Extreme Cracking/Shrinkage in Tiles

[Caroline E]

Hi everyone, 

I am hoping to get some advice regarding a students tile work. Her large tiles had severe cracking/shrinkage from the glaze ^6 oxid. firing (standard ^6 medium speed firing). Please see photo. The student was using commercial clay without grog and used Mayco glazes. For her next set of tiles, I advised the student to use clay with grog and to compress the front/back of tiles. In the firing, we plan to use sand and/or waste slabs underneath the tiles. Is there anything else we should do? Should I be using a firing schedule that has a cooling ramp? 

Thank you in advance for any help.

 

Caroline

[neilestrick]

A waster slab should do the trick, or put some silica sand on the shelf. Make sure you put kiln wash or alumina wax on the waster slab so the tile doesn't stick to it. If you need a cooling ramp it would be during quartz inversion, so from about 1100 to 900, but it's rare that you would need that.

[Caroline E]

@neilestrick Thanks very much. I will try that. 

[Denice]

I see that there are some cracks running from one lump to another.  I make a lot of tile,   a thicker area next to a thinner slab can cause cracks to form.   She might try removing the clay from the back of the tile in the thick areas  and try to keep the clay a even thickness.  Denice

[Caroline E]

@Denice Good point. Thanks!

[Ben xyz]

On 3/14/2024 at 6:30 PM, neilestrick said:

A waster slab should do the trick, or put some silica sand on the shelf. Make sure you put kiln wash or alumina wax on the waster slab so the tile doesn't stick to it.

Having the same problem with large 14"x20" slab(s), 3/8" thick. I am using recycled studio clay (medium and hi-fire clays  mixed together), rotating and flipping the slab often when using the roller (16lbs at a time before trimming). Compressing after with metal rib for further compression before drying. It only occurs when glaze firing (c5) stage and not on bisqued slab (c04).
1. Also making grooves on the back to counter warping (see photo). Perhaps this is making thinner areas against thicker and causing the cracking in half?                                                                  
2. How do "waster slabs" work? Slightly larger and thicker than initial slab? Placed under bisqued slab? Or under greenware from the beginning? A new process for me.                        
3. Since these slabs exist as elements in a sculptural piece, I 'm now tempted to glaze fire at a bisque temperature (c04), using a low-fire clear gloss glaze over the underglazed slab and calling it a day. Not as sturdy (w/o full vitrification), but wasting a lot of time, energy and clay from these large slabs cracking in half at c5. Thoughts?

 

Edited April 9 by Ben xyz
Clarification

[neilestrick]

1 hour ago, Ben xyz said:

2. How do "waster slabs" work? Slightly larger and thicker than initial slab? Placed under bisqued slab? Or under greenware from the beginning? A new process for me.                        
3. Since these slabs exist as elements in a sculptural piece, I 'm now tempted to glaze fire at a bisque temperature (c04), using a low-fire clear gloss glaze over the underglazed slab and calling it a day. Not as sturdy (w/o full vitrification), but wasting a lot of time, energy and clay from these large slabs cracking in half at c5. Thoughts?

The grooves are probably not the issue, and also probably not doing much to prevent warping.

Waster slabs can be very thin, and can even be broken/in pieces, but should fully support the piece on it. It's just something that will shrink with the tile and keep the tile from catching on the kiln shelf. Put some kiln wash or alumina wax on the slab so it doesn't fuse to the clay. Waster slab does not need to be bisque fired.

No guarantee it won't crack in the second low-fire firing, but it's worth a shot if you don't need the durability of cone 5.

It's also possible that your tile is cracking from cooling too quickly, and therefore unevenly, through quartz inversion. You could try a controlled cooling from 1100F down to 850F.