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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 tile. 
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 the tile.

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.
Tips on fusing: 
Co Efficient of Expansion: 
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 c.o.e. glass.
Tack Fuse:
To fuse the glass while keeping the shape of the different pieces and dement ional shape of the glass. Usually between 1400° and 1425°.
Squeeze the glass: 
To hold the glass at 1100° to 1275° to reduce bubbles.
Further notes: 
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.)
  • Devitrify:  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.)
  • Anneal:  To make harder or strengthen . You can't over anneal your glass!
Now go and have fun!
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  © 2002 M. Bolser, All rights reserved
dicroic pendant instructions 
Click the pendant above to learn how to make a dichroic pendant. (.pdf)