I've been frustrated of late with high failure rates in my Raku firing. Raku is- of course, a process laden with thermal shock and awe. Pieces do break. I've walked away from the kiln cursing like a sailor on more than one occasion.
However, I've been firing some fairly complex forms lately (lanterns, fountain components, etc.)... and the shatter rate has approached 50%.
The problem- this particular form doesn't cool at a uniform rate. I've taken these out of the kiln, and the edges cool practically to the touch point, while the interior glows like Smaug's lair.
However, many of these have shattered with chunks lost in the kiln (see repair job below)... and I certainly can't sell water features with gaping cracks in the form.
So- I spent the past month experimenting with sagger firing. A sagger is a closed vessel that creates it's own atmosphere. You can fill them with combustible materials. The 'real' sagger experts often fill their saggers with everything from fertilizer to copper wire, all of which leaves a scrim of wildly unpredictable color on the surface.
My main goals in trying sagger firing were 1) to obtain Raku-like glaze results in a 'gentler' atmosphere, and 2) to 'smoke' my pottery without doing a post-fire reduction step. In particular, I needed the lines on my carvings to blacken.
It's worth noting that my Raky glazes came out looking nothing like they usually do. The grey glaze- usually white crackle. The coppery-looking glaze on the base- usually turquoise.
There's manifestly a heavy reduction atmosphere in the kiln... and my standard glazes are way off the reservation.
So- I don't think I'll be doing any sagger firing as a Raku surrogate.