Changing the shape of a glass bottle, be it wine, Coke, or any kind of soda, is done through a melting process called "slumping." It's a fun way to recycle glass and create quirky, interesting new uses for bottles. You can make spoon rest, wall hangings, even Christmas tree ornaments, or elaborately twisted vases. A kiln is necessary, and can be expensive, so if you're a beginner glass slumper, it helps to start by borrowing one from a friend before you make the investment.
Study glass melting basics. This includes how to use a kiln.
Clean your soda bottle by washing it in the dishwasher or rinsing it with soap water and letting it dry on a dish rack. Use a hairdryer or soak the bottle in warm water to remove any labels.
Lay kiln paper in the shelves of your kiln to avoid your bottle sticking to the kiln. Place your bottle in the kiln on the paper. If you would like to change the shape of your glass to something specific, place it in a mould for that thing. If you would like to merely slump your glass, or make it flat, then leave it on the paper as is. If you're using a mould, you need to coat it in kiln wash first. This can be done by painting the wash on with a paintbrush.
Heat the kiln to 593 degrees Celsius. This should take a little over two hours. Once it's hit that heat, let it rest there at the same temperature for 10 minutes. Then, increase the heat to 0-10.556 degrees Celsius with an increase of around 250 degrees per hour. Lastly, crank the heat up to 1,425 degrees.
Lower the temperature to 1,100 degrees after hitting the peak of 1,475 degrees and open the door to your kiln. Be very careful with the open kiln. Once you've hit 1,100 degrees, lower the temperature to 1,030 degrees and close the door for one-half hour. Then lower the temperature to 850 degrees. After two hours, allow your glass to cool to room temperature.
If you'd like to change the shape of your bottle into something specific, a mould is commonly used but not necessary. Thicker soda bottles will take slightly longer to slump and cool.
Be sure to wear all necessary protective gear when operating your kiln, including safety goggles, tinted glasses for looking into the kiln, long sleeved clothing, heat resistant gloves. A welders helmet works too. If the bottle crack during the process, slow your heating or cooling rate.