what would cause a bright yellow haze on kiln shelves and fired ceramic? steel?

[kelly witmer]

I have this strange sulphur-ish yellow to dark greenish haze that's been getting stronger (each time I fire) on some of my shelves, and it's showing up on my ceramic pieces occasionally as well. Like it's off-gassing or something, leaving faint yellow areas on the fired work.

I'm guessing this happened because I did some experiments firing glass with steel molds I made, but it was over a month ago and has been fired 5 or more times since. Because this shelf keeps getting worse, I'm wondering if I should stop using it, or if covering it with kiln wash might help.

It's an issue because I often make white unglazed ceramic sculptures, and I'm also just really curious what this is!

[Min]

Are the other shelves getting fumed from this one? Does look like something fumed out of the metal molds you fired. How high are you firing? Is there kiln wash on the other side of the shelf? 

[Callie Beller Diesel]

The times I’ve slumped with mild steel moulds they tended to spall but not fume anything like that. What metal was the mould made of? Stainless? Roof flashing from the hardware store? Something galvanized?

It’s less likely it was the glass, but it isn’t impossible. Was there frit or stained glass involved? How hot were you working, and was the glass you were using rated for that purpose?

[kelly witmer]

17 hours ago, Min said:

Are the other shelves getting fumed from this one? Does look like something fumed out of the metal molds you fired. How high are you firing? Is there kiln wash on the other side of the shelf? 

A few others had milder yellow spots. I jump around, highest firing is cone 5, but I also do cone 06 and then the glass slumping at around 1275 F.

[kelly witmer]

9 minutes ago, Callie Beller Diesel said:

The times I’ve slumped with mild steel moulds they tended to spall but not fume anything like that. What metal was the mould made of? Stainless? Roof flashing from the hardware store? Something galvanized?

It’s less likely it was the glass, but it isn’t impossible. Was there frit or stained glass involved? How hot were you working, and was the glass you were using rated for that purpose?

I was using cold rolled steel 20g sheets (not stainless or galvanized). I'm cutting the metal with a plasma cutter and then slumping glass into it - most of the glass is fusible 96 coe, but not all. I had been experimenting with the finished surface of the steel, and someone lent me some muriatic acid that I painted on one of the sheets of metal to remove the protective oils it comes with, and I'm thinking that's what did it (the muriatic acid has the same bright yellow color). 

Probably not too many idiots firing stuff like this in their kilns! Wonder if it will eventually burn off? The odd thing is that it's gotten worse, despite it being a while since I've fired the metal, it's just been ceramic at either cone 5 or 06 a bunch of times.

[Callie Beller Diesel]

Okay, it’s not the glass. Series 96 should be fine at the slumping temps you mention. 

If you didn’t rinse the muriatic acid off, that could be a culprit. It could in theory react with the carbon steel to form iron chloride, depending on how long it was left or if you put kiln wash over the piece. But I’d expect that to be more orange/pink, and it doesn’t explain the green to me.

Other questions would be around how are the shelves stored, and are they second hand?

If the green is getting worse and transferring to other pots, I’d maybe reserve those shelves for something like pit fire or raku.

[Min]

1 hour ago, kelly witmer said:

and someone lent me some muriatic acid that I painted on one of the sheets of metal to remove the protective oils it comes with, and I'm thinking that's what did it

I agree.