dunting

[Mudfish]

I have been using a white slip and have recently noticed some dunting going on, especially along edges of the surface of my wares. I use a white stoneware, and fire to cone 8 - 9 reduction.  My question is, are some glazes better over slips than others?  would a lower expansion glaze dunt more over slip? . Are there certain glazes that are more adaptable over slips with less issues?

[Callie Beller Diesel]

Dunting is something that happens to a piece of pottery where the clay cracks all the way through, mostly because of heating and cooling stresses. Sometimes the stresses start when the piece is formed, sometimes it’s because you go through quartz inversion too quickly, especially if your clay has been fired multiple times and has formed a bunch of crystobalite. Usually that last one is an issue for lower fire clays.

If I understand you correctly, you’re using a white slip on a white stoneware. If you’re applying the slip to the piece while it’s on the dry side of leather hard, you could be re-wetting the clay, causing adsorption cracks that don’t show up until the piece is fired. If your work is too thin, or if your slip isn’t deflocculated (more water), this can exacerbate the problem. 

If you have an image of the dunting cracks, that will help us pinpoint what’s going on. The shape of the cracks will tell you a lot about how it happened.

[neilestrick]

Post some pictures of the problem so we can better define what's happening.

[Mudfish]

https://m.facebook.com/story.php?story_fbid=10224709182830336&id=1634468103  had to post to my Facebook and send that way, otherwise the file was too big. The little shards are what chipped off from the piece

[Mark C.]

Your  link did not work-downsize your photo to a smaller size and post it here or fix link so we can see.

[Mudfish]

[Mudfish]

Another attempt here to post a photo. I can sure see it, and it looks huge so I've probably done something wrong. 

[Min]

Shivering.
Looks like it’s including the glaze and slip through to the claybody. Which claybody? Only with this glaze? Slip recipe or claybody used for the slip? Does the glaze shiver on the pots without slip?

Shivering is dangerous, slivers of glaze coming off rims could get into food/beverage. Also, shivering doesn’t always happen straightaway, I wouldn’t use or sell pots with this clay/slip/glaze combination even if they aren’t showing shivering now.. 

[Mudfish]

the glaze is a high clay content glaze.   

Amount
Ball Clay	40.00
Potash Feldspar	30.00
Dolomite	20.00
Ferro Frit 3134	10.00
Total base recipe	100.00
Tin Oxide	5.00
Bentonite

 

 

slip formula:         I used EPK for the grolleg and OM4 for the sagger                                                                                                                                                                                                                                                                                                                                                                                                                                                               

 

Amount
Grolleg Kaolin	33.33
XX Sagger Ball Clay	33.33
Neph Sye A270	20.00
Tin Oxide

 yes, it only happened on pots with this slip/glaze combination

 

[Mudfish]

Also, the claybody is a white stoneware with about 10% grog

[Mudfish]

Sorry everyone, for using the wrong terminology and thanks so much for your help !

[Min]

Your slip recipe adds to 86.66, are you missing something, maybe silica?

[Babs]

You didn't by chance touch up with slip o n an already too dry join?

This can cause slip and so glaze to break away from rest during glaze firing

[Mudfish]

so far, the shivering has only happened at the attachment area but  actually, this piece I used as a test to see how the slip behaves on bisqueware.

[Min]

From your image it looks like the slip and some of the body has come off. Is this accurate? Slip on bisque ware needs to shrink less than slip applied to leatherhard clay. You want to get slips onto the pot as soon as possible when the pot is as damp as possible.  Engobes contain less plastic ingredients than slips. Calcining some of the clay will reduce the shrinkage of the slip/engobe to prevent the slip/engobe shelling off the pot. Adding some borax or frit helps it bond too. 
 

The terms slip and engobe have different interpretations so this can add some confusion.

[Babs]

To clarify.

You are joinIng pieces at leather hard stage with a slip?

You are applying slip to entire pot at Leatherhard stage?.

If first option, use a thIck slip thinned with vinegar when oth pieces just strong enough, or a deflocculated slip. If applying slip to entire body, spray entire piece with water then apply a deflocculated slip to lessen cracking.

Slip on bisque...another kettle of fish....