Spots on my glass?

[BennettS]

I've been fusing glass for about a year using kilns at a makerspace. Sometimes when stuff comes out after a full fuse, they have little marks on them, usually like black dots. The spots are usually smooth and feel the same as the rest of the piece.

What causes this? Is it something in the kiln or something about my firing schedule?

My glass is mostly Oceanside (the rainbow wedges were Y96 but that's the only non-Oceanside glass I've used and this has happened a half a dozen times) and the frit I used is old Uroboros that's available at the Makerspace.

My schedules have varied but for these two pieces it was:

250 to 500 hold 20
250 to 1000 hold 5
250 to 1250 for 45
450 to 1450 for 20
Anneal
AFAP TO 960 for 60
100 to 700 for 10
300 to room temp

Bonus question: why does the frit sometimes stay grainy, like on the scrap piece? It's fine clear frit. I don't mind it in this piece but if someone has suggestions on how to avoid it, I'd really appreciate it!

Thanks in advance!

[The Hobbyist]

Those brownish circle spots look like what I got many years ago when I started making my own frit. I made the mistake of using galvanized pipe in the process. Even though i used a magnet to remove any metal it didn't take out the non-frerrous zinc used as galvanization. When melted in the kiln is showed up like yours.

When I switched to non galvanized pipe and metal parts the problem went away. The only purchased frit I use is Bullseye, never a problem.

If you are buying that frit you have no way of getting the zinc out (nonmagnetic) and the tinyest particle melts at fusing temps and gasses off as you see there.

Jim