What Is Annealed Glass? Understand the Glass Annealing Process to Avoid Heartbreak

Have you ever spent hours working on a kiln formed glass art project only to have it shatter when you tried to remove it from your kiln?

If you have, then you probably already understand the importance of annealing glass.

But failing to anneal your glass doesn’t always result in such dramatic heartbreak. Sometimes your glass art project will crack months or even years down the line – or fail to fuse together in the first place.

We’re here to help you avoid that! In addition to defining annealed glass, this article will help you understand how to use the glass annealing process to keep your projects (and your spirit!) intact.

What is Annealed Glass?

Annealed glass is glass that has been cooled in a slow, controlled manner. This controlled cooling phase is known as the “glass annealing process.”

If you just allow high-temperature glass to air cool, it retains thermal stress, making it more susceptible to cracks, breakage, and failed fuses. Holding the glass at a steady temperature during cooldown gives it time to stabilize, resulting in stronger, more durable glass that’s less susceptible to thermal shock.

Applications for Annealed Glass

In this article, we’ll be talking about annealed glass in the context of kiln forming and glassblowing. But in a broader context, annealing is used for almost every glass object that’s fabricated, machined, or manufactured.

This includes:

• Windowpanes
• Drinking Glasses
• Kitchenware
• Tabletops
• Mirrors
• Bottles
• The sheet glass and accessory glass you purchase for your glass art projects

However, annealing isn’t the only process for cooling glass. Glass can be “quenched” – or cooled rapidly with high-pressure, high-powered fans.

This technique is known as “tempering glass.” Just like when it comes to heat treating metals, quenching glass results in harder, more impact-resistant glass that’s used for windshields, safety glass, and phone screen protectors.

The Importance of Glass Annealing

For glass art projects that involve kiln forming, glass fusing, or glassblowing, annealing is a must. Unlike most solids, glass has an irregular – or amorphous – molecular structure.

The same unique structure that gives glass its transparency also makes it susceptible to thermal shock. Sudden changes in temperature cause the molecules to expand and contract at different rates.

This leads to more internal stress and less stability, which is why glass art that hasn’t been annealed is notorious for shattering or fracturing.

Annealing Glass for Kiln Forming and Glassblowing

The good news is that glass annealing is easy! It’s just a couple steps you have to program into your kiln firing schedule – which a programmable digital kiln controller like TAP II or TAP II Pro can automatically execute.

However, there are some key differences between annealing for kiln forming and glassblowing. So, we’ll be covering those below!

Straight Annealing vs. Reannealing

For kiln formed glass, heating and controlled cooling happen within a single firing schedule. This process is known as straight annealing. The glass never leaves the kiln between heating and annealing, and the glass annealing process is built right into your firing schedule.

During glassblowing, glass often cools down on its own before it’s had the chance to anneal. In that case, you will need to place your project in an annealer kiln. There, you’ll reheat the glass slightly past the annealing point and slowly cool it. This process is known as Reannealing.

(If you see a term you don’t know, don’t worry – we’ll be defining them in the section below!).

Powering your annealer kilns with TAP II or TAP II Pro provides precise cooldown – as well as the convenient option to remotely start your annealer from your smartphone instead of having to make a separate trip!

Important Glass Annealing Concepts to Understand

Pretty straightforward, right? But before getting into the glass annealing process, it’s important to understand a few concepts:

• Softening Point (or Deformation Point): This is the temperature at which glass becomes too soft and liquefied to fully maintain its shape. If you’re reannealing, you want to make sure you don’t exceed this temperature drastically or for a prolonged hold.
• Annealing Point: This is the setpoint at which glass partially liquefies, quickly relieving internal stress inside the glass. We recommend holding your kiln at this setpoint for at least 30 minutes, though for bigger projects, you might want to hold longer.
• Strain Point: This is the temperature where glass transitions from a partial liquid to a solid with no detectable liquidity. During glass annealing, you’ll want to slowly cool your kiln at a controlled rate past the strain point before allowing it to cool to room temperature.

Once you understand these concepts, you’re ready to start annealing! But first you need to know those temperatures for the kinds of glass you’re working for.

Annealing Temperatures for 90 COE Glass

Each type of glass has its own unique temperatures for each of these setpoints. For example, let’s take a look at Bullseye 90 COE Glass – a popular option for kiln forming and glassblowing. Below are the temperatures to keep in mind:

• Softening Point: 1200°F (649°C) – For kiln forming, every project will exceed the softening point. That’s what makes them slump, cast, or fuse (find setpoints for different glass art techniques on this chart). However, if you’re annealing blown glass, you’ll want to be careful not to risk deformation by exceeding this temperature.
• Annealing Point: 900°F (482°C) – This is where the magic starts to happen! Glass gets hot enough to release internal stress and become resistive to thermal shock. You’ll want to hold your project here to make sure it fully anneals.
• Strain Point: 850°F (454.44°C) – This won’t be a setpoint you program into your firing schedule. But you’ll want to be mindful of this number and slowly ramp down your kiln past the strain point.

Again, I want to reiterate: These are the corresponding temperatures for Bullseye 90 COE glass. Different types of glass have different temperatures for each of these setpoints. Make sure to review manufacturer recommendations prior to annealing.

5 Steps for the Glass Annealing Process

Now that you have the full context, let’s take a look at the steps for annealing glass:

1. Heat your project past the annealing point: If you’re kiln forming glass, your glass will already be above the annealing temperature to slump, cast, or fuse – so this part will already be taken care of by your firing schedule! If you’re glassblowing and your project has already cooled, you’ll want to reheat it slightly past the annealing point while making sure you don’t risk deformation.
2. Ramp down to the annealing temperature and set a hold: On cooldown, set a hold at the annealing temperature. This hold time can be as short as 30 minutes for smaller projects – or it might last weeks for some glass castings. It’s impossible to anneal for too long, so when in doubt, set the hold a little longer.
3. Slowly ramp down past the strain point: Once your project has been annealed, you’ll want to slowly ramp down the temperature of your kiln through the strain point. How fast you’ll set your ramp rate varies greatly based on the size and thickness of your glass. For extremely general guidelines, Delphi Glass recommends a ramp rate of 200-300°F/hr for larger projects and 500°F/hr for smaller projects.
4. Allow your kiln to cool to room temperature: Once your project has passed the strain point, you can safely let your project cool to room temperature. For 90 COE glass, 700°F is typically far enough past the strain point to safely cool your project at the maximum ramp rate. However, other artists might be more or less conservative.
5. Safely remove your project from the kiln: Once your kiln has reached room temperature, it’s safe to open your kiln. Before removing your project, you may want to leave the kiln door open and allow the piece to cool even further – it might still be quite hot! As always, we recommend following proper safety precautions any time you’re operating or opening your kiln – even if it’s at room temp!

Follow these 5 steps consistently for more durable glass art that stands up to thermal shock. Some glass artists have to experience heartbreak, watching a project they spent hours working on shatter into a million pieces (or even just two!), to understand the importance of annealing glass.

But I promise you they never forget it after that!

Understanding the importance of annealing glass is critical. With the TAP Ecosystem, it’s easy to build annealing into your schedules, whether you’re on your phone or with easy-to-use touchscreen controls.

And you can save unlimited firing schedules, so when your glass project turns out just right, it’ll always be easy to repeat it.