Glass is a tough material, but it's still under constant stress. If you've ever seen the bottom of a glass container that's been sitting for too long, you can see how the glass slowly deforms under its own weight. This is because glass contains stresses from being cooled—the same way that cooling steel causes it to become brittle. To relieve those stresses and make your glass stronger, we use annealing ovens or kilns to heat up our glass and then cool it down slowly over a period of time.
Many people don't even know that glass can be annealed.
Did you know that most glass can be annealed?
When you think of glass, you may be thinking of the type that is used in windows or drinking glasses. However, there are many other types of glass and each one has its own unique set of properties and uses. Glass is a solid material that does not have a crystalline structure. Instead, it is composed of atoms whose bonding is based on interatomic forces (Van der Waals forces). This makes it possible to cool down the material and reheat it without losing its shape.
Engineers, chemists, and scientists develop glass for specific purposes.
When you think of glass, you probably imagine a window pane or a drinking glass. That's because these are two of the most common uses for this versatile and often misunderstood material. But how do engineers, chemists, and scientists develop glass for specific purposes?
- Glass is made from silica (SiO2), which is found in sand. Silica has an atomic structure that makes it very strong and resistant to breaking (think quartz).
- Glass is a non-crystalline solid substance that doesn't have the same internal order as metals or rocks. This means that even though it can be melted at high temperatures like other materials, it will not re-form into crystals when cooled down like metals do when they melt; instead it remains amorphous (without distinct shape) forever! That's why you can see through windows but not through bottles filled with water—the molecules are arranged differently inside each type of container so light passes easily through one but reflects off another instead."
Not all glass needs to be annealed.
- Annealing is a process that makes the surface of your glass smoother and more uniform, as well as reducing defects like cracks in the surface.
- Some types of glass are not subject to these issues, so they don’t need to be annealed before use. This includes:
- Glass for light bulbs (e.g., halogen or fluorescent)
- Blown glass for tableware
If you want to make a light fixture or other object that requires strength from its material, it’s best not to use unannealed glass because it can easily crack under pressure.
Annealing is the process of cooling glass slowly to relieve stresses.
It can be done in an oven or kiln, but you can also anneal glass without a kiln using the following steps:
- Clean and dry your piece of glass completely.
- Place the piece on a flat, sturdy surface in an area where it can remain undisturbed for 24 hours or more.
- Turn on a fan and point it toward your work surface so that you don't get any dust particles in the air near your project during this time period. This will create a negative pressure zone around your project, which helps prevent dust from settling on it during its cooling phase after annealing has been completed successfully (you'll see why later).
Kiln Schedule Example
The kiln schedule is based on the glass and the kiln. For example, if you're using a large piece of glass, it will take longer to anneal than a small piece of glass. The same goes for thickness: thicker pieces take longer to anneal than thinner ones. And larger pieces take longer than smaller ones—so if you're making several different sizes of windowpanes, plan accordingly!
And finally, due to temperature variances in your oven, your actual results may differ from those predicted by our kiln schedules. In most cases however this won't be a problem because all our schedules are conservatively calculated for safety reasons; that is why we recommend that you always keep an eye on your glass during any extended periods at high temperatures (such as overnight).
Annealing Schedule - Hot Glass (Borosilicate or Soft Glass)
- Soft Glass
- Anneal at 1550°F for 1 hour
- Borosilicate Glass
- Anneal at 1500°F for 15 minutes, then ramp back down to 1425°F and hold until cool.
- Soda Lime Glass (window glass)
- Anneal at 1140°C / 2116°F - 1250°C / 2192°F from a cold start, depending on thickness. Hold this temperature for 3 hours before quenching in water or oil.
Do-It-Yourself Annealing Oven
If you have a glass dish or pan, a thermometer and some small pieces of glass to anneal, you can make your own do-it-yourself annealing oven. The most important thing about this method is that the temperature must be kept very low, so using a kitchen oven with its much more advanced controls will not work.
- A heat source (stovetop burner or electric heating element)
- A thermally conductive material like stainless steel or aluminum foil that will allow you to set up the oven in a safe place where it won't get knocked over easily
- A small pot/pan with a lid that fits inside the dish/pan
You can anneal a small amount of glass in your oven.
If you don't have a kiln, or if you're just looking for an alternative way to anneal glass without using one, you can use your oven. While it's not quite as good as a kiln (you'll have less control over the process), it does work in a pinch.
First, get some sort of tray that will fit inside your oven and is large enough to hold the pieces of glass you want to anneal. You can find these at the dollar store or hardware store—just make sure they're made out of steel and not aluminum! Then lay down some newspaper on top of the tray so that there's no chance any hot glass will break through onto your countertop when you pull out the tray after heating up in the oven.
Now place your sheet(s) of glass inside this tray and set it aside while preheating your oven to 400°F (200°C). Once heated up (about 10-15 minutes), turn off all fans/ventilation systems/etc., open door slightly more than usual (so gases don't build up too much), then place trays with sheets inside for about 15 minutes before checking them periodically until they reach around 1120°F (~600°C). This should take anywhere from 25-45 minutes depending on how thick each sheet is and how much time has passed since starting heating process; keep checking back until desired temperature has been reached!
Annealing is a process that can be done at home, but it must be done slowly to ensure that the glass does not crack. The best way to anneal large pieces of glass is by using a kiln. If you want to try annealing at home without investing in a kiln then I recommend using an oven or slow cooker as described above.