Brown spots

[Doug Zawodny]

These brown spots appeared on the shelf side of the piece fired on Thinfire at 300dph to 1500 no hold afap to 900 hold 60m 100dph to 700 no hold afap to roomtemp. As you can see they didn’t appear on the teal and the turquoise colors. Is there such a thing as bad Thinfire? I just started using a new batch. Anyone experienced this?

[Brad Walker]

Looks like silver stain. Have you used silver in the kiln recently?

[Kevin Midgley]

Recently? More likely the question is if you ever have used silver stain. It has the habit in my experience of returning to show itself in direct relationship to the time and value of the piece you are firing. Murphy's Law at its best.

[Doug Zawodny]

I don't even know what silver stain is much less use it. Any other guesses?

[Brad Walker]

I don't even know what silver stain is much less use it. Any other guesses?

It's the residue from using silver foil in the kiln. Ever used any?

[Doug Zawodny]

Never

[Doug Zawodny]

Ran another piece on the new Thinfire and got more streaks. Having never used any silver or foil it’s gotta be the Thinfire? Anyone ever heard of such a thing?

[Brad Walker]

I don't see how it's the thinfire, but that's easy to test. Just do a firing on a kiln washed shelf.

Also, try the other side of your shelf.

[JenniferB]

If you've ever fired Bullseye 0116 on that shelf you might want to check put the information on the Bullseye web site. They write: "During processes that require greater heatwork, such as pattern or flow bar techniques, Turquoise Blue Opalescent has the potential to deposit trace amounts of copper on the surface of the kilnshelf. These deposits may react with sulfur-bearing glasses in subsequent firings. Such deposits may not be visible and can react even when the shelf has been properly scraped and reprimed or, alternatively, when used ThinFire has been removed and new ThinFire is applied. This type of contamination is impermanent and may be burned out/fired out over the course of subsequent firings. A contaminated shelf can be fired with glasses—other than sulfur-bearing glasses—and no reaction will take place. In our studios, we've observed the greatest contamination in subsequent firings with sulfur-bearing French Vanilla Opalescent (000137) and Spring Green Opalescent (000126). For a burnout firing, we recommend a rate of 300°F/hr to 1525°F, with a hold of 1:00 hour.

[Doug Zawodny]

All my firings for the last 8 years have been on thinfire. Never on a treated shelf. I have used a lot of turquoise, but always on thinfire. Ill retreat the shelves and see what happens. Thanks.

[Brad Walker]

All my firings for the last 8 years have been on thinfire. Never on a treated shelf. I have used a lot of turquoise, but always on thinfire. Ill retreat the shelves and see what happens. Thanks.

Thinfire wouldn't stop any contamination from getting through.

[DonMcClennen]

suggestion.. flip your shelf over.. underside should not be compromised!