One of the most fascinating things about fusing glass is how it transforms in the kiln at different temperatures. If you've ever wondered why your glass behaves the way it does during firing, this guide will break it down for you! Here’s what happens inside the kiln as the heat rises.
Below 1000°F (538°C): No Visible Changes
At this stage, the glass looks exactly the same as when you put it in the kiln. It’s still solid, with no movement or change—just hanging out, waiting for things to heat up!
1000°F to 1250°F (538°C to 677°C): The Softening Stage
As the temperature climbs, the glass starts to soften slightly. This is where slumping happens—glass can slowly conform to a mold, making things like bowls and plates. If you’re working with certain colors, especially ones with gold in them, holding the kiln at around 1225°F (663°C) for a couple of hours (a “Gold Hold”) can help develop the right color.
1250°F to 1350°F (677°C to 732°C): Tack Fuse
At this temperature range, pieces of glass begin to stick together, but they keep their shape and texture. If you’re looking for a design with dimension—like raised patterns or layered effects—this is the sweet spot. The edges soften, but they don’t completely melt into each other.
1350°F to 1450°F (732°C to 788°C): Contour Fuse
Now we’re getting into contour fusing, where the edges of glass pieces round out and smooth over, but the overall texture isn’t completely lost. This is a great technique if you want a slightly melted look while keeping some definition in your design.
1450°F to 1550°F (788°C to 843°C): Full Fuse
Here’s where the magic happens if you want a smooth, unified piece. The glass completely melts together, losing all texture from separate pieces. If you’re aiming for a polished, flat surface, this is your temperature range.
Above 1550°F (843°C): The Molten Stage
At this point, glass starts behaving more like a thick liquid than a solid. This is used in kilncasting, where molten glass fills a mold to create detailed shapes. Be careful, though—going too hot can lead to devitrification (a rough, cloudy surface) or bubbling from trapped air escaping.
Other Factors to Consider
- Color Development: Some glasses, called "strikers," look different before firing and need to reach a certain temperature to reveal their true colors. For example, some pinks and purples only develop after hitting the right heat level.
- Special Effects: Certain glasses react in fun ways at specific temperatures. Steel Blue Opalescent, for instance, can develop a metallic sheen at a full fuse (around 1490°F or 810°C) or even at a tack fuse (1375°F or 746°C) with a short hold time.
Understanding these temperature changes is key to getting the results you want in fused glass. Whether you're aiming for a lightly textured tack fuse or a smooth, fully fused piece, knowing what happens at each stage helps you control the outcome.
Happy fusing!