This is based on Graham Stone’s work with float glass. The temperatures are applicable to float glass, and so need to be adjusted for any other glass, but illustrate the principle of how heating temperatures affect the glass. Temperatures in degrees Celsius.
10-250 Slow rate heating up. Risk of thermal shock. Venting often done in this phase.
250-500 Medium rate heating. Risk of thermal shock diminishing.
400 + Many glasses now tolerate fast heating up ramp rate.
550 Glass surface beginning to soften slightly
600 Safe from thermal shock above this temperature
610 Glass bending slightly, picking up texture.
680 Glass begins to stick to itself. Tin bloom becomes iridescent.
690 Fusing glasses reaching their softening points.
715 Glass beginning to stretch. Tack-fired pieces adhered by now.
720 Subtle devitrification and iridisation burn off becoming a factor with some glasses.
730 Softening point of float.
750 Edges no longer sharp. Tin bloom stretching becoming "frosty".
760 Tack fuse range for fusing glasses.
770 Float glass fused, but still "sitting up".
790 Trapped air can cause bubbles under sheet glass at this temperature.
800 Full fuse for most fusing glasses.
820 Fused float glass nearly flat.
825 Full fuse for float glass. Devitrification more pronounced.
850 Glass flowing.
950 Glass soft enough to "rake".
1000 Approximate liquidus temperature.
Based on Firing Schedules for Glass; the Kiln Companion, by Graham Stone, Melbourne, 2000, ISBN 0-646-39733-8, p24
Post revised 5th March 2014
Wednesday 5 March 2014
Metal Framing Materials
Lead
is a very weak metal. Therefore various other metals are often
considered for the perimeter of the panel to strengthen the whole.
Zinc
is a metal often used for strengthening the perimeter of panels. It
is stronger than lead – by about 8 times. It is relatively easy to
solder. However it is subject to more rapid corrosion than lead.
So an
alternative is aluminium which is about about the same strength as
zinc. However it does not accept soldering, so professional joining
or cold fixing solutions are required to make the frame.
Copper
is over 10 times the strength of lead and can be considered as an
alternative to zinc. It accepts solder well, but as a came is
extremely expensive. It does corrode to a verdigris unless protected
and maintained. However, because of its strength, copper wire - as a
single strand or several twisted - can be used inside other came such
as lead or zinc to provide strong support.
Brass
is about 19 times stronger than lead. It is available in came
profile as well as “U” and “L” profiles. It accepts solder
well and resists corrosion. It is more expensive than lead, but
similar in price to zinc.
Mild
steel strength varies but is at least 27 times stronger than lead.
It does not accept solder easily, and does corrode without painted
protection, but is a less expensive option than aluminium, zinc or
copper. As an angle or “T” shape, mild steel and iron have been
used for centuries to support leaded glass panels.
Stainless
steel is at least 37 times stronger than lead. It is difficult to
weld and does not accept solder at all. It is very resistant to
corrosion.
When
considering framing solutions for panels, the main factors to
consider are relative strength, corrosion, and joining methods
possible.
Brass,
Copper, Lead and Zinc all can be joined by solder. Aluminium and
stainless steel cannot be joined with solder. Although mild steel
can be joined with solder, a good strong joint is difficult.
The
stronger the metal, the thinner profile required, which can make
metals that are more expensive by weight an economical solution, as
metal prices are most often by weight rather than shape.
It
also is possible to combine a stronger metal with a weaker metal,
such as including copper wire or steel rods in the lead came.
It is
not absolutely necessary to solder the panel to the framing material.
A frame can be made and the panel fixed within it by other than hot
soldering methods. In this case the frame takes the whole weight of
the panel.
Wednesday 26 February 2014
Metal Strengths
Metal
Strengths
The
strength of metals is most often compared by their tensile strengths.
These numbers are Newtons per square millimetre and represent the relative strength of each metal compared to another. The range of numbers indicates the variations caused by various alloys.
Tin 19
Lead 14
– 32
Solder 60/40 48
Zinc 120 – 246
Zinc 120 – 246
Aluminium 120
– 246
Copper 220
– 270
Brass 340
– 540
Mild
steel 500 – 750
Stainless
steel 740 – 970
These
figures may be of interest in considering what frame to place around
a free hanging stained glass panel.
Wednesday 19 February 2014
Panel Framing Options
Some
framing options for free hanging stained glass panels are given here. They are not exhaustive, of course, but do give some principles to be considered when making frames. Wood and metal are the two traditional materials for framing panels to be hung.
Wood
A wood
frame requires joints of some kind. These joints are important to
the durability of the frame. The two main kinds of joints are glued
and screwed.
Glued joints
Lap joints seem to be
strongest. An odd element relating to the strength of this joint is
that placing a wooden pin in the joint weakens, rather strengthens
the lap joint.
Mortice and tenon is also
a strong joint. It requires considerable skill to make a good joint.
A mitred is among the
weakest, but can be strengthened with a biscuit or fillet in the
joint.
A mitred joint with biscuit ready for glueing. |
Screwed joints
These have a lot of
movement before failure, but do give a lot of resilience to the joint
as they can stretch rather than immediately give way. They also can
be used with any of the glued joints if appearance is not of prime
importance.
Frame style
The
width and thickness of the frame are interrelated – thicker
frames (front to back) can be narrower, thinner frames need to be
wider. So the desired appearance of the frame width has a
significant effect on the dimensions of the frame.
Metal cames or angle
Lead can be an
adequate framing material, but if strengthening is required, you can
use copper wire within the came and fold the leaves closed over it. You can also use steel rod within the came, as shown in the posting.
Zinc is a stronger
metal than lead – about 8 times, but still has a weak tensile
strength. I corrodes easily, but accepts solder as a joining method.
It is more expensive than lead.
Some of the variety of zinc came available |
Aluminium is a
little stronger than zinc, but does not take solder. It has similar
costs to zinc.
Some of the aluminium profiles available |
Copper is about 1/3
stronger than zinc and also takes solder. It corrodes to a
verdigris, but can be protected by clear varnish or paint. It is
more expensive than zinc, but can be used as wire which is less
expensive than other forms of copper.
Brass is over two
times stronger than zinc and also takes solder. It resists corrosion
well, and is a little cheaper than copper.
Some of the brass came options. |
Mild steel is over
3 times stronger than zinc, but does not take solder at all well. It
is relatively cheap and welds easily, making it a good framing
material, although a method of fixing the panel into the frame is
required.
Stainless steel is
about 4.5 times stronger than zinc, but does not take solder and
needs special welding. It resists corrosion very well, but is
expensive in relation to zinc.
Hanging and fixing
options
Two point hangings are the
most common as they prevent twisting and distribute the weight to the
sides of the panel.
The hanging material is straight up from the zinc framed sides to the fixing points |
The hanging material
whether line, wires or chains should be straight up from the sides to
two separate fixing points. A triangle shaped hanging puts a bowing
stress on the panel or frame.
A variation where the chain is taken to the corner of the window, is less secure, as it stresses the joint away from the sides |
Loops or holes for screws
should be placed in the frame rather than the panel.
The hanging is from reinforced corners directly to fixing points on the overhead beam |
Ensure the fixing points
for the hanging wires are sound and secure.
If the panel is fitted
tight to the opening, consider ventilation requirements to reduce
condensation between the primary glazing and the hung panel.
Wednesday 12 February 2014
Grinder maintenance
There
are several elements in maintaining one of the work horses of many
glass studios.
Water
Ensure
there is enough water to supply the pump or sponge that wets the
grinding bit before starting any grinding. Too
little water reaching the bit, fails to lubricate the diamonds and
keep the glass cool. If you are getting a white paste or a powder on
or near the glass, you need to increase the water supply.
Empty
the reservoir daily. This keeps the water from producing a smell, and
allows you to clear the glass residue from around the grinding bit.
If you are changing to a finer grit, it is important to change the
water, clean the resevoir, and thoroughly clean the sponge each time
you make that change. Otherwise, you risk bringing coarser grit to
scratch the finer grinding surface.
You
can also buy a additive for the water – often called a diamond
coolant – which is intended to provide a kind of lubrication for
the diamonds. This may extend the life of the bit a little.
Bit
maintenance
Periodic
removal of the bit
and lubrication of the shaft should
be part of the regular maintenance of the grinder. You
should make sure that the socket for the grub screw is clear of glass
residues before attempting to turn it. I do this by using a needle or
other thin sharp object to clear out all the glass powder. When the
socket is cleaned, I push the key into the socket very firmly and
hold it there while turning. Prevention maintenance is to fill the
socket with vaseline or thick grease after tightening the screw.
Inspect
your bit carefully for smooth areas showing that the diamonds have
been worn away. Also look for dents, and other irregularities on the
surface, indicating that the bit is damaged. In these cases, the bit
should be replaced.
Before
putting the old or new grinder bit back, ensure the shaft is smooth
and without corrosion. Then coat the shaft with Vaseline or a
proprietary anti seize-compound. This will ease the removal of the
bit later. If the shaft is corroded, use a strip of fine wet and dry
sandpaper to shine the shaft.
Sometimes
bits need to be dressed – removing protruding diamonds, or cleaning
and exposing new ones on a worn bit. To dress the bit you can grind
some scrap glass, brick, or use a dressing stone to lightly grind
some of the abrasive material away. This can extend the life of the
bit.
Adjustment
of height
If
your grinder bit is too low or too high the
diamond surface will not grind the whole of the glass edge. This can
lead to chipping of the surface of the glass at the edges.
A
good practice is to start with the bit as high as possible to allow
for differing thicknesses of glass. As high as possible is with the
bottom of the diamonds just below the platform of the grinder. This
will ensure that you can deal with varying thicknesses of glass
without immediate adjustment. You can then reduce the height of the
bit as it wears.
Wednesday 5 February 2014
Kiln maintenance
Before
or after each use
Vacuum
the inside of the kiln. Use a low suction setting, especially on
fibre walls and ceilings. Stronger suction is possible when cleaning
the brick floor.
Check
on the kiln furniture – including shelves, boards, supports. Are
they kiln washed and without scrapes, scratches, gaps? Has the kiln
wash been fired to full fuse temperature?. In both cases, clean the
used kiln wash off the shelf and renew.
Check
that the shelves and other kiln furniture are without cracks.
Clean
kiln furniture of dust and debris.
Check
the level of any item placed in the kiln, e.g., mould, with a spirit
level.
Example of a small 2-way spirit level |
Monthly
Electrical
parts: check the elements and their connections (normally at back or
side). The screws on the connectors for the element tails should be
tight. If they are badly corroded , they need to be replaced.
Any
support pins or wires should be firmly seated in the brick work or
supported by sound hangers.
Check
the level of the kiln and internal shelves on a a regular basis and
every time the kiln and its internal furniture is moved.
Wednesday 29 January 2014
Stretch Marks in Slumping
Occasionally
a slumped piece will develop faint lines beginning about half to
two-thirds of the way from the centre and radiating toward the edge.
My
experience leads me to think that these marks come from the glass
moving too quickly at too hot a temperature. The glass softens as it
reaches its slump point. If the temperature is taken above that, the
glass conforms to the mould and then begins to slide downwards. The
mould is by its nature not perfectly smooth and so the high points
make marks on the glass as it moves.
This
is re-enforced by the fact that the glass at the centre of these
slumps does not have those marks. It deforms less than the edges of
the piece and so (at whatever temperature) does not get so marked as
the sides and edges.
To
avoid these stretch marks you need to slump at the lowest possible
temperature and ensure the glass is the same temperature throughout
by the time it gets to its slumping point.
Temperature
Finding
the lowest temperature for the slumps in a particular mould requires
experimentation and observation. A simple curve – circular, oval
or rectangular – requires less heat than one with a flat bottom and
much less than one with angles. For a simple curve you can set your
slumping temperature at say 620ºC with up to an hour soak. The
important element to remember is that each shape and curve of mould
will require different schedules. To determine this you need to make
observations.
From
about 600ºC you need to make periodic observations of the progress
of the slump. Note the temperature at which the glass begins to move
– the reflections in the glass will begin to be curved. This is
the minimum temperature you can use for this span and thickness of
glass on this mould. The length of time required to get a complete
slump may be so long as to make using this temperature impractical.
Slump not quite complete |
Now
observations need to become more frequent – possibly every 10
minutes or less. When you reach a temperature where the glass is visibly
distorting, it is time to cease the temperature advance and begin the
soak. Record this temperature and continue to observe, recording the
time it takes at this temperature to fully slump. Continue to the
anneal.
Inspect
the piece when cool. If you have the result you want, you have the
temperature and soak time needed for this thicknesses and size of
glass on this mould. Record this information. If it is not fully
slumped you can try either extending the time (if that is practical,
it is the best option) or increasing the temperature on another
piece. This increase should be by no more than 10ºC, so that you do
not over fire the piece.
Glass conforms to the bottom of the mould |
Of
course, it is possible that the piece was slumped at too high a
temperature as evidenced by stretch marks, mould marks, uprisings in
the centre, distortions on the edges. Then you need to reduce the
temperature on the next slumping of a piece of the same dimensions.
Start with 10ºC less than your first piece, and programme the same
amount of time. Observe, record and inspect as on the previous one.
This
process shows why it is important to have a kiln with observation
ports to be able to follow the progress of your work. In some ways,
it is more important to have observation ports than whether the kiln
is front or top loading, coffin or clam shell opening. But that is
by the way.
Heat
The
second important element in avoiding stretch marks is to enable the
glass to be at the same temperature throughout its thickness. This
involves the concept of heat work. In
general terms it means you can achieve the same result by putting the
heat in fast and at a high temperature or slowly and at a low
temperature. The “slow and low” approach
allows more control and allows the glass to be the same temperature
on top as on the bottom.
It is
important to heat the glass slowly and steadily all the way up to the
slumping temperature. The temptation to increase the temperature
rapidly after the strain point needs to be resisted. Getting the top
too hot can at the worst, cause a split
on the bottom of the glass as the tension from slumping glass on the
top splits the stiff glass at the bottom.
This
means there is no need for a soak at the strain point, nor a speed up
in the rate of advance up to the slumping temperature. Exactly the
opposite is indicated. Choose a rate of advance for the glass
according to its thickness – at 6mm a rate of 150ºC will be
adequate. Maintain that rate of advance all the way up to the slump
temperature. This also is required when you are making observations
to determine what the slump temperature should be. The moderate rate
of advance all the way to slumping temperature ensures the whole
thickness of the glass is at the same temperature.
Heating
the glass slowly to enable all of it to be at the same temperature,
allows the glass to change shape at the lowest possible temperature
and avoid picking up so much of the mould texture. The glass at the
edge and upper sides is in contact with mould longer than central
parts as it changes shape and slides along the surface of the mould
at elevated temperatures. The lower the temperature used with a long
soak, means that the glass is less likely to slide along the mould
and so adds to the avoidance of stretch marks.
Sunday 26 January 2014
Annealing Range
Among the critical temperature ranges is the temperature around the annealing point. These are known as the strain points. The higher one is the highest temperature that annealing can begin and is the softening point. The lower one is the lowest point at which annealing can be done and is called the strain point. Soaking at any lower temperature will not anneal the glass at all. The ideal point to anneal is the annealing point, because annealing occurs most quickly at this temperature. This temperature is defined by various characteristics both mathematical and observational. The manufacturer can give this temperature, and most do on their websites.
This annealing range is traditionally calculated as being 40ºC either side of the stated annealing temperature. Your aim should be to spend as little time in the temperatures above the annealing range to reduce the chances of devitrification.
Most glass kilns are not really accurate in recording the temperature within the glass. They are measuring the air temperature. The glass on the way down in temperature is hotter than the recorded temperature. If you do a soak at 515°C for example, the glass is actually hotter and is cooling to the 515° point during the soak. So, long soaks at the annealing temperature are required. Longer for thicker is required. The slow cool to at least 5C below the lower strain point does the annealing, and reduces the risk of inadequate annealing.
Recent research at Bullseye indicates that the use of the fact that temperature readings are above the actual temperature of the glass indicates lower annealing points for thick glass. This has been based on temperature probes at various points within the glass, comparing the glass temperature with the air temperature. The results of their research suggest an annealing soak at about 30C below the annealing point with a long soak, and slow anneal cool for the next 55C.
It is still possible to give the glass a thermal shock at temperatures below the lower strain point, so care needs to be taken in the continued cooling. But no further annealing will take place. If you do not anneal properly, the glass will break either in the kiln or later no matter how carefully you cool the glass after annealing.
This annealing range is traditionally calculated as being 40ºC either side of the stated annealing temperature. Your aim should be to spend as little time in the temperatures above the annealing range to reduce the chances of devitrification.
Most glass kilns are not really accurate in recording the temperature within the glass. They are measuring the air temperature. The glass on the way down in temperature is hotter than the recorded temperature. If you do a soak at 515°C for example, the glass is actually hotter and is cooling to the 515° point during the soak. So, long soaks at the annealing temperature are required. Longer for thicker is required. The slow cool to at least 5C below the lower strain point does the annealing, and reduces the risk of inadequate annealing.
Recent research at Bullseye indicates that the use of the fact that temperature readings are above the actual temperature of the glass indicates lower annealing points for thick glass. This has been based on temperature probes at various points within the glass, comparing the glass temperature with the air temperature. The results of their research suggest an annealing soak at about 30C below the annealing point with a long soak, and slow anneal cool for the next 55C.
It is still possible to give the glass a thermal shock at temperatures below the lower strain point, so care needs to be taken in the continued cooling. But no further annealing will take place. If you do not anneal properly, the glass will break either in the kiln or later no matter how carefully you cool the glass after annealing.
Saturday 25 January 2014
Maintaining a Single Colour on the Edge
The piece in the middle distance shows the different colours of the two layers |
Keeping
the edge one colour on a two or multiple colour piece can be done by
cutting the upper layer larger than the lower one(s). If you are
making a 6mm thick piece, the upper piece needs to be 3mm larger all
around. So if you have a 300mm diameter base piece, the top will
need to be 306mm in diameter. This allows a coloured top to bend
over a clear base, giving the appearance of a single colour
throughout.
However
if you are building thicker than 6mm and are willing to allow the
glass to flow, you do not need to add the full thickness of the glass
to the size of the capping piece. I find that at 9mm thick, I need
only 4mm all around to cover the layers. This may be because the
outer edges are nearer 6mm than 9mm thick.
Of
course, if the final piece needs to be a pre-determined size the
lower layers can be cut 3mm smaller than the top all around (for 6mm thick pieces). The top is cut to the final size of the piece.
Wednesday 22 January 2014
Glass Dust
This
is from Greg Rawls' website. He is a glass worker and a certified
industrial hygienist. A huge amount of practical information on
safety in glass working is available on his web site:
Ground
Glass
OSHA
classifies glass dust as a “Nuisance Dust”. Ground glass does
not cause silicosis. You can wear a respirator if you are concerned
about exposure.
Glass
is made from sand, which contains silica - a naturally occurring
mineral silicon dioxide (SiO2). Crystalline forms of silica, also
known as “free” silica, can contribute to the development of
silicosis under prolonged exposure conditions.
It
is important to understand the difference between glass and
crystalline silica because exposure outcomes are extremely different!
Glass is a silicate containing various other ingredients which have
been melted and upon cooling form an amorphous, or non-crystalline
structure. While silica (SiO2) is a primary ingredient in the
manufacturing of glass, when glass is formed under heat, the
crystalline structure is changed to an amorphous structure and is no
longer considered crystalline.
Ground
glass is rarely respirable because the particle is too big. Always
use wet methods when grinding glass! Water captures the dust.
Sometime other chemicals are used to add colour to glass such as
arsenic, lead, cadmium. These are usually present in low
concentrations and are bound to the glass and not readily available
but could present an exposure issue under some circumstances.
Labels:
Dust,
Safety,
Silica,
Stained Glass Supplies Ltd,
Verrier
Wednesday 15 January 2014
Observation Ports for Kilns
Observation Ports for Kilns
When choosing a kiln, an often
overlooked element is the observation ports. These openings in the
side or top of the kiln enable you to observe the progress of your
work during a firing without opening the kiln lid or door. They have
ceramic or fibre plugs to keep the heat in the kiln when you are not
using them to observe what is happening.
A kiln with a very large quartz observation panel |
Some newer kilns are built
with quartz observation panels in the kiln. These serve the same
purpose as the ports, but without the (small) additional heat loss.
When doing any new work it is important
to observe the progress of work, rather than just hope for the best
and see what has happened after the whole process is finished.
Observation can tell you when the piece has reached the desired stage
and progress to the next part of the programme.
A port located too high to be of use for observation of the interior. It is sealed with a ceramic fibre plug. |
The location of the port is important. You need to be able to see the relevant part of the kiln
or they are useless.
Although a small kiln, the observation port at the top is not so useful as one at the side. |
A popular kiln with an appropriately placed observation port. Often these have an additional one on the side opposite the controller. |
Some kilns have multiple ports to make
observation of various parts of the kiln easier.
There are a variety of shapes of these
ports. The shape is not so important as the location and what can be
viewed within the kiln through the ports.
A round port, but probably too low to be of much use |
A rectangular port viewed from the inside showing the field of view that can be allowed |
A kiln with multiple square ports |
If your kiln has come without a port or one that is not placed where most suitable for your use, you can drill the casing and brick or fibre to provide another viewing port. Make a ceramic plug or wad up ceramic fibre blanket to fill the hole when it is not in use.
Wednesday 8 January 2014
Boiled Glass
This
is a technique that will obtain a random, organic feel to glass that
would otherwise be scrap (cullet) – remembering that you have to
use compatible glass throughout. The principle is to take the
temperature up high enough for the glass to begin to flow easily and bubbles to blow through and burst.
The
results can be used as they come out, or they can be cut to provide
points of interest in other work, or the glass pieces can be damed
before firing to obtain thick pieces which can be cut into slices for
other work. And I am sure, there are numerous other ways to use the
resulting glass too.
The
effects are rather like colourful molten rock with gases bubbling
through. These bubbles mix the glass colours. So you need to be
sure you do not use a wide variety of colours, or your result will be
similar to the molten rock - muddy. Use a few contrasting colours,
and ensure you include a significant proportion of white to maintain
bright colours. Also remember that the hot colours – reds,
yellows, oranges – opalise at high temperatures, so the
transparents can be used as opals.
You
can use whole sheets of glass or scraps. In either case, it is
useful to start with a clear base to help avoid picking up kiln wash
when the glass is moving about. The glass must be clean to reduce
the incidence of devitrification. Stack you glass on top of the base
glass in what ever order you like. Contrasting colours alternated
give a strong result.
You
can put shelf paper of 0.5 mm or thicker on the shelf or simply kiln
wash the shelf with several layers of wash until the shelf surface is
no longer visible through the wash. Use of thinfire is not
recommended as the powder can be pulled into the glass.
If
you do not dam the area to contain the glass calculate how far the
glass will expand on the shelf, so that you do not put down too much
glass and have it spill over the edge of the shelf.
You
can use bubble powder onto the base layer to promote the bubbling
during the firing. However, if you are using cullet, you can just
take the temperature up rapidly without a bubble squeeze, which will
give you plenty of air pockets to burst through the layers of glass.
You
can take the temperature up at about 300ºC per hour to 925ºC with
no bubble squeeze and soak for 10 – 15 minutes. Then allow the
kiln to drop the temperature as fast as possible to about 815 and
soak there for around 30 minutes to allow the little bubbles to rise
to the surface an burst too. Then reduce to the annealing
temperature and soak for the thickness you calculated in preparation
for the firing.
Precautions
You
need to be careful in firing and annealing pieces using this glass.
Any glass that has been fired to a high temperature tends to begin
changing compatibility. So you need to be careful on your rates of
advance, and on the annealing and cooling portions of the firing when
using the glass in other projects. You may want to consider using a
schedule for twice the thickness of the piece on subsequent firings.
There
may be devitrification on the surface. You should sandblast or
abrade away this devitrification in some way to be able to get a
shining surface when you fire polish.
There
may also be a number of pin hole sized bubbles at or just below the
surface. These will close with a fire polish also.
Wednesday 1 January 2014
Lead Corrosion in Acids
Lead
forms a protective film, which if undisturbed preserves the metal
below this layer.
The
corrosion resistance of lead is based on its ability to readily form
a tenacious coating of a reaction product. This then becomes a
protective coating. Protective coatings on lead may form as the
result of exposure to sulphates, oxides, carbonates, chromates, or
chemical complexes.
Handbook
of Corrosion Data, by Bruce D Craig, p26
Lead
is resistant to corrosion especially “with solutions containing
sulphate ions, such as sulphuric acid.”
However, the new or bright metal reacts quickly with a variety of alkalis and many organic (although not most inorganic) acids. “...Lead is not stable in nitric and acetic acids, nor in
alkalis. The metal does not resist nitric acid. Lead corrodes rapidly
in acetic and formic acids.” (Handbook
of Corrosion Data, by Bruce D Craig, p.29)
Lead
has very limited resistance to acetic acid.... Dilute [acetic acid],
even at room temperature attacks lead at rates exceeding 1.3mm/year.
These rates increase rapidly with increasing aeration and velocity
However … acetic acid … has little effect at strengths of 52% to
70%.
The
corrosion rate in acid increases rapidly in the presence of oxygen
and also in oxygen in combination with soft waters such as rain and
distilled water. Corrosion increases at the rate approximately
proportional to the oxygen content of the water.”
Handbook
of Corrosion Data, by Bruce D Craig, p.26, 29
This another good reason to avoid vinegar as a cleaning agent for leaded windows.
Lead
dissolves in organic acids (in the presence of oxygen). Lead also
dissolves in quite concentrated alkalis
(≥10%) because of the characteristic of the lead salts that can act
as either an acid or an alkali. These salts are soluble in the
presence of water and oxygen.
Alkali
salts are soluble hydroxides of alkali metals and alkali earth metals, of which common examples are:
- Sodium hydroxide (often called "caustic soda")
- Potassium hydroxide (commonly called "caustic potash")
- lye (generic term, for either of the previous two, or even for a mixture)
- Calcium hydroxide (saturated solution known as "limewater")
- Magnesium hydroxide is an example of an atypical alkali since it has low solubility in water (although the dissolved portion is considered a strong base due to complete dissociation of its ions).
Although this has been a rather technical posting, these
data show that lead is subject to rapid attack by both organic (and
some inorganic) acids and alkalis in relatively low concentrations
when in the presence of aerated water. However in normal
environmental conditions the protective reaction layer avoids much of
this vulnerability.
Wednesday 18 December 2013
Tack Fusing Considerations
1 – Initial Rate of Advance
Tack fuses look easier than full fusing,
but they are really one of the most difficult types of kiln forming. Tack
fusing requires much more care than full fusing.
On heat up, the pieces on top shade the heat from the base glass leading to uneven heating. So you need a slower heat up. You can get some assistance in determining this by looking at what the annealing cool rate for the piece is. A very conservative approach is needed when you have a number of pieces stacked over the base layer. One way of thinking about this is to set your initial rate of advance at approximately twice the anneal cool rate. More information on this is given in this entry.
2 – Annealing
The tacked glass can be considered to be laminated rather than fully formed together. This means the glass sheets are still able, partially, to act as separate entities. So excellent annealing is required.
On heat up, the pieces on top shade the heat from the base glass leading to uneven heating. So you need a slower heat up. You can get some assistance in determining this by looking at what the annealing cool rate for the piece is. A very conservative approach is needed when you have a number of pieces stacked over the base layer. One way of thinking about this is to set your initial rate of advance at approximately twice the anneal cool rate. More information on this is given in this entry.
2 – Annealing
The tacked glass can be considered to be laminated rather than fully formed together. This means the glass sheets are still able, partially, to act as separate entities. So excellent annealing is required.
Glass contracts when it's
cooling, and so tends to pull into itself. In a flat, symmetrical fuse this
isn't much of a problem. In tack fuses where the glass components are still
distinct from their neighbours, they will try to shrink into themselves and
away from each other. If there is not enough time for the glass to settle into
balance, a lot of stress will be locked into the piece that either cause it to
crack on cool down or to be remarkably fragile after firing. In addition, in
tack fusing there are very uneven thicknesses meaning it is hard to maintain
equal temperatures across the glass. The tack fused pieces shield the heat from
the base, leading to localised hot spots on cool down.
On very difficult tack
fuses it's not unusual to anneal for a thickness of four to six times greater
than the actual maximum thickness of the glass. That extended cool helps ensure
that the glass has time to shift and relax as it's becoming stiffer, and also
helps keep the temperature more even throughout.
So in general, tack fused
pieces should be annealed as though they are thicker pieces. Recommendations
range from the rate for glass that is one thickness greater to at least twice
the maximum thickness – including the tacked elements – of the whole item.
Where there are right angles - squares, rectangles - or more acutely angled
shapes, even more time in the annealing cool is required, possibly up to 5
times the total thickness of the piece.
It must be remembered
especially in tack fusing, that annealing is much more than the annealing soak.
The soak is to ensure all the glass is at the same temperature. The anneal cool
over the next 110ºC is to ensure this piece of different thicknesses will all
react together. That means tack fusing takes a lot longer than regular fussing.
3 – Effects of
thicknesses, shapes, degree of tack
The more rectangular or
pointed the pieces there are in the piece, the greater the care in annealing is required. How you
decide on the schedule to use varies. Some go up two or even four times the
total thickness of the piece to choose a firing schedule.
A simplistic estimation of
the schedule required is to subtract the difference between the thickest and
the thinnest part of the piece and add that number to the thickest part. If you
have a 3mm section and a 12mm section, the difference is 9mm. So add 9 to 12
and get 17mm that needs to be annealed for. This thickness applies to the heat
up section as well.
Another way to estimate
the schedule required is to increase the length the annealing schedule for any
and each of the following factors:
·
Tack fusing of a single additional layer on a six millimetre base
·
Rectangular pieces to be tack fused
·
Sharp, pointed pieces to be tack fused
·
Multiple layers to be tack fused
·
Degree of tack – the closer to lamination, the more time required
The annealing schedule to
be considered is the one for at least the next step up in thickness for each of the factors. If you have all five factors the annealing schedule that should be used is one for at least 21mm thick pieces according to this way of thinking
about the firing.
4 –
Testing/Experimentation
The only way you will have
certainty about which to schedule to choose is to make up a piece of the
configuration you intend, but in clear. You can then check for the stresses. If
you have chosen twice the thickness, and stress is showing, you need to try 3
times the thickness, etc. So your annealing soak needs to be longer, if stress
shows. You can speed things by having your annealing soak at the lower end of
the annealing range (for Bullseye this is 482C, rather than 516C).
You will need to do some
experimentation on what works best for you. You also need to have a pair of
polarisation filters to help you with determining whether you have any stress
in your piece or not. If your piece is to be in opaque glasses, you need to do
a mock up in clear.
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