Slumping Questions

Slumping Questions

Post by Mark Charl » Wed, 18 Feb 1998 04:00:00



How critical is it to flash cool after achieving the proper slumping
temp? How crucial would annealing be on a 12"x12"x1/8" piece? Ideally
what I'd like to do is just set the kiln sitter to the right cone & have
it done by morning. I know ideally I should use a ramp controller but I
can't afford one right now.  Thanks for any help, Mark

 
 
 

Slumping Questions

Post by Stev » Wed, 18 Feb 1998 04:00:00


Quote:

> How critical is it to flash cool after achieving the proper slumping
> temp? How crucial would annealing be on a 12"x12"x1/8" piece? Ideally
> what I'd like to do is just set the kiln sitter to the right cone & have
> it done by morning. I know ideally I should use a ramp controller but I
> can't afford one right now.  Thanks for any help, Mark

  Flash cooling isn't critical at all.  Normally this is done when a
piece is taken to "visual maturity" and one wishes to halt the process
at a particular point.

  Annealling *is* critical on a piece that size, but it really depends
on the insulating characteristics of, and thermal mass inside the kiln
as to whether or not a simple powerdown will allow slow enough cooling.

  If you don't have a pyrometer to be able to check the cooling rate,
then it will be necessary to use a polariscope (a pair of polarizing
filters) to check for stress at least on your first piece to see if the
process is yielding a relatively stress-free product.

  Check out the Kiln page at my site, and the Warm Glass FAQ.

--

Metamorphosis Glassworks Page         http://people.delphi.com/stackman
GLASS HOST & Assistant Manager    of     The Delphi Arts & Crafts Forum
                     http://www.delphi.com/crafts

 
 
 

Slumping Questions

Post by Bob Duchesne » Wed, 18 Feb 1998 04:00:00


Quote:

>How critical is it to flash cool after achieving the proper slumping
>temp? How crucial would annealing be on a 12"x12"x1/8" piece? Ideally
>what I'd like to do is just set the kiln sitter to the right cone &
have
>it done by morning. I know ideally I should use a ramp controller but I
>can't afford one right now.  Thanks for any help, Mark

You likely will be able to get away with flash cooling (also called flash
venting) a piece only 1/8" thick. The best way would be to find the cone
that would shut the kiln off as you state. For slunping, 1100'F is about
right for many projects. Flash cooling is generally used to stop the
*action* when fusing at higher temps and checking the progress by eye.
Flash venting is usually stopped above anealing temps and anealing
allowed.
Good luck, Bob
 
 
 

Slumping Questions

Post by Rod Christe » Wed, 18 Feb 1998 04:00:00


Quote:


>> How critical is it to flash cool after achieving the proper slumping
>> temp? How crucial would annealing be on a 12"x12"x1/8" piece? Ideally
>> what I'd like to do is just set the kiln sitter to the right cone & have
>> it done by morning. I know ideally I should use a ramp controller but I
>> can't afford one right now.  Thanks for any help, Mark

>  Flash cooling isn't critical at all.  Normally this is done when a
>piece is taken to "visual maturity" and one wishes to halt the process
>at a particular point.

>  Annealling *is* critical on a piece that size, but it really depends
>on the insulating characteristics of, and thermal mass inside the kiln
>as to whether or not a simple powerdown will allow slow enough cooling.

There is a theory too, that flash cooling or the "kiln crash" prevents
crystals from forming (devitrification) on the surface of the piece.   Maybe
this is more relevant at casting temperatures.

The thin-ness of your piece will help it cool evenly, but small differences
will probably build up stresses fast in a 12 x 12 sheet.  I recommend using
a pyrometer to ensure the ramp through the annealing range.  Just turning my
kiln off results in a cooling rate that is much too fast.  I'm sort of
working on a controller for my home-made kiln, but in the mean-time I can
get a reasonable profile with manual control.  It is very tedious, but it
works.

-Rod Christel

 
 
 

Slumping Questions

Post by MikeFir » Thu, 19 Feb 1998 04:00:00



Quote:
(Mark Charles) writes:
>How critical is it to flash cool after achieving the proper slumping temp?

 Excuse me if I missed something, but I thought flash cooling served the
purpose of
defeating devitrification - the white surface defects caused by spending too
much time in the temperature range above 1100-1200F.  
  It is my understanding that after taking the glass slowly to above the
annealling point (about 900F) then take it as quickly as possible to the peak
temperature (or effect wanted) with soaking if necessary to drop everything
into the mold or thoroughly fuse all layers.  Then the temperature is dropped
quickly to just above the annealling point to avoid devitrification.
  Anything wrong with that?

 Mike Firth, Hot Bits furnace glassblowing newsletter

  Home Page: http://ourworld.compuserve.com/homepages/MikeFirth

 
 
 

Slumping Questions

Post by TGLASSA » Thu, 19 Feb 1998 04:00:00


Quote:
>I should use a ramp controller

Actually I have my controller hooked up to a drip irragation timer so I can use
it to a certain degree and I think they are under 50.00.  I my timer to come on
when I am not in my studio setting  my kiln at med for 2.5 hours till 900
degrees then I turn the kiln to high for the appropriate amount of time
depending on how high I want it to go then  i flash vent, then set the kiln on
low for 3 hours then the timer shuts it off after 4 hours. At least I can start
and shut off the kiln when I'm not around.
 
 
 

Slumping Questions

Post by Stev » Thu, 19 Feb 1998 04:00:00


Quote:

> >  Flash cooling isn't critical at all.  Normally this is done when a
> >piece is taken to "visual maturity" and one wishes to halt the process
> >at a particular point.

> >  Annealling *is* critical on a piece that size, but it really depends
> >on the insulating characteristics of, and thermal mass inside the kiln
> >as to whether or not a simple powerdown will allow slow enough cooling.

> There is a theory too, that flash cooling or the "kiln crash" prevents
> crystals from forming (devitrification) on the surface of the piece.   Maybe
> this is more relevant at casting temperatures.

  Yes, and sometimes even at fusing temps, depending on the glass.
In my experience, devitrification isn't generally a problem at
slumping temps.

Quote:
> The thin-ness of your piece will help it cool evenly, but small differences
> will probably build up stresses fast in a 12 x 12 sheet.  I recommend using
> a pyrometer to ensure the ramp through the annealing range.  Just turning my
> kiln off results in a cooling rate that is much too fast.  I'm sort of
> working on a controller for my home-made kiln, but in the mean-time I can
> get a reasonable profile with manual control.  It is very tedious, but it
> works.

> -Rod Christel

  If your kiln won't give you a slow enough cooling rate with a simple
power-down, often a ceramic fiber blanket thrown over the top of the
kiln will slow cooling enough to give you a usable annealing rate for
a piece this size.


Metamorphosis Glassworks Page         http://people.delphi.com/stackman
GLASS HOST & Assistant Manager    of     The Delphi Arts & Crafts Forum
                      http://www.delphi.com/crafts