said:

This is my version of ^6 G1215U with a lower expansion and 0 frits. It's going to be thick in the bucket from all the epk plus gerstley borate. Just try a 100 grams of it and see if it crazes. You can always use the original recipe with the frit 3249 but it's expensive. Gerstley Borate can be swapped out with a frit if you do find this fits so it's not as thick a glaze in the bucket. I'ld just try a small test batch of this to see if it fits first.

That is the other clear I use.  So far no problems.  With my clays.  Porcelain, stoneware, buff and dark.  Got that from Min!  I gave the recipe to my friend and she loves it with her clay bodies, mostly Continental clay, stoneware.  We call it Madeleine's Clear.  That way we know where it came from.  I use a lot of underglaze as well.  It is friendly with UG.  

Roberta

[Danielle Caron]

Ah... didn't see "spodumene" as the last ingredient. Highly toxic. I prefer to avoid using it. Unfortunate.

[Min]

19 minutes ago, Danielle Caron said:

Ah... didn't see "spodumene" as the last ingredient. Highly toxic. I prefer to avoid using it. Unfortunate.

Spodumene does contain lithia but not in sufficient amount to be an issue. This glaze contains 12.16% spodumene, the lithia content in spodumene is 8% therefore the actual lithia in the glaze will be 0.9728%.

It is definitely not one of the materials I would classify as "highly toxic".

[Danielle Caron]

Ok. That's good to know. I was told the 'story' of spodumene in Pyrex by a potter and why they stopped using it, and I must say that it was at the least worrysome. 

Thanks for the precision.

D