Mystery contamination discolors piece

[LindaS]

During a tack fuse of my piece I was firing on thinfire laid on a kilnwashed ceramic tile (used many times before, but not for some time now). I suspect there was something on the tile because underneath the middle of the piece the thinfire turned gray. There is a difference in the surface of the glass on the underside over this gray patch, and beyond it the bottom of some blue glass is now glazed with a reddish film (oxidation of the copper in the glass???). There was silver foil laid on top of the piece, but there's no glass that should be silver-reactive and the top of the piece doesn't show any effects. Bottom of the piece is BE grays, blue, and clear, previously fused (on thin fire, different shelf) without anything odd happening. I did a bunch of tests before making this piece (on smaller tiles) and nothing happened there, so seems like it was something about this particular tile. I don't know what could have been on the tile - possibly something picked up during a move, because as far as I can see from my records I haven't fired anything on that that would have reacted - other gray stuff, for example.

My question (which might have the answer "you'll have to try and see!") is how to proceed. Before this happened I had been planning to square up the edges, fire polish, and then slump. I am concerned that the red haze might be enhanced or spread (right now it isn't really visible from the top) or something worse might happen if I go up to fire polish temp and maybe I should just slump and be done with it. Alternatively, while I don't have a sander or sandblaster, I might try to feebly and forlornly scrub the bottom of the piece with some wet/dry sandpaper and see if I can get anything off, then go ahead as planned. I don't know whether I should be concerned that I might transfer some contamination to the mold, either. In the interest of not mucking up my one real kiln shelf, if I do firepolish, I might use the tile again and perhaps test it first with some similar glass after cleaning it.

Any insight anyone can offer would be appreciated.

[Barry Kaiser]

all glass is silver reactive

[Barry Kaiser]

Any time you have silver contacting a mold or a shelf, it contaminates the mold and shelf

[LindaS]

Thanks, Barry! I do know about the potential for silver contamination, and I was worried about that when I started planning this piece (which was why I used the cheapo piece of tile for a shelf). But in this case I don't think it was the silver - or if it was, it was something pretty indirect in which the silver reacted with something else that caused the effect. I'd never fired silver on this tile shelf, so it wasn't silver contamination left over from a previous firing. I'd done several tests with pieces of these same glasses, some under the exact same conditions, but using a different, smaller piece of tile as a shelf. In the tests did not see anything remotely like this, in spite of using that test shelf repeatedly, at most only yellowing where the silver laid on top "disappeared" into the clear glass when I went too hot with it. Nothing affecting the blue glass at all, even when silver was placed in direct contact. Also, in all of these cases the silver was encased and on top, so wasn't directly contacting the shelf at all (I kept it away from the edge), especially not in the very center of the bottom, which I'd think would be the least silver-y place in the kiln even if I was getting contamination during the firing (and the center of the reaction is not under the silver, so I don't think it was from silver traveling through the glass). The reaction is confined to the bottom. It might be something that required low oxygen levels to occur, since it aligned so well with the central axis of the piece. But silver or not, whatever the contamination is, it was reactive, so I am still concerned about what to do next. I think I'm going to try to put some of the test pieces on this shelf and see what happens to them, but not sure where to go from there. I'd really like to be able to save this piece, if possible, but I'd also not want to muck up anything valuable in doing that. Thanks again for your time and help!

[LindaS]

One heartening thing is that most of the red stuff is coming off with the sandpaper. Not sure I can get it out of the deepest areas. Does look like whatever is was fumed from a point in the center of the piece - can see radial streaks clearly as it is sanded off.
I noticed the lid to my kiln wash jar has a few spots of corrosion. Maybe a metal flake got into the KW suspension....