People ask about
whether it is possible to tack fuse additional elements without affecting the
profile of the existing piece.
It is as though glass has a memory of the heat it has been subjected
to. For example, a sharp tack will become a slightly rounded tack,
even though refired to a sharp tack again. So, it is impossible to
refire a piece to the same temperature or higher without affecting the existing
profile. But it is possible to fire a piece with differing profiles
if you plan the sequence of firings.
Tack fuse onto existing profile
It is possible to add pieces to be tack fused with little
distortion to the existing piece through careful scheduling and
observation. There are several requirements.
• A
moderate rate of advance to the working temperature is required, rather than a
fast one. This is because the piece is a single thicker piece with uneven
thicknesses. Also, a slow rise in temperature allows completion of
the work to at a lower temperature. This means there will be less
change to the existing profile.
• A minimal bubble squeeze - or
none at all - is required on this second firing. The added pieces
generally will be small, so if possible, eliminate the bubble
squeeze. The requirement is to add as little heat work as possible.
• The
working temperature should be to a low tack fuse temperature with a long
soak.
• Observation is required from the
time the working temperature is achieved. Peeking at 5-minute
intervals is needed. This to be certain that the current tack fuse
can be achieved without much affecting the existing profile. It will
be a compromise that you will be able to choose during the
firing. The decision will be whether to retain existing profile and
have a sharp tack. Or a slightly rounded tack and more rounded
profile on the original piece.
Planning for multiple levels of tack
It is possible to design a piece with multiple profiles within the
completed piece. You need to plan out the levels and degrees of tack you
want before you start firing.
To do this planning, you need to remember that all heat work is
cumulative. In simple terms it means that on a second firing you will start
where you left off with the first one. The texture in the first firing will
become softer, rounded, or flatter than the second or even the third firing.
Three degrees of tack can be achieved with a little
planning. It works similarly to paint firings. Some
paints fire higher than enamels, and enamels hotter than stain. You
have to plan to fire all the tracing and shading first. Then you add
the opaque enamels, followed by the transparent enamels. Finally,
you add the silver stain. This is unlike painting on canvas where
you build up the image all together.
The same principle is true of a multiple level tack fuse
piece. When creating various profiles in glass, you proceed from
firing the areas that will be the flattest first. Then proceed to the areas
which will have the least tack last. This is a consequence of the
cumulative effect of heat on re-fired glass.
Plan out the areas that you want to have the least
profile. Assemble the glass for those areas. I suggest that a 6mm
base is the initial requirement for anything that is going to be fired multiple
times. Add the initial pieces that will become a contour fuse or a
very rounded tack.
First firing
Put this assembly in the kiln and
schedule. Do not fire to the contour profile temperature. Instead,
you will be scheduling for a sinter or sharp tack. This depends on how many
textures you plan to incorporate. Start with a sharp tack. Fire at
the appropriate rate with a bubble squeeze to about 740°C for 10 minutes and
proceed to the anneal cool. Different kilns will need other
temperatures to achieve a sharp tack.
You do not fire to the contour fuse temperature, because the base
will be subject to more firings. Each of these firings will soften
the base layers more than the previous one. This is the application
of the principle of cumulative heat work. When you fire a piece for
a second time, there will be little effect until the softening point of the
glass is reached. Once there, the glass further softens, giving the effect of a
contour fuse.
Any glass that had already achieved contour profile from the first
firing will flatten further. This can be used in cases where the
working temperature was not high enough. Just fire again to the
original schedule’s temperature. Take account of the need for a
slower ramp rate to the softening point.
Second firing
Once cool and cleaned, you can add your
next profile level of tack fusing to the base. Note that “level of tack”
does not refer to thickness being built up. It is about the amount of
roundness you want to impart to the pieces. You may be placing this
second - sharper – level of tack in the spaces left during the first
firing. Again, schedule to the original approximate 740°C. But remember
the base is now a single piece. You need to slow the ramp rate to the
softening point, after which the speed can be increased. You will not
need to retain the bubble squeeze unless you are adding large pieces, or into
low areas.
The second firing will show the pieces added for the second firing
to have the profile of the original pieces. Those pieces having their
first firing will have a sharper appearance.
credit:
vitreus-art.co.uk
This
is a piece where the flower petals and leaves could have been placed for the
second firing to give a softer background with less rounded flower details.
Third firing
Clean well and add the pieces for the
final level of tack. Schedule the initial rate of advance a little slower
than the second firing. The piece is growing in thickness and
complexity. Once the softening point is reached, the original rate of
advance can once again be used up to original temperature.
Final firing
Clean well and add the pieces for the
final level of tack. Schedule the initial rate of advance a little slower
than the second firing. The piece is growing in thickness and complexity.
Once the softening point is reached, the original rate of advance can once
again be used up to original temperature.
Further notes on multiple firings
It is a good idea to observe the firing, once the working
temperature is achieved. This is to ensure enough roundness is being
given to the final pieces being tacked to the whole. Be prepared to
extend the soak if the final pieces are not rounded enough.
Although you should have a good idea of the degree of tack for the final pieces
from the previous two firings.
You may need to experiment a little with the temperature and
length of soaks at the working temperature. For example, if the degree of
tack is too sharp in the first firing, you can extend the soak or increase the
temperature for the next ones.
If you are firing at 740°C, you may feel you can afford to extend
the soak for the subsequent firings, because you are in the lower part of the
devitrification range. Consider the risk of devitrification increases with the
number of firings of the glass. The preference is to increase the
temperature a bit for subsequent firings to ensure you are not spending a
cumulatively long time in the devitrification range but still be able to get
the final tack level desired.
The preference is to increase the temperature a bit for subsequent
firings to ensure you are not spending a cumulatively long time in the
devitrification range but still be able to get the final tack level
desired.
Because most of your heat work is happening in the low end of the
devitrification range, the cleaning regime must be very thorough. Any
chemicals or soaps used must be completely washed off with clean water.
The piece must be polished dry to ensure there are no water marks left on the
glass.
You can, of course, have more levels of tack. One approach
would be to start with a sinter, or tack to stick, firing. And repeat that four
or more times. Another is to increase the working temperature and reduce
the length of time soaked there. The shorter time means there is less
rounding of each level, allowing the build-up of many levels of tack. All
of these require some experimentation.
