surely a lot depends on where the elements are? For a kiln with elements right at the top, in the lid, two shelves would mean that the top shelf is near the elements, hence the full fuse for the top shelf and a lesser fuse for lower shelves. But if you have a kiln with side elements, wouldn't that be quite different?
Yes, and no. A ceramics kiln will have a series of elements up the sides, but heat rises, so it still tends to be hotter at the top than at the bottom. At Uni, I made some ceramics wall art that was an assemblage of 37 different shaped 'tiles'. Because it was terracotta clay, which goes deeper in colour the hotter/more heat work is applied, I had two choices - fire the kiln three times with all the pieces in the same position, utilising only one shelf. Or mix up which tiles were on which shelf so that there was a subtle range of shades across the whole piece, such is the heat difference from top to bottom of the kiln (and in this case, I ignored the very top and very bottom shelves, only using the middle three and there was still a difference in temperatures, which we confirmed with the use of cones)
But you're completely right, a top elements only kiln will definitely have uneven heat with more than one layer of shelves. This is why dedicated glass kilns are often larger area and flatter in design, so that everything is the same distance away from the heat source and only needs to be on one layer.
Interesting to read on the Paragon site, Sarah, this:
The newest SC-2 has two separate heating elements in the left and right sides of the firing chamber. The back of the chamber has no element. Extensive testing shows that the heat distribution in the current SC-2 is more even throughout the firing chamber, because heat does not build up in the back of the kiln.
That reads to me as though they used to have a rear element too (which is what I thought they had and why I just went to check it out from Shefeenan's comment) - that, added to the heat loss of a bead door would definitely give you an uneven fuse.
I am sure I have read of others using SC2 two shelves and although getting a full fuse on both, one shelf is more rounded than another, I think that person planned on making two pairs of coasters, twice (so pairing the two top shelf ones, and two bottom shelf ones if that makes sense)
