The main reasons that kiln wash sticks to glass are:
1. Firing at too high a temperature. The higher the temperature, the more likely the kiln wash will stick to the glass.
2. Firing with opalescent glass against the shelf. Kiln wash sticks to opalescent glasses more easily than to transparent glass.
3. Re-using kiln washed shelves that have been to fusing temperatures already.
4. Using kiln wash with high amounts of china clay makes for more sticking. Thus some brands stick more frequently than others.
Firing at too high a temperature is probably the worse culprit. The second is using opalescent directly on the kiln shelf.
Strategies to avoid this sticking are:
1. Fire at the lowest temperature you can to get the result you want. This often requires slow rates of advance and extended soaks at the working temperature
2. Use Bullseye kiln wash. It is among the best.
3. Have a transparent glass as the bottom layer.
4. Use iridised glass, with the iridised side down to the shelf, as the iridisation acts as a separator. Do not do this with Thinfire, as it can lead to large cavities in the glass. Fire onto kiln wash.
There are ways to get the kiln wash off but it's easier to avoid it. Using an iridised sheet on the bottom is probably the most effective prevention.
Saturday 22 December 2018
Removing Kiln Wash from Shelves
There are at least three ways to remove kiln/batt wash from mullite kiln shelves.
One quick way is to use a broad wallpaper scraper held at a very acute angle to the shelf. This rapidly removes the separator. One down side to this method is that any uneven pressure can put a gouge into the surface of the shelf.
So a more gentle way to remove the wash is to use a drywall/plaster board sanding sheet or other open weave sanding material. This allows the powdered wash to come through the sanding material rather than clog up the material. The disadvantage to this is that it takes longer to remove the wash, although it does leave a very smooth shelf after many sandings.
A third way is to wash off the kiln wash. This is relatively quick, but it gets the shelf wet and requires a longer period before the shelf becomes dry. You can, of course put the next application of kiln wash on as soon as the shelf is clean. They both can dry off at the same time.
Power tools used to clean kiln wash from the shelves can induce low points in the shelf which will promote bubbles during fusing. It is recommended to avoid power tools in removing kiln wash.
One quick way is to use a broad wallpaper scraper held at a very acute angle to the shelf. This rapidly removes the separator. One down side to this method is that any uneven pressure can put a gouge into the surface of the shelf.
So a more gentle way to remove the wash is to use a drywall/plaster board sanding sheet or other open weave sanding material. This allows the powdered wash to come through the sanding material rather than clog up the material. The disadvantage to this is that it takes longer to remove the wash, although it does leave a very smooth shelf after many sandings.
A third way is to wash off the kiln wash. This is relatively quick, but it gets the shelf wet and requires a longer period before the shelf becomes dry. You can, of course put the next application of kiln wash on as soon as the shelf is clean. They both can dry off at the same time.
Power tools used to clean kiln wash from the shelves can induce low points in the shelf which will promote bubbles during fusing. It is recommended to avoid power tools in removing kiln wash.
Uprisings at the Bottom of a Slumped Bowl
“I just finished slumping a dish and I got a big lump in the center of the bottom. This is not an air bubble, just a lump. What should I do to avoid this again?”
Several suggestions are possible.
Ensure there are holes at the bottom of the mould that allow air to get out into the kiln. Prop the mould up on stilts if the hole does not go directly out of the mould. Alternatively, drill a hole in the side to allow the air to escape from under the mould.
Firing for too long or at too high a temperature will cause the glass to continue sliding down. Having nowhere else to go, the weight causes the bottom to begin rising. This is a consistent experience across several kilns and with multiple users.
So keep the temperature down to the minimum required. To find that out, watch the slumping in stages (do not stare!). Look at the piece for a second or two every five minutes after you reach your desired temp.
If it already has slumped adequately, you are firing too high. Reduce your temperature in subsequent firings and watch to find what the required temp and time is. There is absolutely no substitute in slumping but to watch and learn what your mould and glass require.
If you are slumping at such a temperature to seal the glass to the mould, you are firing too hot anyway. Or put more positively, use a low temperature slump, that is, a slump at the lowest temperature to achieve the desired result over an extended period of your choice.
A low temperature slump will allow the glass to conform to the shape of the mould without softening so much that it takes up all the markings of the mould. Therefore, there are spaces for the air to escape from under the glass all the way to the top as well as through the air holes at the bottom. It also gives the most mark-free slump possible for your shape.
Several suggestions are possible.
Ensure there are holes at the bottom of the mould that allow air to get out into the kiln. Prop the mould up on stilts if the hole does not go directly out of the mould. Alternatively, drill a hole in the side to allow the air to escape from under the mould.
Firing for too long or at too high a temperature will cause the glass to continue sliding down. Having nowhere else to go, the weight causes the bottom to begin rising. This is a consistent experience across several kilns and with multiple users.
So keep the temperature down to the minimum required. To find that out, watch the slumping in stages (do not stare!). Look at the piece for a second or two every five minutes after you reach your desired temp.
If it already has slumped adequately, you are firing too high. Reduce your temperature in subsequent firings and watch to find what the required temp and time is. There is absolutely no substitute in slumping but to watch and learn what your mould and glass require.
If you are slumping at such a temperature to seal the glass to the mould, you are firing too hot anyway. Or put more positively, use a low temperature slump, that is, a slump at the lowest temperature to achieve the desired result over an extended period of your choice.
A low temperature slump will allow the glass to conform to the shape of the mould without softening so much that it takes up all the markings of the mould. Therefore, there are spaces for the air to escape from under the glass all the way to the top as well as through the air holes at the bottom. It also gives the most mark-free slump possible for your shape.
Changing Grits on a Belt Sander
When to Go to a Finer Grit on Belt Sander?
If you change the direction the belt moves along the glass with each step it will be easier to see if you have removed each of the previous step's scratches. If you see marks at another angle than your present direction, you need to continue the current step a little while longer.
A second method that I use more often is to use a paint marker to show up the areas not ground. You need to dry the piece before applying the paint marker and let the paint dry before continuing with the next grit. The paint should be applied thinly to avoid transferring the paint to the belt, which reduces the efficiency of the grits. Any low spots or scratches will show up as white areas or lines on the surface.
The process of drying and painting also helps to provide a break in the sometimes tedious process of grinding and polishing.
These two methods can also be used for cold working by hand.
If you change the direction the belt moves along the glass with each step it will be easier to see if you have removed each of the previous step's scratches. If you see marks at another angle than your present direction, you need to continue the current step a little while longer.
A second method that I use more often is to use a paint marker to show up the areas not ground. You need to dry the piece before applying the paint marker and let the paint dry before continuing with the next grit. The paint should be applied thinly to avoid transferring the paint to the belt, which reduces the efficiency of the grits. Any low spots or scratches will show up as white areas or lines on the surface.
The process of drying and painting also helps to provide a break in the sometimes tedious process of grinding and polishing.
These two methods can also be used for cold working by hand.
Wednesday 19 December 2018
Striking glass
Yes, much glass is striking in its effect. But the term is used in a technical sense to
indicate the glass has not reached its intended colour without further firing.
A striking glass is one that changes to its true colour. Not
one which takes up a different colour.
There seem to be differing ideas on how striking works, but it is an
intentional process.
Several glasses coloured with copper or silver strike to Their final colour when heated. It seems
that copper when used to make red (rather than blue or green) can undergo a
chemical change during the heating. The
copper oxide used is normally Cu2O.
When heated the copper and oxygen molecules can separate and form bonds
with other molecules. The rapid cooling
that is used in glass prevents the copper and oxygen from combining in the Cu2O
formation. The extent of this
dissociation determines the degree of colour change. Thus, the colour is affected by the heat work
given to the glass – assuming the starting proportions of materials are the
same. This can occur with some other
colouring metals too.
Another form of striking is caused by the growth of crystals
within the glass. In these cases, usually in silver bearing glass, the metals
separate from the silica and form small crystalline structures which are also
fixed by the rapid cooling required for glass.
There is another theory that the colour change is due to the
orientation of the colouring molecules within the glass matrix. The idea is that the molecules will change
from the clearer state to the struck colour due to the orientation caused by
reheating and cooling.
The actual process seems to be unknown in a definitive sense. What is known is that temperature, a reducing or oxidising atmosphere, and heat work will vary the intensity of the strike in colour. This means that where the project is especially sensitive, you must undertake experiments to help predict the colour that will be achieved with the conditions you choose to use.
Wednesday 12 December 2018
Break Diagnosis in Slumping
The usual advice in looking at the reasons for breaks in
your pieces must be considered in relation to the process being used. Breaks during slumping need to be considered differently to those occurring during fusing.
The advice normally is that if the edges are sharp, the
break occurred on the way down in temperature. Therefore, the glass must have
an annealing fracture or a compatibility break.
It continues on to say if the edges are rounded it occurred on the heat up,
as it broke while brittle and then rounded with the additional heat.
This is true, but only on rounded tack and fused pieces.
When the process is a slump, there is not enough heat to
round the edges. So, the edges will be
sharp whether the break was on the heat up or the cool down.
How can you tell in a
slump process when the break occurred?
If you can put the pieces of the slump back together and
they fit perfectly, the break was on the cool down, as the piece was already
fully formed.
If the pieces do not fit together perfectly, the break was
on the heat up. This is because the
break occurred, and then the two (or more) pieces slumped independently, thus
leaving slightly different shapes at the break line.
There is a special case here, of course. Sometimes the break is only a split in the
bottom, that does not come all the way to the top of the piece. This split (or splits)
occur when the heat up is too fast. The
top becomes plastic while the bottom is still brittle/stiff. The weight of the hotter, more pliable glass
overcomes the strength of the cooler and heat stressed bottom, causing it to
split. More information is given here: Diagnosis of Breaks
There is also more extensive information on diagnosis of
breaks in this blog entry on slumping cracks.
Wednesday 5 December 2018
Slip cast moulds
Hard spots in some moulds are the result of the method of
creating the moulds. Most of the ceramic moulds we use in kilnforming are slip
cast.
This diagram shows the main stages of slip casting |
Slip casting is a way of quickly producing multiples from a
mould. The original shape is surrounded
by a one - or multiple - part plaster mould.
This mould is used to contain the clay slip which is poured in.
The plaster absorbs water from the slip,
stiffening the clay in contact with the plaster. After a defined time, the
remaining slip is poured out of the mould.
The clay remains in the mould a short time until it begins to contract
from the plaster mould and is described as leather hard.
It is then de-moulded, trimmed and cleaned
before it is further dried. When
appropriately dry, it is fired.
Some moulds we receive show a spot where the kiln wash does
not cover the surface in the same way as the rest. This is a result of the method of pouring the
slip into the mould. Slip that is hand
poured does not fall in the same place for long. But industrially poured slip often falls in
the same place for the whole of the pour.
This creates a hard spot - an area where the slip is more compacted than
the rest of the object.
This hard spot does
not affect the appearance or performance of the object. However, it does not absorb the water from
the kiln wash as well as the other areas. And this is when the hard spot
becomes apparent. It will still have enough separator to keep the glass from
sticking, although visually it appears bare. If concerned, you can coat that
area more than the rest after the kiln wash has dried a little. You need to be careful that you do not
introduce an unevenness into the kiln washed surface, as that might appear on
the slumped surface of the glass.
Wednesday 28 November 2018
Float Annealing Temperatures
Float glass annealing temperatures vary quite a bit from one
manufacturer to another; and even within one manufacturer’s product line.
Comparisons of various
float glasses
Some companies are more informative that others. Pilkington are one of the more open of European
glass manufacturers on various bits of information.
Pilkington Float
CoLE 83 *10-5
Softening point: 715°C
annealing point: 548°C
strain point: 511C
Pilkington Optiwhite ™
Softening point: ca. 732°C
annealing point: ca. 559°C
strain point: ca. 526°C
There is a difference of 11C between two of the Pilkington
product lines for the annealing points.
The softening and strain points are slightly wider.
Glaverbel, a
Belgian company, restricts their information to CoLE and the softening point.
CoLE 91 * 10-5
Softening point: 600°C
Saint-Gobain, a
French company, shows some more of the variation in the product lines, although
they do not give specific annealing points for the different products.
CoLE 90 * 10-5
annealing range: 520 - 550°C
Low E glass
softening – 840°C
strain - 617°C
R glass (sound reducing)
softening – 986°C
strain - 736°C
D glass (decorative)
softening point – 769°C
Compatibility
Even this small sample of float glasses shows there is a
significant difference between manufacturers for the softening, annealing and
strain points. This means that, unless
you are sure of the glass merchant’s source of glass, you will need to test
each batch of glass for compatibility with previous batches, if you are
combining from different suppliers.
I included the CoLE numbers (which all the manufacturers
specified as an average change in length for each degree C increase in
temperature from 0 to 300°C) to show the variation and to challenge anyone to
find Bullseye and Saint-Gobain or Glaverbel compatible with each other. My experience has shown that the Optul coloured
frit and confetti is more likely to be compatible with Pilkington than the
other two.
I have been beginning my annealing of float glass at
525°C. This little bit of literature
research shows that my annealing soak should be starting higher, possibly at
540°C, certainly no lower than 530°C.
Other areas of the world may find their float glass has significantly
different annealing ranges.
Wednesday 21 November 2018
Broken base glass
Firing a piece with a partially covered base layer requires
more care than two even layers to avoid the fracture of the glass during the
heat up stage of a firing. Slower rates
of advance need to be used.
Glass is a poor conductor of heat and electricity. This can
be good in certain circumstances but is usually one of the limitations in
kilnforming. The poor conductivity of
glass means the top layer of glass will need to be heated before it begins to
transmit heat to the glass below.
A while back an example was shown that is a special case,
but also illustrates the general principle (apologies to the poster, as I
didn’t take down the name at the time and can’t find the original post now).
This sheet of clear glass was covered by an arrangement of
stringers, with a border of clear exposed.
I don’t know positively, but I presume this was done in the knowledge
that the single sheet of clear glass would become smaller, and the border would
be cut down to the appropriate size.
Be that as it may, the exposure of the clear allowed the
edges of the clear to heat up faster than the covered part of the sheet. The stress of the temperature differential
between the centre and the edges led to the fracture of the glass during the
heat up. This can be confirmed by the
rounding of the broken edges. It is
further confirmed, by observing the relative straightness of the stringers, that
the break occurred before the stringers became sticky enough to even laminate
to the base glass - the clear glass broke underneath, leaving the stringers
relatively undisturbed. It is also an indication that the glass broke earlier
than the slumping temperature, as the stringers would have been sticky enough
to break with the clear otherwise.
One speculation given for the break was that it was affected
by the size. You can see the size is
relatively large for the kiln. This may
have had some influence on the fracture as well. But it is not so much the size as the
shielding of the heat from above for a large part of the base sheet. We don’t
know if this was a side fired kiln, but if it was, there would be an increased
exposure of the edges of the glass to the heat and so increase the likelihood
of temperature differentials leading to too much stress for the base glass.
The rate of advance for partially covered sheets needs to be
reduced to be slower than for evenly covered base sheets. Even on evenly covered base sheets, there is
a risk of breakage of the bottom sheet, if the rate of advance is too
quick. Slower heating reduces the
temperature differentials, as the gradual rise in heat allows the glass to be
closer in temperature from top to bottom.
Labels:
Schedules,
Single Layers,
Stephen Richard,
Thermal Shock,
Verrier
Wednesday 7 November 2018
Specific Gravity, CoLE, and Colourants of Glass
I’ve been asked the
question “is there is differential in
specific gravity as related to COE or colorant used in the glass (white opal v
clear)”?
Using the typical compositions of soda lime glass (the stuff
we use in fusing), both transparent and opalescent and combining the specific
gravity of the elements that go to make up the glass, I have attempted to
answer question - the last part of the question first.
Difference in specific
gravity between transparent and opalescent glass
Transparent glass
|
There are, of course minor amounts of flux and metals for colour in addition to these basic materials.
The specific gravity of typical soda lime glass is 2.45.
Opalescent glass
Initially
opalescent glass was made using bone ash, but these tended to develop a rough
surface due to crystal formation on the surface. The incorporation of calcium phosphate (bone
ash) and Flouride compounds and/or arsenic became the major method of producing
opalescent glass for a time.
The
current typical composition by weight (with specific gravities) is:
Silicon
Dioxide (SiO2) – 66.2%, 2.648 SG
Sodium
Oxide (Na2O) – 12%, 2.270
Boric
Oxide (B2O3) – 10%, 2.550
Phosphorus pentoxide (P2O5) – 5%, 2.390
Aluminum
Oxide (Al2O3) – 4.5%,
3.987
Calcium oxide (CaO) – 1.5%,
3.340
Magnesium oxide (MgO) - 0.8%,
2.320
The combined specific gravities are within 0.03% of each
other - a negligible amount. So, the specific gravity of both opalescent
and transparent glass can be considered to the same. For practical purposes, we
take this to be 2.5 rather than the more accurate 2.45.
Other glasses exhibit different specific gravities due to
the materials used, for example:
Lead Crystal Glass
Lead Crystal glass contains similar
proportions of the above materials with the addition of between 2% and 38% lead
by weight. Due to this variation the
specific gravity of lead crystal is generally between 2.9 and 3.1, but can be
as high as 5.9.
Borosilicate glass
Non-alkaline-earth borosilicate glass (borosilicate glass 3.3)
The boric oxide (B2O3)
content for borosilicate glass is typically 12–13% and the Silicon dioxide (SiO2)
content over 80%. CoLE 33
Alkaline-earth-containing borosilicate glasses
In addition to about
75% SiO2 and 8–12% B2O3, these glasses
contain up to 5% alkaline earths and alumina (Al2O3). CoLE 40 – 50
High-borate borosilicate glasses
Glasses containing
15–25% B2O3, 65–70% SiO2, and smaller amounts
of alkalis and Al2O3
All these borosilicate glasses have a specific gravity of
ca. 2.23
Correlation between
CoLE and and specific gravity?
This comparison of different glasses shows that the
materials used in making the glass have a strong influence on the specific
gravity. However, there does not appear
to be a correlation between CoLE and specific gravity in the case of
borosilicate glass. If this can be
applied to other glasses, there is no correlation between specific gravity and
CoLE.
Correlation between
specific gravity and colourisation minerals and CoLE?
The minerals that colour glass are a very small proportion
of the glass composition (except copper where up to 3% may be used for
turquoise). The metals are held in
suspension by the silica and glass formers.
That means the glass is moving largely independently of the colourants
which are held in suspension rather than bring part of the glass structure.
There is unlikely to be any significant effect of the metals on the Coefficient
of Linear Expansion. The small amounts
of minerals are unlikely to have an effect on the specific gravity. So, the conclusion is that there is no
correlation between CoLE, specific gravity, and colouring minerals.
The short answer
This has been the long answer to the question. The short answers are:
·
The specific gravity of soda lime transparent
glass and opalescent glass is the same – no significant difference is in
evidence.
·
There appears to be no correlation between
specific gravity and CoLE.
·
There is unlikely to be any correlation between
colourant minerals and CoLE or specific gravity.
Wednesday 31 October 2018
Lubrication for cutters
You can cut
glass without oil. It has been done for
a long time. But it has been found that
there are advantages to using oil on a score line.
The purpose of oil:
·
A
minor element is to oil the cutter wheel.
·
A
major element is to oil the score line.
An oiled score line stays open longer than a dry one.
·
An
oiled score line reduces the amount of visible chipping from the score line.
The kind of
oil
·
Mineral
oil does not oxidise to gum up the scoring wheel.
·
Any
light mineral oil from sewing machine oil to WD40 is acceptable. Some use very
light oils such as turpentine or white spirits.
·
There
are cutting oils that are synthetic and easier to clean than the standard oils
and spirits, in that less residue is left when the oil is wiped off.
·
Vegetable
oils might appear to be a good substitute.
But they oxidise and become sticky, attracting dust and other particles
which soon block the turning of the scoring wheel. This requires frequent checking and cleaning. Avoid vegtable oils.
Methods of
applying
·
The
oil can be put into the cutters that have a reservoir.
·
The
cutter can be dipped into a container of oil with or without an oil-soaked
material.
·
The
oil can be painted onto the glass before scoring.
Wednesday 24 October 2018
Frit by thermal shock
Frit can be created by thermal
shock. You will still need to do some
manual breaking up. The principle is that you heat the glass and then cool it
rapidly, causing the glass to break into pieces.
Place the glass in a stainless steel bowl
and heat as fast as possible to 300C – 400C. Turn the kiln off and pull out the
bowl, using heat resistant gloves and dump the hot glass into a large bucket of
water. Once the glass is cool, pour off the water and dry the glass. When dry, you can break the crazed glass into
smaller bits just as you would with other glass. Note that pouring water over the glass has
two disadvantages – one, it does not completely thermal shock the glass, and
two, the large amount of steam released is very dangerous.
The advantages of this quenching method of
obtaining frit are that you can create frit with less effort. You also get less fines and powder with this
method. And less effort is required to smash up the glass.
Some indicate that ice cold water to
quench the glass is a good idea. This is
because warm water will not provide enough of a shock to the glass to craze it
throughout. But if you have a large
bucket of water, there is no necessity, as the volume of water will cool the
glass quickly enough. Of course, if you
are planning another quenching, you need to renew the water, as it will not be
cold enough to thoroughly craze the glass.
You can, in part, control the size of the
resulting frit. Firing at 300C results
in larger frit than firing at 400C.
However, firing at 500C does not provide even smaller frit. The best results are between 300-400C,
although frit can be made at 200C as well.
Experiment with temperatures to get the frit you want.
Once you have dried the frit, you can
begin to break it up. Some can be done by hand, but the pieces are often sharp,
so gloves are essential. The other
standard methods of breaking up glass to make frit are applicable. But it does
not take as much effort as breaking from cullett.
Wednesday 17 October 2018
Annealing vs toughening
The
statement “annealing stained glass makes it stronger” appeared on the internet
some time ago. Of course, without
annealing there is no glass, it would simply crumble. Annealing is the process of allowing the
glaseous state to be achieved.
I
think the statement is more about the difference between annealed and
toughened/tempered glass. In summary, it
relates to the amount of stress within the glass. Well annealed glass has less stress than
inadequately annealed glass and so is more stable. Toughening is a process that balances stress
and tension in the glass.
The
processes are for different purposes and follow different processes.
Annealing
Annealing of glass is a process of slowly cooling hot glass to relieve residual
internal stresses introduced during manufacture. Annealing of glass is critical
to its durability. Glass that has not been properly annealed retains thermal
stresses caused by rapid cooling, which decreases the strength
and reliability of the product. Inadequately annealed glass is likely to crack
or shatter when subjected to relatively small temperature changes or to minor mechanical
shock. It even may fail spontaneously from its internal stresses.
To anneal glass, it is necessary to soak
it at its annealing temperature. This is determined mathematically as a
viscosity of 1013 Poise (Poise is a measure of viscosity). For most soda lime glass,
this annealing temperature is in the range of 450–540°C, and is the so-called annealing point or temperature
equalisation point of the glass. At such a viscosity, the glass is too stiff
for significant change of shape without breaking, but it is soft enough to
relax internal strains by microscopic flow.
The piece then heat-soaks until its temperature is even throughout and the
stress relaxation is adequate. The time necessary for annealing depends on its
maximum thickness. The glass then is cooled at a predetermined rate until its
temperature passes the strain
point (viscosity = 1014.5 Poise), below which even
microscopic internal flow effectively stops and annealing stops with it. It
then is safe to cool the product to room temperature at a rate limited by the thickness of the glass.
At the annealing point (viscosity = 1013 Poise), stresses
relax within minutes, while at the strain
point (viscosity = 1014.5 Poise) stresses relax within hours. Stresses acquired at temperatures
above the strain point, and not relaxed by annealing, remain in the glass
indefinitely and may cause either immediate or delayed failure. Stresses
resulting from cooling too rapidly below the strain point are considered temporary,
although they may be adequate to promote immediate failure.
But annealed glass, with almost no internal stress, is subject to
microscopic surface cracks, and any tension gets magnified at the surface,
reducing the applied tension needed to propagate the crack. Once it starts
propagating, tension gets magnified even more easily, causing it at breaking
point, to propagate at the speed of sound in the material.
In short, the aim of annealing
is to relieve the stress to create a stable piece of glass. The above describes
when and how that occurs.
Toughened/Tempered Glass
Toughening or tempering glass starts with annealed
glass to form one type of safety glass.
This done through a process of controlled thermal or chemical treatments
to increase its strength compared with normal glass. Tempering puts the outer
surfaces into compression and the interior into tension. Such stresses cause
the glass, when broken, to crumble into small granular chunks instead of
splintering into jagged shards as annealed glass does. The granular chunks are
less likely to cause injury – thus safety glass.
Toughened glass is stronger than normal glass. The greater contraction of the inner layer
during manufacturing induces compressive stresses in the surface of the glass
balanced by tensile stresses internally. For glass to be considered toughened,
the compressive stress on the surface of the glass should be a minimum of 69
megapascals (10,000 psi). For it to be considered safety glass, the
surface compressive stress should exceed 100 megapascals (15,000 psi).
It is the compressive stress that gives the
toughened glass increased strength. Any cutting or grinding must be done prior
to tempering. Cutting, grinding, and sharp impacts after tempering will cause
the glass to fracture.
Toughened glass is normally made from annealed sheet
glass via a thermal tempering process. The glass is placed onto a roller table,
taking it through a furnace that heats it well above its transition temperature
of ca. 540°C (depending on the glass concerned) to around 620°C. The glass is
then rapidly cooled with forced air drafts while the inner portion remains free
to flow for a short time.
An alternative chemical toughening process involves
forcing a surface layer of glass at least 0.1 mm thick into compression by
ion exchange of the sodium ions in the glass surface with potassium ions (which
are 30% larger), by immersion of the glass into a bath of molten potassium
nitrate. Chemical toughening results in increased toughness compared with
thermal toughening and can be applied to glass objects of complex shapes.
This blog entry is largely based on Wikipedia
https://en.wikipedia.org/wiki/Toughened_glass
and other sources.
Wednesday 10 October 2018
Slumping Different Glasses in the Same Firing
The question has arisen as to whether it
is possible to slump Bullseye and Spectrum in same slump firing.
Yes, it is possible.
But precautions are necessary.
Different temperatures are generally recommended for
Spectrum and Bullseye. Spectrum is
generally expected to do the same slump as Bullseye at 25C less.
This implies that Bullseye should be put
in larger or easier slump moulds than Spectrum and fired to the lower
temperature required by Spectrum. The thinking behind this is that smaller
spans require longer or more heat to slump.
Steeper moulds require more time and heat than less steep ones.
In general, shallow slumps will work
better for both glasses together than more steep or textured ones.
To be certain of a good result, you
should fire as low as practical for an extended soak. Follow this with an extended annealing and a slower
cooling rate than normal for Spectrum.
This applies to almost all the glass that
is being produced with the aim of being compatible with these two glasses. It is not possible to get a good result for
float glass if it is put into the same firing as for Bullseye or Spectrum.
Wednesday 3 October 2018
Tack Fuse vs Fire polish
Are tack fuse and fire polish the
same thing?
Maybe
They both occur
in the same temperature same range, depending on the degree of tack fuse you
want.
What you are
doing in the fire polish process is heating the top surface enough to appear
polished. Very little time is needed in a fire polish at top temperature as
opposed to a tack fuse.
In a tack
fuse, you want the bottom of the upper pieces to be hot enough to stick to the
bottom layer. This requires a higher temperature or longer soak than a fire
polish.
At around
730C, depending on your kiln, you will be softening the upper surface of the
glass enough to give a polished appearance.
To determine whether the polished surface has been achieved, you can
peek into your kiln at the chosen temperature to see if the polish is complete.
This is also
the temperature at which sintering, or a lamination of the glass pieces
occurs. The edges will still be sharp,
but cannot be pulled apart. This kind of
fusing needs careful annealing – long soaks and slow cools.
Tack fusing
of various degrees occurs in the temperature range from 730C to 770C. To determine which temperature and soak time
will give you the result you desire will require experimentation and
observation. Generally, you can achieve the desired level of fuse with lower temperatures and longer soaks, as you can at higher temperatures and longer soaks.
It is also possible to give a fire polish to your glass at a really low temperature, such as 550C, with a very long soak. This will avoid significantly flatening the surface of your piece. This is the effect of heat work.
Further information is available in the ebook Low Temperature Kiln Forming.
Wednesday 26 September 2018
The relative order of kiln forming events
When
preparing for multiple firings of elements onto a prepared piece, you need to
consider the order and temperatures of events so that you do not harm an
earlier stage of the project. This blog
entry will not give definitive temperatures, as that varies by glass and by
kiln. Instead, it indicates what happens
in progression from highest to lowest temperatures in approximate Celsius
degrees.
ca. 1300C - Approximate
liquid temperature
ca. 850 – 1000C - Glass
blowing working temperature
ca. 950C - Raking and
combing
ca. 850C - Casting
ca. 810C - Full fuse
ca. 790C - Large bubble
formation
ca. 770C - High tack,
low contour fuse
ca. 760C - Tack fuse
ca. 750C - Fire polish
ca. 700C – 760C - Devitrification
range
ca. 700C - Lamination
tack
ca. 600C – 680C - Slump and drape
ca. 650C - Vitreous
paint curing temperature
ca. 600C - No risk of
thermal shock above this temperature
ca. 540 – 580C - Glass stainers enamel
curing temperature
ca. 520 – 550C - Silver stain firing
temperature
ca. 550C - Glass
surface beginning to soften
Slow rates
of advance needed from room temperature to ca. 500C
These temperatures are of course, affected by the soak times. The longer the soak time, the lower temperature required. The rate at which you achieve the temperature also affects the effective temperature. Slower rates of advance require lower temperatures, than fast rises in temperature. These illustrate the effect of heat work.
The table shows
for example you need to do all the flat operations and firings before slumping
or draping. It also shows you can use
vitreous glass paints at the same time as slumping and draping. This emphasises that the standard practice is
to plan the kind of firings you will need for the piece and do them in the
order of highest temperature first, lowest last.
In general,
you do need to do the highest temperature operation first and lowest last. But there are some things you can do with
heat work. For example, if you needed to
sandblast a tack fused piece, but did not want to risk reducing the differences
in height there things you can do. From the list above, you can
see the glass surface begins to soften around 500C. It is possible to soak the glass for a long
time around 500C to give it a fire polish, instead of going to a much higher
temperature. You will need to experiment
to find the right combination of temperature and soak length, but it can be
done.
This article
is to show that knowledge of what is happening to the glass at different
temperatures, can help in “fooling” the glass into giving you the results you
want without always following the “rules”.
This may also be what it is to be a maverick glass worker. Use the behaviour of glass to your advantage.
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