Devitrification in New Kiln/Kiln Shelf

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misteroldhouse
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Devitrification in New Kiln/Kiln Shelf

Post by misteroldhouse »

Hi All,

I'm the brand new owner of a Denver kiln and Western Industrial Ceramics 60" x 30" kiln shelf...

I did my first test....the very first firing after hooking it up and doing one brief test to 400F. The test involved placing 2" x 2" double-stacked tiles around the kiln...looking for hot and cold spots, etc.

All fired and fused well, but every one of my tiles have devit on them...and I never, ever get devit! Interestingly, I fired a smaller (also brand new, first firing) kiln shelf of the same material in my 21" x 21" GL-24 and also go devit...!

My only conclusion is that the organic material (or whatever) in the kiln shelf is burning off and causing this devit.

My question is "how long until it goes away" and "is there any way to make it go away faster"?

Any thoughts or advice would be most appreciated.

L~
Larry Pile
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www.kesslercraftsman.com
Rick Wilton
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Re: Devitrification in New Kiln/Kiln Shelf

Post by Rick Wilton »

Yep, the binders in the fibre will do that.

Fire that kiln 100-200 degrees hotter than any normal temps to once and for all burn off the binders.
Rick Wilton
rosanna gusler
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Re: Devitrification in New Kiln/Kiln Shelf

Post by rosanna gusler »

and vent throughout that firing.
artist, owner of wanchese art studio, marine finisher
adrian
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Re: Devitrification in New Kiln/Kiln Shelf

Post by adrian »

HI Folks
Interesting to see this topic - and very timely....

I've recently built a 4ft x 2ft x 1ft tall clamshell / ceramic fibre-insulated kiln - to allow me to make 'big stuff' (TM).
It's replacing a Skutt hotstart - which was limited to about 13" square.

The new home-built kiln has 7kw of resistive heating in the lid, and I've been firing on thinfire paper on the ceramic board kiln-base
(so far so good!) - and a Skutt Hotstart /Bartlett controller (because I had one spare!)

One of the items that I make fairly regularly is a 12" roundel of clear 3mm glass, with a big, stylised fuchsia (cut from 3mm red glass) full-fused into it.

I must have made hundreds of these in the old Skutt kiln (on a ceramic shelf & thinfire paper) - without problems.
I fired three of these in the new fibre kiln yesterday, using the same schedule, and the clear glass all suffered from slight devit -
not the 'rough, sugary' type - but small patches of devit that looked like a very light etch on the top surface..

I've just re-fired them with a borax-based 'anti-devit' coating - and they look better.. can't really tell 100% as they're still at 250c!

However - the big question (for me) is "Why should they devit in the new kiln and not in the old".
- as I'd prefer to make them in one hit - rather than having to correct things later.

FWIW - the glass is bog-standard Spectrum SP100w waterglass (stained-glass glass rather than fusing glass), and it's thoroughly washed both sides
in water & washing-up liquid, then dried, before being fired.
Schedule is the default 'fast, full fuse' with a top temperature of 780c - all settings that are tried and tested in the old brick kiln.
The only change to the schedule is to lengthen the time @ process temp to 15 mins - as a 10 min fuse in the new kiln doesn't quite give a full-fuse
(I'm guessing that the old brick kiln takes longer to heat up, so there's more heat in the glass in the old kiln by the time it reaches process temp?)

I've read up on the subject of devit - seems that the length of time spent in the devit temperature range on the way up & on the way down is important,

It appears that the 'built-in' schedule I'm using is

500f/hour to 300f (150c) hold 5
800f/hour to 1436 (780c) process hold 15
max cooling (I'm not venting) to 950 (510c) hold 30
175f/hour to 800f (425c) hold 10
then cool naturally.

I'm considering increasing the ramp speed for the second segment to 9999 ? (fairly convenient)
and maybe crash-cooling once the process is done ? (less convenient as it means being there at the right time)

Any thoughts, please ?
Thanks in advance
Adrian
Buttercup
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Re: Devitrification in New Kiln/Kiln Shelf

Post by Buttercup »

Adrian, did I miss something? You're using art glass as opposed to a fusing-formulated glass, waterglass plus a fuchscia-coloured glass? What are the specs for the fuchsia glass? Could this be a compatibility issue, even though you seem to have done it many times before with untested glass? Jen
adrian
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Re: Devitrification in New Kiln/Kiln Shelf

Post by adrian »

HI Jen
Thanks for the reply - and, no, you didn't miss anything <grin>

All of the glass involved in these suncatchers is Spectrum Waterglass.
My local supplier said, quite a few years ago, "You're fine fusing waterglass to waterglass"
- and I must say that he was correct (I've done stress tests etc)

On the odd occasions where I've needed to break the 'fuse waterglass only' rule, I've made up fused test strips,
and been guided by the results.. so I don't think compatibility is the issue here.

I've just checked the kin, and the two pieces that I re-fired on the same schedule overnight (with the borax anti-devit) have turned out fine,
the devit has gone - so that's a good thing.... but I'd like to know what caused it in the first place...

I guess I could fire these pieces in future with borax anti-devit- but clearly there's some difference between the two kilns,
and it would be nice to understand it...

Maybe I should take the empty kiln up to 880c or thereabouts (100c above my normal process temperature - as Rick suggested) and see if that burns out any remaining binders in the ceramic fibre / ceramic boards - might work ?

Thanks
Adrian
Brad Walker
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Re: Devitrification in New Kiln/Kiln Shelf

Post by Brad Walker »

adrian wrote: but I'd like to know what caused it in the first place...
Unlike your old kiln, which was brick with a ceramic shelf, your new kiln has a fiber board shelf and fiber construction. That will give off a fair amount of fumes and gases during the first firings. Combine that with thinfire -- which also outgases -- and you have a recipe for devitrification.

I would follow Rick's recommendation -- fire the kiln up to 50 to 100 degrees higher than normal process temperature in order to burn the binders out of the shelf and insulation. If you don't do that, the binders will still burn out, it will just take longer.

Also, you don't mention ventilation in your kiln, but you may want to prop the lid open a few cm during the initial firings to better allow the fumes to escape.
adrian
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Re: Devitrification in New Kiln/Kiln Shelf

Post by adrian »

HI Brad

Thanks for the comments.
Odd thing is, I've probably done 12 or so firings to 780c in the last two weeks or so,
but this is the first time I've had the problem....

I did a firing this morning up to 880c - with the kiln cracked open about half an inch -
so I'll have another try at a 'proper' firing later on and see how it goes.

There's no 'built-in' venting...

At least I know that, should there be any slight devit, I can 'cure' it with another firing with the borax painted on...

Thanks
Adrian
Buttercup
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Re: Devitrification in New Kiln/Kiln Shelf

Post by Buttercup »

There you go Adrian, seems the new kiln needed a good 'burn-out' firing.

I'm really curious about the compatibility of any colour waterglass with another. Can you indeed fire glass from different coloured sheets together? At what temperature do they lose their 'dimples'? Fascinating! Thanks, Jen
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