Warm Glass Primer:

 The Elements of a Firing Schedule,
An AzGC Informational
 Hannelore Dean *

You arrange your glass project lovingly on a kiln shelf, pop it into the kiln, close the lid, and then what? Do you punch in the firing schedule you stumbled across on the Internet? Use one of the factory-set programs your kiln came with, even though it doesn't quite fit this project? Shorten the annealing schedule a little because you really, truly need the project finished sooner? Cross your fingers and hope for the best? Make a burnt offering to the kiln gods?

Well, put away the Ouija board there is a better way. Believe it or not, firing schedules are more science than art. That's not to say that there's a simple, one-size-fits-all schedule; quite the opposite. Many factors come into play, and you need to carefully consider all of them when you get ready to fire a project. I'll talk more about those factors in a future post, but for right now I just want to acquaint kilnforming newcomers with the basic science of what happens in there, in the dark, after you close the kiln door and hit "Start."

Thermal Shock and the 5-Degree (or 10-Degree) Rule

Glass is unforgiving. It demands to be heated and cooled so gradually that it is roughly the same temperature throughout its entire mass at all times. If there is too great a temperature variation between one part of a glass piece and any other part, the glass will fracture. This is called thermal shock, and firing schedules are designed to avoid it.

So what does "roughly the same temperature" mean? Different glasses vary in their susceptibility to thermal shock, but there's a rule of thumb that will cover you in most situations you're likely to encounter in your kilnforming career. It's called the 5-Degree Rule (if you're measuring your temperatures in Celsius) or the 10-Degree Rule (for Fahrenheit). In a nutshell, you want to keep all parts of your glass piece within about 5 degrees Celsius, or about 10 degrees Fahrenheit, of all other parts of your glass piece to avoid thermal shock. Obviously, 5 degrees Celsius is not exactly the same as 10 degrees Fahrenheit it's actually 9 degrees but this is just a rule of thumb, remember! And you're not going to be hiding out inside your kiln and keeping a running log of what the temperature is over all parts of your glass project. Your well-designed firing schedule is going to take care of all that for you.


COE and Glass Compatibility

I said above that different glasses vary in their susceptibility to thermal shock. Let me explain that a little further. All materials expand or contract somewhat at different temperatures, and glass is no exception. Different kinds of glass have different expansion and contraction characteristics. The glass you use in the kitchen for cooking and baking is likely (but not necessarily) made from borosilicate glass, a special type of glass that doesn't expand or contract very much; that's why it can withstand more thermal variation than any other type of commonly used glass. Bottle glass and window glass (made from soda-lime glass) expand and contract more than your borosilicate measuring cup, which means they are somewhat more susceptible to thermal shock. Much of the commonly used art glass used for kilnforming has an even higher rate of expansion and contraction, which means it is even more susceptible to thermal shock.

There's a shorthand way to describe how much a particular glass will expand or contract within a given temperature range. It's called COE, which stands for Coefficient of Expansion. (You will sometimes see it written CTE, for Coefficient of Thermal Expansion. It's the same thing.) The COE of any particular glass is a very, very small number as in, a decimal point, followed by a string of zeroes, followed by some non-zero numbers. Another way of putting it would be that "COE 96" glass has a COE of 96 times 10 to the minus 7th power. That's a mouthful, so we lop off the decimal point and the zeroes and just say "COE 90," "COE 96," "COE 104," etc.

Borosilicate glass is used to great effect by glass artists in some applications, but when you're starting out in kilnforming, you'll probably be using glass with a relatively high COE (compared to borosilicate, whose COE is in the 30's). Bottle glass and window glass have a COE in the 80's; the major brands of art glass meant for kilnforming mostly have COE's in the 90's. Certain glass rods, used mainly in beadmaking, have a COE closer to 104.

Why does this matter? Because when you are melting different pieces of glass together, they must be closely matched in terms of their respective COE's. Here's another rule of thumb: You cannot repeat, cannot successfully melt together different glasses whose COE's are more than about 1 to 3 points from each other. And this applies to small projects only; the larger the project, the less wiggle room you have to mix glasses with different COE's. To be more accurate, you can melt them together just fine; but if you do, your project's failure is assured after the glass has cooled down again. "Failure" can be as benign as the piece being already broken when you open the kiln, or as I have described elsewhere as catastrophic as a potentially dangerous explosion (think: flying glass shards ) even months later.

In a later tutorial I'd like to talk about how you can do your own compatibility testing when you have two unknown pieces of glass that you'd like to use together in the same project. For now, though, there's a very simple solution. Starting out, you can buy tested compatible glass from major manufacturers. All System 96 glass, for example, has been thoroughly tested so that you can combine all the different colors of the product line with confidence. Similarly, any piece of Bullseye tested compatible glass can be used with any other piece of Bullseye tested compatible glass. (The same manufacturers also produce glass that is not tested compatible. This glass is meant more for applications such as stained glass, where compatibility is not an issue. Be sure you buy tested compatible glass. It costs a bit more, but it's an absolute necessity when you are melting different glasses together in the same fused, slumped, cast, blown, or flameworked project.)

A word of caution here: glasses from different manufacturers may not be compatible, even if they carry the same COE designation. It's always safer to test glasses from different manufacturers before you use them together. One thing you hear all the time is that Bullseye makes COE 90 glass, and Bullseye glass is invariably listed in the COE 90 category of suppliers' websites and catalogs. The fact is, Bullseye doesn't claim to be making COE 90 glass, even though the COE of their compatible glass is in that range. What they do say is that any of their tested compatible glass can be used with any of their other tested compatible glass. They make no claims about whether any of their glass can be successfully combined with any other manufacturer's glass. If you don't know how to perform compatibility tests, stick with one manufacturer's glass for now.

What about bottle glass and window glass? The answer is, they may or may not be compatible with each other. You must test first before you combine pieces of unknown glass that didn't start out being part of the same piece. If you want to get started right away slumping bottles, that is just fine, because one more rule of thumb here every piece of glass is compatible with itself. If you are organized enough to keep track of which pieces of glass came from which bottle, you can smash a bottle and recombine the pieces of that bottle. If you are smashing two different bottles, you can't be absolutely guaranteed that they will be compatible with each other. Yes, it's likely that two bottles that are the same color, and once held the same product, and were purchased from the same store at the same time will be compatible with each other. But it's not guaranteed. The two bottles may have come from different batches of glass, and the manufacturers of glass containers have no particular interest in whether or not all their bottles can be melted together. If "probably" isn't good enough for you and it shouldn't be, given the risks I've just described then you must test any unknown glasses before you combine them in a fused, slumped, cast, blown, or flameworked piece.

Understanding Firing Cycle Terminology

Because of glass's stringent requirements, not to mention its complete lack of a sense of humor, it needs to be heated for a while, then held (or "soaked") at certain temperature points so that the temperature has time to equalize over all areas of the glass, as well as all areas of the kiln, before proceeding with the remainder of the firing cycle. This is true both when the glass is heating up, and when it's cooling down again. That's why the kiln-sitter method of firing, which can work perfectly for ceramics, is not right for glass.

Instead, glass is fired using something called a ramp-hold firing program. The program is divided into several parts called segments, and each segment is defined by three measurements:

  1. How fast the kiln is heating or cooling during this segment; this is called the ramp rate.

  2. What temperature the kiln is heating to or cooling to in this segment; this is called the target temperature.

  3. How long the kiln holds this temperature before proceeding to the next segment; this is called the hold time, or the soak time.


A typical firing cycle will be written down something like this:

Segment # Ramp Rate (F/Hr) Target Temp. (F) Hold Time(HH/MM)
 1 500 1250  01:00
 2 150 1465 00:10
 etc. etc. etc. etc.


But you'll also see it greatly abbreviated, like this:

  1. 500/1250/1:00

  2. 150/1465/:10

  3. etc.


It's important to understand the concept of heatwork in kilnforming. The idea here is that what happens to your glass in the kiln depends not only on how hot it gets, and not only on how long you hold it there, but rather on the combined effects of heat and time. Vary one, and you will probably want to vary the other. Depending on what you want to achieve, you may decide to fire a particular glass project to a higher temperature for a shorter time, or to a lower temperature for a longer time. Either schedule might be appropriate, but each might have somewhat different results. To illustrate the concept, think about cooking a piece of beef. You could roast it at 450F for an hour and a quarter and it would come out of the oven nicely browned and crispy on the outside, but juicy and rare inside. Or you could take the same piece of beef and braise it with a little liquid inside a covered Dutch oven for four hours at 300F. It would still be delicious, but now it would be the same color all the way through, very tender, and falling off the bone. With experience and good record keeping, you'll come to know whether you want to cook your glass hot and fast, or long and slow, and what effects you can expect from either schedule. This helps to explain why there is no one "right" schedule for firing a project.

What Goes on Behind Closed (Kiln) Doors

OK. Now you know why you need to heat and cool your glass at the appropriate rates, you understand what glass compatibility is, you know about firing program terminology, and you have at least a glimmer of understanding about why different firing schedules may be suitable for the same size, shape, and type of glass. Finally we're ready to talk about what is actually happening to your glass inside the kiln. Here, then, is a general description of the stages your glass goes through when it's on its own in the kiln. I'm providing approximate numbers that would apply to Bullseye glass not because I am recommending Bullseye glass over others, rather because they are known for their scrupulous testing and the accuracy of their published information. Stating temperatures in terms of one particular type of glass will keep things simple and give you a general idea of what kinds of temperatures we're talking about here. I'm also using some of their terminology in describing the various stages of a firing cycle. Other people may use different terms to describe the same things, but this is a good place to start. The numbers I've stated here would be a bit lower for softer glass such as System 96, considerably higher for window glass or bottle glass, and even higher for borosilicate glass. For even more detailed information on a variety of different types of glass, see the reading list at the end of this tutorial.

Initial Heat, and Optional Pre-Rapid Heat Soak:

During this stage of firing, you are bringing the kiln temperature up slowly from room temperature to at least 1000F, or somewhat higher (up to 1250F or so, depending on a variety of factors, including what you are trying to accomplish). Firing too fast at this point may cause the glass to shatter from thermal shock. The actual temperature below which there is danger that the glass may undergo thermal shock is somewhat lower than this (around 850F), but the temperature of the glass always lags behind the temperature your kiln is recording at its thermocouple. Heating slowly until the kiln is reading at least 1000F will ensure that the glass is at least hotter than the threshold at which it can undergo thermal shock. This is also the reason why experienced kilnformers may tell you, "Never open the kiln below 1000F." In my own practice, I go even further and won't open my kiln below at least 1100F, and even at that temperature I don't dilly-dally. I open the kiln, peek, and quickly shut the kiln again. An open kiln will cool off rapidly, and I want to avoid accidentally cooling it too much.

The ramp rate during this phase can vary (a ballpark range is 300F/hour to 600F/hour), but when in doubt, slower is better. There is no downside to heating more slowly during this phase (except, of course, that it takes longer . . . ) You may want to get your project in and out of the kiln quickly, but when you consider the expense, time, and effort that you have put into the project just to get it into the kiln, it's a false economy to hurry here.

Below around 1000F, your glass will look the same as when you put it into the kiln. It won't have started to soften yet. Corners won't have started to round yet. It won't have slumped yet. At the upper end of this range say, around 1250F -- the glass will most likely still be the same shape, but there will be some visible rounding and softening of the edges. This is getting into the slumping range, but for the most part two pieces of glass won't stick together unless held for a very long time.

Somewhere in this range you may choose to soak the glass for a while shorter (or not at all) for small, thin pieces, and longer for large, thick pieces. A hold here provides a chance for the glass temperature to equalize over the entire mass of the piece. Another reason for inserting a hold here is to allow air bubbles a chance to escape through the edges of stacked layers. A hold that is inserted for this purpose is called a bubble squeeze. Bubbles are a fact of life for kilnformers, but bubbles where you don't want them can detract from your piece. Again, patience at this stage will be rewarded with a better result. A bubble squeeze is useful when you have stacked layers of glass that have never been fused together before. If you are doing a second firing (for example, to slump a previously fused slab), you wouldn't need a bubble squeeze because presumably any bubbles that were going to escape would have done so during the first firing.

Rapid Heat to Process Temperature, and Optional Process Soak:

Above around 1000F, your glass is no longer subject to thermal shock. You can heat the kiln more rapidly during this phase. Your target temperature also called process temperature -- and hold during this phase will depend on what you are trying to accomplish. There is some overlap between the temperature ranges at which things happen to the glass, because (remember the concept of heatwork?) a longer soak at a lower temperature may accomplish the same purpose as a shorter soak at a higher temperature.

Some of the possible processes you may want to accomplish include:

  1. Slumping. Somewhere between 1000F and around 1250F the glass begins to soften enough to sag or drape into or over a mold or other support that you have placed under it. The glass doesn't get completely molten, only soft enough to bend. This is one method of making bowls, dinnerware, and so forth from a flat piece of glass. Although some fusers occasionally use firing schedules that allow them to fuse and slump glass in the same firing cycle, by far the more common and accepted practice is to fuse your flat slab of glass first, do any required coldworking, then slump it in a separate firing cycle. For the most part, separate pieces of glass will not stick together at slumping temperature except after a very long hold, if then.

  2. Fire polish. In this type of firing schedule, the glass gets just hot enough so that the surface begins to melt. This will heal tiny nicks and scratches, such as those caused by grinding or sandblasting. The surface of the glass will become glossy. For Bullseye glass, this happens around 1250F - 1350F. Glass will also slump more readily. It may begin to stick together. At the upper end of this range the glass is just beginning to become vulnerable to devitrification, an undesirable occurrence where crystals begin to grow on the surface of the glass, so it's good not to linger between here and around 1450F longer than necessary.

  3. Tack fuse. If you go a little hotter than the fire polishing stage of your glass, two or more pieces of glass get hot enough to fuse together completely while still retaining much of their shape. For Bullseye glass this happens somewhere between 1350F and 1400F. The glass will be softer and will slump more easily perhaps too easily -- in this range. Devitrification can start here; or, if already started, will get worse and can penetrate into the interior of the glass, where it will be more difficult to remove later without ruining the piece.

  4. Full fuse. Between around 1400F and 1500F, Bullseye glass softens enough to flow and seek a common level. When not restrained by dams or molds, glass seeks a thickness of about ". Two sheets of 1/8" glass, therefore, can be fused together without the use of a mold or other restraint. The edges of the glass will be gently rounded, the surface will be flat, and in many cases no coldworking will be needed to finish the piece. Glass thinner than " will shrink up and become rounded, or it may bulge at the corners and become thin and very weak in the middle. Shape distortion will occur. Glass thicker than " inch, if not restrained, will spread out and, if given enough time, will eventually reach a thickness of " which may mean that it will flow right off your kiln shelf and make a puddle on your kiln floor. This is the stage at which kilncarving can be accomplished. (Kilncarving is where sheet glass fully slumps over a refractory form placed in the mold typically fiber paper, but other materials can also be used.)

  5. Casting into a detailed mold. Above 1500F glass becomes even runnier. Temperatures in the mid-1500's will allow glass to run into small mold cavities and pick up a lot of detail. Full fusing can also happen at this temperature. (Even within the same manufacturer's product line, some glasses will be "stiffer" than others i.e. they will be harder and will reach the various degrees of softening at different temperatures, which is another argument for great record keeping.)

  6. High-temperature processes. You may have thought we were already talking about high temperatures, but if we get into the 1700-1750F range, other techniques become possible. The glass will be runnier at this higher temperature, and multicolored slabs fired in this range will have lots of movement, interesting swirl patterns, and unexpected color blending. This is the range in which the kiln can be opened and the molten glass combed with steel implements. (This is an advanced technique requiring many safety precautions, and could be the subject of another tutorial if anyone is interested.) Crucible casting can also be performed at these temperatures.

Rapid Cool:

Once you've achieved the effect you were trying to achieve which may require a hold at process temperature to give the glass time to move in the way you intended there's no reason to hold it at maximum temperature any longer. This is the one place in the firing cycle where faster is better. Since the glass won't undergo thermal shock above around 850F or so, you can safely cool it as rapidly as possible until the kiln controller reaches your target temperature, which will be below 1000F. Remember, the glass temperature lags behind the temperature detected by the kiln's thermocouple, so on the way down it's safe to set your target temperature below 1000F. The manufacturers of System 96 glass recommend 960F. Bullseye has recently lowered their recommended target temperature for this segment of the firing cycle from 960F to 900F.

Besides just saving time, there's another reason to cool the glass as quickly as possible through this range, and it has to do with avoiding devitrification. I mentioned devitrification briefly above, but I'll go into a little more detail here. Glass is often referred to as a supercooled liquid because of the way in which its molecules are arranged. Even in its stiff, solid, room-temperature state, the molecules of glass are arranged in a random, amorphous way, quite unlike a crystalline structure. This amorphous arrangement is what makes glass "glassy." In a certain temperature range, however, glass can devitrify, or come out of its amorphous, "glassy" state and begin to form crystals. These crystals will appear as a scummy, unattractive haze on the surface of the finished piece, and can sometimes penetrate into the interior of the piece. This haze can usually be sandblasted off or ground off if it has not gone too deep, but it's better not to have it in the first place. By cooling the glass as fast as possible between process temperature and the target temperature of 900F - 960F, you can reduce the chances of devitrification occurring.

However, "as fast as possible" doesn't mean you should throw the kiln wide open for a while. This is known as crash cooling, and while there are a few situations in which it may be a good idea (more on this perhaps in a future tutorial), for the most part it's a good way to crash your kiln. The electrical elements aren't meant to be subjected to such rapid temperature changes. Molds, kiln shelves, and kiln bricks can also be damaged this way. Glass artists aren't in complete agreement on this subject, but my practice is almost always to leave the kiln closed and program it to cool at a ramp rate of "FULL" or "AFAP" or "9999" or whatever designation a particular kiln's controller uses. What will happen is that no electrical current will be sent to the kiln elements, and heat will be dissipated gradually through the kiln walls until the target temperature is reached. This will take a lot longer than throwing the lid open, but it is a gentler way to treat your kiln, and it reduces the risk of accidentally going too far and causing your glass, kiln bricks, kiln furniture, or molds to break or at the very least be weakened from thermal shock.

Anneal Soak:

After the rapid cooling phase, your glass piece needs to soak at its new, lower temperature long enough to equalize the temperature over the entire piece and to allow most of the internal stress to be resolved. Stress occurs in glass when it expands upon heating and contracts upon cooling. Stress in glass is relieved by a controlled cooling process known as annealing. Annealing begins at the temperature that you have just dropped to, and continues throughout the annealing range of the particular glass you are using. The low end of the annealing range is typically around 700F. The lower end of the range is called the strain point.

Annealing occurs in two steps, the anneal soak and the anneal cool. The recommended anneal soak temperature varies by glass type. For example, for System 96 glass it's 960F. The Bullseye folks, as I mentioned earlier, lowered their recommended anneal soak temperature to 900F. The length of the anneal soak varies greatly, with the most significant factors being the thickness of the glass piece, the variation in thickness between the thickest and thinnest parts of the piece, and how the glass is situated in the kiln (i.e., on a kiln shelf on posts, directly on the floor of the kiln, in an open-faced mold, in a closed mold, etc.) For small, thin pieces you may need to soak the glass for only half an hour, while very large cast sculptures may have to stay in the anneal soak for months. There is no downside whatsoever to annealing longer (except that, obviously, it takes longer . . . ) At this temperature you are safely out of the devitrification range. However, you are now back into thermal shock range, so don't open the kiln, even briefly.

Anneal Cool:

After the anneal soak, it's time to cool the kiln down gradually to the strain point -- around 700F -- which is the lower end of the annealing range. Residual stress in the glass continues to be relieved through this gradual cooling process. The goal of annealing is to relieve ALL the stress in the glass. Going too fast here will increase the risk that the glass will fail later at room temperature. The ramp rate during the anneal cool will depend on the same factors that governed the duration of the anneal soak.

Cool to room temperature:

For small or medium sized pieces that are " thick, it is normally fine at this point to allow the kiln to cool on its own to room temperature. Larger and/or thicker pieces may require a more controlled cooling from 700F to room temperature.

I know you're eager by now to see how things turned out but room temperature doesn't mean the temperature that a room without air conditioning in the middle of the desert at midday in summer might be. It means the actual temperature of the room your kiln is situated in, right now. If that room is an unheated garage and the ambient temperature is 40F, then that's room temperature for you today -- sorry. Most conventional wisdom says you can take your piece out when the display reads 100F. I suffer from an abundance of caution, and I wait until it reaches actual room temperature. I haven't had a piece explode yet.

Tink-Tink: A Cautionary Tale

I'll leave you today with an anecdote from my first days in the warm glass arena. I was taking a glass casting class at our local community college. There were perhaps a dozen students in the class, all with different interests, all working on different sized pieces. The teacher was trying to balance the needs of all students in a single firing schedule that would allow us to put our projects in the kilns each Thursday, and remove them the following Thursday. This was overkill for the small projects, but they weren't harmed by being fired slower than they needed to be.

One Thursday the kilns hadn't gotten to room temperature by the time class began. This can happen when a kiln contains a lot of material, or when the ambient temperature is high. Our teacher propped the kiln lids open a couple of inches. Over the course of the morning, students pestered him: Can we take our projects out now? Not yet????

When the kilns got to around 100F, we were given permission to retrieve our projects. The molds were still warm to the touch. We covered our projects with our sweatshirts or whatever came to hand and let them cool on our work tables for a while. Then we were permitted to plunge our projects into buckets of water to loosen the molds (a practice I've since learned is a no-no). One woman, who had made a large plaque, cleaned up her project and set it on her table for us all to admire. It was beautiful; we ooh-ed and aah-ed. About an hour later, we heard it -- perhaps the most disheartening sound to a kilnformer's ears: TINK! TINK-TINK! This woman's beautiful glass plaque was now sporting several large cracks. She had well over $200 worth of glass billets in this piece, not to mention all her time and hard work., and she'd been planning to sell it for at least $1500.

Queue up the Supremes (I'm dating myself here, I know) and prepare to lip-synch (paraphrasing slightly): You can't hurry glass no, you just have to wait.

Further reading:

There are so many books to choose from it can be confusing. These are a few of my favorites:

  • Firing Schedules for Glass: The Kiln Companion by Graham Stone. This book, considered by many to be an indispensable studio reference, has been out of print for many years. Happily, it can now be ordered from www.igneousglassworks.com . It's $50 plus shipping. If you like to delve into the how's and why's of things, buy this book.

  • Glass Fusing Book(s) One, Two, and Three by Boyce Lundstrom.

    • Book One: Kiln Firing Glass

    • Book Two: Advanced Fusing Techniques

    • Book Three: Glass Casting and Moldmaking

  • Pte de Verre and Kiln Casting of Glass by Jim Kervin and Dan Fenton

  •  Users of Bullseye glass can't go wrong with their Bullseye TechBook, which provides a concise, expert guide to a number of warm glass topics. You can download the same information for free from the website, but if you want a hard copy you probably couldn't print it all out for $15.


*Note- This entire article is Copyrighted by Hannelore Dean, and licensed for use by A.Z.G.C, 10-12-2010. Reproduction in any form is by permission only.