Sudden shivering issue - same glaze as usual different batch of slip

[kel-kay]

Good day, I am suddenly getting shivering in earth-ware mugs ::: My scenario is I coned fired 11 mugs, 5 mugs from the per-made liquid slip I purchased from my supplier in December21, the other 6 mugs were from the per-made liquid slip I purchased at the beginning of May 2022.  I use Mayco's Stroke and Coat glaze which is pretty versatile and can tolerate a lot, never had any other issue with it other then color moving on highly textured bodies (best fix was add another coat of glaze).  I even fired the clay to cone 05 bisquewith it and it still has never defected. I called the supplier to let them know this suddenly became an issue and asked how to fix it.  They suggested to rack up the firing temp from cone 03 (1950) to 2000 using the ramp hold and re-fire any pieces not already glazed.  Then glaze one of those pieces and test again for shivering.  I am doing that now but I have doubts that will fix the issue since  again my knowledge that Stroke and Coat on an 05 fired bisque previously did not shiver.  They said they test all their slip mixing using a clear coat, which is also a Mayco brand also just not Stroke and Coats Colors (obviously), they fire at 1950-2000 degrees with this new slip they changed to due to the talc shortage.  Doing my own online research nothing ever states firing at a high temp of the clay body will fix shivering it is based on silica and other ingredients of the clay and/or slip, correct?

Since I have over 30 gallons of slip to use, would adding an X amount of felspar to it provide correction in the clay expansion? Or does the felspar need to be mixed in with the initial creation of the slip to its liquid form?

Edited May 27, 2022 by kel-kay

[Min]

Ask your supplier how the slip recipe has changed due to talc replacement. Since we don't have the slip recipe this is going to be difficult to spitball a fix for.

[Hulk]

background/info/definitions and related links/articles Glaze Shivering (digitalfire.com)

Edited May 27, 2022 by Hulk

[kel-kay]

I gave Evan's Ceramics in Kansas a call.  He let me know it basically is an error in the slip recipe of the batch I received and not a kiln or firing issue especially so with Mayco's Wonderglaze since it it extremely forgiving - so much so it be painted on greenware and then fired.  I called him to see if one can add feldsar to already mixed slip and that was a no, so I have 30 gallons of slip I have to use for none glazed works of ceramic art   Since my main product line is mugs it will be awhile, like 2 years, to use 30 gallons yet at least I got a straight answer about this issue.

[Jeff Longtin]

Hello Kel-Kay,

Do you have a gram scale? I would weigh out 100 grams of slip and set it aside to dry. After it has completely dried, I would weigh it again. Then determine how much powder makes up each 100 gram batch. Then you can conduct simple small tests.  

In my own situation I determined that the water is about 50 grams per 150 grams slip. (In other words: 50 grams water to 100 grams powder.) I use this simple ratio when I add mason stains to my premade porcelain slip. (I further determined that 150 grams slip equals 3 oz liquid. )

Recently I tried adding 3% Nep Sye to my clay to see if it would reduce glaze crazing. I measured out 9 oz slip and added 9 grams Nep Sye.  It didn't achieve the desired effect so I tried firing the clay to cone 8. That worked much better so I chose to go that route instead.