Another Pinging Question

[cracked pot]

I know this has been covered many times before but this is a new wrinkle for me.  Glazes and clay have been used many times before with no problems.  This time I made two casserole dishes which were thrown on bottomless  the wheel, stretched oval then had a bottom added with a coil for strength on the  inside and smoothed.

 

Now pieces are pinging and I can see crazing if I look closely around the coil added area.  Can the extra thickness and smoothing cause this?

Thanks.

 

[High Bridge Pottery]

Has any of your older stuff with this glaze/clay shown up crazing? I would think the thickness could speed it up but not be the cause of the crazing. Is the glaze thicker than usual as they are bigger items?

[Tim T]

Yes, as Joel says, a thicker glaze is more prone to crazing. Also, I think a faster rate of cooling will encourage crazing.

[Guest JBaymore]

If a clay and body combo is very close to a fit (well matched COEs) but not actually "there", almost any variation in things can cause the existing mismatch to start to show up.  Stuff like thicker glaze layer, faster cooling, slightly different finish firing profile, poor oxidation of organics in bisque firing, variations in wall thickness profile, and so on.

 

If this is the case, it possibly also means that the glaze will exhibit delayed crazing in use over time.... even if you are not seeing it when you unload the kiln and send work off to its intended destinations.

 

Make a test cup.  When the glazed work comes out of the kiln, paint the surface with india ink.  Let it sit for a couple minutes, and then wipe it off the surface well.  Look for crazing lines getting stained in.  Take the same test piece, and put it in your freezer.  Leave it there for a few hours.  Immediately fill it with hot tap water .  Repeat the india ink test.  Repeat this sequence a number of times.  That will tell you a lot about the actual "fit" of the glaze to the body.

 

best,

 

..................john

[cracked pot]

Never had any crazing before with this glaze(Falls Creek Shino) but may have put it on thicker as I wanted it to look more white than brown.  Other bowls in load did not have this problem.  The crazing is very fine and can only be seen in very direct sunlight and then, only if you are looking for it and only at the join.  One of the casserole dishes sold almost out of my hands from the kiln.  Think I will keep the other to monitor it. 

Pieces still ring true when tapped so don't think there is any compromise of the clay.  Thanks for your responses.

[curt]

Could be that your construction method has introduced structural stresses into the pot which are manifesting themselves in the glaze in certain 'weak' areas - say around joins - as you have observed. I have seen this kind of localised, fine glaze cracking on wood fired pots where the heat work produced by the flame path (as evidenced by ash deposits on the pot) appeared to set up uneven stresses in an otherwise normal pot.

 

If this is the case the pots will likely be weaker than usual and more prone to sudden breakage such as shattering or breaking, even after a relatively minor knock.

[cracked pot]

Curt,

Thank you for your reply.  This sounds like a likely cause.  The crazing is only around the joined area.  Guess I will keep these baking dishes for myself and observe. 

Thanks