kilnfailFiring schedules are probably the single biggest source of confusion in kilnforming glass. Over the years (and a bunch of research, testing and listening to smarter-than-me glassists), I’ve developed strategies for schedule management; this series will share a bit. In part I, I talk about “The Rules.” Later, I’ll do some schedule construction.

As always, this stuff is based on MY experience, with my designs, my technical considerations and my understanding. I don’t promise that everyone will agree with all (or any of it)…or that it will work exactly the same way for you. Consider it a starting point for your own explorations.

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[dropcap]NEVER[/dropcap] say “always” (or “never”), at least where firing schedules are concerned. But join any gathering of glassists, and the alwayses and neverses just start a-rollin’ in. NEVER do this, or ALWAYS do that…

[separator top=”40″ style=”none, single, double, dashed, dotted, shadow”]Well, forget “The Rules.” They get you into trouble, stop you from thinking, and just generally mess up your day.

I’m a natural contrarian; when I hear glassists solemnly parroting “The Rules” as if breaking them will earn a visit from the kiln police, my first impulse is to argue. So, as a first foray into discussing the ways to develop a firing schedule, let’s start with…The Rules.

They’re mostly wrong. Click on any of these absolute, etched-in-stone “rules” and you’ll see what I mean.

[accordian][toggle title=”NEVER use pre-programmed firing schedules” open=”no”]Sure you can. If you’re a new glassist with a brand-new kiln, you may have noticed that it comes with a set of schedules built into the controller. Go ahead and use them for awhile.

Experienced glassists, stop screaming: Remember how scary (and expensive) glassmaking seemed when you started out? There’s a lot to think about, and a not-inconsiderable safety concern (1500 degrees in my KITCHEN? Are you KIDDING?).

My first fused glass plate

My first fused glass plate

If a pre-programmed schedule helps new glassists get over that scary hump, why not? We want the new artist to make a few successful pieces and hunger for more, not think that kilnformed glass is too complicated for words.

Besides, there’s a good chance that a preprogrammed schedule won’t fail on those first projects. Small hobbyist kilns hold small hobbyist pieces, so there’s just not that much that can go wrong with the schedule. Eliminating the need to develop a good firing schedule lets the newbie focus on the basics: Glass compatibility, pattern, cutting, assembly, mold releases, etc.

When is it time to leave pre-programmed schedules behind and start building your own? When your artwork starts breaking.

If you’ve successfully fused nine plates and the tenth blows up in the kiln…congratulations! You’re no longer a newbie! Your work has progressed to the point that a one-size-fits-all schedule doesn’t work anymore. You need to begin learning about what actually happens (versus what you want to happen) at each segment of your firing programs.

Then, and ONLY then, is it OK to tell folks they should throw out the pre-programs and think for themselves. [/toggle]
[toggle title=”ALWAYS make each new firing longer than the last” open=”no”]Why? Annealed is annealed and, last time I checked, glass couldn’t count. So why would it care whether this was the first, second or third firing…as long as each previous firing left it well-annealed?

There ARE reasons to lengthen successive firing schedules, but they have more to do with the fact that the first firing usually combines lots of smaller pieces of glass into a single, thicker whole. Thicker or oddly shaped glass needs a longer anneal. Or sometimes the glassist suspects that something–too many firings, abusive coldwork, previous firing misses–has added stresses of its own. So, the schedule tiptoes into the heat in hopes of avoiding a break.

But past firing history by itself should have diddly-squat to do with picking a firing schedule.[/toggle]
[toggle title=”ALWAYS add a hold at every firing segment” open=”no”]This is kinda like running a stoplight and stopping twice at the next one.

The theory is that, since all parts of the glass need to stay within 5 degrees C (a bit less than 10F) to avoid stressing/cracking, you stop at every segment of a ramp up or cool down schedule to let hotter (or cooler) parts catch up.

This is also known colloquially as “evenivity.” Better term: Thermal equilibrium, because that’s what it is.

Personally, I’d rather keep the glass at thermal equilibrium throughout the whole schedule. If you need to add holds to keep the glass from breaking, you’re not controlling the temperature well enough. Glass that’s heating and cooling evenly shouldn’t need a catch-up hold. Slow down to a more gradual schedule.

There are plenty of good reasons, of course, to stick a hold in a firing segment: When you’re squeezing out air between glass particles, processing the glass, performing an anneal soak, giving the glass time to relax after processing, drying out a mold, etc. And some kiln controllers require a hold at every segment.

But adding a catch-up hold to make up for a bad schedule? Nope.[/toggle]

[toggle title=”Low and slow is ALWAYS the best” open=”no”]amberbaskettopsideview1Given how often I write “go low and slow,” you’d think I’d agree with this one and, mostly, I do. Rapid changes in glass temperature lead to more violent changes; the glass will trap more air, slump less evenly, flop instead of settling gently into place, and need higher temperatures to achieve the desired result. Higher temperatures, in turn, can exacerbate devit, cause undesirable color changes, or (if you do it a lot to the same glass) shift compatibility.


There ARE times when low and slow is simply a waste of time, however, such as when you’re bringing a bunch of frit up to process temperatures. There’s not much mass to thermal shock, so ramping up at 100dph doesn’t make much sense (unless there’s some other reason).

And there are cases where low and slow might actually hurt the piece, especially if the glass has a tendency to devitrify. Or, in this tack-fused bowl, for example, I want just enough heat to tack the components together so that they lock, then get them the heck out of the heat. The softer and floppier the glass gets, the less likely it is to hold its shape, and the harder it will be to get this effect.[/toggle]

[toggle title=”You can NEVER fuse/slump/firepolish in the same firing” open=”no”]logcabinplateGee, sorry to hear that. If I’d realized I could never do it, I wouldn’t have made this plate (left) or that bowl (right). Both were made in a standard slumping mold, in a single firing that managed fusing, slumping and firepolishing.lotusbowl

Under normal circumstances, trying to accomplish a fuse and a slump in the same firing is a waste of time. The level of heatwork required to fully fuse glass will also turn it into a puddle in your mold…but that’s under normal circumstances.

There are times, however, when it works. I used Bullseye’s square slumper mold for the pate de verre plate at left. It’s shallow enough that a controlled schedule can fuse, slump, and firepolish in the same firing.

Of course it would have turned out just the same if I’d fused it flat, then slumped, so there’s no particular advantage to doing it in a single firing. And since combining those functions in one firing is a lot more persnickety, with a much higher failure rate, we’re generally better off sticking to flat AND slump firings.

It would be extremely difficult, however, to flat-fuse the lotus bowl (right) and then slump it into exactly that configuration in a mold. Instead, I assembled the pieces directly in the mold and tack-fused them into position, using gravity to slide everything into its final position.

It certainly didn’t save any time–the bowl you see was my second attempt after a LOT of tests with scrap glass, and the annealing schedule was so long and slow that it added days to the kilntime.

I do combine firepolish and slump in a single firing, mostly because it cuts down on firing exposure for the piece. It’s a fairly easy process; just coldwork the firepolish area to at least 400-grit and then slump a bit hotter/longer than you normally would. Once you figure out how your kiln behaves, it won’t take long to work out the schedule. [/toggle]

[toggle title=”Glass ALWAYS slumps/fuses at X degrees” open=”no”]Nope. Glass is an amorphous solid, like chocolate or wax, and doesn’t have a well-defined melting point, i.e., at one temperature it’s a hard solid and one degree higher–whoosh–it’s dripping off the kilnshelf.

Crystalline solids have molecules arranged in orderly arrays, connected by the same types of bonds. Applying energy (heat) to those bonds causes them to break. Since the bonds are the same, they all pretty much break at the same temperature, freeing up the molecules to move around, i.e., become a liquid. That temperature is the melting point of the solid.

Glass molecules aren’t arranged in an orderly fashion, and the bonds between molecules can be longer, shorter…whatever. Apply one level of heat, and only SOME bonds break. Keep applying the same level of heat, and more break. Raise the temperature you’re applying, and break even more bonds. As more and more bonds break, the glass gets softer, flows a bit more…and will keep on doing that until it’s as fully liquid as it’s gonna get.

That gradual softening is what lets us do with glass what we do with glass. (Imagine trying to slump a piece of ice.)

It’s also why two schedules, one that zooms 500dph to 1350F and holds for 30 minutes, and one that creeps up 200dph to 1225F with a 5-minute hold, might accomplish similar results. The AMOUNT of time the heat was applied is just as important as the temperature(s).

You can literally shift glass transition from the solid to liquid phase by the way you apply “heatwork,” i.e., total amount of heat energy applied over a period of time.

From a thermodynamics perspective, the concept of “heatwork” is a bit fuzzy; I’ve tried shoving X degrees per hour times X hours times whatever to build some sort of heatwork calculus and never managed it. There’s probably someone out there who has (so please PLEASE speak up).

So…if you ever hear someone say “this glass fuses at 1475F,” they’re either making a LOT of assumptions about the heatwork that led up to that temperature…or they don’t know what they’re talking about.[/toggle]

[toggle title=”But that schedule ALWAYS worked before!” open=”no”]Those are the saddest words ever spoken by a glassist.

Whether it worked before or not, it’s not working now. Your glass cracked/didn’t slump/wound up in a puddle/stuck to the mold/exploded. So…what changed?

I asked that a lot when I started. I was absolutely certain that the disaster in my kiln had NOTHING to do with the schedule. Oh sure, I’d tacked on some frit balls, stuck another couple layers of glass underneath, stacked another kilnshelf on top to fire two pieces at once, used a new mold…but nothing CHANGED…right?

Wrong. Every so often the change is using the wrong glass, or a problem with the kiln. Most of the time, though, you’ve changed what’s inside the kiln. It may not be obvious, but something has changed…so you must change the schedule to compensate. I’m gonna get into that in part II.

[separator top=”40″ style=”double”]So that’s it for mine; how about yours? And if you’ve got data to the contrary please, please share–I love learning this stuff.

In the meantime, here are some of my favorite resources for basic firing schedule management.

[title size=”1 to 6″]Firing schedule resources[/title]

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[content_box title=”Why did it break?” icon=”” image=”” image_width=”35″ image_height=”35″ link=”” linktarget=”” linktext=”view” animation_type=”snake” animation_direction=”left” animation_speed=”.3″]Fabulous diagnostic video, requires Bullseye subscription ($39/yr)[/content_box]
[content_box title=”Heat and glass” link=”” linktarget=”” linktext=”read” animation_type=”fade” animation_direction=”down” animation_speed=”.3″]Bullseye again, probably one of the best basic heat/glass explainers [/content_box]
[content_box title=”Annealing Glass” link=”” linktarget=”” linktext=”read” animation_type=”slide” animation_direction=”left” animation_speed=”0.3″]From Corning, good basic info on annealing actions and terminology[/content_box]
[content_box title=”Basic firing info” link=”” linktarget=”” linktext=”read” animation_type=”flash” animation_direction=”uright” animation_speed=”.3″]Nice primer on schedules and thermal shock from [/content_box]