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Hi,

I have made large female busts with Standard Ceramics sculpture clay that I have made into paper clay.  It fires at ^2-^6 and the paper is burned out at the bisque stage.  Because it is heavily grogged and I need a smooth finish for my glaze decoration, I rib it with a toothed rib when leatherhard to pull out the surface grog.  I then sieve the same clay slip to remove grog and paint over the scored surface to create a smooth surface for glazing.  I also brush on a layer of Amaco white underglaze to give a brighter background surface for the glazes.  After bisque firing, I wipe off any loose dust etc. on the surface and then paint elaborate designs with Mayco's Stroke and Coat glazes, 3 coats.  I then refire the piece only to ^06.  As I open the kiln (at 100 degrees F, or less) I can hear the glaze pinging like crazy and on inspection can watch long cracks forming where the glaze is trying to fit the clay.  On the curved areas, small sections of glaze lift up and I can see that much of the  glaze surface is starting to shiver off the piece.  I am exceedingly frustrated, can anyone give solutions? 

Penelope front, email version.jpg

Penelope, back, email version.jpg

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Crazing from a too great a glaze vs clay coefficient of expansion issue like Bill said, I would fire some test tiles hotter and see if that helps, if not then you will need to change either the claybody or the glazes to ones that "fit" together without crazing.  Since you are using commercial glazes this is going to be a trial and error process.

I'm wondering if the shivering is due to a poor bond between the slip you brushed on  and the piece. How dry was the piece when you brushed on the slip? Fairly thin slip or ? Also, when the glaze lifts off is it taking the underglaze with it? It's uncommon to have both crazing and shivering with the same glazes / clay.

Going forward I really strongly suggest doing some small test tiles / scrap pieces to test with. Include some corner angles and edges with the slip & underglaze and glaze as shivering often occurs in those areas. Tap the edges / corners with something like a knife handle, if there is a loose bond it should shiver off then if it's going to.

 

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thank you for taking time to assess my problem.  I usually make test tiles but had had so much success with stroke and coats on smaller projects, I just forgot to do it on this clay body.  I did have another large piece  with the same issue that I refired to cone 6 and the stroke and coats tried to crawl off the piece.  I also wondered about the adherence of the slip on the piece.  Or maybe the underglaze primer.   I applied them when the sculpture was firm leatherhard and should have probably applied them when the clay was more moist?  I think some of the slip also lost adherence to the sculpture.....taking the stroke and coats with it.  I have a show coming up next fall and need to figure this out so I can make the next 6 'ladies'.  I've done 3 successfully so far using mostly underglazes for the decoration on a mixed stoneware body.  (I mix my scrap clays together to make paperclay)  Curved test pieces are currently drying to further test glazes on the sculpture clay.  I just thought I could save a firing using stroke and coats, as I apply the underglazes on a bisqued piece, then bisque-fire the underglazes and then glaze fire the whole piece.

 

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15 hours ago, Ann E. V. said:

I did have another large piece  with the same issue that I refired to cone 6 and the stroke and coats tried to crawl off the piece.

This sounds like an application issue as Stroke and Coat can go to cone 6. Perhaps dusty or oily bisque or thick a glaze layer or coats not dry between layers.

15 hours ago, Ann E. V. said:

I applied them when the sculpture was firm leatherhard and should have probably applied them when the clay was more moist? 

Yes, get the slip on there as soon as you possibly can. To reduce the water content in the slip you can add a tiny amount of darvan to an overly thick slip to "thin" the slip to brushing consistency. You might want to consider adding zircopax to your slip to skip the white underglaze. Again, test tiles are your friends.

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