Intriguing surface defect
Posted: Sun Aug 24, 2003 7:57 am
I'm a bit puzzled by a surface flaw that has developed in a couple of pieces recently fired in Spectrum 96 glass. I've noticed that sometimes a very thin mark develops about 1 mm adjacent to areas where two pieces of different colors butt together. It's always where there is cut edge, but it doesn't happen along the entire cut. For example, I've been making a series of pieces where a half circle of one color is butted against the complementary arc cut into a piece of black, and then both are laid over a clear base and full fused. In a recent firing, this surface defect developed along a section of the arc where the black and other color adjoin. Sometimes the defect can be felt by rubbing a finger nail over it, sometimes it's less deep and appears as a ghost line. I've been able to eliminate it by sandblasting and then firepolishing. BUT, it developed during a slump firing on a piece that had enough surface detail and other sandblasted areas that I cured to a matte finish, that it couldn't be sandblasted and refired without obliterating some of the design. I always inspect pieces thoroughly between firings, and I did not see the defect in this piece until the slump firing which went to only 1125/10 min.
It appears almost as though the line forms back at the spot where the edge retracts to during the shrinking phase of a full fuse, before the glass flows out and fuses to the adjacent piece. As I said, it's about 1 mm back from the joint between the adjacent pieces, not necessarily along the entire cut. Might this be devit? My full fuse program is: 400 dph/1000/0; 60 dph/1250/0 (bubble squeeze for large pieces); 400 dph/1430/10 min; AFAP 1000/10; 300 dph/955/60 min; 100 dph/800/kiln off to room temp. All subsequent firings are to process temps of 1300 or less with similar ramps and soaks.
Any ideas about what's going on and methods I might use to prevent it would be greatly appreciated.
Thanks,
Cliff
It appears almost as though the line forms back at the spot where the edge retracts to during the shrinking phase of a full fuse, before the glass flows out and fuses to the adjacent piece. As I said, it's about 1 mm back from the joint between the adjacent pieces, not necessarily along the entire cut. Might this be devit? My full fuse program is: 400 dph/1000/0; 60 dph/1250/0 (bubble squeeze for large pieces); 400 dph/1430/10 min; AFAP 1000/10; 300 dph/955/60 min; 100 dph/800/kiln off to room temp. All subsequent firings are to process temps of 1300 or less with similar ramps and soaks.
Any ideas about what's going on and methods I might use to prevent it would be greatly appreciated.
Thanks,
Cliff