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How to fuse glass
Basic Guide to Fusing Glass
In order to fuse glass you have to know at least some basic properties of the
glass. The first thing that is the most important is that all glass is not created equal. Glass expands when
heated. This expansion can & is measured. This expansion is called Coefficient of Expansion (C.O.E. - the
rate at which glass expands). The glass you will fuse has to match in expansion or it will crack upon cooling.
Bullseye glass, Wasser glass & Uroboros glass make tested compatible at 90 C.O.E.. Spectrum System 96 &
some Uroboros glass is tested at 96 C.O.E.. Moretti is 104 C.O.E. & Pyrex (Borosilicate) is 32 C.O.E. Never
mix glass that you don't know the C.O.E. of unless you run compatibility tests.
In this project you will fuse a 4 inch
You must prepare your kiln for firing glass. To do this apply kiln wash
(following manufacturers directions) to the bottom of the kiln. Use a haik brush to apply the wash. Make sure
you go in 1 direction only. (The wash should flow off the brush easily as in pushing a puddle.) Then apply
another layer flowing the wash at a 90 degree angle from the previous application. Apply 5 or 6 coats in this
manner. Fire the kiln to 500° to evaporate the water in the kiln wash being sure to vent the kiln lid with a
small kiln post. Let your kiln cool naturally. Next apply glass separator (Or kiln wash) to your kiln shelf in
the same manner as before. Fire the shelf to 500° same as before) & let the kiln cool naturally (Do not
open the lid wide this can crack your shelf!)
You will need: a glass cutter, compatible glass 2 pieces of standard thickness
(approx. 1/8ths inch thick [3mm]) glass (clear is best to start with), plus colored glass for decoration. I
recommend thin glass. Optional: Frit (small broken up particles of glass), Confetti (little pieces of very thin
glass flakes) &/or stringers. Some Elmer's glue (thin it just a little with water) or use honey (The kind
you eat.), & Windex.
First cut your 2 pieces of glass & clean them well (finger prints can leave
ugly marks on fired glass). Lay your bottom piece down on your kiln shelf (Hold the edges of your glass to
reduce fingerprints.) Then lay your top piece on it. Next cut designs out of thin glass, &/or frit,
stringers, confetti etc. & lay them on top of your glass. You can hold them in place with a drop of the
thin Elmer's or the honey. Do not pile up your decorations on top of each other, you can overlap some of the
pieces but just don't pile it up.
Now you are ready to fire. Your target (Set point) temperature is 1450°. You can
heat your kiln at 900° per hour to 1,000° for top fired kilns, for side fired kilns slow the heating to 600°
per hour. Then fire as fast as your kiln can go up to 1450°. It is important to get past the 1300 -1400° range
This is where glass will devitrify* (ugly white stuff on the surface of the glass). I recommend a hold of 30
minutes at 1175 degrees to relax the glass & to reduce bubbles between layers. At the 1450° set point hold
the kiln. You can remove the kiln peep hole plug after 10 minutes to check your glass. & then replace the
plug. Keep checking the glass every few minutes until the glass surface is flat & the sides are rounded
(like the alphabet letter "f" without the cross line!). Your glass should be fully fused with in 20 minutes,
but kilns do vary. So watch your glass. Do not let it stay so long at 1450° that you loose the square shape of
Once your glass is 'done' pull out the peep hole plug & open the kiln lid all
the way to "flash" vent your kiln. Vent until the temperature drops to 1,000°, (if you wish you can rest the
glass for 5 minutes or so at 1,000° to distribute the heat evenly). then close the lid & replace the plug.
The kiln will heat back up to approx. 1175°. That's OK to let it do that. Once the kiln drops to 950° hold it
there to anneal the glass for 30 minutes. If your kiln has thick fire bricks & cools slowly you can turn it
off & let it cool to room temp. If it is a ceramic fiber kiln you will have to fire down at a rate not
faster than 300° per hour. I recommend an approximate 6 - 8 hour cool down for all projects of up to 8 inches
across & 2 to 3 layers thick. Never take you glass out of the kiln until it is actually room temperature.
If you do you will thermal shock your glass by exposing it to cooler air while still brittle & hot.
Co Efficient of
Don't mix glass with different co-efficient of expansion (COE). It will just
break up. This is due to glass such as (Moretti 104 c.o.e.) heating and cooling at a different rate than say
Bullseye which is 90 coe. The coe can be no more than 1 point in difference.
There is more to it than that but, it's sufficient to say don't mix different
To fuse the glass while keeping the shape of the different pieces and dement
ional shape of the glass. Usually between 1400° and 1425°.
To hold the glass at 1100° to 1275° to reduce bubbles.
Glass is brittle between room temp & 1,000°. Do not open your kiln under
1,000°. (You can't see the glass anyway.) At over 1,000° the glass is hot enough to view it. Please wear eye
protection when viewing your "hot glass". The air escaping from the kiln can damage unprotected eyes!
If you want to re-fire your glass a second time to slump it or add more
decoration, remember that it is now twice as thick & you can only heat it up at 450° per hour. (heat half
as fast as when you first heated it up. Cool down is the same way.)
Surface of glass has crystallized due to remaining to long in the
temperature range just before it get molten. (Some dark colored glass & some opal glass is subject to
devitrification more than others. Such as transparent cobalt.)
To make harder or strengthen . You can't over anneal your
Now go and have fun!
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© 2002 M. Bolser, All rights reserved