Gray scum

[Jodi Longobardo]

I have been making a jewelry piece for a customer that has a white (wissimach white) flower on a pink (bullseye petal pink opal) background. I fused the background separately with no problem and then tack fused the flower with the following schedule:

AFAP ---> 1050 10
100 ---> 1250 10
AFAP ---> 1400 10
AFAP ---> 960 10
300 ---> 750 0
500 ---> 114 0

I have a small Paragon kiln and this schedule has always worked great for me for full fusing (with the top temperature raised to 1500) so I just amended it for tack fusing. I got exactly the amount of fusing I wanted with this schedule.

When I took the piece out, I noticed that one of the petals had a gray scum all across the top. Even though it looked different from the scum you get on glass when you haven't cleaned your edges well enough, I thought maybe I hadn't washed the grinder scum off of that petal, so I sanded it with a diamond hand block, washed it well with vinegar and water, and put it back in the kiln with the same schedule.

Well, I just took it out and now there is a little bit of the gray scum on the side of the pink glass that definitely wasn't there before. It is really close to the surface, since a few seconds of sanding with the hand pad took it off both times.

Now I need to put the piece back in the kiln to fire polish out the scratches from the sanding, but I am afraid just putting it back in will result in more of this scum elsewhere on the piece. Does anyone have any idea what is causing this? Any suggestions for this firing? I know I can take it to a lower temp this time (and could have last time) since I am not tack fusing any more, just fire polishing.

Thanks so much in advance for any help.

--jodi

[Nicole Hanna]

Are you using thinfire shelf paper? I've never had it happen, but others have said it can cause a discoloration on glass. If so, you need to vent the kiln till the paper binders burn off.
Are you scrubbing the pieces really well? Like, with a toothbrush or scrub pad....some times you think you've gotten all the grinder scum off but it can be very persistent. Also, what are you cleaning with? Some products can cause problems, like Windex.
Some glasses are just very prone to devit though also. You may need to coat with Spray A to avoid it.

Nicole

[Jodi Longobardo]

I used thinfire for the initial firing of the pink glass, but not for all subsequent firings, so I don't think it's that.

I grind my pieces then put them immediately in a vinegar bath. Then I scrub once with the vinegar, and once with clean water to get the vinegar off. I use a toothbrush to scrub.

If it were the glass, I would have thought the gray scum would have confined itself to either the white or the pink glass, not to both on separate occasions, but maybe it is devit. I am out of spray A or I would try that.

Thanks for the suggestions!

--jodi

[Morganica]

Fiber paper scum is a nice way of saying devit (based on my experience, not anything official). I'd expect it to show up first on paler opal glasses; I haven't seen a lot of difference between the really pale colors and the whites. And it's a cumulative thing. Even if you don't notice it on first firing, it'll get worse with subsequent firings unless removed.

What makes diagnosis harder is that the scum can be difficult to spot when there's not much of it, so you think you've gotten away clean. Then it continues to grow in the next firing and THEN you notice it.

[Jodi Longobardo]

Thank you, Cynthia, for the advice. I am thinking it was probably devit too. I took off the second wave of scum by sanding and fired again to 1330. I also vacuumed my kiln and cleaned the piece with alcohol after sanding. This time no more scum showed up. Don't know if that was luck or something I did helped. Oh well, at least I have my piece done with no more scum!

--jodi