You can melt glass bottles in a kiln to create novel glass bottle trays. When the bottle is hot enough, the top will slump down to the bottom and create a flat surface. Glass must be put through a specific heating and cooling cycle to protect it from becoming over stressed, leading to cracking or fracturing. Slumped bottle trays are great for serving snacks of all kinds. They make perfect housewarming gifts, too.
Take off any labels still on the bottle. Soak it in water until the label peels off. Apply white spirit to a cotton ball and clean off any remaining glue from the side of the bottle.
Set the bottle aside to thoroughly air dry inside and out. Mix up a solution of devitrification of spray. This is a simple water and borax mixture. This spray will keep the bottle glass looking clear while going through the slumping process. Add a teaspoon of borax to a cup of distilled water.
Spray the devitrification spray on the outside of the bottle. Allow it to air dry.
Set the prepared bottle on its side directly on a kiln shelf or place it on a slump mould. A slump mould is made of fired clay and has a definite shape. The bottle will take on this shape when it slumps. Both the kiln shelf and slump mould must be coated with kiln wash, which keeps the bottle from sticking to the surface when it gets hot. Mix up kiln wash per manufacturer's directions and paint it on with a paint brush.
Turn on the kiln and heat it to 593 degrees Celsius at 315 degrees C per hour. Upon reaching 593 degrees Celsius, hold the kiln at that temperature for 10 minutes.
Take the temperature of the kiln up to 704 degrees Celsius. Leave it there for a half hour.
Raise the temperature to 774 degrees Celsius as fast as possible. Stay at this temperature for up to 20 minutes. After the first 5 minutes start looking through the peephole in the wall of the kiln to see if the bottle has slumped.
As soon as it slumps, quickly bring the inside temperature of the kiln down to 538 degrees Celsius. Do this by repeatedly opening and closing the lid or door of the kiln until the temperature drops to 538 degrees Celsius. Hold at 538 degrees Celsius for 10 minutes.
Bring the temperature down to 427 degrees Celsius at a rate of 82.2 degrees C each hour.
Turn off the kiln and don't open it until the inside temperature has returned to room temperature. This could take 8 to 12 hours depending on how well insulated the kiln is.
Bottles come in different thicknesses, which means they will take varying amounts of time to melt or slump.
Slump moulds have air vent holes in them. Make sure after kiln washing the mould that the hole is open and clear for air to flow through. Wear long-sleeved shirts and wear heat-resistant gloves when flash cooling the kiln to protect against the heat of the kiln. Protect your eyes when looking into a red-hot kiln, wear tinted glasses.