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Heat the kiln at a rate of +500F (+275C) per hour, until it reaches 1100F (590C). This will just start to heat the bottles. If you are using ceramic molds, you may want to use a slower heating rate to reduce the risk of cracking the molds. "Soaking" the glass at this temperature ensures that every part of the glass reaches the correct temperature. Pay close attention to the steps below to find out how long to keep the kiln at each temperature. This time, heat the kiln at a rate of no more than +250F (+140C) per hour, for a little more than an hour. At this point, the glass will just begin to lose its shape, especially in the middle. You can hold the temperature here for 20 minutes if you're aiming for a flatter, wider middle, or move on after a few minutes if you want the middle to keep more of its shape. Heat at a rate of +300F (+165C) per hour if using ceramic molds, or faster if not. Stay at this temperature until the bottles have slumped to the desired appearance. This is the step that changes most, based on your bottles, kiln, and desired appearance. Consider these numbers a starting point for your first project. Always wear eye protection while looking through a peephole. If your kiln doesn't have a window or peephole, you won't be able to check on the bottles. Lift the lid of the kiln taking care to protect yourself from the heat to cool the kiln rapidly until it reaches a temperature between 900 and 1100F (480 to 590C). The less time the bottle spends at high temperatures, the lower the chance of devitrification, or the forming of cloudy surface textures. Glass takes on significant stress when heated, and can crack or become brittle if it is not "annealed", a process in which the glass molecules are rearranged in a more stable pattern before they cool. There are two common ways to do this: The simplest method, which is usually sufficient for bottles, is to let the kiln cool gradually, never by more than -150F (-80C) per hour. If you kiln cools faster than this, you'll need to briefly fire it occasionally to counteract the cooling. For more effective annealing, leave the kiln at 900F (480C) for a full hour. Different types of glass have different optimal annealing temperatures, so to be extra-safe you could also leave it at 1000F (540C) and/or 800F (425C) for an hour each, starting with the highest temperature first. The bottles should have slumped flat. If you used kiln paper and the fibers stuck to the bottle, wear a respirator mask while cleaning them off the glass.
Fire the kiln to 1100F (590C). Hold this temperature for ten minutes. Heat more slowly to 1300F (700C). Heat rapidly to about 1450F (790C). Flash vent the kiln to about 1000F (540C). Anneal the glass. Let the kiln cool to room temperature.