Glass 101 |
What is a Firing
Program? |
Type of Glass |
Heatwork |
Size and Mass |
Critical Temperature
Ranges |
Heating Range |
Process Range |
Pre-Annealing
Cooling Range |
Annealing Range |
Firing Processes |
Fusing |
Slumping |
Loading Glass |
Can I Fuse Glass in My
Ceramic Kiln? |
How Long Will It Take to
Fire My Kiln? |
Programming GlassFire Mode |
Programming Ramp/Hold Mode |
Glass Firing
Basics |
Glass 101
The following guidelines just scratch the surface of the
knowledge base associated with firing glass. We highly suggest
you take a firing class from your local distributor. If classes
are not available in your area, there are numerous books
available on the subject that can be found at bookstores and on
the internet.
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What is a Firing Program?
Glass is very sensitive to changes in temperature below 1000 °F.
If it is heated or cooled too quickly through certain
temperature ranges it creates stress within the glass which can
cause breakage. Firing programs are used to control these
temperature rates and limit the amount of stress created within
the glass as well as create the desired effect on the glass. A
firing program is composed of one or more firing segments that
dictate the heating or cooling rate throughout the program. Each
one of the lines in the chart represents a segment or hold time
within a segment and the slope of the line represents the rate
of firing. A firing program is either entered into a kiln
controller or on kilns without controllers it is replicated by
turning up and down temperature switches.
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Type of Glass
The art of firing glass has been around for centuries however,
comparatively speaking, it has only been recently that companies
have begun manufacturing glass specifically designed to fuse
together. Glass, like most everything on earth, expands when
exposed to heat and contracts when it is cooled. It expands at a
measurable rate, known as the COE, or coefficient of expansion,
and as it becomes liquid it flows at different rates which is
referred to as it’s viscosity level. These variables and a host
of others must be carefully managed to create glass that can be
fused together without crazing, cracking, warping, or breaking.
Always consult with your supplier of glass to determine if the
glass you wish to fuse is compatible.
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Heatwork
Heatwork is a term used to describe the relationship of time and
temperature and their combined effects on glass. To a certain
extent the two are inversely related. This means that the higher
the temperature the less time is needed to create the same
effect and likewise, the lower the temperature the more time is
needed. This concept becomes most useful at the "Working"
temperature range of glass. This is the temperature range where
the glass is fused, slumped or sagged. Most fusing glass will
fuse between 1450 F and 1480 F. It is possible to get the same
results (or the same amount of heatwork), by bringing the kiln
to 1450 F and holding it at that temperature for 30 minutes as
you would by bringing the kiln to 1480 F and holding it for only
10 minutes. There may be other factors that make you choose one
working temperature over the other such as the thickness of the
project.
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Size and Mass
The size of the piece is one of the most influential factors for
creating a firing program. One of the keys to successful
heatwork is having the entire piece go through critical
temperature ranges at the same moment. When a piece is thick it
takes longer for the center to heat up than it does the outside
of the piece. When it is a large diameter, slight differences in
temperature throughout the chamber of the kiln can cause the
piece to expand at different rates. The key to firing larger and
thicker pieces is to slow the firing rates through critical
temperature ranges. Determining how slow is often a trial and
error proposition therefore it is best to start with a
conservatively slow program. More projects are ruined by going
too fast than too slow.
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Critical Temperature Ranges
A "Critical Temperature Range" is any temperature or temperature
range in the firing cycle that has a high level of potential for
limiting the success of the project. Limited success can be
expressed as overfired, underfired, breakage, devitrification,
or bubbles just to name a few. It can be argued that there are
numerous critical temperature ranges. To keep things simple we
are going to discuss the primary four: Heating Range, Process
Range, Pre-Annealing Cooling Range, and Annealing Range.
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Heating Range
The Heating Range goes from room temperature to the first set of
data in the Process Range. The only concern during this range is
heating the pieces too fast without adding steps to the program.
Steps are hold periods at designated temperatures that allow the
piece to balance out during the firing. Small pieces can
normally be heated as fast as 800 F./Hr. as long as steps are
added. With larger pieces you will want to slow the rate and
possibly add additional steps depending on the size of the
piece.
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Process Range
The Process Range is the temperature range where the material
begins to visibly change. It is this stage that determines the
final shape of the piece. It is often a good idea to add a pre-
Process Range segment to slow the kiln down before entering the
Process Range. If the kiln is firing too fast into the process
range it is possible to overshoot your goal temperature. During
the Process Range temperatures and hold times are key. If you
are unsure of the desired peak temperature you may want to start
on the low end of the range with a longer soak. This will help
insure that thicker pieces receive the proper heatwork
throughout the entire piece.
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Pre-Annealing Cooling Range
After the process range is through, it is desirable to cool the
piece quickly for several reasons. The first reason is to stop
the heatwork. This is especially important on a project such as
a less then 100% fuse or a drop mold. The second reason is that
an undesirable reaction known as devitrification can occur
during this cooling period if the kiln is cooled to slow.
Devitrification is a scummy white crystallization on the glass
surface that is difficult if not impossible to remove. Be sure
to slow down the cooling before you enter the Annealing Range.
Opening the kiln lid to increase the rate of cooling, while
practiced, is not always recommended. On certain models the
thermocouple is in the rear of the kiln and the temperature from
front to back can vary greatly causing part of the piece to
enter the annealing phase before the part in the rear.
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Annealing Range
The final critical range is the Annealing Range. Every piece of
glass has an annealing point, this is a point in the cooling
cycle where the molecules in the glass realign themselves into a
solid and stable form. It is very difficult to know exactly
where that specific point will be, so during this period it is
critical to fire the kiln at a slower rate throughout the range.
Our pre- programmed firing schedules in the Glass Fire Mode
anneal from 1000 °F to 750 °F which should be adequate for most
stained glass. By incorporating such a broad range the risk of
breakage is limited. Be sure to keep the lid or door of the kiln
closed until the kiln reaches room temperature. Opening the lid
too soon can cause pieces to break.
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Firing Processes
There are many different processes or techniques used for
manipulating glass with heat inside a kiln. In this
manual we will focus on two, Fusing and Slumping. Other
techniques include but are not limited to Drop Molds,
Pate de Verre, Casting, Painting, and Combing. For more
information on using your kiln with these techniques
please consult your glass supplier.
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Fusing
Fusing is the process of joining 2 or more pieces of glass
together by the application of heat. This glass can be in the
form of sheets, stringers, frit or a host of other forms. There
are different degrees of fusing. You may want to fuse glass so
it sticks to another piece of glass without deforming. This is
known as a "Fuse to Stick". If you were to apply more heatwork
to the piece the edges would round slightly. This is known as a
"Tack Fuse". A "Full Fuse" is created when the pieces have
melted completely together and are 1/4" thick. A "Texture Fuse"
is any point in-between a Fuse to Stick" and a "Full Fuse".
There is a temperature range at which glass can be fused. The
point at which it begins to fuse is influenced by the rate at
which the temperature is climbing when it reaches the fusing
range. Most fusing glass will begin to fuse between 1400 F and
1480 F. Remember that heatwork is a function of time and
temperature. Starting with glass that has been determined to be
compatible is only the beginning to a successful fusing or
slumping project. The temperature and various temperature rates
in a firing program must be designed to the specific needs of
the project you are creating. The size, thickness, shape, and
type of glass all must be considered when designing a firing
program. As a precaution you may want to provide a dam or
barrier around the glass when fusing more than 2 layers. With
more than 2 layers, the glass will spread until it finds a level
of 1/4" and could possibly flow into another piece or off the
shelf.
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Slumping
Slumping can be defined as the controlled bending of glass under
the influence of heat and gravity within a kiln. This is
generally done over or into a mold. Molds can be made out of a
variety of different materials and can be found at art glass
supply businesses. When slumping, it is necessary to take into
account the shape of the mold, the thickness of the piece, and
the degree of heatwork desired. Gravity plays a very important
role in slumping, especially slumping over a mold as opposed to
into a mold. If the shape of the mold dictates that the unbent
glass is largely unsupported, the weight of the unsupported
glass will pull the glass over the mold quicker than if only a
small portion is unsupported. A thin piece of glass will bend
quicker than a thick piece of glass. A thick piece of glass
requires more Hold time in the final segment of the process
phase. In some cases the artist may want to control the amount
of bend by visually inspecting the kiln. When the proper amount
of heatwork is reached the artist can begin annealing. Slumping
projects that receive too much heatwork can take on unwanted
texture from the mold or in extreme cases fuse to a puddle.
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Loading Glass
How you load the kiln can really affect how your pieces come
out. Traditionally, glass kilns have elements in the lid. This
allows the heat to radiate evenly across the whole shelf instead
of coming from the side where it hits the edge of the shelf
first and works it’s way into the center of the kiln. This
allows you to fire faster without getting large temperature
differences within your project. Unfortunately, this also means
you cannot fire on multiple layers because the top shelf would
block the radiant heat from reaching the shelves below it.
Here are some general rules to loading a traditional glass kiln
with elements in the lid:
* Post your shelf at least 1 inch off the floor of the kiln.
* Make sure the kiln wash on your shelf is in good shape and any
fiber paper you use is free of creases and holes.
* Maintain 2 inches of clearance from your kiln elements.
* Maintain 2 inches of clearance from your thermocouple.
* Anticipate how slumped pieces will slump and fused pieces will
flow.
The last one is the most difficult to judge but as you gain
experience you will develop a sense of how glass moves. Thick
fused pieces will search a level of two layers of glass. If you
have more than two layers they will flow beyond their original
footprint so be sure not to put any pieces too close on the
shelf.
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FAQ's |
Can I Fuse Glass in My
Ceramic Kiln?
As a matter of fact you can. For years it was believed that
glass could only be fused in a kiln with elements in the lid.
Artists worried that if you tried to fuse glass in a side-fired
kiln the temperature difference between the edges of the glass
near the elements, and the center of the glass would become to
great and the piece would crack. We then found out that if you
slow your firing program down you can keep this temperature
difference to a minimum and the fused pieces came out just fine.
When you fuse in a side fired kiln you have the advantage of
being able to load pieces on multiple layers so you can fire an
entire class's projects at one time. Take a look at this article
that tells you how to load and program your kilns for glass.
Fusing
Glass in Your Ceramic Kiln
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How Long Will It Take to
Fire My Kiln?
The amount of time your kiln takes to fire depends on a number
of factors:
* What program you are running.
* The size of the kiln.
* The amount of mass loaded in the kiln.
* The insulation of the bricks or fiber in your kiln.
* The power rating of the kiln.
* The temperature and airflow of your kiln room.
The chart below gives you the estimated time of each of the
GlassFire Mode programs found on the GlassMaster controller. In
the calculations we are assuming a starting temperature of 70 F
and an average free fall cooling rate of 330 F/Hr from peak
temperature to 1000 F. These times do not include the time it
takes to cool from 750 F to room temperature which varies
greatly and can be a considerable amount of time on kilns that
are large and well insulated.

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Programming GlassFire Mode
GlassFire Mode is by far the easiest and fastest
way to fire glass in you kiln. All of the programs were designed
by Gil Reynolds who literally wrote the book on Fusing. He
designed them based on months of testing using both 90 COE glass
and 96 COE glass.
All you need to to is enter the process you want (Full Fuse, Tac
Fuse or Slump) and the Speed at which you want to fire. That’s
it! When you press start the kiln will fire the program and all
you need to do is come back and admire your perfectly fired
pieces. Download the firing instructions for GlassFire Mode
below.
Click
Here to View GlassFire Mode Instructions
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Programming Ramp/Hold Mode
If you have a GlassMaster controller it will have
two firing modes, GlassFire and Ramp/Hold. GlassFire mode is by
far the easiest and most common firing mode used in Glass.
Ramp/Hold firing mode is used primarily when you are firing a
project that is outside of the norm. Below is a list of possible
reasons you may want to use Ramp/Hold Mode:
Your glass project is larger in diameter than the dimensions in
the GlassFire Mode Firing Speed Chart allow for.
Your glass project is thicker than the dimensions in the
GlassFire Mode Firing Speed Chart allow for.
You are firing a piece that involves a process not found in
GlassFire Mode like casting or glass painting.
You understand the risks and would like to run a faster program.
Programming a kiln may seem intimidating, but once you do it a
few times you will find that it is really quite easy. We put
together a video which takes you step by step through
programming a ceramic kiln using Ramp/Hold. While the controller
looks slightly different, the steps are the same. If after
watching the video you still have questions feel free to call us
and we would be happy to walk you through your first firing. You
can also download a copy of the instructions for programming
Ramp/Hold Mode by clicking the link below.GlassFire Mode is by
far the easiest and fastest way to fire glass in you kiln. All
of the programs were designed by Gil Reynolds who literally
wrote the book on Fusing. He designed them based on months of
testing using both 90 COE glass and 96 COE glass.
All you need to to is enter the process you want (Full Fuse, Tac
Fuse or Slump) and the Speed at which you want to fire. That’s
it! When you press start the kiln will fire the program and all
you need to do is come back and admire your perfectly fired
pieces. Download the firing instructions for GlassFire Mode
below.
Click
Here to View Ramp/Hold Mode Instructions
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