Shown in the image is the back slab of a white oven I built 3 years ago.
Clearly visible right of centre, is a vertical steam crack.
 
I fired the heater several times without the door, then put a couple of medium fires with the door in place. During the third big fire, at which time I assumed all mechanical water had been evacuated from the slab, I added more softwood to whiten off the fire box ceiling, which was at this point only partially white.
 
As the soft wood burnt, I looked into the oven and actually saw the crack appear. I could see steam being emitted from the crack along its entire lenght, and the material each side of the crack throbbing with darkness, as steam on its way out of the crack was absorbed into the dry surface of the slab, and then almost instantly evacuated through the porosity of the surface.
 
This image was taken 3 years after I observed the event, and there has been no change whatsoever. Seen here hot, the fissure is invisible when cold. I looked up into the firebox to make sure the crack was not just along the inner surface of the slab. The slab is indeed cracked side to side.
 
The castable is Heatcrete 24 ESC  with 10 mm aggregate.
 
Obviously I brought the heater up to temperature too fast. And may have had a slight exess of water during mixing ?
 
Neither myself nor the client are concerned that the slab will fail, and mechanical strenth is not an issue with the back slab, as the 28 inch wide slab only bridges the 10 inches of the opening to the under hearth smoke path.

Of the four castable elements  that form the oven, this one, being in the throat, is affected most by thermal shock.

I have observed fissuring of this slab in several other white ovens I built.  I now take extreme care during the curing process and have not since observed the phenomenon.
 
 
Marcus Flynn.