Flattening glass bottles requires the use of a kiln. The bottle must be heated to an extreme temperature and then cooled slowly.
Flattening a glass bottle is also called “slumping.” A slumping project requires a bottle that is clean, dry, and label-free. To remove the label, the bottle should be soaked in a water and dish soap solution, and scrubbed until the label adhesive is completely gone. Nail polish remover can help if the label is especially stubborn. The bottle should be cleaned inside and out, and allowed to air dry thoroughly.
The next step is to prepare the kiln. Kiln wash or kiln paper should be applied to the kiln shelf. Some kilns are programmable while others have to be controlled manually. The kiln should be set to warm at a rate of 500 degrees Fahrenheit per hour. Any faster, and thermal shock could occur. After the kiln reaches 1100 F, it should be “soaked,” or kept at the same temperature, for 10 minutes. This allows temperature uniformity in the glass.
Next, the temperature in the kiln should be increased again slowly, at a rate of 250 F per hour. After reaching 1300 F, the kiln can be heated more quickly. Slumping should occur around 1425 F, if not earlier. The kiln should then be “crash cooled” back to 1100 F. Opening the lid slightly is recommended. Then, slow cooling should occur, at 150 F per hour. The bottle is ready when it is completely cool.