Polarised light filters are used to detect stress in a non-destructive testing method in kilnforming. The use of the filters is described in this blog. To produce consistent reliable results, there are certain conditions.
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| Stress halos from broken and fused bottles |
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| Stress points in a drawing square illustrating the concentrated stress at corners |
Fire for Your Kiln and Objectives
Credit: FusedGlass.org
It is important to evaluate each segment. What is the apparent purpose? How does it fit with the principles of applying heat and the characteristics of glass?
Creating samples or tests provides both references on firing profiles and knowledge of the characteristics of the kiln.
Of course, these tiles must be labelled with glass types and code numbers and the temperature used. This is not all the information required though.
Sample making gives confidence in preparing work for the
kiln and scheduling to get the desired profile.
Sometimes kiln wash does not seem to want to stick to the mould. There are several possible reasons. The main two seem to be a hard spot in the slip cast moulds that we use. Another is the previous use of boron nitride or other sealant of porous surfaces.
The
remedies are different for these two causes.
For hard spots you can add a bit extra kiln wash to the area. Normally enough separator adheres to the spot
to avoid sticking. This is so even though
you can see the spot more clearly than the rest of the mould.
Sealed surfaces present a little more difficulty. It is possible to carefully sand blast off the boron nitride from the surface using low pressure and very little abrasive. This works well for textured surfaces, if you are careful. You can also manually sand the sealant off which works better for regularly shaped smooth surfaces. The object of both these processes is to remove the sealed surface to reveal the porous material again. You must remember that you are removing some of the surface of the mould in these abrasive processes. Once removed kiln wash can be applied as before.
If neither abrasive method works, it does not mean the mould is ruined. You can continue to use boron nitride. Or, if you want to avoid the costs of boron nitride, you can sprinkle fine dry kiln wash over the mould. You should give the mould a final application of boron nitride before using the dry kiln wash.
There seems to be a view that the exact consistency of the kiln wash mix is important. Within limits the mix proportions are not vital. The general recommendations from manufacturers is one part powder to five parts water – both by volume. This is a good guide for general use.
It is possible to make the kiln wash mix too thick. If it goes onto the shelf or mould in a pasty
fashion it is too thick. A thick mixture
leaves definite streaks and uneven levels that are difficult to smooth and
level. If you get these effects, scrape
it off and put it into a jar with more water.
Mix until it is creamy to avoid lumps.
Then add more water until you have a very liquid mix. It needs only be a little less runny than
plain water.
Is it possible to have too thin a mix of kiln wash? I suppose it is, but not likely. If you feel it is too thin, you only need to
add more coats of the mix until the shelf surface is obscured. Often when the
mix seem thin, it is because the powder has separated from the water. It is necessary to stir the kiln wash
thoroughly to get all the solids in suspension.
Then frequent stirring during the application is necessary to keep the
mix even at both the top and the bottom of the container. Storing the mixed kiln wash in a clear
container will enable you to see if kiln wash is still settled on the bottom.
The object of the kiln wash is to provide a separator between the
supporting surface and the glass. It
needs to be only a film of separator to be effective. In fact, if the kiln wash is too thick, it
will flake and stick to the back of the glass.
In the case of kiln wash - more is definitely worse.
For very absorbent materials such as vermiculite or fibre board, I mix
kiln wash thicker – about 1:3. The idea
behind this is to reduce the amount of water the mould absorbs. With less water in the mould, less drying
time is needed, especially with a vermiculite mould, where steam pressure could
break the mould.
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.
Dennis Brady has done a lot of work on predicting the size of circles resulting from stacking squares of glass and taking them to full fuse for enough time to allow flattening of the stacks. This may be up to half an hour at 815°C for Bullseye. Some observation will be required.
Stacks of 12mm/ 0.5" squares arranged at 45° to each other and taken to a full fuse:
I had a few queries about this regular progression and wondered if it applied to opalescent as well as transparent glass. I set up a few tests in my kiln. I fired them at 400C to 815C for 10 minutes.
You can see that the opalescents require more heat work than the transparent. If you are making circles with both transparent and opalescent you will need more time at the top temperature - perhaps 30 to 45 minutes. This results from the greater viscosity of the opalescent colours.
I also tried making ovals from rectangular pieces oriented at about 25 degrees to each other. You can see they were not successful with a 10 minute soak.
Any suggestions on how to avoid getting the oblong bubble under the neck of the bottle? This was my first try and I’m really happy with clarity, no devitrification in these.
I used this schedule:
Fahrenheit Celsius
300/1150/30 167/620/30
200/1370/20 111/740/20
400/1450/20 222/787/20
AFAP/950/60 AFAP/510/60
150/800/0 63/427/0
300/100 167/55/off
The bubble is kind of cool but not sure what it will do when I put it in a bottle mould.
To minimise the bubble, you need a bubble squeeze. There isn't one of sufficient length or at the right temperature in the schedule. The softening point of bottle glass is approximately 720C. Starting the bubble squeeze at ca. 670C/1240F and progressing slowly (ca.50/90F or less) to 720C/1340F may give a better bubble squeeze.
Also, the anneal soak is a bit low. Bottle glass and float glass both have annealing points of about 550C. You might make use of a lower annealing soak temperature to reduce the cooling time. It is usually possible to anneal 30C below the published annealing temperature. In this case that would be 520C.
There is pretty thick glass in some places due to the way the bottom and neck of the bottle form. You may want to extend your anneal soak to one for 12mm/0.5”. The soak time for this is 2 hours. The first cooling segment would be 55C/100F per hour to 475C/888F if you use 520C/970F as the annealing soak. The second cool segment should be at 99C/180F per hour to 420C/790F. And the final rate at 330C/600F to room temperature. It is important to include all three stages of cooling. The research for my book Low Temperature Kilnforming (Or directly from stephen.richard43@gmail.com) has shown that to get the best stress-free results use all three stages of cooling.
Bubbles at the shoulder of the bottle are common. The change in circumference of the bottle at the shoulder means there is a greater amount of glass to “compress”. Bottles with tapered circumference at the top of the bottle have fewer problems with creating bubbles. The abrupt change in size at the shoulder causes bubbles to be more common. A long slow bubble squeeze will allow the shoulder to form more closely in line with the neck.
There are other things you can do to
help avoid the bubbles. One thing is to insert a thin kiln washed wire into the
neck of the bottle. This gives a path for the air to escape and allows you to
pull it out, although a mark will be left. You could also think of
drilling a hole in what will be the underside at the shoulder to allow air out
to the shelf. It does not need to be a big hole.
Bubbles at the shoulder of a slumped bottle are a common problem. It results from the greater amount of glass that has to slump into the space. This leaves a cavity. Slower bubble squeezes can help, as well as various venting methods.
When making a drop vase in opalescent glass, the design needs to be on the outside. This will require ensuring the design will be on the bottom when suspended on the drop ring.
It is possible to build the whole piece as normal with the design on the top and fire it. Then you can turn it over to make sure the design is facing downwards.
To get a crisper design for the outside the flip and fire technique can be used. Build from the outer layers to the inner layers. You are building upside down. Place the design to be seen on the outside of the drop vase down on the prepared shelf first. Follow this up by placing the inner layers in order from the most outside to the most inside layers.
These instructions rely upon firing the blank first rather than building on the drop ring.
However, you can build on the ring if you need to save one of the two long firings. Only one modification is required. Place a sheet of clear down first. Assemble the design as for a flip and fire technique, i.e., outside layers first, inside layer last.
This will require a slow heat up to ensure you have allowed enough time for the air to be squeezed from between all the layers and that all the glass at the same temperature before the drop begins. Sprinkling a fine layer of clear powder over the clear is a good way to assist allowing the air out. Place the design pieces down before applying the powder.
This is not the best way to make drop vases, but it can work with care in placing the decorative pieces and applying the powder.
Recently, when looking for a small frit maker, no shop had one
in stock. Having heard of making one
from plumbing pipes, I went in search of material. I came across stainless steel pipe and caps.
A threaded 25mm pipe and cap can be fitted loosely into the
larger one, and so forms the plunger or piston.
There needs to be a handle. It could be a turned piece of wood to fit the inside of the pipe. In this case, I obtained a reducing connector to fit a 12mm pipe to the plunger and topped it with another cap.
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| The completed frit maker |
Galvanised pipe would be cheaper but carries the possibility
of introducing zinc into the frit.
Stainless steel risks introducing non-magnetic particles into the frit. As I sieve out powder from my own frit making
before washing, I am not too concerned about steel contamination. If you want powder, use uncoated mild steel so
the contamination can be drawn out with a strong magnet.
