Curious about kiln wash

[Cliff Swanson]

What specifically happens to kiln wash during full fuse processing that renders it unsuitable, or at least of questionable value, as a separator for subsequent firings?

Cliff

[Bert Weiss]

What specifically happens to kiln wash during full fuse processing that renders it unsuitable, or at least of questionable value, as a separator for subsequent firings?

Cliff

Cliff

I'm not a techie, but the kaolin goes through a shift at 1400ºF that causes it to deposit scum on subsequent firings.

[Brock]

What specifically happens to kiln wash during full fuse processing that renders it unsuitable, or at least of questionable value, as a separator for subsequent firings?

Cliff

Sometimes nothing. If you fire at the lower end of the fusing range say 1430 - 1450, you can get many firings out of your shelf.

I use black as my bottom layer, and as it is a soft glass, compared to most opals, I get 6, 7, sometimes more, firings from a properly prepared shelf.

If your shelf doesn't have cracking, flaking, or other obvious signs of wear, you can probably use it again.

If you do, you may get a heat signature from the area where a piece was previously fired. Brock

[Lani McGregor]

What specifically happens to kiln wash during full fuse processing that renders it unsuitable, or at least of questionable value, as a separator for subsequent firings?

Cliff

Cliff, I’m not sure if anyone here (BE) truly understands the mechanics of how kiln wash works, but this is what we *think* happens with the ones made of kaolin/alumina hydrate:

Water of hydration (aka chemical water) is given off of both the alumina hydrate and the kaolin. These gasses released during heating to elevated temps act like a sort of air hockey table, keeping the glass – which we all know is quite sticky above ~1450F - from bonding to the shelf.

Once the kilnwash is dehydrated, this effect is no longer in play and stuff sticks.

One theory.

L

[Bert Weiss]

What specifically happens to kiln wash during full fuse processing that renders it unsuitable, or at least of questionable value, as a separator for subsequent firings?

Cliff

Cliff, I?m not sure if anyone here (BE) truly understands the mechanics of how kiln wash works, but this is what we *think* happens with the ones made of kaolin/alumina hydrate:

Water of hydration (aka chemical water) is given off of both the alumina hydrate and the kaolin. These gasses released during heating to elevated temps act like a sort of air hockey table, keeping the glass ? which we all know is quite sticky above ~1450F - from bonding to the shelf.

Once the kilnwash is dehydrated, this effect is no longer in play and stuff sticks.

One theory.

L

Lani

Interesting theory.

I use a powder made of alumina, diatomeceous earth and plaster. I fire it over and over as a coating over blanket or LD board. I get good results. I blame the scum thing on kaolin which I keep out of my reusable mix. In my observation those powders are much less likely to stick than the alumino silicate fibers used in blanket and board (which aren't too sticky themselves).

[Ralph]

Some thoughts:

I think it's the glass - or at least the flux content of it - that contaminates the kiln wash and renders it less refractory for subsequent uses. Art glasses (float too) contain significant quantities of reactive metal oxide fluxes, especially sodium. Even at the comparatively low temperatures used in glass fusing the fluxes are active, even volatile, and can migrate on to (or into) refractory surfaces in contact with hot glass.

At higher temperatures - such as in glass-making furnaces - flux contamination of refractories is an ongoing issue. At slumping temperatures the effect is not so pronounced, but at fusing temperatures it definitely is.

After many experiments with different kiln washes and separators I've noticed that colored glass frits fused in contact with separator materials can visibly color them. Because the colorants used are thermally stable (ceramic stains, not oxides) the colorant is simply acting as a marker for something much more thermally active. The most heat-reactive components in glass are the fluxes. These can readily volatilize at fusing temperatures. The volatiles - colored by the ceramic stains - can migrate or "fume" on to nearby surfaces. The volatiles will even penetrate some porous separator materials.

So kiln wash works really well first time but, after picking up some flux from the glass, the kiln-wash surface is less refractory next time and may not work satisfactorily.

It's possible that there is enough flux at the kiln-wash surface to visibly affect the next piece of glass. This can cause Brock's heat signature effect.

Kiln wash is not really an issue for me right now, but it would be relatively simple to test the theory. Fuse some glass tiles on kiln wash that has previously had glass fused on it and fire other tests on kiln wash that has been fired, but not in contact with glass. I think that kiln wash that has not been contaminated with flux will work well after many firings, but so what? No one wants to fire kiln wash without glass on it!

Ralph

[quill]

I think that kiln wash that has not been contaminated with flux will work well after many firings, but so what? No one wants to fire kiln wash without glass on it!

I just tested (inadvertantly) this theory. I have had a kiln shelf I had prepared for fusing for a few months ago just sitting. I was waiting to take a class before using it. I did take a class a few weeks ago but the instructor used shelf paper & since I had some, that's what I used too... but I laid it over this shelf & fired it this way for oh, maybe 10 firings at 1450.
Yesterday I ran out of paper so fused directly on shelf. Worked great. This morning I used the shelf again. Lots of kiln wash stuck. Some pieces were covered entirely on the back. Luckily, the lime-away tip worked!

[Brian and Jenny Blanthorn]

What specifically happens to kiln wash during full fuse processing that renders it unsuitable, or at least of questionable value, as a separator for subsequent firings?

Cliff

Cliff, I’m not sure if anyone here (BE) truly understands the mechanics of how kiln wash works, but this is what we *think* happens with the ones made of kaolin/alumina hydrate:

Water of hydration (aka chemical water) is given off of both the alumina hydrate and the kaolin. These gasses released during heating to elevated temps act like a sort of air hockey table, keeping the glass – which we all know is quite sticky above ~1450F - from bonding to the shelf.

Once the kilnwash is dehydrated, this effect is no longer in play and stuff sticks.

One theory.

L

I have not noticed any difference with new or old kiln wash

Sometimes I clean off n use new wash

But usually I just patch up the old stuff

Some of my kiln wash must go through 20 or so firings

One point tho I usually grind so I am not 2 fussy about the surface

If I do get any oxide contamination I either paint over or remove it