Expert: Leisha Hiester - 2/2/2010

Question
QUESTION: Hello..   we made recently some experiments for drawing and painting white glazed sinks and toilets using overglaze coloring techniques then refire them for fixing the colors .. some trials gave beautiful results, while others failed due to cracks(breakages) of the products after being refired..
Are these breakages due to probable relative fast cooling ? noting that we do not open the klin before cooling down below 100 degrees..  or that is because the glaze does not have the same expansion factor as the mud beneath ?  and how can we overcome that ?
thank you in advance..

ANSWER: Dear Raouf,
Can you re-send the photo? It didn't come through for some reason. That's unusual. I got the box, but no picture.
thanks. Leisha

---------- FOLLOW-UP ----------

QUESTION: with pleasure..

Answer
Raouf,
For some reason I cannot open your photo. I'll try without seeing them. Yes, rapid cooling can cause cracking, also glaze incompatibility. It sounds like the original product, the sinks %26 toilets may have come from different firings, different suppliers maybe %26 possibly not completely vitrified can cause this too.
I would first ensure you're firing properly and cooling properly.
Is there any consistency between the ones that cracked and the ones that didn't? i.e., all the toilets versus the sinks or all one firing and another not. Any pattern you can identify?
All from one supplier? All from same batch of clay?
I would love to see what you're doing, but cannot see the photos. Never had this happen before. What type of photo are you using?

Leisha