Glass frit is coarsely ground glass used in many glass projects, such as casting. Typically, homemade frit is recycled from broken glass pieces, but it's possible to make glass from its base materials. This instructable describes how to make borosilicate glass, commonly known as pyrex. The basic idea is very simple: mix the raw ingredients, put the resulting powder into coated ceramic mold, and heat to 800° C, cooling slowly back to room temperature.
-Furnace or kiln that can heat up to at least 800° C (1500° F)
-Ceramic mold for glass casting (examples found here)
-Glass separator (also known as kiln wash) (example found here)
-Small scale to measure ingredients
-Proper safety equipment for working with chemicals (ie gloves and a disposable respirator) and with the furnace
-Dremel tool (optional)
Step 1: Mix ingredients
***WARNING: silica powder is extremely harmful when inhaled. Always use proper safety equipment! ***
Borosilicate glass is composed of sodium oxide (Na?O), boron trioxide (B?0?), and silicon dioxide (Si0?). To get this final composition, we will be mixing sodium carbonate, boric acid, and silicon dioxide.
Amounts to make 100 g of glass:
Boric acid 49.11 g
Silica 47.73 g
Sodium Carbonate 42.10 g
Weigh out the proper amount of each material based on how much you want to make. Note that the total weight will be greater than 100 g because some chemicals will evaporate in the melting process.
Mix them together thoroughly (Tip: This process can be automated using a very clean, well sealed rock tumbler and some steel balls)
Material Data Sheets:
Refer to these documents for important safety information
Step 2: Prepare Mold
If the mold isn't properly prepared, the cooled glass will react with and stick to the mold. This won't be a problem for the glass, but will cause you to have to buy a new mold. To combat this, apply several coats of mold release to any parts of the mold that might come in contact with glass. Follow the directions on the bottle or use the following:
0. If in powdered form, mix with water as directed
1. Brush onto mold in long, even strokes, covering the whole mold in an even layer
2. Let dry and rotate 90°
3. Repeat at least 3-4 times
4. Heat empty mold to 300° to ensure there is no remaining mosture
Step 3: Make Glass!
The next step is to melt the powder mixture from step 1 until it melts, then slowly bring it back down to room temperature.
Put the empty mold in the furnace or kiln with the following heat cycle:
1. heat slowly (< 600°/hr) to 800° C
1.1 add a small pile of powder to the center of the mold
2. hold at 800° C for 12 hours
2.1 add small doses of powder to the mold while at 800° C every 20 minutes.
3. cool slowly (~ 30° / hr) to 500° C
4. hold at 500° for 2+ hours
5. cool slowly to room temperature (~ 60° / hr).
Important: Do not open the furnace during the heat cycle, except to add powder. Thermal shock could cause the mold to crack or shatter.
Step 4: Extract Glass
Once the furnace has cooled to below about 50° C (112° F), it is safe to remove the mold. The glass will probably be cracked, because stresses formed in the glass as it cools cause fractures. Annealing, or holding at 500° C, is supposed to prevent this by allowing the almost solid glass mixture to reach eqilibrium before solidifying, but this is very difficult to do correctly.
In theory, the mold release should cause the glass to fall out when inverted, but it often doesn't work that well. Instead, find a weak point in the glass at the corner of the mold and use a small chisel or flathead screwdriver to start prying out the glass. Be careful not to break the mold! Water can be used to wash away the dusty kiln wash away as you go.
Step 5: Finishing
Congratulations! You now have a plate (or several pieces) of glass covered with kiln wash, which can be removed with sandpaper or a dremel with a sanding attachment if necessary. From here, the glass can be crushed into frit or melted down for glass working.
Examples of glass art projects:
Mixing raw powder
Further glass working techniques