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A Glass Draping Technique
by Mary L. Kersey
Introduction
This tutorial is based on a piece we did a
short while ago which I think serves as a great lesson in draping
glass. The piece was a draped coffee table top made from a 3/8" glass
round. We decided to add a little color to our table top to give it
some flare, and so we'll lead you through that process as well in
order to make this tutorial a little more versatile (for the sake
of kewlness). I decided to break the tutorial into two sections, Coloring
the Glass and Draping the glass. This ordering should help to separate
the two processes for those who are only interested in one or the
other. Ya, ya, ya...let's get to work!
Coloring the Top
To color our table top, we decided to use
a crumpled sheet of copper foil we had lying around our studio. We
wanted it to cover the area that would serve as the actual "table
top", so we had to cut it into a circle that would fit the draping
mold, but not actually drape with the glass. Our round was 24" in
diameter to start with, and we planned on letting the folds drape
about six inches. That meant we needed the foil to be 10" in diameter
(24" - (2 x 6")) - 1" inch border. My mother thanked me for being
a math/cs major ;) The art of crumpling foil isn't really a lost art,
so we'll leave the details of that up to you. What we will explain
is how we prepared the bed our glass rested on.
We chose to use 1/4" thick FiberFrax© paper
(which had the rigidizer fired out of it) texture side up. Then we
set our foil down onto the paper, and centered it as best as we could.
Then we centered the squeeky clean glass on top of the foil and waited
for the exclamation point to be appended to this sentence! Gotta love
emphasis every now and then... it helps to break the monotony of writing
;)
With the glass positioned in a nice orderly fashion, we gave it a
last once over with some cleaner and a soft rag. One of the peculiarities
we find with fusing glass is that fingerprints on the glass don't
burn off! Well, the oils burn off, but they leave a finger print pattern
in the glass?! Not what we wanted for this project so we gave it a
last once over.
The actual firing of the glass wasn't so complicated. The goal of
firing the glass was getting it just hot enough to melt to and partially
oxydize the copper. The partial oxydization of copper turns it a brilliant
deep red with some orange hi-lites. To achieve this goal, we brought
our kiln up to 1450 F at a rate of about 10F per minute. This shotgun
approach was good enough to get us through the critical temperature
for our glass (I'll explain this momentarily). Once we got our glass
to 1450F, which took about 2 1/2 hours to reach, we let it sit at
that temperature for about 30 minutes in order to fuse to the copper.
After sitting for half an hour, we then wanted to bring the temperature
down to the annealing temperature, so we crash cooled it by opening
the lid to our kiln until the temperature reached around 1100 F (you
don't want to crash cool any cooler than this!), then we closed the
lid, and brought the temperature down to 1011F which we figured to
be a safe annealing temperature for our plate glass. We then let it
soak at the annealing temperature for an hour, and then brought the
temperature down to 300F over a span of 10 hours. Once it reached
300F, we turned off the kiln and let it cool off. The result? A rather
kewl looking round of copper imbedded glass with an amazing flare
of red! Now, before we proceed to the actual draping process, let's
have a short talk about glass and the critical temperature which I
mentioned earlier.
The Critical Temperature
Glass has two critical temperatures or zones
where the glass is most likely to fracture due to thermal shock. These
two zones are the 500F degrees and 700F for plate glass. At these
temperatures, the glass begins to build up internal pressures due
to the increased molecular motion caused by the heat. Some areas of
the glass build up more tension than others, and because of this,
the glass can fracture between these tension areas. This is called
thermal shock, and it is a bad thing! So let's discuss how we can
avoid it in our second section which deals with draping glass.
Because the process of draping glass involves resting the glass on
top of a form, portions of the glass will be in contact with a surface
(fiberfrax paper in our case), while other portions will be suspended.
Because fiberfrax paper is a heat refractory, it will heat up much
faster than anything else in the kiln. We need to keep this in mind
when heating our kiln, especially when we reach the critical temperatures.
The effects of the critical temperature on the glass are multiplied
by this factor, so steps need to be taken to ensure the safety of
the glass!
So how do we deal with this phenomena? There are generally two approaches,
either of which work just fine. We won't recommend one over the other,
as everyone has their own style. You should get to know your kiln,
and make your decision based on its characteristics (our three kilns
have more personality than they do characteristics ;). But I will
mention that we use what is called a "blanket" method. The other method
we can call the "School Zone" method (to coin a phrase).
The blanket method is simply a method where we go really slow throughout
the entire ramp time, as opposed to the "School Zone"e; method
where you take the glass up slowly to 500F and soak for 20 minutes,
then ramp up slowly from there to 700F and soak again for 20 minutes,
and then slowly ramp up to 1020F and soak for 20 minutes before shooting
up to 1350F. It seems an eternity while you are waiting but patience
is the most important lesson to learn when working with glass.
We prefer our method simply because we got used to using it early
on, and it works fine for us. We tend to average out the time essential
for the glass to soak, and distribute it equally throughout our ramp
time. We determined that a good blanket rate for our project would
be to take our kiln up to 1020F in a time frame of about 3 1/2 hours.
Preparing to Drape
the glass
At this point, you may or may not have a round
like the piece which we prepared in the previous section. If you prefer
to test your skills on a plain piece of flat 3/8" glass, you can start
from here, and still come up with a really neat table top. The point
of the previous section, was to add an element of color using a fusing
technique that will give you experience in fusing glass, as well as
adding color! From here, we begin by preparing the form which will
suspend our glass over the bed of the kiln.
For our table top form, we used four bricks, a sheet of fiberfrax
paper, and a round piece of cold rolled steel which we found in the
"leftover" pile at a metal yard near our studios.
The bricks are standard 2300F rated soft bricks. These fire bricks
are normally 9 1/2" by 4 1/2" by 2". We stood four of these
bricks up on end in such a way that they would support our forming
surface (you can think of it as a little table to rest our table top
on). We then centered our piece of 12" diameter, 1/8" thick
cold rolled steel on top of the bricks to form a small table. Make
sure that your bricks don't stand out past the steel! We don't want
the glass to stick to or form around the brick when they drape (at
least not for this tutorial ;).
We then took our FiberFrax paper and cut a circle that measured 1/8"
past the edge of the steel circle. This way, when the glass drapes,
the paper will bend over the edge of the steel. This will ensure that
the glass will not come in contact with the steel. We then centered
our incredibly clean glass (no fingerprints allowed!) on top of our
form. If you want, you can move the glass off center for a variation
in the drape.
Before we closed the lid, we checked the glass to make sure it was
level by resting a clean level onto it. If it seemed to be uneven,
then we carfully shimmed the form with pieces of fiberfrax paper.
The glass doesn't need to be too incredibly level, but we chose to
be a little extra careful for the sake of Murphy's Law. Now that all
is ready, we then close the lid, and begin discussion of firing times.
Firing the glass
Below is the program we used to fire our glass table top. This may vary slightly from
kiln to kiln, but it is a good starting point
You'll notice that you should take a peek
to see if your glass has started moving at around 1200F. This is when
your glass will start to move so you need to lift the lid just enough
to look in about every ten minutes to see if your piece is draped
to your liking. Your target temperature should be around 1350F. This
is NOT a guarantee that you will want to stop your slump when you
reach this temperature. Your queue to stop the drape should be "When
it looks good". 1350F is just a general guide to get you there!
Once your piece has draped to your liking, it's time to crash cool
to stop the slump. Open your lid and let your kiln cool down to 1100F
AND NO LESS! Then program your kiln to take it the rest of the way
to the annealing temperature, which for plate glass is around 1011F.
Then let your glass anneal for an hour. After an hour, set your kiln
to take twelve hours to reach 500F. After your kiln reaches 500F,
turn it off and let it cool by itself. Be sure not to lift the lid
until your kiln gets under 100F for precautionary measures.
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