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:
-
How fast the kiln is
heating or cooling during this segment; this is called the ramp
rate.
-
What temperature the kiln is heating to or cooling to in this segment;
this is called the target temperature.
-
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 (ºF/Hr) |
Target Temp. (ºF) |
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:
-
500/1250/1:00
-
150/1465/:10
-
etc.
Heatwork
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 450ºF 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 300ºF. 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 1000ºF, or somewhat higher (up to
1250ºF 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
850ºF), 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 1000ºF 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
1000ºF." In my own practice, I go even further and won't open my
kiln below at least 1100ºF, 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 300ºF/hour to 600ºF/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 1000ºF, 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 1250ºF -- 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 1000ºF, 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:
-
Slumping. Somewhere
between 1000ºF and around 1250ºF 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.
-
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 1250ºF - 1350ºF. 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 1450ºF longer
than necessary.
-
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 1350ºF
and 1400ºF. 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.
-
Full fuse. Between around 1400ºF and 1500ºF, Bullseye glass
softens enough to flow and seek a common level. When not
restrained by dams or molds, glass seeks a thickness of about
¼". 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 ¼" 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 ¼" inch,
if not restrained, will spread out and, if given enough time,
will eventually reach a thickness of ¼" – 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.)
-
Casting into a detailed mold. Above 1500ºF 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.)
-
High-temperature processes. You may have thought we were
already talking about high temperatures, but if we get into the
1700-1750ºF 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 850ºF or so, you can safely cool it
as rapidly as possible until the kiln controller reaches your
target temperature, which will be below 1000ºF. 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 1000ºF. The manufacturers of System 96
glass recommend 960ºF. Bullseye has recently lowered their
recommended target temperature for this segment of the firing
cycle from 960ºF to 900ºF.
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 900ºF - 960ºF, 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
700ºF. 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 960ºF. The Bullseye folks, as I mentioned
earlier, lowered their recommended anneal soak temperature to
900ºF. 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 700ºF -- 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 ¼" 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
700ºF 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 40ºF, then that's room temperature for you
today -- sorry. Most conventional wisdom says you can take your
piece out when the display reads 100ºF. 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 100ºF, 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
-
Pâte 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.
http://www.bullseyeglass.com/products/bullseye-techbook.html
