Cracks when double-bisquing large work ?

[nuna]

I usually paint underglaze on greenware so that the glaze application does not affect the painting. Occasionally, I paint on bisqueware for logistical reasons, eg the work is too fragile to risk a lot of handling; in these cases, I send the piece back through the bisque with no issues. At least, I DID... until I had a disaster where a large (2' long), thick piece got huge cracks in the second bisque firing. This second bisque was done at a different studio, and I don't know what schedule they used or how it was fired, so I'm not sure if anything strange happened.  Needless to say, I am now very wary about double-bisquing very large pieces. 

I am currently working a large round tabletop (made with a 1/4" thick, 2' diameter slab, and a 1-1/2" rim and a grid underneath).  I bisqued it facedown and am now painting the top with a lot of intricate details. I don't want to risk glazing it without "setting" the underglaze first before I pour or brush the glaze on, but I'm scared! Especially because  it will have to be fired right-side-up now due to the underglaze (I've noticed some fluxier colors will stick to the kiln shelf).

What would be the safest way to bisque this again? Options I've considered:

• Bisque super low at cone 020 or so. In this case, would it be safer to fire it fast or slow? 
• A friend swears by a custom schedule that goes extra slow during quartz inversion for double-bisquing risky pieces. Has anyone heard of this?
• Bisque it normally. I don't know how the previous work was fired during its fatal second bisque, so maybe the studio fired it in a strange way, or the stress of transporting it damaged it and the issue has nothing to do with re-bisquing.

Thank you so much for your help! And apologies for the long post. 

Edited March 16 by nuna

[Bill Kielb]

One thing that comes to my mind for large pieces,  is it will shrink and grab on the shelf. The rim and likely the grid if flush at the bottom will tend to drag on the kiln shelf. The grid itself if not reasonably even could also cause this to warp and bend depending on variance in thickness and construction.  A waster slab and thin even silica beneath and between comes to mind to keep it smoothly moving and from grabbing. Same for glaze firing, although predicting distortion I think is tough without having tested a few shapes and if glazing only one side could deform the shape. Sounds like you can glaze both to a large extent. Quartz inversion (IMO)  is almost not a thing as every piece in every firing goes through it twice each firing the world around (probably millions of pieces). Having said that a reasonable slow speed helps with the rate it shrinks and there is no reason to go to cone 04 so neither of those suggestions sounds silly. We do multiple bisques with layer applications of underglaze but always bisque to lesser temps just to set each layer of underglaze.  There is no reason to waster the energy or stress the piece.

Edited March 16 by Bill Kielb

[Pres]

I also think that your worst enemy in this firing will be the shrinkage factor. I have fired flat slabs in the past using several extruded biqued "rollers" 1/4 " thick. They crack sometimes during firing, but difference in length is not a problem lay them out in the area where the slab will set. Another option is a layer of grog. This can be sprinkled well with a spice shaker with larger holes.  Use one of these on your glaze firing also.

best,

Pres

[nuna]

@Pres @Bill Kielb I did put a significant amount of alumina oxide sand beneath the face-down tabletop for the first bisque firing. I'm happy to put more down again when it is fired face-up in the second firing, but since it has already shrunk in that first 06 bisque, will it actually shrink again when re-bisqued?  I assumed it was done shrink as far as bisque went. 

The suggestion of a waster slab is a great idea, especially since my grid is definitely not perfectly even/flat. I will definitely make one for the eventual cone 6 firing, and maybe for the second bisque as well.

[nuna]

8 hours ago, Bill Kielb said:

Same for glaze firing, although predicting distortion I think is tough without having tested a few shapes and if glazing only one side could deform the shape. Sounds like you can glaze both to a large extent. 

I'm actually not too worried about the table warping a bit since this is part of a sculptural installation and not actual functional use. Do you still think glazed both sides would be safer? Also, I've never used sand/grog in a glaze firing (if I'm understand your suggestion correctly); I've heard it can blow around in the kiln during firing and I wouldn't want to get stuck in the glaze - thoughts?

[Bill Kielb]

32 minutes ago, nuna said:

will it actually shrink again when re-bisqued?  I

Yes it will

28 minutes ago, nuna said:

Do you still think glazed both sides would be safer?

Yes  I do, but it is complex so testing may have helped here. My guess is it would help. Total guess though.

 

28 minutes ago, nuna said:

I've never used sand/grog in a glaze firing (if I'm understand your suggestion correctly); I've heard it can blow around in the kiln during firing and I wouldn't want to get stuck in the glaze - thoughts?

Pres used grog, I think I said silica, alumina is great too. Pres had the super excellent idea of applying thin and  evenly with a salt shaker though. All can work, all require care not to spread around the kiln. Grog is probably neatest, then silica, then alumina.

Edited March 16 by Bill Kielb

[nuna]

5 minutes ago, Bill Kielb said:

Yes it will

Wow ok I didn't know this! Thank you. 

[PeterH]

A memorable analogy
 

 

[Min]

15 hours ago, nuna said:

Bisque super low at cone 020 or so. In this case, would it be safer to fire it fast or slow? 

Slow. Both for firing up and cooling down.

These pictures are from Digitalfire of lowfire ware, pots have cracked from uneven cooling, edges will have cooled faster than the middle area of the pieces. The way to avoid this is to have the cooling down go as evenly as possible. Slow the cooling down between 1150F - 950F, I go at 100F/hour through this zone with suspect work.

 There is negligible shrinkage, ie less than 1% between bone dry greenware and bisque firing to ^06- ^04, would be interesting to see data that shows refiring to a lower temp causes more shrinkage.

edit: when having your piece in the kiln put it between shelves, not at the top or bottom of the kiln, this helps the heat even out also. I put rectangular or triangular kiln posts around the outside edge of the shelf also. (round posts can roll)

 

Edited March 16 by Min
typo

[Bill Kielb]

Just perhaps an easy thing to remember - Flat plates with a hole in them start off with a pretty distinctive stress concentration. Have seen many a clock face crack just like in the pictures above. Reinforcing around a hole can help with clay, just like many other materials, this pattern and fix are similar to help many materials survive this typical stress concentration around a hole - pictured below. We reinforce all holes in flat plates with some added uniform thickness or washer if you will. Since, along with good practices no longitudinal cracks in these clock face figures.

Edited March 17 by Bill Kielb

[nuna]

OK so my current plan for the second bisque will be to fire it face up/grid down to 022 on slow, atop a very even layer of alumina (the L&L manual said not to use silica which is why I switched), possibly also on a waster slab on its own layer of alumina.

I may also use alumina and a bisqued waster slab for the eventual firing to 6.

Thanks all.

[neilestrick]

17 hours ago, nuna said:

(the L&L manual said not to use silica which is why I switched

I believe that recommendation is to prevent the material from getting into the element grooves. I don't think that switching to another material is necessarily any better in that regard. If you're going to do it, use the material that works the best which would be silica sand, and just be sure not to get any into your elements, and vacuum them out after firing.

There is very little shrinkage in a bisque firing, so it's more about even heating/cooling and/or expansion/contraction during the heating/cooling.

[oldlady]

i have not read this entire post but would like the original poster to consider spraying glaze to avoid disturbing the underglaze.

a very simple siphon sprayer costs about $30 and is branded EZsprayer.  it is very fast and a kiln load, big kiln, can be done in an hour or so depending on how many colors you use.

just a thought.

[Callie Beller Diesel]

Chiming in a little late on this one, but you mentioned the crack in your piece happening at another studio, where the bisque cycle is unknown to you. I strongly suspect the cause wasn’t the fact that it was bisqued twice, more that the cycle used was likely not suitable for your piece. Two foot pieces are going to need a slower cycle than regular pieces, just due to sheer thermal mass.  If the studio’s usual bisque cycle didn’t take that into account, or the kiln pack wasn’t a particularly heavy or even, that would be the most likely explanation to me.

Side note: I just spent the morning going through Ceramic Materials Workshop’s new commercial clay body analysis resource. It’s free if anyone wants to check it out for themselves. So far the submissions are limited to assorted clays in the US that are in the roughly cone 6-10 range. Bill’s statement that clays will shrink upon a second bisque depend greatly on what the first bisque temperature was, and which clay body is in question. If you’re bisquing to a common 06 temperature, the statement of a less than 1% increase is likely enough. If your usual bisque is higher or lower, it might not be. After about 1000*C (roughly cone 04) many clay bodies hold steady for a time before continuing to shrink. If you bisque very low to cone 020, the shrinkage charts actually decrease for a time, meaning the piece expands a bit after the carbonates and chemical water burn off. 

Again, this is VERY clay body dependent.