Crazing

[Annamarsh]

Hi everyone, 

I have an enquiry about Crazing. Below are two pictures of my students work which have a spiderweb crackle on the surface. They were each glazed firstly with an Amaco underglaze (3 layers as suggested) and a Potclays brush-on transparent glaze (firing temp 1020-1040 and again 3 layers). 

I have read this could be caused by a few different variables. I was wondering if anyone had any knowledge in regards to why this may happen with underglaze and transparent glaze?  The dipping glazes that I used did were even and did not have crazing. 

Could be that too many layers of underglaze and glaze are applied (6 in total), when I applied the transparent glaze I used a hair dryer to dry the glaze in between layers would it make a difference for the transparent glaze to dry naturally? Or is it more likely to be an issue with how the kiln fires the glaze. 

Thanks everyone for your help!

Anna 

[Bill Kielb]

Generally crazing happens when the glaze does not fit the claybody meaning the coefficients of expansion of clay and glaze are different enough,  crazing occurs. Crazing can occur immediately or be delayed by days, weeks, months etc..... so in your case this has occurred because the glaze shrinks faster than the body.

Some things that affect the coefficient of expansion would be: have the items been fired to maturity - meaning is the clay fully vitrified?  It’s hard to know this answer without knowing what cone these were fired to as clay does not respond simply to a peak temperature but rather heatwork which means accumulated time and temperature. Sort of like baking a cake at 175c for thirty minutes. Without the time part (30 minutes), your cake may not be done. Cones are used in ceramics to verify the work done in the firing. We’re these fired with witness cones present? If not it’s sort of like that cake, I fired it to 175c, but maybe for only ten minutes, not sure my timer was not set. So is it done? Maybe, maybe not. With cake we can try it and know, clay not so much. The moral of the story, ceramics requires witness cones to confirm.

So if the clay was not fired to maturity, it may not have the expansion characteristics that it would fully fired to maturity so it’s hard to rule out firing entirely.

Generally crazing occurs because the clay and glaze do not fit, you do have a thick layer of underglaze though which contains its own clay and has a coefficient of expansion of its own. So if this clear normally works with this clay when it’s natural, then the heavy underglaze likely is influencing this fit.

finally slight mismatches in fit are nearly always present and thick layers of glaze will accentuate the difference so a glaze that may have taken months or more to craze on a certain clay might reveal itself immediately when applied very thickly.

Most often the claybody and glaze are a bad fit for each other. I wish I had a simpler explanation. You likely will need to find a clear glaze more compatible. That said applying things too thickly, often not good really.

One thing comes to mind, are all the products: Clay, underglaze, and glaze, rated for the same cone?

[Min]

6 hours ago, Annamarsh said:

They were each glazed firstly with an Amaco underglaze (3 layers as suggested) and a Potclays brush-on transparent glaze (firing temp 1020-1040 and again 3 layers). 

.... The dipping glazes that I used did were even and did not have crazing. 

So you used two different glazes? Sounds like the brush on glaze doesn't fit that claybody and the dipping one does. I notice you are in London, may I ask which claybody you are using? Same clay for both glazes? Reason I ask is it seems very common in England to use wide firing range claybodies which can also be problematic if they aren't fired to their peak temperature/cone.

Welcome to the forum.

3 hours ago, Bill Kielb said:

are all the products: Clay, underglaze, and glaze, rated for the same cone?

I haven't found underglaze needs to be for a specific cone. All the brands I use cover a wide firing range for most of the colours.