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claybandit

transparent glaze crazing over underglaze

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Hi, I make ceramic jewellery, using white earthenware and I'm having issues with crazing.

I use three coats of underglaze (Contem and Amaco) on my pieces to get an opaque finish. I fire to 1000 or 1040, then I glaze to 1040 using a brush-on Transparent glaze (Duncan Envision IN1001)

The crazing appears in the days after the glaze firing and ruins the piece because it is already small and precise. The crazing isn't only of the underglazed areas, it's occasionally on the clean areas too.

This is happening with about 30% of my pieces which is far too high for me.  Also, as I sell online and to shops, I don't want my customers' orders developing crazing after they've bought a perfect piece.

You can see the type of things I make from my profile pic and my homepage banner.

I have tried not opening the kiln until it is under 100, putting the glaze on thinner, and different temperature combinations, but to no avail.

Could it be the glaze? Or is it due to the complicated surface of my pieces causing the delayed stress?

Thank you.

 

 

Edited by claybandit
additional info

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How long has this crazing problem been occurring. Have you been producing pieces on a regular basis before with a greater % of success? If so, what have you changed in the materials or the process?

johnnyK

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Hi Johnny. I've just started making again after a fifteen year break.

All new kiln, clay, glazes etc. I didn't use underglazes like this before, and I've only been making for about two months.

All my current firings have a little bit of delayed crazing.  Driving me crazy too!

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It's not the underglaze at all. 

Crazing happens because the glaze shrinks as it cools at a  different rate than the clay body it's applied to, wether it happens straight out of the kiln or later on. It's related to the chemistry of the two, and not to the kiln cooling rate. If some of your pieces are crazing now, all of them will eventually. 

You need to try a new clear, and you'll have to do a bit of testing to find one that fits the clay you're using. Make a few little pinch pots and glaze them each with your new selections as you would your jewellery, and fire using your usual cycle.  Once they're cool, discard the ones that craze right away. To make sure that the ones that look ok at this point won't develop delayed crazing, take the survivors and do a thermal shock test on them: put the cups in the freezer for 24 hours, then put them in the sink and pour boiling water on them. If they're going to craze at all, this will make it show. 

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This might not be helpful, but I was recently having this problem too with work glazed with exactly the same methods you're using, except I discovered the hired kiln I was using was firing my ^6 clay to ^06. Two suggestions; the pieces from that fiasco that have remained more stable than others were small and chunky. I applied the most idiot-proof underglazes to bisqued pieces and directly applied the clear glaze over the raw underglazes, and did the glaze firing in one step.  Also, ironically some experimental pieces with underglazes over greenware also appear stable, but I didn't try putting clear glaze over those (yet), they are bright but have a 'velvet' look. I believe those were Mayco stroke n' coat over the greenware. 

Since I've had good results firing this kind of work to ^5-6 previously, my plan is to redo all this work eventually at a higher temperature. I can say for a fact that cone 6 clay, Duncan concepts underglazes turn out beautifully at cone 5-6 and don't need a clear glaze, they develop their own glossy finish without having to use it.  I've also tried some Mayco, some Fireshades but they can turn out velvety or eggshell, etc. I'm not sure it would work for your specific style of jewelry as they tend to get a bit more "character" around the edges at that temperature and tend to move/run depending on the color used, but if fired flat that is minimal, also some slight color changes as the clay body tends to interact with the glaze, but this didn't happen with ^6 porcelain or any other ^6 white I tried. Probably your materials just aren't playing nice together for some reason, but I very very rarely had any crazing at cone 5-6 and the work I made at that time came out beautifully. 

Edited by yappystudent

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It can be tricky getting a lowfire clear that doesn't craze. Do you have the ability to fire to cone 6 and is there any way you can get a cone 6 clay, one that matures at close to 1220C? It's much easier to get a well fitting glaze at cone 6 than at lowfire. I know that a lot of claybodies in the UK have ridiculous wide firing ranges but if you can get one like this and fire to maturity, cone 6, approx 1220C it would be the way I would suggest going. I would also ask your supplier for a glaze that fits the clay without crazing, they should be able to point you in the right direction.

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Thank you all for some interesting and good advice. Unfortunately I can't fire these designs to a high temperature successfully. I have tried, but my kiln, although brand new, struggles to get there (it's supposed to go to 1300) but anyway, the designs don't suit the high temp.

I do have to use a transparent glaze over the underglazes, even if they vitrify because of their usage. Being against the skin and being handled a lot, they pick up dirt and can get oily marks on them. Also a glaze makes them smoother and more tactile.

I am buying a couple of different new brush-on transparents tomorrow and will do some testing on them.

I have to say that I find this site so helpful.

When I was in business years ago, the internet just wasn't there at all, so help was hard to find.

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On 4/28/2018 at 4:06 AM, claybandit said:

 

I use three coats of underglaze (Contem and Amaco) on my pieces to get an opaque finish. I fire to 1000 or 1040, then I glaze to 1040 using a brush-on Transparent glaze (Duncan Envision IN1001)

Thought:

I was wondering if you'd tested putting the clear coat directly over the underglaze before it's fired. That is to say: clay bisque-fired, then apply underglaze, let dry (but don't fire it) then apply clear coat, glaze fire them together at the same time.

I managed to do my jewelry work this way, applying both the underglaze and clear with soft brushes quite well without disturbing the underglaze as long as I was careful.  

 

 

Edited by yappystudent

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On 4/30/2018 at 10:30 PM, Rae Reich said:

You didn't say - are you bisque firing your pieces before underglaze? 

Hi Rae, I'm applying the underglaze to leather-hard, then a bisque firing.

Applying the underglaze after bisque doesn't work with my designs as I scrape back some of the surface to reveal raised lines, and need absolute precision.

I'm trying a new firing today, 1150 bisque, then 1040 glaze to see if it makes a difference

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On 5/1/2018 at 7:57 AM, yappystudent said:

 

I have actually tried this. I used a spray gun so as not to disturb the surface, and fired once to the glaze maturity (1040).

Sadly the crazing was still there. All that effort .... grrrrr.

I've just opened a bisque firing at 1150 and they're looking good (see photo) Now for the glaze.

Maybe maturing the clay at a higher temperature will help. I've tried this before but the surface went squiffy. I didn't write down what temperature I fired to previously, but it looks like1150 isn't too high.

1150 bisque.jpg

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"Maybe maturing the clay at a higher temperature will help. I've tried this before but the surface went squiffy. I didn't write down what temperature I fired to previously, but it looks like1150 isn't too high."

 

Now's the time to test different clear glazes.  (A small test kiln would be nice, huh?) I don't think that the same glaze is going to fit any better - crazing is because the glaze has shrunk more than the clay, if your clay has already shrunk its max then the same glaze will surely shrink some more. 

Edited by Rae Reich

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Hi Rae, I'm trying out a new glaze right now.

I have been firing bisque to 1000 or 1040 previously.  I don't think that at that temperature the clay had shrunk to its max, so I'm now trying 1150 bisque which I hope will mean that that the clay body has matured enough. When I said in my previous reply to you that the surface went 'squiffy' (nice technical language) when I fired a higher bisque, I meant the bisque surface. I didn't bother glazing them as they were ruined.

I've just finished firing some work to 1150 bisque (see photo in previous answer) , and I've used my old glaze on half, and the rest with a different new transparent glaze, firing right now to 1040.

We shall see. Cross your fingers for me!

And yes, I'd love a little test kiln.

Andrea

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@claybandit, I would make some test pieces, don't need the underglazes on them, just slabs of clay about 10 cm square and bisque and glaze them. If any appear to be craze free then great, test those for crazing as Callie suggested or if you want to really stress test them put them in the oven at 150C for 20 minutes then plunge them into cool water. Repeat this 3 times then look for crazing. If your test tiles survive this then terrific, if not you could try fixing the glaze from the sample with the crazing lines the furthest apart. (the closer the crazing lines are the greater the mismatch of glaze and clay fit) Sometimes adding a little silica can stop crazing, but only if the crazing isn't too severe. Since you don't have the recipes for the glazes it would take some testing, adding small amounts of silica and repeating the above test. Seems a shame to run tests with your underglazed pieces.

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Thanks for the advice Min.

Unfortunately I'm putting up a display in an artist's open house starting this weekend as part of a local festival (Brighton Festival UK), so I'm pushed for time to get some new work done.

I know I shouldn't test glazes on actual work, but I'm so hard pressed for time at the moment that I'm taking risks where otherwise I wouldn't.

I'm hopeful I'll have something OK to display/sell.

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