Glass Firing Schedules

Why Bother?

We all know that it’s true, so let’s just put it out there:   You don’t want to be here reading a tutorial  about firing schedules when there are so many more interesting topics on the other pages of this website.  So, before I lose you to pressed glass, faux stone, or photos cats with silly captions, consider these two simple facts:

When you consider the cost (in both time and money) of losing your work to a bad firing, it is easy to become frustrated.

Here’s the good news:  Understanding firing schedules and recognizing a bad one requires no math skills.   All it takes is a little information and some common sense.

This tutorial provides the information.

It may also help to put you at ease to know that almost nobody creates firing schedules from scratch.  I certainly don’t and I sometimes fire hundreds of different glass projects every single week.   Engineers who understand advanced mathematics, thermal dynamics, and phrases like “tensile at the mid-plane” and “parabolic variation” create firing schedules.

Our job is much simpler:  
1.    Recognize a good firing schedule.
2.    Adapt it to the project at hand.  

This Tutorial will help you to do both.

Overview

With the exception of firing schedules for casting, which are outside the scope of this article, glass fusing schedules can be broken down into these seven phases*:

Heating Phases:

•    Initial Heating            •    Bubble Squeeze             •    Final Heating

Change Phase:

•    Process Step

Cooling Phases

•    Rapid Cooling            •    Annealing            •    Final Cooling

 

Firing Schedule Overviiew

 

Mastering firing schedules requires no more than a good understanding of what each phase does so that you can adjust it to accommodate your specific needs.

With that in mind, let’s warm things up!

 


* For the purposes of this article, a “phase” is a part of a firing schedule that has a singular purpose.  A “segment” is a single cooling or heating step, plus any temperature hold.  These are the steps we program into our kilns.  As you will see in this article, a single phase might require multiple segments or just part of a segment.

 

Initial Heating

What it does:

Heats the glass from room temperature to about 900° F (482° C).

Firing Schedule - Intial Heating


Things to consider:

For most glass firings, the only thing to worry about in the initial heating is thermal shock.   Thermal shock can happen when glass heats unevenly and part of the glass expands a lot more than another part.  Since solid glass doesn’t stretch, it breaks.

Here are some situations that can cause thermal shock:

When considering the risk for thermal shock, remember that all your risk factors work together.  A thick piece of glass that is close to the side elements and has a large piece of copper foil embedded between layers is going to require much more caution during the Initial Heating.

Reducing Risk:

The best defense against thermal shock is to slow down the Initial Heating temperature ramp.  Fortunately, you can always slow down this step without any negative consequences.

You can also reduce the risk of thermal shock by ensuring that you load your kiln with the largest projects toward the center of the shelf, where the heat will be most even.

When the edge of your large project must be close to the side elements (when that’s the only way it will fit in the kiln), consider building a short wall of kiln furniture between the elements and the glass.  This will act as a heat baffle and protect the glass for direct shots of heat.

 

Bubble Squeeze

What it does:

Reduces number and size of bubbles both between the layers of glass and between the glass and the kiln shelf.  This stage also prepares glass for more even fusing.  Here is an example of the firing schedule step required for a Bubble Squeeze phase:

Firing Schedule - Bubble Squeeze


Things to consider:

Glass is heavy.  By spending extra time in the slumping range, the weight of the glass will help the glass layers settle together, squeezing out air that might otherwise be trapped between them or between the glass and the shelf.  With less trapped air, bubbles become smaller and occur less frequently.

Here are some reasons to consider lengthening the Bubble Squeeze:

A typical bubble squeeze might be only 30 minutes long.  An extreme bubble squeeze can last two hours or more.  Keep in mind that fused glass is rarely completely free of bubbles.

A lesser known bubble squeeze benefit:

In addition to minimizing trapped air, there’s another benefit to holding your glass at 1225° F: Glass will melt more evenly when it fuses together.  Why?  While glass is a great insulator when solid, as it softens it changes into an increasingly good conductor.  Glass molecules that “loosen up” during the bubble squeeze allow the heat required for fusing to move more easily – and thus more evenly – in the steps that follow.

Final Heating

What it does:

Heats the kiln as fast as possible to get the glass to fusing temperatures.

The Final Heating and the Process Stage (more later) share a single firing segment.  Here is an example of a final heating step for the Final Heating Phase:

Firing Schedule - Final Heating


Things to consider:

By the time we get to our final heating segment we are well past the point at which we need to worry about thermal shock so there is no need to slow down.

In fact, our glass has now entered the temperature range in which devitrification can occur. We want to get to our top temperature as quickly as possible, do what needs to be done, and then get out of the devitrification-friendly temperatures.

Process Stage

What it does:

Changes the glass to the desired state.

Firing Schedule - Process Stage


Things to consider:

When fired correctly, this is the only stage of firing that makes permanent, visible changes your glass .  The schedules for firing glass of a given thickness to a tack fuse, soft (dimensional) fuse, and full fuse will all be identical except for this step.

During this critical phase, the best way to achieve the desired results is to look in the kiln and observe the changes as they happen.   At process temperature, the glass is well past the point of thermal shock so the only risk of looking into the kiln now is personal safety.  Ensure you are wearing proper, high-temperature gloves, eye protection (#2 welding glass work well), and natural fiber clothing (your skin preferrs singed clothing over melted clothing).

When looking into a kiln at process temperature, here are some things that will help you understand what you are seeing:

While process temperature is important, process time is equally critical to your results.  You may find, for example, that holding 1475° F for 5 minutes and holding at 1425° F  for 15 minutes both give you the same result.  When in doubt, always choose lower, slower, and longer.  Why?  Glass melts more predictably when it heats evenly.

Rapid Cooling

What it does:

Cools your glass to just above the point at which stress can be created.

Firing Schedule - Rapid Cooling

Things to consider:

As with Final Cooling, this is simply a transition phase – a step to get us from one temperature (process) to another (annealing).  Other than cooling, there is nothing to accomplish and there is little that can go wrong.  For almost any firing schedule, the segment for this phase is the same:  full speed down to our Annealing phase.

 

Annealing

What it does:

Controls the rate of cooling to reduce permanent stress in the glass.

Firing Schedule - Annealing


Why it is important:

Imagine a thick block of molten glass, sitting in a kiln, at a toasty 1500° F.  Suddenly, disaster strikes when a nearby lightening strike causes a power failure.

The temperature of the air around the glass falls quickly – more quickly, in fact, than the temperature of the glass.   Soon we find ourselves with a kiln full of 800° F air surrounding a block of 1200° F glass. This (relatively) cool blanket of air cools the surface of our glass block.
We now have a problem.  The center of our block of glass is much hotter than the surface. The cooler surface is trying to contract, but the middle of the glass isn’t ready to shrink.  Even though the surface is prevented from contracting, it still solidifies.

The heat in the interior glass will eventually escape.  When it does, the glass will contract and solidify.  As it contracts, though, it pulls away from the now solid surface.  This results in a permanent stress in the glass , which may result in the cracking of the work.

To avoid this scenario, we must cool the glass gradually to minimize the temperature gap between the inside and outside of the glass.  We call this Annealing the glass.


How it works:

It is impossible to keep all parts of the glass at exactly the same temperature.  First, there is no such thing as a kiln that heats and cools perfectly evenly.  Second, the glass that touches the air will always cool ahead of the glass that is wrapped in more glass.  Does that mean that glass always has some stress?   Absolutely – and that’s okay so long there isn’t enough stress to break our artwork.

The well-tested strategy for minimizing stress has two parts:

To successfully anneal a specific glass project, we need to know two things:

  1. How long do I need to hold the glass at the annealing point to ensure that the annealing process starts with all the glass at the same temperature?
  2. At what rate can I cool the glass so as not to create too much stress?

The science and math behind these answers is extraordinarily complex, so for answers we need to look to a trusted reference.  Here are some good online places to start:


Things to consider:

There are a number of things to look for that may be reason to extend the anneal hold and anneal cooling.  These include:

When in doubt, anneal for longer than you believe is required.  Unless you are lucky enough to have a good friend working in the labs at Corning, there isn’t a good way to test glass for proper annealing.

Since you cannot "over anneal" glass, an abundance of caution is your best insurance.

Final Cooling

What it does:

Cools your glass to room temperature.

Firing Schedule - Final Cooling


Things to consider:

As with our initial heating, thermal shock is the number one concern during the final cooling.   Not all breaks during final cooling, though, are the result of thermal shock.

Stress in glass has a cumulative effect.  A slightly aggressive annealing may not cause the glass to break.  Ramping the temperature down a little too quickly during final cooling may not, by itself, result in a broken piece.   The stress that results from doing both of these things on the same piece may collaborate to create a broken masterpiece.

If the glass is properly annealed, any break during final cooling is almost always either the result of thermal shock, incompatible glass, or glass sticking to the shelf.

Some Last Thoughts

The task of creating firings schedules has a reputation for being difficult and, if we are talking about creating firing schedules from scratch, the reputation is well deserved.

Fortunately, we don’t need to be material scientists with a deep understanding of things like dynamic viscosity and Fourier's law. What we do need is a starting point (a reliable schedule), a basic understanding of what each stage of a firings schedule is meant to accomplish, and a desire to learn a little more each time we fire glass.

It also doesn’t hurt to have a good sense of humor and an appreciation for learning when, despite our best efforts, things don’t work out quite the way we had planned. Taking the time to understand firing failures is the best way to expand your skills at adapting schedules for difficult projects.