Tea Pots Cracking Violently

[neilestrick]

It could be that the spout and/or handle attachments provide a weak spot for the cracking to begin, or that they create additional stresses to the form that make them easier to crack.

[Min]

I think you need to post your liner glaze recipe. SCM, the Hill recipe, has a COE of 9.52, if your other outside glazes have high COE’s also, admittedly probably not as high as that one, and your liner is a low COE you are going to have problems, which will be compounded by having a thick glaze layer on a thin pot.  

 

I would stop using these glazes until you have done some fit tests if you haven't already done so. Thinly thrown cylinders glazed thickly with your liner glaze alone, liner glaze inside plus your regular glazes outside, your outside glazes on both inside and outside, and also try with a higher COE liner glaze, again to about the same thickness as on the teapots. Frozen overnight then boiling water poured into them.

 

Just because you have other pots that haven't cracked it doesn't mean they won't since this kind of cracking can show up months later. (fingers crossed I’m wrong)

[Ben]

The simplest test would be to use the liner glaze inside and out on one pot and the outside glazes on both inside and out on another pot.

In addition you can test the relative coe of the clay by using a series of glazes with varying coes. Studio potter had an article on this topic. I think it was by Jim Robinson.

 

edit: Here it is. 

 

http://studiopotter.org/pdfs/spBB_01a2_original.pdf

 

These should work for you. 

 

This same technique is outlined in Mastering Cone Six Glazes as well.

[hitchmss]

Sorry it has taken me a couple of weeks to get back with more information. I have calculated the COE's for the glazes used on the teapots and will list below;

Inside Liner Glaze which is poured in- COE 6.21

Outside Glazes- Strontium Crystal Magic which is dipped- COE 9.73

Oribe which is sprayed- COE 6.66

Tenmoku which is sprayed- COE 7.98

Ash glaze which is sprayed (50% Washed ash, 50% redart)- COE 6.19

   The glaze with the highest COE is the SCM, and it has the thickest coat of glaze on the exterior(biggest culprit). The three sprayed glazes (oribe, tenmoku, and ash) are light coats (1-2 passes each with gun) and not over the entire surface, but more in "bands".

     I have yet had a chance to do any other tests involving glaze fit. I will make some spherical forms to test application of different combinations of glazes both inside and out and will freeze and fill with boiling h20 and will provide results. I will be making and glazing one sphere the same as above teapots to see if the handle/spout are providing the straw which breaks the ol' camel(curious because I have a number of speherical forms very similar in construction as teapots, with same glazes, from different clay, that have not had any issues for almost a year and counting).

   Thank you for any help thus far and any more to come!