A power point presentation I made a few months ago to the group Lunch with a Glass Artist.
It is 33 slides long.
A power point presentation I made a few months ago to the group Lunch with a Glass Artist.
It is 33 slides long.
Credit: Marcy Berman |
I have not had much success [with] the Patty Gray mould despite using the recommended firing schedule. I always have holes or bubbles and the edges are not smooth.
The schedule for Oceanside was:
I want to ensure the glass has enough time when it is flowing most freely at the top temperature to level out. This requires scheduling a longer soak at the top and observing how well the glass is levelling out. If more time is required you can add it on the “run,” and advance to the next segment when the surface is as wanted. Read up in your kiln manual how to do both these things.
Yes, the rate is one which will enable devitrification to form on flat glass. The soak at top temperature is even more likely to promote it. However, as the glass is flowing, less devitrification has an opportunity to form. The crystallisation – which is what devitrification is - of the glass takes time to form. The movement of the glass surface is sufficient to reduce the formation of those crystals. It is of course likely there will be some devitrification, but not as much as the slow rates and long soaks would lead you to think.
But for these flows there always is
the possibility of devitrification. You have to plan a method of removing it.
Unless the surface is very flat, grinding the top is not a fast way to remove
it. Sandblasting is a quick way to remove devitrification. Another way is to
sift a thin layer of clear glass powder over the surface. This is an
increasingly popular way to deal with devitrification for those without access
to sandblasting facilities. When fired again, the powder melts and forms a new
shining surface. The piece will need to be fired fire again whether sandblasted
or covered in glass powder.
The summary for flows:
"Do glass molecules actually migrate when they are sintered together? "
Sintering occurs at the atomic level,
where the atoms at the edge of the particles attach to others in other
particles. An analogy occurs to me of Scottish country dancing. In big
gatherings, small groups are formed to perform the dance, say an eightsome
reel. As the dance goes on the groups become more coordinated and eventually
form one large group, held together by the people on the edges of each group.
A more scientific description comes from
Wikipedia:
“Sintering … is the
process of compacting and forming a solid mass of material by heat or pressure
without melting it. … The atoms in the materials diffuse across the boundaries
of the particles, fusing the particles together and creating one solid piece.
Because the sintering temperature does not have to reach the melting point of
the material, sintering is often chosen as the shaping process for materials
with extremely high melting points such as tungsten and molybdenum.”
Applied to glass this means that you can make a solid piece of glass out
of multiple touching or overlapping pieces that do not change their shape. This
uses low temperatures and very long soaks.
Schematic-diagram-for-the-sintering-and-fusion-reaction-of-the-glass-frits-on-a-substrate. Credit: ResearchGate |
The usual process is to take the glass at a moderate rate up to the
lower strain point. The rate of advance is slowed to 50°C or less to a
temperature between slumping and the bottom of the tack fuse range.
The slow rate of advance allows a lot of heat work to be put into the
glass. This, combined with a long soak (hours), gives the atoms of the molecules
time to combine with their neighbours in other particles.
Sintering occurs in the range of 610°C to 700°C (1130°F to 1275°F). The
lower limit is determined by the strain point of the glass and by practicality.
The length of time required at the strain point - 540°C/1005°F - is so long
(days) that it is impractical.
The upper limit is determined by the onset of devitrification. This has
been determined by the scientific studies of sintered glass as a structure for
growing bone transplants. Devitrification reduces the strength of the bonds of the particles at the molecular
level. The process of crystallisation breaks the bonds already formed between
the atomic structures of the molecules. These studies showed that the onset of
devitrification is at 650°C/1204°F and is visibly apparent at 700°C/1292°F
regardless of the glass used.
The lowest practical temperature for sintering is 650°C/1203°F. Indications
are that at least an additional two hours are needed for the sinter soak for
each 10°C/18°F reduction below 650°C/1203°F. This would make for a 12-hour soak
at 610°C/1131°F. For me this is not practical.
More information on
the kilnforming processes and sintering experimentation is available in this eBook: Low Temperature Kiln
Forming.
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.
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.
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.
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.
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.
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.
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.
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.
More information is
available in the ebook Low Temperature Kilnforming.
Three firings to the same sharp tack profile will give multiple profiles in the finished piece.
Grain structure at 650C |
Grain structure at 690C |
Left to right - devitrified surface, powder covering, fired piece Photo credit: Bullseye Glass Co. |
borax in powder form |
An example of a borax washing powder |