A kiln is necessary to raise the temperature of glass up to 1500 degrees Fahrenheit, while a blow torch can raise the temperature of glass to approximately 900 degrees. To slump glass successfully and safely, you must do it in a kiln. Using a propane torch alone would not get hot enough to slump the glass, and using a propane torch on glass larger than a rod thickness will cause the glass to crack under thermal shock. Present-day glass lampworkers use an oxygen-propane torch. Oxygen is mixed with the propane to make the flame hotter.