First
of all we would like to let you know what Glass Fusing is. This warm glass
procedure is the method of taking two or more pieces of glass and heating them
together in a kiln. The heating of the glass makes the glass to be soft and if it
is heated in a higher temperature, it becomes molten and pliable. As it becomes
molten, the two or more pieces start sinking together and become one piece of
glass. The heated pieces of glass must be compatible to each other that it
could be melted together.
What is the difference between a
full and tack fusing?
A full fuse is when two or more pieces of glass are heated in a kiln until the
pieces are melted together to one solid piece of glass. This process usually
occurs around 790 C (1450 F) to 840 C (1550 F).
A tack fuse is when two or
more pieces of glass are heated until the pieces
start melting together. They almost appear to be just glued together. Each
piece maintains its original shape and texture. This process usually occurs
around 730 C (1350 F) to 790 C (1450 F)
Two
problems can occur when you are attempting either of these procedures. You can
lose the shape of your glass, and you can get unwanted bubbles there as well.
Most glasses when heated need to assume a thickness of about 6 mm (? inch), or
about two layers of glass thickness. If you are trying to heat just one layer
of glass, it has a tendency to pull in to obtain this 6 mm (? inch) thickness.
Two or more layers flows out and assumes the ? inch thickness, which is about
the size of two layers of glass.
Annealing
a fused glass is the process of slowly cooling glass so that the internal
temperature matches the external temperature. This allows the glass to reach a
stress-relief point, which relieves internal stress. The piece is allowed to
heat-soak until the temperature is consistent throughout the piece. As glass is
cools, the outside always cools faster than the inside of the glass. Glass
contracts as it is cooling. If the outside of the glass cools and contracts
faster than the inside, this causes stress in the glass. Too much stress and
the glass can break. The slower glass is cooled, the less amount of temperature
difference throughout the glass and less amount of stress develop. The anneal
point reflects the viscosity of the glass but not the COE. This temperature and
the holding time really depend on the size of the piece and the COE of the
glass. Smaller pieces don’t require as much time to achieve this process as
large bodies of glass. A higher COE of the glass requires lower annealing
temperature than glass with a lower COE.
This
process is based on the mass and dimensions of the glass. Thickness is a strong
factor in that the insulating effect of the thick glass is more of a factor
than the size, although the size is still important. For example: if you take a
one inch cube of glass, because of the thickness, needs much more annealing
that say a two by two by one-quarter piece of glass, even though both have the
same mass. There is a lot more wiggle room in a smaller piece of glass. The
larger and thicker the piece, the more time it takes to anneal the piece
properly. You are trying to achieve even heat in the entire glass and holding
it at that temperature. It is held steady and for a period of time to remove
the stress caused from the heating process. This also includes a cooling down
phase. This is done slowly so as to not permit too much stress to build up in
the glass. Although invisible to the naked eye, the strain is present and can
be a possible point of failure in the glass piece unless relieved. You don’t
want your piece to crack or break at some point in time. Any time glass is
heated and cooled, it messes with the molecules. When you anneal the piece, it
brings the molecules back in line. If the molecules don’t get lined up again, the
glass will break. Resist the temptation to peek into the kiln before the
process is complete. Any rapid temperature changes will obstruct the process.
The glass is cooled slowly at a rate until the temperature is below a critical
point when the internal stress is balanced with the surface tension on the
outside of the glass. At this point it can be safely dropped to room
temperature.
Soaking
is when you hold glass at a steady temperature over any span of time. Soaking
the glass at a high temperature runs the risk of being so hot that it will
distort. But this process has the advantage of requiring a shorter soak time
for the built up stress to even out through the piece. Your piece will also
need to cool down through a longer temperature range. Soaking at a lower
temperature has the benefit of a shorter cooling time, but also involves a
longer soaking time to remove any stress, and may not even remove all of the
stress. After the glass has soaked for the required time, all of the stress
will dissolve, but stress can reappear during the cooling phase. The quicker
the glass is cooled down, the more the amount of stress that can be built up in
the glass. This temperature to anneal any glass is a range, and is located at
approximately the center of this range. It is not important to reach an exact
temperature. Keeping the glass temperature steady for an amount of time before
cooling slowly to room temperature is important.
The
temperature recommended to achieve this process for most glass, is around 500 C
(940? F). If you are using borosilicate glass, use around 560 C (1050? F). If
you don't know the COE of you glass, try the shotgun method. The cooling down
of the glass is important between the anneal temperature and the strain point.
Glass can develop stress throughout the entire cooling process. Any stress that
develops below the strain point is only temporary. If stress develops in the
glass above the strain point it can't be removed. You can speed up the cooling
time below the strain point temperature, because the stress that is caused
during that process will not cause the glass to break any time in the future.
You want to avoid thermal shock by not cooling the glass too quickly though.
Don't open the lid of your kiln during this process. If you use a temperature
below the strain point to cool slowly down to before then increasing the
cooling rate, you don't need to know the actual strain point temperature of
every glass. Use approximately 425 C (800? F).
When
the glass has reached the anneal temperature:
You
have to keep soaking the glass long enough to remove any stress. For a smaller
piece (the size of a tiny bead) use about 20 minutes 315 C (600? F) per hour
(10? F per minute). For larger pieces (the size of large bead) use about one
hour. This size may require a rate as slow as 10C (50? F) per hour (less than
1? F per minute). For an even larger paperweight size, it can take about half
of a day. When fusing or slumping glass, for each thickness from ? inch (12 mm)
to 1 inch (25 mm), it should be annealed for 30 minutes. If you are having a
hard time determining the annealing time play it safe and anneal longer. You
can’t over anneal glass. After you soak the glass, cool it down past the strain
point temperature slow enough so it does not allow building up too much stress.
Once the temperature has gone below the strain point temperature, you can
increase the cooling rate without producing permanent stress. But if you
cooling the glass too quick below the strain point temperature can still cause
the glass to break due to thermal shock. You can control the temperature by
using a temperature controller that is programmable. If you are doing this
process manually, use an infinite control switch (thermostat). First you must
soak the glass. This has been explained above. After the soak time has
finished, there are several options you have, depending upon the size of the
glass.
For small glass items just turn off the infinite control. The hot brick walls
have soaked enough heat to keep the kiln from cooling very fast.
For medium to large pieces turn the
infinite control switch to a lower number. Once the temperature drops to about
half way to the strain point, set the dial to low. This setting the dial to a
lower number allows the temperature to cool down at a slower rate. Check the
kiln in about 15 min. and by then the oven temperature should drop to below the
strain point temperature. You can now turn off the kiln and allow it to cool to
room temperature.
Breaks
that occur during the process are typically "S" shaped. If a break
runs along the line of two different glasses, it is usually due to the glasses
not being compatible. Thermal shock causes the broken glass to move apart and
separate. This process is more easily achieved with a uniform mass, such as a
round bead. As the number of edges and depths increase, it comes time for annealing.
So you need to anneal even tack-fused and slumped projects. If you are
extending the time, and it doesn’t seem to work, you can try insulating the
edges of your work with fiber paper, fiber blanket or fiber board. These items
will help to stabilize the cooling rate throughout the entire glass piece.
Always remember it if you can’t over soak or cool your pieces slow. So if you
are soaking and cooling a variety of sizes of glass, use the rate that is
recommended for the larger pieces.
For
more information: www.glass-fusing-made-easy.com.
Glass
