tiny points on shelf side of fired glass

[Laura Walter]

I have been firing for years, using thin fire paper on my shelf to fire all my projects on. I just rebuilt the kiln and now am using a fiber board shelf treated with kiln wash to fire directly on. I used the same firing schedule that I normally use. My test load came out with tiny/tiny sharp points around all the edges of the glass on the shelf side. Any ideas why?
(We did add 1 extra layer of insulation in the bottom of the kiln.)
Laura

[Morganica]

Were you using mullite (ceramic) shelves before? Sounds like you're overfiring a bit. How thick were the pieces in the test load?

[Laura Walter]

I was using the fiber shelves but had never coated them with kiln wash and had always used thin fire paper on top of it. I would just like to get away from always using the thin fire paper.
The pieces were an open sort of weave design just using scraps to test what the kiln would do. Perhaps with the extra layer in the bottom of the kiln I won't need to go as hot. I shall try that.

[Brad Walker]

Firing on fiber is more likely to cause needling than firing on thinfire. One option is to try lowering your top temperature; needling is more likely at higher temps and lowering even a few degrees can help. Another option is to lightly dust the top of the fiber board with dry kiln wash powder. A light dusting only, it can help smooth out the surface of the board and prevent the needling.

[Laura Walter]

Thanks Brad, If I get needling today I will try dusting the shelf with the kiln wash.
Yesterday, I fired to a tack fuse temperature, got less needling but it still is there. My final ramp was only 10 dgm. Perhaps that is part of the problem. Now I am doing a test run with my final ramp at 999.9 to 1450 where I usually stop (unless doing multiple layers that I want to go flat for plates, etc..). I will know by tomorrow what is happening. All the pieces I am testing are just crisscrossed layers of clear scrap and color, so it isn't more than one layer of glass in most of the piece except at the overlapping joints.
I am just wondering if I screwed up when rebuilding the kiln by adding an extra layer of a 1 1/2 inch lightweight ceramic board in the bottom on top of the brick and before the fiber shelf. Any ideas on that?

[Brad Walker]

All the pieces I am testing are just crisscrossed layers of clear scrap and color, so it isn't more than one layer of glass in most of the piece except at the overlapping joints.

If it's only one layer of glass, then needling is much more likely.

I am just wondering if I screwed up when rebuilding the kiln by adding an extra layer of a 1 1/2 inch lightweight ceramic board in the bottom on top of the brick and before the fiber shelf. Any ideas on that?

That shouldn't have anything to do with the needling, but most people would argue that an all brick floor is best.