Intriguing surface defect

Cliff Swanson · Post by **Cliff Swanson** » 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

Mark Kemp · Post by **Mark Kemp** » Sun Aug 24, 2003 12:01 pm

Is the glass opal or cathedral? I haven't had this problem exactly, but the Spectrum opals have a darker layer on the outside than the inside, like glass flashed on both surfaces. Sometimes this shows up after fusing, and can either be an unpleasant or pleasant effect.

Cliff Swanson · Post by **Cliff Swanson** » Sun Aug 24, 2003 4:07 pm

Mark...It's happened with black, and with blue cathedral. I know exactly what you are talking about, because I've seen the affect you mentioned after tack fusing some mosaic-sized frit I made by crushing S96 blue opal. I sandblasted it off and refired to 1275/10 min to fire polish. Wored great.

Cliff

charlie · Post by **charlie** » Mon Aug 25, 2003 10:52 am

maybe devit. were the edges ground to make match better? if so, when the edge pulls back, it cold bring the ground edge to the surface, which is known for devitting because of the very fine bubbles that can be caused by grinding.

Cliff Swanson · Post by **Cliff Swanson** » Mon Aug 25, 2003 9:49 pm

Charle...the edges weren't ground, but you and I are on the same wavelength regarding the mechanism of what's going on. It really looks like the line I'm talking about is like a tide mark on a beach. Only in this case it marks the point to which the glass contracted before it flowed out at full fuse.

Cliff

lissa · Post by **lissa** » Tue Aug 26, 2003 11:01 am

This was happening to me a lot, more with certain colors, on both ground and unground edges, obviously a lot worse with the ground edges. I have found super spray to be helpful (and not grinding!). I don't know if you use anything like that but now I rarely get that haze line. I wondered if it had to do with my firing schedules because I fire hot and hold at full fuse for a long time, but if you are getting it with just a ten min. soak at 1475 that may have nothing to do with it. Vexing and perplexing.

lissa.

Sandpiper · Post by **Sandpiper** » Tue Aug 26, 2003 11:32 am

I just started getting this, too. I'm getting it on black. To me it looks a little like those pregnancy stretch marks. My spectrum black has always fired beautifully, and I had a slow to heat up kiln. I've spent an hour getting from 1300 to 1450!!! No problem. Now, new kiln heats fast, now getting devit. Doesn't make sense. I have one piece where two pieces of black butt up. One has devit the other doesn't. I'm wondering if it is cleaning issue because the lines follow like a wipe mark. Am I making sense?
Sandpiper, alias Robin

Cliff Swanson · Post by **Cliff Swanson** » Tue Aug 26, 2003 2:44 pm

FWIW to those of you having this same problem, it does get cured by sandblasting and refiring to a fire polish temp (1275-1300/10 min in my kiln).

Cliff