The rate at which a kiln's temperature changes is considered the ramp time. This applies to both increasing and decreasing temperatures and is part of the firing schedule of any glass fusing process. A kiln heats similar to an oven only hotter. Your oven needs time to reach your desired temperature for it, and so does a kiln. This rate is measured in how many Fahrenheit or Celsius degrees per hour the temperature changes.
This rate is different depending on many factors. The thickness of glass can influence this rate, as well as the technique or project. Many times though, a slower rate is more desired than a fast rate at least on the initial ramp up to the first temperature stage of a project. This will slowly heat up the glass and reduce the risk of thermal shock from happening, which can cause cracks to occur. There are those times when a fast ramp time is used for glass fusing. This is when you have to depend on the specific directions for the particular technique you are using.
Thermal shock can happen on the initial heating of glass. If the glass does not heat evenly, it can cause part of your piece to expand at a different rate than the rest of it. This could not only cause the glass to crack, but also to break. Preventing thermal shock is a main concern when fusing glass. This is where how fast or slow you ramp up your kiln's heat comes into play on the initial heating. Setting a slow ramp rate will allow the glass sufficient time to heat evenly.
All kilns heat differently from each other. One could reach the heat settings fairly quickly, while another one will reach the desired settings slower. Luckily, the ramp times can be adjusted for this fact if need be. This is especially important when your kiln heats quicker than you need it to for the project you are working on with glass fusing.
Firing schedules are just the ramp rates, temperatures desired at certain stages, soak times, annealing and cooling times. All these things go into the firing process and if any one stage is not done correctly, it can adversely affect the outcome of your project. One ideal thing to do is to make notes each time you fire glass. This way, you can look back at your notes next project to see where any adjustments need to be made. Reading your notes each time helps you learn how to fine-tune all parts of your firing schedule including the ramp rate.
Make sure your ramp times and firing schedules are right to obtain the desired results in your fused glass project. It just take some practice to become proficient at knowing when and how to adjust these things to suit each type of project you create with glass fusing. Make improvements when necessary and your projects will be beautiful.
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