Glass is fragile stuff. That’s not news to most people. But what may surprise you is that glass can be even more shatter prone—if you neglect one key step in your glassworking.

That step? Glass annealing.

If you want to create glass pieces that are both beautiful and durable, you need to get comfortable with the crucial glass annealing process.

Let’s take a look inside glass annealing, exploring what it is, why it matters, and what you need to do to anneal glass safely and effectively.

What Is Glass Annealing?

Annealing is a glass working process that involves slowly cooling glass to relieve internal stresses and prevent it from breaking.

Without annealing, glass objects are likely to crack or shatter, even if they seem to be cool to the touch.

The Science of Annealing Glass

Glass, a type of substance called an “amorphous solid,” has distinctive properties.

It seems like a solid to the touch. But unlike most solids, the molecules of glass are disordered—similar to those of most liquids. But unlike liquids, glass molecules are fixed in position and aren’t moving around.

The unique structure of glass means that it responds to heat in an unusual way.

It doesn’t go through a rapid phase transition like water does, from frozen solid material to melted liquid. Rather, glass quite slowly becomes molten glass when heated.

Glassworking must be done at high temperatures, when glass viscosity is low. Once you cool the material past what’s called the glass transition temperature, it’s hardened into place. If you cool glass too quickly, it stiffens unevenly, trapping stress points into the glass. You’re left with something very brittle and shatter prone.

Annealing glass alleviates these stresses by cooling the material in a much more controlled manner.

Annealing Temperature and Strain Point

There’s a sweet spot called, appropriately, the annealing temperature or annealing point.

That’s when glass is still malleable, so you can help relax those internal stresses. But it’s still hard enough to hold its shape.

Once you reach this temperature range, it’s important to hold the material there for a time. Then cool it gradually down past the glass transition temperature (when it firms up) to another sweet spot, called the strain point.

That’s when the molecules stop moving, fixing your stressed-relieved glass in place.

What If You Don’t Anneal?

Overlooking the critical step of glass annealing could lead to risks for your final product, including:

• Structural weaknesses: If internal stresses are allowed to harden in, you’re likely to find small stress fractures quickly forming in the glass.
• Spontaneous breakage: Glass can shatter at the slightest knocks without annealing.
• Thermal shock: Glass that hasn’t been annealed will give in easily to the strains of expansion and contraction when heated or cooled.
• Compromised artistic integrity: A flimsy product can easily ruin your hard work and cancel out your artistic vision.

Steps of the Annealing Process

The right annealing schedule for glass varies based on factors including glass composition, thickness, and shape.

Different types of glass have specific annealing requirements, so be sure to consult manufacturer recommendations or seek guidance from experienced glassmakers familiar with the type of glass you're working with.

Here are general guidelines for the annealing process.

1. Initial Glass Cooling After Hot Working

While glass is still in its molten state, you can allow it to cool rapidly in the kiln. Drop the temperature until it reaches annealing temperature—usually somewhere around 950 °F.

2. Holding/Soaking at Annealing Temperature

For basic, flat glass, hold at annealing temperature for about an hour per quarter inch of thickness. So, 2 hours for glass that’s a half inch thick.

You may need to anneal longer for complex or highly detailed glass objects to ensure thorough stress relief and stability. You can’t really anneal for too long, so it’s better to do it longer than necessary.

3. Slow Cooling After Holding

After your anneal hold, reduce the temperature slowly down to below the strain point. This process is key to uniform cooling.

Figure on a rate of 5 °F per minute (300 °F per hour) for glass up to 1/8 inch thick. Go slower for thicker glass. The strain point also varies depending on the glass.

One example: the bottom of the strain zone for Spectrum System 96 glass is about 800 °F.

4. Completing Cool Down

Once the glass goes below the strain point, you can cool more rapidly again down to room temperature.

Considerations and Concerns to Ensure Annealing Success

Here are some additional tips for annealing glass:

• Be precise. Even temperature distribution is crucial to effective annealing. So be sure to use a kiln with precise temperature control, such as a Hot Shot glass kiln. Our kilns are ready to ship in a range of sizes ideal for both hot working and annealing of bowls, vases, small castings, and more.
• Be patient. Your effort and artistry as a glassworker is worth taking the time to protect. If you’re not sure, anneal longer than you think necessary. And don’t be tempted to open the kiln while the glass is annealing!
• Be safe. Don’t handle the glass until it has cooled completely. Use equipment designed to prevent burns, such as Hot Shot’s Cool-Touch Technology.

What You Get When You Do Annealing Right

Start with these guidelines. Consult manufacturer and expert recommendations. Proceed carefully in annealing. When you do, the resulting annealed glass will enhance your glass products in several ways:

Stability and strength: Preventing the formation of microscopic cracks and improving overall durability.

Thermal shock prevention: Ensuring more uniform temperature distribution throughout the material.

Dimensional stability: Allowing the glass to relax and settle into its final shape, minimizing any undesirable deformations.

We think you’ll agree your work is worth this effort! If you have any questions along the way, feel free to contact our team anytime.