Raw Glaze Problem Solving

[Margie Cleveland]

Last month I did my first single firing.  I was surprised how easy it was to glaze greenware, and had a great firing.  Didn't lose one piece.  Now working on my second load and 20% of the pots developed cracks after glazing.  Greenware was completely dry.  Pieces that I dipped did not crack, only some of the pieces that I sprayed did.  Waited for glaze to dry before applying another coat, but some still cracked.  Any suggestions for solving this problem would be greatly appreciated.

[George Lewter]

OK, I think we need more information.  When did the cracks appear? - before or during the firing?  It sounds like between glaze applications but we need to be sure. Where are the cracks and how are they oriented?  What is different about your second batch - different clay or glazes?  Thickness of application?  More or less water in the glazes? If the cracks are easily visible, pictures might give us a clue.

Raw glazing puts a lot of stress on the clay as it expands as it absorbs water from the glaze.  It seems very odd that the problem appears for you when spraying rather than dipping.  I've had issues with bloating when I was dipping inside and outside in one application. Pouring the interiors and spraying the exteriors solved that problem for me

[Margie Cleveland]

Some cracks appeared soon after glazing.  Others I noticed the following day (although they could have happened soon after glazing, too).  All but one (1.5" hairline diagonal crack on glaze from lid rim inwards)  are easily visible, started on the glaze and by the next day went straight through to the inside of the pot.  On one lid, the crack was about an inch from the center and followed the curve of the lid, almost a full circle.  Small colanders had a fairly straight horizontal crack (again following the curve of the pot) about an inch from the bottom with several vertical crack going up to punched holes from the main crack. 

I used the same clay (Standard 181), but different glazes.  Specific gravity of glaze used on these pots: 68 and 72 (both cracked).  On first firing, 68 and 69.5 (no cracking).

Thickness was the same--4 separate sprays on each pot, waiting for glaze to dry before next spray.  I glaze interiors on day, and exteriors no sooner than the next day.

Maybe I should do 4 continuous sprays without letting the glaze dry between coats.  I'll test one and see what happens.

[Margie Cleveland]

Correction regarding specific gravity:  I have a 50cc syringe, so compare 50cc of water to 50cc of glaze instead of 100.  So the true specific gravity of my glazes range from 136 to 142.

Margie Cleveland said:

Some cracks appeared soon after glazing.  Others I noticed the following day (although they could have happened soon after glazing, too).  All but one (1.5" hairline diagonal crack on glaze from lid rim inwards)  are easily visible, started on the glaze and by the next day went straight through to the inside of the pot.  On one lid, the crack was about an inch from the center and followed the curve of the lid, almost a full circle.  Small colanders had a fairly straight horizontal crack (again following the curve of the pot) about an inch from the bottom with several vertical crack going up to punched holes from the main crack. 

I used the same clay (Standard 181), but different glazes.  Specific gravity of glaze used on these pots: 68 and 72 (both cracked).  On first firing, 68 and 69.5 (no cracking).

Thickness was the same--4 separate sprays on each pot, waiting for glaze to dry before next spray.  I glaze interiors on day, and exteriors no sooner than the next day.

Maybe I should do 4 continuous sprays without letting the glaze dry between coats.  I'll test one and see what happens.

[George Lewter]

I certainly don't have a sure-fire answer for your problem, but I concur with your decision to try spraying your glazes with less time between coats as one possibility, as the repeated shock of water on bone dry ware may be too much. My latest technique is to place pots in front of a fan for just half an hour or so between coats to evaporate some of the water I just sprayed on.

If the problem comes up with one shape in particular (like lids with flanges projecting out at angles), then test with that shape. Again if it is just one shape giving you problems, you might try drying with a heat gun between coats to drive off water more quickly, rather than letting it accumulate in the body. Another possibility would be to glaze before your pots are totally bone dry, like as soon as they reach uniform color (wet/dry color difference fades away).  A final suggestion if all else fails, is to try a different clay body.  I know the Laguna #80 brown stoneware that I use is less sensitive to the water of raw glazing than is my other main clay, Laguna B-Mix 5 white stoneware.   

[Thomas]

I would wipe my pots off just to give them a little water before spraying. I had this problem, and it worked on over half the peices i had a problem with,  but I just bisque them now.

[Margie Cleveland]

Thank you George and Lewis for your suggestions.  Cracks appeared on lids and small colanders, so I now have 5 application tests to try on them.  Won't fire again until January, but will let you know how it turns out.

Thomas said:

I would wipe my pots off just to give them a little water before spraying. I had this problem, and it worked on over half the peices i had a problem with,  but I just bisque them now.

[George Lewter]

We haven't heard back from you on your raw glaze cracking problem.  Did you resolve it?

[Margie Cleveland]

I don't know why I got the cracks on that firing, and I don't know why I've only had 2 pots crack since.  Part of the trials and tribulations of pottery, I guess.

[John Lowes]

I have recently attended a Steven Hill glazing workshop.  On the first day he threw pots. On the morning of the second day he force dried the pots he would glaze.  My observation was that he hit 250 deg for 3-4 hours and dried them out in an electric kiln.

When he glazed one of his signature melon pitchers, he poured the inside and took it to the first glazing station.  He took a few minutes to talk about spraying and started right in spraying a SCM base, then went right to his secondary colors, ash and accent glazes, and finished with Jen's Juicy Fruit misting over the whole.  There was no interruption of the glazing process at all except for the few minutes explaining the glazing equipment, shooting up or down, across rims and the like.

He then glazed a stingray bowl/platter, starting first upside down on a banding wheel to spray base, then seccondary, ash and accents before turning it over and going back station to station to spray the top side.

Given he has been doing this a long time, and not everyone has 11 color stations to walk through for spraying each pot, but still, no time waitng on anything to dry between "coats" tells me that it may be unnecessary to dry any, and it may even be detrimental.

[Margie Cleveland]

Thank you, John, for sharing this information.  I've been spraying or pouring the liner glaze one day and the exterior the next day.  When spraying, I use 3-4 coats, waiting for the glaze to dry in between coats, which is time consuming.  I'm going to start throwing tomorrow.  When I glaze, I'll spray 3-4 continuous coats.  Will let you know how the firing turns out.

[John Lowes]

One other thing, Steven has switched from Laguna B-mix to Standard 257 porcelain with his throwing body. I meant to ask him if he was concerned about firing a Cone 8-10 body at Cone 6, but never got to ask.  With an absorption at Cone 8 of 1% firing at Cone 6 may not be a concern, as far as absorption is concerned, but testing would answer that question.  None of his display Cone 6 pots appeared to have glaze fit issues.