Follow in devit question! Help with program please - WarmGlass.com

Follow in devit question! Help with program please

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theanimallover
Posts: 48
Joined: Tue Jan 22, 2008 9:19 am
Location: The Sticks, England, UK

Follow in devit question! Help with program please

Postby theanimallover » Thu Jan 24, 2013 7:26 am

Hi,
Further to my previous post. I am having problems with devit on sky blue and spring green transparant glass, it shows up more on the the green I believe as I back with white which illuminates the glass (and the problem).

When I fire on my bubble squeeze program the glass has no bubbles but devit, if I use my preprogrammed 'slow' it has no devit but bubbles! Please can someone help with a new in-between program to try?

The bubble squeeze:

300 750 10
300 1150 30
50 1250 2h
300 1480 15

Slow fuse (no devit but bubbles occur) I use the skutt programmed program. I can't find info on what it exactly is, but I think it is this one:

300 250 25
300 500 25
300 750 25
600 1250 20

I really appreciate your help (in advance :-))

Thanks,
Karen

beaconglass
Posts: 5
Joined: Mon Sep 26, 2011 4:41 pm

Re: Follow in devit question! Help with program please

Postby beaconglass » Thu Jan 24, 2013 3:50 pm

I'm no fusing expert, that's for sure, but it looks like your first program is un-necessarily slow heating from 1250 to 1480 (since you can't thermal shock the glass in that temp range), and it looks like your long hold at 1250 may also be contributing to de-vit, although I always thought de-vit only occurs during the cool down, and above ~1300 (but perhaps the "seeds" of the devit process can be started during a slow warm up). Are you rapid cooling from 1480 down to ~1200 or so before allowing the kiln to cool at it's own rate? If you are relying on the kiln cooling down from 1480, that might be the biggest source of the problem (but that would seem to also affect your other ramp if that is the case).

Stephen Richard
Posts: 302
Joined: Sun Jul 06, 2003 4:36 pm
Location: Glasgow, Scotland
Contact:

Re: Follow in devit question! Help with program please

Postby Stephen Richard » Thu Jan 24, 2013 4:33 pm

I don't remember the size of the piece you are firing, but in a top fired rectangular kiln I regularly fire two layer pieces of up to 450 mm dia. at:
200ºC/hr to 677ºC, soak for 30 mins
300ºC/hr to 804 (flat fuse) for 10 mins
9999(AFAP)to 516ºC, 30mins
80ºC to 370ºC, 0 mins
250ºC/hr to 100ºC, 0
off

An alternative is to go at ca 50ºC/hr between 625º and 677ºC still retaining the 30 min soak.

Your programme did not seem extremely slow to me, so I still think it may have something to do with cleaning.

Another possibility is, if you are using thinfire, many recommend venting the kiln until about 500ºC
Steve Richard
You can view my Blog at: http://verrier-glass.blogspot.com/

theanimallover
Posts: 48
Joined: Tue Jan 22, 2008 9:19 am
Location: The Sticks, England, UK

Re: Follow in devit question! Help with program please

Postby theanimallover » Fri Jan 25, 2013 7:13 pm

Hi yes rapid cool and I use thinfire. The pieces are just 9cm. I don't think it's a cleaning issue as I am thorough and clear capped pieces come out well.

Thanks.

Stephen Richard
Posts: 302
Joined: Sun Jul 06, 2003 4:36 pm
Location: Glasgow, Scotland
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Re: Follow in devit question! Help with program please

Postby Stephen Richard » Sat Jan 26, 2013 4:48 am

It has been recommended many times on the list here that when using thinfire, you should vent the kiln to a temperature above the burnout of thinfire, somewhere above 500ºC.
I am fortunate, and do not have that problem, so this is just reporting on others' experience.
Steve Richard
You can view my Blog at: http://verrier-glass.blogspot.com/

theanimallover
Posts: 48
Joined: Tue Jan 22, 2008 9:19 am
Location: The Sticks, England, UK

Re: Follow in devit question! Help with program please

Postby theanimallover » Sun Jan 27, 2013 9:17 am

Thanks Stephen. I have never vented before, I will look into it and give it a go. I still find it odd that clear capped glass always comes out fine but if the top piece is coloured I get devit.

I would prefer not to use thinfire but when I started fusing, using kiln wash, the kiln wash stuck to the bottom and ruined the glass.

Stephen Richard
Posts: 302
Joined: Sun Jul 06, 2003 4:36 pm
Location: Glasgow, Scotland
Contact:

Re: Follow in devit question! Help with program please

Postby Stephen Richard » Sun Jan 27, 2013 11:12 am

theanimallover wrote:Hi,
Further to my previous post. I am having problems with devit on sky blue and spring green transparant glass, it shows up more on the the green I believe as I back with white which illuminates the glass (and the problem).
When I fire on my bubble squeeze program the glass has no bubbles but devit, if I use my preprogrammed 'slow' it has no devit but bubbles! Please can someone help with a new in-between program to try?
The bubble squeeze:
300 750 10
300 1150 30
50 1250 2h
300 1480 15
Slow fuse (no devit but bubbles occur) I use the skutt programmed program. I can't find info on what it exactly is, but I think it is this one:
300 250 25
300 500 25
300 750 25
600 1250 20
I really appreciate your help (in advance :-))
Thanks,
Karen


I have looked at your schedule again, and noted how long a soak you had at 677ºC. After a slow advance from 620ºC a 2 hour soak is excessive. It is at the bottom of the devitrification range so can prepare the conditions for devitrification. A maximum of 30 mins is all that is required at the bubble squeeze.
In your second schedule I find the soaks unnecessary. Glass performs best with regular steady advances in temperature. So, you can alter the schedule to give that steady rise by thinking about how long the kiln takes to achieve the temperature you require. You are essentially climbing in temp at 300ºF up to 750ºF, then advancing rapidly to the top temperature, but without a bubble squeeze. Also 750F is below the strain point of the glass, so you are taking risks with thermal shock to the glass.
In the schedule above you are taking 3.75 hours to get to 750F. you can get there in the same amount of time by using 200F/hr and this will have a smaller chance of shock. It is of course, relatively slow - you could take it up at 200C (360F) per hour safely.
However, you have no bubble squeeze of any note in this schedule - which occurs at the softening point of the glass - between 620 and 677ºC (1150 and 1250F). 333ºC/hr is simply too fast to allow the glass to reach the softening point satisfactorily. The slow rate of advance in your first schedule would work with only a 20 min soak, or simply a stop for 30 mins after a 200ºC rate of advance to 677ºC.

If you are using Bullseye kiln wash and firing below 1500F for 10mins at top temperature, you should not have any big problems. But remember you need to renew your kiln wash each time you go above about 730ºC. A chemical change occurs in the kaolin - a constituent of the kiln wash - at around that temperature. There are other kiln washes, of course. Primo works well in my experience, so will a solution of alumina hydrate, CMC and water.
Steve Richard
You can view my Blog at: http://verrier-glass.blogspot.com/

Steve Immerman
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Joined: Sun Mar 09, 2003 4:55 pm
Location: Eau Claire, Wisconsin
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Re: Follow in devit question! Help with program please

Postby Steve Immerman » Sun Jan 27, 2013 1:18 pm

I would go slow from 1150-1250 (like 60 dph), then very fast to full fuse (like 800 dph), and keep your top temp as low as you can (like 1470).

Here are some other thoughts on preventing devit:
http://www.clearwaterglass.com/Tutorial ... ation.html

Steve

theanimallover
Posts: 48
Joined: Tue Jan 22, 2008 9:19 am
Location: The Sticks, England, UK

Re: Follow in devit question! Help with program please

Postby theanimallover » Tue Feb 05, 2013 8:44 pm

Thank you both Steve's.
I will carefully consider schedules. My bubble squeeze has gone from 30min to 2 hours over the last couple of years, perhaps it excessive.

You mentioned the second schedule could cause thermal shock. This is actually the program that is set on the kiln which is a worry.

I will take a look at devit info now, thanks.


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