Skarva - paper porcelain tile work

[Belgin]

Hi All,

I have been getting cracks/ splits during  both bisque and glaze firing and I dont understand how I can prevent  from happening in future.

The slabs are from Skarva paper porcelain tiles size 40  x 35 x 1cm thick . They are fired 1200 C with 40 min soak. Both firings were slow ( 13 hrs each)

I used slab roller to roll out the slabs hoping that I will not face cracks but I was wrong.

 

Any advice will be greatly appreciated.

 

Thank you

Belgin

 

[neilestrick]

That's a big tile! They're probably catching on the shelf, unable to expand and contract as needed during the firing process. Try firing them on a little bit of silica sand, something like 70 mesh. You don't need much, just a dusting. It will allow for more movement, but you'll still have an even, level surface for them to rest on.

[glazenerd]

Belgiin:

I fire large format porcelain all the time: about the only thing I make.some are larger than the example you give. 

1. Large format tile means large are surface contact with the kiln shelf- which equals drag as it shrinks. Let's say the shrinkage rate is 13%; that means it will move 13% as it is fired. You have to deal with that drag/ friction. 

2. Because of its large size and footprint; it is susceptible to stress cracks/ fissures at the quartz inversion temperature. (563C). This means the kiln needs to be slowed down to 50C an hour climb from  520 to 600C; then back to your regular ramp.

3. Putting large mesh sand under the tile when firing will help it move/ reduce drag.

if firing tile is an ongoing production item in your studio: then make your own custom tile sitters.

I purchased regular tile setters and cut them into sections to fit large format tile. Now the surface contact is limited to a few points. I will dust the slots with sand sometimes: I fire much larger pieces than shown here. The largest porcelain pieces I have fired for counter tops were 28" x 28" x 1" thick. I lost 1 out of every 4.. Not too bad of failure rate given the size.

i commonly single fire 16 x 18" x 1/2" custom tiles as seen below.. It can be done.

T