Expert: Leisha Hiester - 4/29/2010

Question
I am making Celtic goblets for my son's wedding for the groomsmen. I fire them to 04 and when I glaze them with Amaco ST23 stone texture to 05. When finished 2 out of 3 cracked.  What am I doing wrong? They are fine when bisque fired.  Could I have poured them too thin?  Please help he gets married the 15th of May and I need 8.

Answer
Jim,

Isn't that the way it always goes? When you're under pressure and you want to do it so right...I've done lots of wedding favors over the years and now I just plan for at least a 10% over run.

Are you sure that glaze doesn't call for cone 06? I recall using that stone look once and having trouble with it too, but I think mine was Mayco. Try a test at Cone 06.

If it's a goblet I would pour it a little thicker, but if you've got your slip right you should be o.k. Check your poured pieces for uniformity. A goblet is a hard piece (depending on the mold) to get uniform. You can test by cutting a piece in half at leather hard stage to see the thickness all over.

Did you check viscosity? Your flow rate should be under 30 seconds. A viscometer and a hydrometer are invaluable tools for pouring. A hydrometer helps test the gravity or weight of the slip.

The other thing to check for is that your bisque is maturing. If you have an electric kiln use some cones on each shelf to test. Sometimes an element is going out. Is there any consistency to where the goblets were in the kiln to the cracking?

There's never an easy answer in ceramics. My mantra is test, test, test and then test, test, test. A process of elimination is the best way to determine causes.

Hope this is helpful. Please write again. Hope you make the wedding and Congratulations!
Leisha