I am amazed by the effort put into
ramp up rates, bubble squeezes, and top temperatures in comparison to
annealing. The emphasis on social media groups
seems to be to get the right ramp rates for tack fuses and slumps, bubble squeezes,
etc. Most of the attention is on the way
up to processing temperature.
The treatment of annealing and
cooling is almost cavalier by comparison. The attention seems to be on what temperature,
and how long a soak is needed. Then some
arbitrary rate is used to cool to 370ºC/700ºF.
Annealing, in comparison to firing to
top temperature, is both more complex and more vital to getting sound, lasting
projects completed.Skimping on
annealing is an unsound practice leading to a lot of post-firing difficulties.
Annealing is more than a temperature
and a time. It is also the cooling to
avoid inducing temporary stress. That stress during cooling can be large enough to break the glass. This temporary stress is due to expansion
differentials within the glass.
People often cite the saving of
electricity as the reason for turning off at 370ºC/700ºF. My response is that if the kiln is cooling off
slower than the rate set, there will be no electricity used. No electricity demands. No controller intervention. No relay operation.
Annealing at the lower end of the range
with a three-stage cooling provides good results.The results of Bullseye research on annealing
are shown in their chart for annealing thick items. It applies to glass 6mm and much larger. It results from a recommendation to anneal at
the lower end of the annealing range to get good anneals. Other industrial research shows annealing in
the lower end gives denser glass, and by implication, more robust glass. Wissmach have accepted the results of Bullseye
research and now recommend 482ºC/900ºF as the annealing temperature for their
W96. The annealing point of course
remains at 516ºC/960ºF.
Bullseye research goes on to show
that a progressive cooling gives the best results. They recommend a three-stage cooling process. The first is for the initial 55ºC/º100F below
the annealing temperature, a second 55ºC/100ºF cooling and a final cooling to
room temperature.
It is a good practice to schedule all
three cooling rates. It may be considered unnecessary because your kiln cools
slower than the chart indicates. Well,
that is fine until you get into tack and contour fusing. Then you will need the three-stage cooling
process as you will be annealing for thicknesses up to 2.5 times actual height.
Of course, you can find out all the
reasons for careful annealing in my book "Annealing; concepts, principles,
practice" Available from Bullseye at
A schedule was
presented for a slumping problem of a 6mm/0.25” blank. It consisted of three segments each of a rate
of 277C/500F with short holds up to 399C/750F and then a rapid rise to 745C/1375F.
The cool was done with two long holds at
537C/1000F and 482C/900F followed by cooling rates for 12mm/0.5”
My response was
that, yes it was fired too high. Not
only that, but the firing strategy, as shown by the schedule, is odd.
Strategy
The general
strategy for slumping follows these ideas.
·Glass
is slow to absorb heat, and in one sense, this schedule accepts that by having short
soaks at intervals.As glass is slow to
absorb heat, it is necessary to use slow ramp rates and without pauses and
changes in rates.This should be applied
all the way to the slumping temperature.
·Holds
of short durations are not effective at any stage in a slumping firing.The objective is to allow the glass time to
form to the mould with as little marking as possible.This implies slow rates to low temperatures
with significant holds at appropriate stages.This about putting enough heat work into the glass that higher
temperatures are not needed.
·This
kind of firing requires observation for new moulds and new arrangements of
glass to ensure the slump is complete.Once
you know the mould requirements and are repeating the layup of the glass, the
firing records will tell you what rates and times to use to get a complete
slump with minimum marking.
·The hold
at annealing temperature is to equalise the temperature throughout the glass to
produce a stress-free result.Any soaks
above are negated or repeated by the necessary soak at the annealing
temperature.The hold there must be long
enough to complete the temperature equalisation that is the annealing.
·My work
has shown that annealing for one (3mm/0.125”) layer thicker produces a piece
with less stress.This indicates that a
6mm/0.25” piece should be annealed as for 9mm/0.35” to get the best result.
The summary of the
firing strategy for slumping is:
·A
single ramp of a slow rate to the slumping temperature.
·Observation
of the progress of the slump to determine the lowest practical temperature and
hold time.
·Annealing
for one layer thicker that being slumped.
·Three
stage cooling of the piece at rates related to the annealing hold.
Critique
This is a critique of the schedule. For comparison, my schedule for a
full fused 6mm blank would be different.
·140ºC/250ºF
to 677º/1250ºF for 30 to 45 minutes.
·9999 to
482ºC/900ºF for 1.5 hours
·69ºC/124ºF
to 427ºC/800ºF, no hold
·125ºC/225ºF
to 371ºC/700ºF, no hold
·330ºC/600ºF
to room temperature, off.
The rate of the
published schedule is fast for a full fused blank and extremely fast for a tack
fused blank. This needs to be slowed. The
schedule provides a single (fast) rate of heating, but with unnecessary holds. The holds are so short as to be ineffective, anyway. There
is no need for the holds on the way up to the slumping temperature.In general slumping schedules are of fewer
segments. This is because glass behaves
well with steady slow inputs of heat.
Then strangely, the
schedule increases the rate to top temperature. It does so with a brief soak at 593ºC/1100ºF. This fast rate of 333ºC/ 600ºF begins at 400ºC/750ºF.This is still in the brittle phase of the
glass and risks breaking the glass.The
brittle stage ends around 540ºC/ 1005ºF.
This rapid rate
softens the surface and edges of the glass without allowing time for the
underside to catch up. This explains uneven
edges. It also risks breaking the glass
from too great expansion of the top before the bottom.
Additionally, the
schedule uses a temperature more than 55ºC/100ºF above what is a reasonable highest
slumping temperature.The top
temperature of this schedule is in the tack fusing range.
There is no need
for a hold 55ºC/100ºF above annealing soak. It is the annealing soak that
equalises the temperature before the cool begins.The higher temperature equalisation is
negated by the cooler soak at annealing temperature. So, the hold at the higher
temperature and slow cool to the annealing temperature only delays the firing
by about two hours.It does not have any
effect on the final piece.
The schedule is cooling for a piece of 12mm/0.5”.This is slower than necessary.As noted above, cooling for one layer thicker
than the piece is advisable to get the most stress free result. The annealing soak could be 1.5 hours
following this idea. Cooling with a
three stage schedule reduces the risk of inducing temporary stresses that might
break the glass. Although the initial
cooling rate I recommend is very similar to this schedule, it safely reduces the
total cooling time.
·69ºC/124ºF
to 427ºC/800ºF, no hold
·125ºC/225ºF
to 371ºC/700ºF, no hold
·330ºC/600ºF
to room temperature, off.
Using my kind of
schedule for the first time will require peeking once top temperature is
reached to determine when the slump is complete. It may take as much as an
hour. Be prepared to either extend the hold, or to skip to the next segment if
complete earlier. The controller manual will explain how.
More information is given in Low Temperature Kilnforming, An Evidence-based guide to scheduling. Available from Etsy and Bullseye
In the fast-paced world of semiconductor manufacturing, where precision and reliability are paramount, choosing a suitable passivation material is critical to ensuring the optimal performance of electronic devices. Among the library of viable materials, glass has gained significant attention for its unique properties and versatility. This article looks at how glass is used for passivation and what properties make it highly suitable for the job.
Understanding Passivation in Semiconductors
Before unpacking the specifics of glass as a material for passivation, it is essential to understand the concept of passivation in semiconductor manufacturing. Passivation involves depositing a protective material onto the surface of metals or metal alloys to enhance their resistance to environmental factors.
The layering material can be organic or inorganic and should exhibit excellent electrical insulation and strong substrate adhesion, as well as block the ingress of chemical species. In the case of semiconductors, passivation is crucial to preventing degradation and ensuring long-term reliability.1,2
Why Use Glass for Passivation?
Glass has emerged as a compelling choice for passivation due to its unique combination of properties. For example, glass can be formulated in numerous ways, with common types including Pb-Si-Al, Zn-B-Si, and Pb-Zn-B. This allows manufacturers to produce glass capable of meeting low and high-voltage electrical specifications; matching the coefficient of thermal expansion of semiconductor materials; and meeting the low temperature processing requirements.3,4
Glass is chemically durable and thus can provide an inert barrier against external elements, such as moisture and contaminants, which might otherwise compromise the semiconductor’s performance. Moreover, the high transparency of some glasses, such as borosilicate glass, makes them ideal for applications with critical optical properties, such as photovoltaics. This transparency enables efficient energy transmission and absorption, contributing to the overall performance of semiconductor devices and solar cells.5,6
How are Semiconductors Passivated?
Glass can be deposited onto semiconductors in a variety of ways. Choosing methods for passivation depends on factors such as the semiconductor device’s specific requirements, the passivation layer’s desired properties, and the overall manufacturing process. Methods for achieving glass passivation in semiconductor manufacturing include:7
Chemical vapor deposition (CVD), including plasma-enhanced CVD (PECVD)
Physical vapor deposition (PVD), including E-beam deposition
Sputter Coating
Atomic Layer Deposition (ALD)
In manufacturing, the process of glass passivation is frequently succeeded by chemical procedures, such as the etching of contact windows or the electrolytic deposition of contacts. These procedures may pose a threat to the integrity of the glass.
The chemical resistance of different passivation glasses varies significantly and serves as a crucial factor in determining the suitable glass type and the accompanying etching process.8
Comparing Glass to Other Materials
While various materials can be used for passivation, glass stands out for its exceptional stability over temperature, humidity, and time. Literature searches reveal a lack of head-to-head comparisons with other common passivation materials; however, general comparisons can be drawn.6
Amorphous silicon (a-Si) films utilized in solar cells present numerous advantages. These include a lower deposition temperature, in contrast to the temperatures commonly employed in cell manufacturing. However, it is essential to note that a-Si films exhibit sensitivity to subsequent high-temperature processes, which are frequently necessary in industrial manufacturing technology.9
Similarly, AlOx passivation films can be applied at relatively low temperatures but can be limited by slow deposition speeds when using specific application methods. This can generate problems for high-throughput techniques, such as solar cell production.9
Polyimide, a common passivation material lauded for its strength and thermal stability, is also susceptible to moisture absorption. This can impact the strength and dielectric properties of the protective coating, risking the integrity of the semiconductor.10
Applications of Glass Passivation
Passivation glasses demonstrate outstanding performance in wafer passivation and encapsulation processes, providing advantages to a diverse range of semiconductor devices, including:8
Thyristors
Power transistors
Diodes
Rectifiers
Varistors
Glass also has applications in solar cell passivation. In a recent study, researchers developed a method for enhancing borosilicate glass (BSG) passivation using high temperatures before lowering the temperature to accommodate the metallization process. In doing so, they notably improved the solar cell’s efficiency.11
In another study, phosphosilicate glass (PSG) was found to significantly enhance the practical lifetime of minority carriers and improve the overall performance of solar cells, particularly in structures involving nanocrystalline silicon and crystalline silicon.12
Mo-Sci’s Expertise in Glass Thin Films
Fueled by the increasing prevalence of smart devices and advancements in the automotive and aerospace sectors, the semiconductor passivation glass market is anticipated to grow consistently in the next few years.3
Mo-Sci’s expertise lies in leveraging the unique properties of glass to create tailored solutions, ensuring the reliability and performance of many applications, including glass seals and glass coatings. Contact us for more information.
References and Further Reading
Pehkonen, S.O., et al. (2018). Chapter 2 – Self-Assembly Ultrathin Film Coatings for the Mitigation of Corrosion: General Considerations. Interface Science and Technology. doi.org/10.1016/B978-0-12-813584-6.00002-8
Lu, Q., et al. (2018). Chapter 5 – Polyimides for Electronic Applications. Advanced Polyimide Materials. doi.org/10.1016/B978-0-12-812640-0.00005-6
Zhong, C., et al. (2022). Properties and mechanism of amorphous lead aluminosilicate passivation layers used in semiconductor devices through molecular dynamic simulation. Ceramics International. doi.org/10.1016/j.ceramint.2022.07.191
Hansen, U., et al. (2009). Robust and Hermetic Borosilicate Glass Coatings by E-Beam Evaporation. Procedia Chemistry. doi.org/10.1016/j.proche.2009.07.019
Korvus Technology. [Online] The Revolution of PVD Systems in Thin Film Semiconductor Production. Available at: https://korvustech.com/thin-film-semiconductor/ (Accessed on 05 January 2024).
Liao, B., et al. (2021). Unlocking the potential of boronsilicate glass passivation for industrial tunnel oxide passivated contact solar cells. Progress in Photovoltaics. doi.org/10.1002/pip.3519
Imamura, K., et al. (2018). Effective passivation for nanocrystalline Si layer/crystalline Si solar cells by use of phosphosilicate glass. Solar Energy. doi.org/10.1016/j.solener.2018.04.063
I have just
taken a large piece, with uneven layers out of the kiln, it went in … and fired
for double thickness. A small piece has flipped and is showing the white side. …
If I cover this with a thin layer of coloured powder frit, does the piece need
the long anneal process when I fire it again, please. I will be taking it up to
the lowest tack fuse temperature possible [my emphasis], so the rest doesn’t change
too much.
When considering the re-firing of a fused piece, even with
minimal changes, the schedule needs re-evaluation of both ramp rates and
annealing. In this case, the major change is using a sinter firing – “the
lowest tack fuse temperature possible”.
Ramp Up Rates
Previously the piece was in several layers.
The piece is now a thicker single
piece and needs more careful ramp rates.
It is also of uneven thicknesses.
And you intend to fire to a sharp
tack or sinter.
These things make a requirement for more cautious firing. You
cannot fire as quickly from cold as forthe original unfired piece.
Previously, the sheets could be heated as though separate. They were not hot
enough to stick together until beyond the strain point. They now could
experience the differential expansion from rapid heating, which can cause breaks.
The previously fired piece will need a slower initial ramp
rate this time. This is because you are firing for a sharp tack. This is also
known as fusing to stick, or sintering. It is not because of a second firing.
It is because of the differences in the glass for this firing. You are firing a
single thicker piece of uneven layers to a sharp tack.
Looking at Stone* and the Bullseye chart for Annealing Thick Slabsindicates that in general, the first ramp rate should be
halved for each doubling of calculated thickness. This is for full fused items.
However, this is going to be a more difficult fusing profile - sintering. The
calculation for sintering is as for 2.5 times the thickest part of the piece.
This factor of 2.5 was determined by a series of experiments that are detailed
in the eBook Low Temperature Kilnforming.
You started with firing two layers of 3mm/0.125” at possibly
330°C/595°F. You are now firing the fused 6mm/0.252 piece to a sharp tack. This
means you should be looking at firing for 2.5 times or 15mm/0.625”. This
implies 240°C/435°F as the maximum first ramp rate. A more cautious approach is
to fire to 300ºC/540ºF at a rate of 72ºC/130ºF, as most heat-up breaks occur
below that temperature. You should maintain that rate to 540°C/1005°F afterwards.
Annealing
The annealing time and cool rate will be affected in the
same way as the change to a sharp tack firing. Without that fuse profile change,
and no change in the profile or thickness of the piece, it could have been
annealed as previously. However, changing to a sharp tack means a longer anneal
soak is required. This sharp tack annealing is for 2.5 times the thickness or
150 minutes.
Cooling
The cooling rates for this piece are not the same as for the
first firing. A sharp tack firing will require cooling rates of:
40ºC/73ºF to 482°C/900°F.
72ºC/130ºF f427ºC/800ºF.
240ºC/435ºF to room temperature
This applies regardless of the fusing glass you are using, as it is the viscosity which is the important factor in cooling. Viscosity is primarily related to temperature.
Refiring with Significant Additions.
Ramp rate
If there are additions to the thickness, a slower first ramp
rate will necessary. If an additional 3mm layer is placed on top of a 6mm base
for a rounded tack, you will need to schedule as for 19mm/0.75” (twice the
thickest part). This will be 150°C/270°F for the first ramp rate. For a sharp
tack, it will be as for 22.5mm/0.825”. The maximum rate will be reduced to
120ºC/216F for the first ramp. This shows the additional caution required for
sharper fusing profiles.
Annealing
The annealing will need to be longer than the first firing.
The thickness has changed with the additions of pieces for a rounded tack
firing. Instead of annealing for 6mm/0.25” you will be annealing as for 19mm/0.75”.
This requires a hold of three hours at the annealing temperature and cooling
over three stages:
The first cool rate is 25°C/45°F
per hour to 482°C/900°F.
The second rate is 45°C/81°F per
hour to 427ºC/800ºF.
The last rate is at 90C°C/162°F
per hour to room temperature.
If there are additions, plus firing to the lowest possible
tack temperature – as in the example - the firing must be as for 2.5 times the
actual thickness. Annealing as for 25mm/1” gives rates of:
The first cool rate is 15°C/27°F
per hour to 482°C/900°F.
The second rate is 27°C/49°F per
hour to 427ºC/800ºF.
The last rate is at 90C°C/162°F
per hour to room temperature.
These examples show how dramatically later additions in
thickness can add to the length of the firing to re-fire a well-annealed piece
without breaking it on the heat-up. It also shows that changing the profile to
a sharper tack affects the annealing and cooling times and rates.
*Graham Stone. Firing Schedules for Glass; the Kiln
Companion. 2000, Melbourne. ISBN 0-646-397733-8
As a side note Stone’s book has become a collectable.
This is a frequent statement
in response to a firing that has gone wrong.
You don't always
fuse the same thing, or the same design, or the same thickness, etc. So why always
use the same schedule?
The schedule for the
firing each piece needs to be assessed individually. It may be similar to
previous firings. But it may have differences. Assess what those differences
mean for the firing. Some factors to consider.
Addition of another
layer to a stack in tack fusing makes a difference to the firing requirements.
Even if it is only on part of the piece. It needs to have a slower ramp rate
and a longer anneal soak and slower cooling.
A different design
will make a difference in firing requirements too. For example, if you are adding
a design to the edges of the glass, you will need different bubble squeeze
schedules than when you do not have a border. It will need to be slower and
longer than usual.
The placement of
the piece in the kiln may require a re-think of the schedule too. If the piece
is near the edge of the kiln shelf, or in a cool part of the kiln while others
are more central, the same schedule is unlikely to work. You need to slow the
schedule to account for the different heat work each piece will receive during
the firing.
If you have introduced a strong contrast of colour or mixed transparent and opalescent glass in a different way, you may need slower heat ups and longer cools.
These are some
examples of why the same schedule does not work all the time. It works for
pieces that are the same. But it does not work for pieces that are different.
And we should not expect it to.
There are sources to
help in developing appropriate schedules. Bob Leatherbarrow’s book FiringSchedules for Kilnformed Glass is an excellent one.
A statement was
made on a Facebook group that transparent glass absorbs more heat than
opalescent glass. And it releases more heat during cooling. The poster may have
meant that the transparent heats more quickly than the opalescent, and cools
more quickly.
Yes, dark transparent
glass absorbs heat quicker than most opalescent (marginally), and it releases the
heat more quickly (again marginally) than opalescent. The colour and degree of
transparency do not absorb any more or less heat, given appropriate rates. They
gain the same heat and temperature, although at slightly different rates due to differences in viscosity.
An occasional table
The rate of
heating and cooling is important in maintaining an equal rate of absorption of
heat. The temperature of both styles can become the same if appropriate lengths of heating,
annealing, and cooling are used. The slightly different rates of heat gain can
give a difference in viscosity and therefore expansion. This slight mismatch during rapid ramp rates, might set up
stresses great enough to break the glass. This can occur on the quick heat up
of glass during the brittle phase (approximately up to 540ºC/1005ºF). In fact,
most heat-up breaks occur below 300ºC/540ºF.
The main impact of
differential heat gain/loss is during cooling. Annealing of sufficient length
eliminates the problem of differential contraction through achieving and
maintaining the Delta T = 5C or less (ΔT≤5C). It is during the cooling that the
rates of heat loss may have an effect. The marginally quicker heat loss of many transparents over most opalescent glass exhibits different viscosities and rates of contraction. The
stresses created are temporary. But they might be great enough to cause breaks
during the cooling. Slow cooling related to the thickness and nature of the glass takes care of the differential contraction
rates by maintaining small temperature differentials.
Significance of Differential Heat
Gain/Loss
Uneven thicknesses
and the tack fusing profile both have much greater effects than the differential
cooling rates of transparent and opalescent glass. It may be that strongly contrasting colours (such as purple and white) are also more important factors in heat gain and loss than transparent and opalescent combinations. Cooling at an appropriate
rate to room temperature for these factors will be sufficient to remove any
risk of differential contraction between transparent and opalescent glasses.
These two substances
are useful means of removing kiln wash and refractory mould material from
glass. They are important where abrasive methods such as sand blasting are not
available or appropriate.
My recent
experience with both citric acid and trisodium citrate shows differences in
performance. This makes each more suitable in different contexts.
credit: Amazon
Trisodium
citrate is the safest option when long soaks are required to remove refractory
mould material. The trisodium citrate removes any risk of etching the glass on
long soaks. It has been shown by Christopher Jeffree that two-day
soaks in this will not etch the glass. It is most suitable for casting work.
Items cleaned with citric acid and vinegar credit: Christopher Jeffree
Citric acid
acts quickly on kiln wash, making long soaks less necessary. Depending on the
thickness of the stuck kiln wash and the amount of agitation of the stuck kiln
wash, the time required may be only a dozen minutes. It rarely takes more than
a few hours.Citric acid does not work
quickly on refractory materials. This makes the trisodium citrate the better
choice for long soaks.
The most popular and easily available ceramic shelves are made
from Mullite, Cordierite, and CoreLite. Other hard specialist kiln shelves are
available. They are made of other materials. Shelves are also made from other
materials such as refractory fibre board, vermiculite, and fire-resistant
ceiling tiles. This concentrates on the care of ceramic shelves.
Composition and Characteristics
This table gives
some information about the characteristics of the materials involved in these
shelves.
Name
Thermal Shock Resistance
Brittle
Strength
Composition
CoreLite
Low
Yes
Moderate
Ceramic with a high silica content
Cordierite
High
Yes
Strong, but heavy
Magnesium, iron, aluminium oxide, silica
Mullite
High
Yes
Strong. but heavy
Silica, Aluminium oxide
CoreLite is a trade name for an extruded ceramic
shelf. It is strong, but brittle. It is subject to thermal shock below 540ºC/1000ºF.
This suggests the ceramic has a high silica content as the quartz inversion is at
573°C/1063°F,
where the ceramic has a sudden expansion on heating and an equal contraction on
cooling. The cooling rate at this temperature is normally slow enough to avoid breakage.
credit: Clay Planet
cordierite - composed of magnesium, iron,
aluminium oxide, and silica. hard, brittle, and with low expansion
characteristics.
credit: refractorykilnfurniture.com
Mullite –composed largely of silica and
aluminium oxide. It is strong, brittle, and has good thermal shock resistance.
Care
There is enough information from considering the composition
of these shelves to indicate they are all brittle and have differing vulnerabilities.
These have implications for storage, use and cleaning.
Storage
If storing vertically, take care to avoid setting down on hard
surfaces. If they are in a rack, have a separate slot for each shelf. This
avoids friction between shelves and possible surface scratches. The most useful
material for these racks is wood, or harder materials covered with wood. These
racks can be horizontal or vertical.
If it is not possible to have a separate rack for each shelf,
do not lean them on each other. Shelves leaning against others or against hard
surfaces can become scratched. Provide a cushion against scratches such as
cardboard, or thin plywood.
When moving the shelves, avoid setting them down on their corners,
or bumping the shelf anywhere against hard structures.
Use
Reduce firing speeds to less than 220ºC/430ºF per hour up to
540ºC/1005ºF, especially for CoreLite shelves. Cordierite and Mullite shelves
are not as sensitive, but still can be broken by fast firing rates in this temperature region.
Cover a large portion of the shelf at each firing to avoid
uneven heating of the shelf. It is best to evenly distribute moulds and other
things that shade the heat from the shelf around the shelf to help avoid thermal
shock breaks.
If you cannot or do not want to cover the whole shelf, elevate
the mould(s). This helps to keep the whole shelf at the same temperature when only
small parts of shelf are covered. It does not seem to matter so much when flat
glass is in contact with the shelf. But continue to observe the moderate ramp
rates below 540ºC/1005ºF.
It is even more important to elevate damp or heavy moulds
from the shelf. These kinds of moulds shade the heat from the shelf immediately
below them while the rest of the shelf heats rapidly. This difference in
expansion over parts of the shelf becomes too great for the shelf to resist.
Another thing to avoid is cutting fibre or shelf paper on
top of the shelf. It often creates long shallow scratches in the shelf. These
can be the source of bubbles, but more often, flaws on the back of the fired
pieces.
Cleaning
Care is needed to avoid mechanical damage during cleaning. Scraping
can create scratches in the shelf. These are difficult to remove or fill smoothly.
So, scraping needs to be done carefully.
Any sanding also needs to be done carefully. If you use
power tools, it is very easy to create shallow depressions that will be the
source of bubbles in future firings. It is slightly more time consuming to manually
sand the kiln wash with a sanding screen with or without a holder. But it preserves
the flatness of the surface.
If it is decided to wash the shelf primer off the shelf,
consider how difficult it is to wash a very persistent baked on substance. It
requires thorough scrubbing to remove all the hardened material. Power washers are not advised since the high water pressure can abrade the surface of the shelf. But if you do
decide on washing, you need to air dry for several days afterwards. Then kiln
dry slowly to just below boiling point of water. Soak at that point for several
hours, or until a mirror held above the open port does not fog up.
There is more information on removing kiln wash here and
here.
Summary
Ceramic kiln shelves are hard, but subject to scratches,
impact breaks, excess dampness, failure due to uneven temperatures, and to
rapid rises in temperature below 540ºC/1005ºF.
Powder may appear
to disappear after firing as Donna Brown found out with the pieces of her work
shown here. Glass powder is finely ground glass sheet. The full colour of glass
sheet is seen only when the glass is 3mm thick. So, to get the same intensity
of colour you need to have the powder nearly 3mm thick.
This image shows the powder application before firing. Picture credit: Donna Brown
There is not
enough powder applied to the honeycomb. Everyone needs to run some tests to see
how much powder is needed for strong colour. By running some tests of different
thicknesses of powder you will be able to see how shading effects can be
produced with powder. You should run the tests on both light coloured and dark
coloured bases. Opalescent glass requires more powder than transparent. Opaque powders
are better than transparent colours to show on dark colour.
This image shows the result of the firing, showing a thicker application of powder was required to give the full effect. Picture credit: Donna Brown
In this particular
application, I would put the powdered colour down and then the honeycomb grid on top for
better definition of the honeycomb.
I had a piece
crack due to an annealing oops. I put powder on it and put it back in at a
higher temp with a much longer anneal time. It looks great on the front, but I
can still see where the crack was on the back. Is it supposed to be like that?
I didn't think to put powder on that side.
If you think about
why you get crisp lines at the bottom of a strip construction and a more fluid
appearance on the top, you will be near the answer of why a repair looks ok on
top but shows the crack on the bottom. The
temperature on the bottom of the glass is less than on the top at the working
temperature. And less again than the air temperature which we measure. This
means that the bottom part of the glass has less chance to fully recombine.
This, combined with the resistance to movement of the glass along the shelf,
results in evidence of the crack being maintained.
Credit: Clearwater Glass Studio
There are some things that can
be done to minimise the evidence of the crack. Make sure you know why
your piece cracked before you try to mend it. An annealing crack will need
different treatment than a thermal shock crack or a compatibility crack. Simply
refiring the piece may only make the problem worse.
One approach is to place a sheet
underneath. Make sure the broken glass is well cleaned and firmly pushed
together. Dams may be useful to keep the glass compressed together. Glass
expands both horizontally and vertically during the fusing process. Confining the
glass will transfer most of the expansion in a vertical direction. This
additional (small) vertical movement may help in forming the glass seamlessly. The
broken glass now being supported by an unbroken sheet will enable the movement
required to “heal” the crack.
If you do not want to change
the surface, you can fire upside down. To do this you need to have a loose bed
of powdered kiln wash, or whiting (a form of chalk) that is thick enough to press
the textured side fully into the separator. Make sure the glass is pressed
together without any separator getting into the crack. One way to ensure the crack does not open is to use a small amount of cyanoacrylate (super) glue which will burn away during the firing. Put a sheet of clear glass over and fire. Thoroughly clean the face after this repair
firing. The ultimate top needs to be fire polished to remove the evidence of the crack, and if it has picked
up any marks from the powder.
You could, of course, fire
upside down in this way but without the additional sheet, to avoid making the piece any thicker. This
may or may not work well. If the base layer is one layer thick, it may pull in
at the sides and pull apart at the crack where it is one layer thick. It is also possible that bubbles will develop in the thin parts
of tack glass because of the uneven thicknesses.
A final note. Placing powder on the back will not improve
things. The powder will not fully incorporate with the glass and so leave a
rough surface without concealing the crack.