DANGER!
MELT GLASS
Harness the superheat of fire.
Firewood typically burns at around 900°F but bottle glass doesn’t melt until almost 1,400°F, so we must force the fire to burn hotter by feeding it extra oxygen and constraining the heat to a small area.
1. Prepare your fire area. It must be clear of all combustible material so that you can let your fire burn itself out naturally.
2. Build an oven. Place 2 large logs side by side with a gap between them just large enough to lay the bottle in. Align the logs so that the wind blows straight through the gap. This is going to be the combustion zone.
3. Ignite. The upwind side of the gap is the front of your fire; the downwind side will be the exhaust. Place paper and kindling in the front third of the gap and get your fire started.
4. Build up the fire. Once the kindling is really starting to burn, blow gently on it. Feed kindling and larger pieces into the fire until the logs on either side are burning on their own.
5. Add the bottle. Put on goggles and use a stick to place a bottle at the hottest part of the fire. Place some small firewood across the top of the combustion area to seal in the heat, but make sure the upwind end remains open.
6. Increase the heat. Blow gently and feed medium-sized sticks into the combustion area to keep it well fed until the bottle slumps. Slumping happens when the glass becomes so soft that it cannot hold its shape.
WARNING: Glass that is heated to the melting point and then cooled quickly can shatter explosively. Make your fire somewhere that you can let it burn itself out rather than dousing it with water.
Gever Tulley
REQUIRES
Fire
Water bucket
Glass bottle, soda- or pill-sized
Safety goggles
Adult supervision
DURATION
2–3 Hours
DIFFICULTY
Difficult
SUPPLEMENTARY DATA
A bellows is designed to draw air into a chamber and then force it out through a small opening pointed at the combustion zone. The extra air makes the fire burn faster and hotter. This is useful for melting glass or metal.
Natural glass is made in volcanoes, and by lightning strikes and meteor impacts. Crude forms of glass can be made by heating sand mixed with the ash created by burning certain kinds of plants. So a fire made on a beach might create a small amount of glass, given the right conditions — which is how many archaeologists think the process was first discovered.
7. Cool down. Allow the fire to burn itself out slowly to prevent the glass from shattering due to sudden temperature changes. Once the fire is cool to the touch, use a stick to gently lift the melted bottle from the ashes. It may be very fragile, so be careful when handling it. 
Excerpted from Fifty Dangerous Things (You Should Let Your Children Do) by Gever Tulley with Julie Spiegler (fiftydangerousthings.com). Gever is co-founder of Brightworks, a new K–12 school in San Francisco (sfbrightworks.org).