Needling is a description of the fine points emerging from the edges of glass.
This occurs in two conditions mainly.
The one that is most commonly seen is in the fusing of single layers of glass. The surface tension of the glass pulls the glass in from its original size, trying to achieve the 6-7mm that is a thickness equilibrium at full fusing temperatures. If the surface the glass is resting on has any rough areas, and most surfaces do, some of the glass will stick and the rest retract. This leaves short, thin and extremely sharp “needles” extending from the edges.
Two common surfaces allow these sharp edges. Fibre paper of 0.5mm and greater is rough enough to allow the hot glass to stick to tiny depressions in the paper. Kiln wash is often not smooth enough to prevent this kind of sticking either. You can smooth powdered kiln wash or aluminia hydrate over these surfaces to reduce the grabbing of the surface by the hot glass. However, the powder is often drawn back with the contracting glass. Thinfire or Papyros paper is fine enough to avoid the needling most of the time without any addition of powders.
The other main condition is in casting, mainly box casting or damming. In this case, the stack of glass sheets or cullet is higher before firing than its final thickness. This means the glass flows out to the dams and sinks down to its final thickness during the firing process. As the glass touches the fibre paper or other separator it behaves just as the single layer of glass does. Some sticks to the surface while the rest is dragged away by the surface tension and reducing thickness of the stack of glass.
Prevention of Needling
Lining dams
Separators for dams
Saturday 15 July 2017
Thursday 13 July 2017
Quartz Inversions and Conversions
You need to know about this in both casting and when using ceramic pots in the kiln.
Quartz
Crystalline solids are rather temperamental and quartz is no different. Quartz is a crystalline form of silica in that it has a three dimensional regular pattern of molecular units. These form naturally in nature because lengthy cooling times allow arrangement. Quartz is made of a network of triangular pyramid (tetrahedron) shaped molecules of silicon combined with four oxygens.
Unfortunately, the quartz delights in changing the orientation of the tetrahedron shaped molecules with respect to each other, thus loosening or tightening the whole mass (and thus changing its total size). It exhibits twenty or more “phases”. A change to another phase is called a “silica conversion”. The most significant phases are quartz, tridymite, crystobalite, and glass.
Inversions
Changes which occur between these are reversible, that is, the change which occurs during heat-up is inverted during cool down. These changes are thus called “quartz inversions”. These inversions, unfortunately, often have associated, rather sudden, volume changes. That means that quartz conversions are something to consider when optimizing the fired properties; quartz inversions are something to consider when firing to prevent cracking losses. There are two important inversions you need to know about because of their sudden occurrence during temperature increase and decrease.
Quartz
The first is simply called ‘quartz inversion’ and it occurs quite quickly in the 570°C range (1060°F). In this case, the crystal lattice straightens itself out slightly, thus expanding 1% or so. This is therefore an important temperature in casting as it is an expansion on the heat up and a contraction, “grabbing” the glass on the way down. This is the reason for various modifiers when silica or flint is used as the strengthener.
Crystobalite
The second is crystobalite inversion at 226°C. This is a little nastier because it generates a sudden change of 2.5% in volume. This material has many more forms than quartz, so it is complex to say the least. However, while all bodies will have some quartz, you won’t have a problem with crystobalite inversion unless there is crystobalite in your body. Crystobalite forms naturally and slowly during cooling from above cone 3 (1104-1149°C). It forms much better if pure crystobalite is added to the body to seed the crystals or in the presence of catalysts (e.g. talc in earthenware bodies). Thus, this element exists in most ceramic moulds and moving slowly around 226°C should be observed when firing containers made of ceramic materials.
Quartz
Crystalline solids are rather temperamental and quartz is no different. Quartz is a crystalline form of silica in that it has a three dimensional regular pattern of molecular units. These form naturally in nature because lengthy cooling times allow arrangement. Quartz is made of a network of triangular pyramid (tetrahedron) shaped molecules of silicon combined with four oxygens.
Unfortunately, the quartz delights in changing the orientation of the tetrahedron shaped molecules with respect to each other, thus loosening or tightening the whole mass (and thus changing its total size). It exhibits twenty or more “phases”. A change to another phase is called a “silica conversion”. The most significant phases are quartz, tridymite, crystobalite, and glass.
Changes which occur between these are reversible, that is, the change which occurs during heat-up is inverted during cool down. These changes are thus called “quartz inversions”. These inversions, unfortunately, often have associated, rather sudden, volume changes. That means that quartz conversions are something to consider when optimizing the fired properties; quartz inversions are something to consider when firing to prevent cracking losses. There are two important inversions you need to know about because of their sudden occurrence during temperature increase and decrease.
Quartz
The first is simply called ‘quartz inversion’ and it occurs quite quickly in the 570°C range (1060°F). In this case, the crystal lattice straightens itself out slightly, thus expanding 1% or so. This is therefore an important temperature in casting as it is an expansion on the heat up and a contraction, “grabbing” the glass on the way down. This is the reason for various modifiers when silica or flint is used as the strengthener.
The second is crystobalite inversion at 226°C. This is a little nastier because it generates a sudden change of 2.5% in volume. This material has many more forms than quartz, so it is complex to say the least. However, while all bodies will have some quartz, you won’t have a problem with crystobalite inversion unless there is crystobalite in your body. Crystobalite forms naturally and slowly during cooling from above cone 3 (1104-1149°C). It forms much better if pure crystobalite is added to the body to seed the crystals or in the presence of catalysts (e.g. talc in earthenware bodies). Thus, this element exists in most ceramic moulds and moving slowly around 226°C should be observed when firing containers made of ceramic materials.
Wednesday 5 July 2017
Simple Investment Mould Materials
There are a
lot of differing recipe options for making plaster moulds. A simple general purpose
investment mould making material and method follows:
Equal parts of powdered silica (sometimes called silica flour or flint), plaster of Paris and water by weight. For example:
1 kilo silica
1 kilo Plaster Paris
1 kilo water
(Do not measure by volume)
Mix silica and plaster of Paris dry in separate bucket by hand. If you can use a closed container that is best. Otherwise use breathing protection and do the mixing outside. Silica is very bad for your health.
Mix silica and plaster of Paris dry in separate bucket by hand. If you can use a closed container that is best. Otherwise use breathing protection and do the mixing outside. Silica is very bad for your health.
Measure the
water into a separate bucket with enough volume for three times the amount of
water. Slowly sprinkle the entire contents of the dry mix into the bucket of
water. Do not dump it in!
Let the mixture sit for 2 minutes (slaking). Then mix by hand slowly to prevent bubbles. Using your hands allows you to feel any lumps that are present and break them down gently. Depending on temperature and amount of water, you have 15-20 minutes before the mix begins to become solid.
Let the mixture sit for 2 minutes (slaking). Then mix by hand slowly to prevent bubbles. Using your hands allows you to feel any lumps that are present and break them down gently. Depending on temperature and amount of water, you have 15-20 minutes before the mix begins to become solid.
When mixed thoroughly, pour carefully and slowly into a corner of the mould box or container to reduce the occurrence of bubbles within the investment material or against the master.
When the pour
is finished, tap the mould container to encourage any bubbles to the surface.
You can take the investment and master from the container once it is cold to the touch. Remove the master from the investment material carefully to avoid damaging the surface of the investment.
You can take the investment and master from the container once it is cold to the touch. Remove the master from the investment material carefully to avoid damaging the surface of the investment.
For pate de
verre, you can use the mould almost immediately. For casting, it is important to have a dry
mould.
Let the whole
air dry. Depending on the temp, humidity and density this can last from several
days to several weeks. A way to tell how dry the investment is, is by weighing
the mould when it has just hardened. When it has lost on third of its weight (the
water component), it is ready for kiln drying. This removes the chemically
bound water from the investment material.
This is only an outline of what to do. Investment moulds are extremely complicated in their chemistry, physics, and use.
Wednesday 28 June 2017
Stencils vs. Saw
Saw
Frequently when people want to make a complicated shape they
resort to a saw to create the shape.
This is used in both stained glass and fused glass work. Although it may be necessary in stained glass
applications, it is not as necessary in fusing.
One of a variety of saws |
Stencils
There is an alternative to an expensive saw – stencils and
frits. You can make a stencil from stiff
card. Place the stencil in the appropriate place. Then sift powder or sprinkle
frit over the stencil. Lift carefully
and the shape is there ready for fusing.
Example of sifting powder over a complicated stencil |
To get the depth of colour obtained from sheet glass, you
need to apply the powder or frit to at least the thickness of sheet glass. This
also means that you need to go to a full fuse with the powder or frit on the
top surface. You can, of course, later
cap and fire again.
Example of the cutting of a stencil |
More guidance on stencils is available here:
Wednesday 21 June 2017
Mica
What it is
Mica is widely distributed throughout the world and occurs in igneous, metamorphic and sedimentary rocks. Mica
is similar to granite in its crystalline composition. The nearly perfect cleavage, which is the
most prominent characteristic of mica, is explained by the hexagonal sheet-like arrangement of its atoms.
Mica can be composed of a variety of minerals giving various
colours and transparency. Purple,
rosy, silver and grey colours come from the mineral called lepidolite.
Dark green, brown and black come from biotite.
Yellowish-brown, green and
white come from phlogopite.
Colourless and transparent micas are called muscovite.
All these
have a pearly vitreous lustre.
The melting point of mica depends on its exact composition, but ranges
from 700⁰C to 1000⁰C.
Glass has a specific gravity of about 2.5, and mica ranges
from 2.8-3.1, so it is slightly heavier than glass.
Tips on uses of mica
powder and flakes
The naturally occurring colours are largely impervious to kiln forming
temperatures. Other added colours have
various resistances to the heat of fusing. This is determined by the
temperatures used to apply the colour to the mica. Cosmetic mica is coloured at low temperatures and will not survive kiln forming with their colour in tact.
Mica does not combine with glass, but is encased by glass as
it sinks into the glass surface. You can
use various fluxes to soften the surface of the glass. Borax is one of those. The cleaving of the mica results in only the
layer in contact with the glass sticking.
The upper layers brush off. This
applies to both powder and flakes. One solution is to fire with mica on top in
the initial firing and then cap for the final one.
When encasing mica exercise caution. Micas flakes must be
applied thinly, as air is easily trapped between layers which leads to large
bubbles from between layers of glass.
This is the result of the shearing of layers of the flakes allowing air
between layers. Although powdered mica
is less likely to create large bubbles, air bubbles are often created for the
same reason. This is the reason it is
most often recommended to fire the mica on top.
Of course, one use of the mica to make complicated designs
is to cover the whole area and fuse.
Then sandblast a design removing the mica from areas of the glass. You
can then fire polish, or cap and re-fire to seal the mica.
Mica safety
MSDS for mica only mentions the
inhalation of the dust as a risk. Mica is resistant to acid attack and is
largely inert. Inhalation of the dust is a (low level) risk. Any significant health and safety problems
relate to the coloured coatings.
Wednesday 14 June 2017
Deep slumps
One of Karl Harron's deep slumped bowls |
Deep slumps require
multiple stages to get even drops without thinning the sides. There are several makers of staged slumping
moulds which allow progressively deeper slumps in a series of firings into
deeper moulds.
If you have a steep-sided mould, you will find slumping directly into the shape difficult. There will be uneven slumps, thinning of
sides, hang-ups, etc., among your attempts to achieve the slump in one firing. It is possible to mimic this series of moulds without buying the whole set.
To avoid these difficulties, you can build up the inside bottom of the mould by placing powdered kiln wash in the bottom and smoothing it to a gentle curve. You should aim for a gentle shape as in a ball mould.
After the first
firing, remove some of the powder, placing it in a clean container. Shape the remaining powder into a deeper
slump than the first one.
It takes some time
and practice to achieve a smooth even curve.
You can ease the shaping process by cutting the intermediate shapes from
stiff card. This can be rotated to
achieve an even curve in the powder.
Remove any excess powder and do a final rotation to give the powder a
final smoothing. Place the glass back on
the mould and fire.
It may be that you will
need to repeat this several times to get the full slump. Separate template curves need to be cut for
each slump if you are doing more than one intermediate slump. It does depend on
the steepness of the mould sides and the depth of the slump as to how many
stages are required. Sometimes the slump
can be achieved in only two stages.
After firing the
powder, pour it back into your kiln wash container, as it still is good for
mixing to apply to shelves, moulds etc.
This method is
useful for any mould that is too deep for achieving the slump in one firing,
and without buying intermediate moulds. Remember the final result will be smaller than the size of the deep mould, as the span of the glass becomes less with each deeper slump.
Wednesday 7 June 2017
Effects of Annealing at the Top End of the Range
It is possible to begin your annealing at any point in the
annealing range.
The annealing point is the temperature at which the glass
most quickly relieves the stress within.
This occurs at the glass transition point.
The annealing range is between the softening point and the strain point of the glass. No annealing can be achieved above the
softening point, nor below the strain point.
This range, for practical purposes can be taken to be 55°C above and
below the published annealing point. For
thick slabs, Bullseye has chosen to start the anneal 34°C below the published
annealing point of 516°C.
High Annealing Point
They could have chosen to use a higher point, even up to 571°C,
the approximate strain point of the glass. The effect of this is an extended anneal
cool. The reasons are as follows.
The anneal soak does not need to be extended, as the purpose is to get all the glass at the same temperature in preparation for the annealing cool.
The anneal soak does not need to be extended, as the purpose is to get all the glass at the same temperature in preparation for the annealing cool.
The cooling rate must be slower (approximately one third the
rate) than an anneal soak at a lower temperature, as the glass must be
maintained at the same temperature throughout the long cool.
Also, the initial rate of cool needs to be maintained down to the strain point, which is 110°C below the softening point. Of course, after that initial cool, the speed of cooling can be increased.
Also, the initial rate of cool needs to be maintained down to the strain point, which is 110°C below the softening point. Of course, after that initial cool, the speed of cooling can be increased.
Low Annealing Point
Starting the anneal cool closer to the strain point requires
a longer soak to ensure the glass is all at the same temperature (+/- 5°C)
before the anneal cool begins.
Typically, this initial soak would be for an hour before the initial
cool begins (for a 6mm to 9mm thick piece).
Effect of the Differences in Approach
The advantages and disadvantages centre around these needs
to
- soak long enough to get all the glass to the same temperature and secondly, to
- cool slowly enough to maintain the even temperature distribution throughout the glass.
Example
If you think of an example of a
piece of Bullseye glass 12mm thick, it will show the differences in approach.
High temperature soak
A soak of 30 minutes at 571°C (the highest possible start for an annealing soak) is
required to even the temperature. To
ensure the temperature differentials in the glass do not deviate from the +/-5°C,
the cool needs to be at 18°C per hour down to 461°C. It is possible then to increase the speed to
36°C down to 370°C. This gives you a
total annealing cool of just over 5 hours.
Low temperature soak
Starting the anneal at 482°C
requires an hour soak followed by a decrease in temperature of 55°C per hour to
427°C, and an increased rate of 110°C to 370°C.
This gives an anneal cool time of 3 hours and 30 minutes.
The example shows how, although the annealing result may be
the same, there is considerable time saved (for thicker pieces) in using the lower part of the
annealing range to begin the annealing.
It also will save some electricity.
However, an anneal of 30 minutes at 516°C with a cool of 80°C per hour to 370°C will still give a perfectly adequate anneal for 6mm thick pieces.
However, an anneal of 30 minutes at 516°C with a cool of 80°C per hour to 370°C will still give a perfectly adequate anneal for 6mm thick pieces.
Wednesday 31 May 2017
Breaks after the Piece is Cool
People sometimes fire a piece only to have it break after it
is cool. They decide to re-fire with
additional decoration to conceal the break.
But it breaks again a day after it has cooled. Their questions centre around thermal shock
and annealing. They used the same CoE from different suppliers, so it must be
one of these elements that caused the breakage.
Thermal
Shock
This is an effect of a too rapid heat change. This can occur on the way up in temperature
or on the way down. If it occurred on
the way up to a fuse, the edges will be rounded. If it occurred on the way up to a slump the
edges may be sharp still, but the pieces will not fit together because the
slump occurred before the slump. It the
break occurs on the way down the pieces will be sharp. The break will be visible when you open the
kiln. More information is here.
If the break occurs after the piece
is cool, it is not thermal shock.
If the break occurs some length of time after the piece is
cool, it can be an annealing or a compatibility problem. They are difficult to distinguish apart sometimes.
The annealing break usually crosses through the applied pieces
and typically has a hook at each end of the break. If the piece has significant differences in
thicknesses, the break may follow the edge of the thicker pieces for some distance
before it crosses it toward an edge. This kind of break makes it difficult to
tell from an incompatibility break.
An incompatibility break may occur in the kiln, or it may
occur days, months or years later. Typically,
the break or crack will be around the incompatible glass. The break or crack may follow one edge of the
incompatible glass before it jumps to an edge.
The greater the incompatibility, the more likely it is to break
apart. Smaller levels of incompatibility
lead to fractures around the incompatible glass pieces, but not complete breaks.
There is more information about the diagnosis of the causes
of cracks and breaks here.
Annealing
Another possible cause of delayed breakage is inadequate annealing. Most guidelines on annealing assume a flat uniform thickness. The popularity of tack fused elements, means these are inadequate guides on the annealing soak and annealing cool. Tack fused items generally need double the temperature equalisation soak and half the annealing cool rate. This post gives information on how the annealing needs modification on tack fused items.
Compatibility
The user indicated all the glass was
of the same CoE. This is not necessarily
helpful.
Coefficient of Linear Expansion (CoE) is measured between
0°C and 300°C. The amount of expansion over this temperature range is measured
and averaged. The result is expressed as a fraction of a metre per degree Celsius.
CoE90 means that the glass will expand 9 one-thousandths of a millimetre for
each degree Celsius. If this were to
hold true for higher temperatures, the movement at 800C would be 7.2mm in
length over the starting size. However,
the CoE rises with temperature in glass and is variable in different glasses,
so this does not tell us how much the expansion at the annealing point will
be. It is the annealing point expansion
rate that is more important. More information is here.
Compatibility is much more than the
rate of expansion of glass at any given temperature. It involves the balance of the forces caused
by viscosity and expansion rates around the annealing point.
Viscosity is probably the most important force in creating
compatible glasses. There is information on viscosity here. To make a range of compatible glass the forces of expansion
and viscosity need to be balanced. Each
manufacturer will do this in subtly different ways. Therefore, not all glass that is claimed by
one manufacturer to compatible with another’s will be so.
All is not lost. It does not need to be left to chance.
Testing glass from different sources is required, as you can
see from the above comments. It is
possible to test the compatibility of glass from different sources in your own
kiln. The test is based on the principle
that glass compatible with a base sheet will be compatible with other glasses
that are also compatible with that same base sheet. There are several methods to do this testing,
but this is the one I use, based on Shar Moorman’s methods.
If you are investing considerable effort and expense in a
piece which will use glass from different sources or manufacturers, and which is simply
labelled CoE90, or CoE96, you need to use these tests before you start putting
the glass together. The more you deviate
from one manufacturer’s glass in a piece, the more testing is vital.
In the past, people found ways of combining glass that was
not necessarily compatible, by different layering, various volume
relationships, etc. But the advent of
manufacturers’ developing compatible lines of glass eliminated the need to do
all that testing and experimenting.
While the fused glass market was small, there were only a few companies
producing fusing glass. When the market
increased, the commercial environment led to others developing glass said to be
compatible with one or other of the main producers of fusing compatible glass.
If you are buying by CoE you must
test what you buy against what you have.
The discussion above shows that even with the best
intentions, different manufacturers will have differences that may be small,
but can be large enough to destroy your project. This means that unless you are willing to do
the testing, you should stick with one manufacturer of fusing compatible
glass.
Do not get sucked into the belief
that CoE tells you anything important about compatibility.
Wednesday 24 May 2017
Sticking Kiln Wash
Sometimes people
experience kiln wash sticking to the bottom of their glass.
You need some
understanding of what kiln wash is to know why the wash
sticks. It is largely due to the chemical changes in the kaolin at fusing
temperatures.
Opalescent glass does
tend to pick up kiln wash more easily than transparent, and does it more at
higher temperatures. It is the case that at higher temperatures and longer soaks, the kiln wash is
more likely to stick to any of the glasses than at lower temperatures and with
shorter soaks. This re-enforces the mantra of "low and slow" to avoid problems in kiln forming.
To achieve the same
effects at lower temperatures as at higher temperatures, your rate of advance needs to be slower from the
slump point to the top temperature. This
additional heat work will achieve the desired effect with a lower temperature.
One kiln wash, Primo,
does not contain china clay. If you use
this and it is sticking to the bottom of the glass, you may be firing too high.
Try a lower temperature with a longer soak to reduce the kiln wash pickup.
Wednesday 17 May 2017
Compatibility Tests
These procedures are based on the observation that glasses compatible with the base glass are compatible with each other. This means that you can test opaque colours’ compatibilities with each other by testing each of them on clear strips.
Annealing test
These tests must be combined with an annealing test. This consists of putting two pieces from the same sheet of glass together - so you know they are compatible - and firing them along with your compatibility test.
Viewing the results of your annealing through the polarised filters shows whether there is stress left in your annealing. If there is, the compatibility tests are inconlusive as there is no difference in appearance of stress whether from incompatibility or from inadequate annealing. Once you have the annealing right, you can then interpret the compatibility tests done at the same time.
Strip test
Cut a strip of base glass 75mm/3" wide and as long as convenient for you or your kiln.
Cut clear glass squares of 25mm/1" to separate the colours.
Cut 25mm/1" squares of the colours to be tested.
Start with a clear square at one end of the clear strip and alternate colours and clear along the strip finishing with a clear square.
Annealing test
These tests must be combined with an annealing test. This consists of putting two pieces from the same sheet of glass together - so you know they are compatible - and firing them along with your compatibility test.
Viewing the results of your annealing through the polarised filters shows whether there is stress left in your annealing. If there is, the compatibility tests are inconlusive as there is no difference in appearance of stress whether from incompatibility or from inadequate annealing. Once you have the annealing right, you can then interpret the compatibility tests done at the same time.
Strip test
Cut a strip of base glass 75mm/3" wide and as long as convenient for you or your kiln.
Cut clear glass squares of 25mm/1" to separate the colours.
Cut 25mm/1" squares of the colours to be tested.
Start with a clear square at one end of the clear strip and alternate colours and clear along the strip finishing with a clear square.
Place two strips 25mm/1" wide either side of the clear and coloured squares.
Add a stack of two layers of clear to the kiln before firing as a test for adequate annealing. If the annealing is inadequate, then the whole test is invalid.
Test the result with polarising filters. Start with the clear annealing test square. If no stress is apparent, go to the test strip. But if stress is apparent in the annealing test, look to your annealing schedule as something needs to change. Usually the requirement is a combination of a longer soak at the annealing temperature and a slower annealing cool.
To test for compatibility, look carefully for little bits of light in the clear glass surrounding the colour. These are indications of stress – the more light or the bigger the halo, the greater the stress. Really extreme stress appears to be similar to a rainbow, although without the full spectrum.
You can use this test to determine if you annealing is satisfactory for larger pieces. In this case you should use at least 100mm squares. Stack them to the height of your planned project and dam them with fibre board or other refractory materials to prevent spread. Fire to full fuse and anneal. When cool check for stresses.
The tile method looks at compressive factors too.
Cut a 100mm/4" square clear tile
Cut two strips of glass 25mm/1" wide and 100mm/4" long for each test
Cut two rectangles of 25mm by 50mm (1" by 2") of the same glass for the two remaining sides
Cut a square of 50mm/2" for the centre. The glass in the middle is normally the test glass. To be very certain of what has happened you can do the reverse lay up at the same time. You put coloured glass around the outside, but in this case the inside needs to be clear or transparent. At least one element needs to be transparent enough to view the stress patterns, if any. So you could have a clear middle and black exterior, and vice versa.
This test is a more time consuming process and you may wish to use it only for larger projects.
Also look at the use of polarising filters
Add a stack of two layers of clear to the kiln before firing as a test for adequate annealing. If the annealing is inadequate, then the whole test is invalid.
In this test there is very mild stress showing from the dark brown and dark green. This is well within the limits for compatibility. |
To test for compatibility, look carefully for little bits of light in the clear glass surrounding the colour. These are indications of stress – the more light or the bigger the halo, the greater the stress. Really extreme stress appears to be similar to a rainbow, although without the full spectrum.
You can use this test to determine if you annealing is satisfactory for larger pieces. In this case you should use at least 100mm squares. Stack them to the height of your planned project and dam them with fibre board or other refractory materials to prevent spread. Fire to full fuse and anneal. When cool check for stresses.
The tile method looks at compressive factors too.
Cut a 100mm/4" square clear tile
Cut two strips of glass 25mm/1" wide and 100mm/4" long for each test
Cut two rectangles of 25mm by 50mm (1" by 2") of the same glass for the two remaining sides
Cut a square of 50mm/2" for the centre. The glass in the middle is normally the test glass. To be very certain of what has happened you can do the reverse lay up at the same time. You put coloured glass around the outside, but in this case the inside needs to be clear or transparent. At least one element needs to be transparent enough to view the stress patterns, if any. So you could have a clear middle and black exterior, and vice versa.
This test is a more time consuming process and you may wish to use it only for larger projects.
Also look at the use of polarising filters
Charges for Repairs
Repairs always cost more than the owner or artist expects on
initial inspection. The cost is very
similar to, or more expensive than, the cost of a new panel if the whole has to
be taken apart and renewed.
If it is a repair to part of the window or object, you need
to be careful that you do not under price.
The cost elements you need to consider are these at minimum:
- Glass
- Materials
- Time
- Overheads
- Travel
- Installation
- Contingencies
- Profit
Glass - and the
cost of obtaining it. Can you obtain the
same or very similar glass to the original?
If you can’t, is the client willing to have the repair in different
glass? If you get approval, you need to
cost it – whether you already have it or not.
If you do not have it in your stocks, you need to add in the cost of
getting it whether that is travel or postal order. You need to include the time either or both methods
involve in the costs.
Materials – The materials
you will use in addition to the glass need to be considered. These include solder, Foil or lead, flux,
patina, cleaning materials, etc.
Time - labour and
admin. You need to assess how much time it will take to do the repairs. Then multiply that by your labour rate. You
do have one, don’t you? If not, get down
to it and create one. Use steps one and two of this description. You also need to take into consideration the time to recreate a pattern for the broken area if extensive.
Overheads – If your
overheads are not included in your hourly rate, this is the time to include
them in the pricing.
Travel = Your mileage
rate + time to get there and back. If
you don’t have a mileage rate, look at what your local authority allows. This will be lower than what businesses
allow, but are reasonable, and publicly available. (At the time of writing the allowance in Scotland
is approximately £0.50 per mile.) It takes
time to get to the location, so this needs to be included in the cost too. Of
course, if they are willing to bring the item, it reduces the cost to the
client.
Installation – If
you are expected to install the piece, you need to include travel (there and
back at least twice) and time. You also
need to include the estimated time to remove and install a substitute (and its
cost) as well as installation of the repaired piece.
These seven elements added together give you the cost of doing
the repairs. That is the bottom
line. But there is one more element to
consider:
Profit – You do
expect to get a profit from all this work, don’t you? If not, why do the repair at all? You are not a charity. Of course, you can decide to give away your
profit. Before you do, think about what
you have to pay for repairs – to your car, your plumbing, etc. You deserve some profit on everything you
have invested in this craft that you love.
The love will die without profit.
This may all sound like it is too much trouble for a simple repair. Yes, it does take a bit of consideration to start with. But once you have established the basic labour, travel, overhead and profit levels, the rest is pretty straight forward. You will have an idea of how long it takes to do the work, to travel, the glass costs, etc., and the profit level. You only need to multiply by the rates you have established to give you the price. I should warn you - it will be much higher than you initially thought.
Tuesday 9 May 2017
CMC as a Medium
There is a lot of use of powdered glass in a medium supplied in an applicator bottle. This is convenient and expensive. You can approach the consistency of the commercial product by use of CMC
CMC is carbyl methyl cellulose. It is a binder (glue) and it helps keep solids suspended in liquid. You can buy it at a pottery supplier as it is often used in glaze mixtures and known as a glaze binder. It is also used a lot in the food processing industry and is available as food grade CMC. It normally is supplied in powder form.
To make use of it you need to dissolve a teaspoon or two in a half pint of water and let it stand. It does take a while to wet and in doing so it expands. By the next day it will be a thick mass. It can be further diluted, but it is often best to make it to the consistency you will need for the current purpose. It will require a little experimentation to get the proportions right for various purposes.
Put on a dust mask. Sift the powdered glass to remove the coarsest particles and mix the result with the CMC, 1 to 2 of powder or more. Mix thoroughly and leave to stand overnight. Then dilute to the desired flow if required and mix well. Again let it stand overnight to completely incorporate the powder. If it is too liquid, it is desirable to add more powder than more CMC to avoid diluting the colour.
CMC is carbyl methyl cellulose. It is a binder (glue) and it helps keep solids suspended in liquid. You can buy it at a pottery supplier as it is often used in glaze mixtures and known as a glaze binder. It is also used a lot in the food processing industry and is available as food grade CMC. It normally is supplied in powder form.
To make use of it you need to dissolve a teaspoon or two in a half pint of water and let it stand. It does take a while to wet and in doing so it expands. By the next day it will be a thick mass. It can be further diluted, but it is often best to make it to the consistency you will need for the current purpose. It will require a little experimentation to get the proportions right for various purposes.
Put on a dust mask. Sift the powdered glass to remove the coarsest particles and mix the result with the CMC, 1 to 2 of powder or more. Mix thoroughly and leave to stand overnight. Then dilute to the desired flow if required and mix well. Again let it stand overnight to completely incorporate the powder. If it is too liquid, it is desirable to add more powder than more CMC to avoid diluting the colour.
Wednesday 3 May 2017
Channels in Jewellery Items
The principle in forming channels in fused glass is to keep the space open with something that will survive the firing and can be easily removed.
You can use kiln washed wire, mandrels, or tooth picks which you can pull out after cooling. These tend to leave a residue of the kiln wash behind. So this is best used on opaque items.
You can use rolled or cut fibre paper, which can be washed out after cooling, leaving a clean hole. This is works well on transparent items.
Both these methods tend to leave bumps over the channel. So you can make a three layer piece. Cut the middle layer short enough to allow the element to keep the hole open (toothpick, cut piece of fibre paper, wire etc.) to be placed with enough overlap of the top layer to catch the bottom layer. In this kind of setup you need to make the top layer a bit longer than the bottom layer. Make sure you are generous in the length of the "hole keeper" so if the glass (now possibly 9mm) does expand you do not trap the material inside.
Of course on a three layer set up like this you could use thin glass which would give you about 6mm of thickness thus eliminating the spread due to volume. In this case you would need to use fibre paper or wire that is about 1.5mm high/thick. It is probably best to have a thin piece of glass on each side of the “hole keeper” to ensure the glass does not retreat due to lack of volume.
You can experiment with a layer of standard and two of thin in various combinations to find the one you like best.
You can use kiln washed wire, mandrels, or tooth picks which you can pull out after cooling. These tend to leave a residue of the kiln wash behind. So this is best used on opaque items.
You can use rolled or cut fibre paper, which can be washed out after cooling, leaving a clean hole. This is works well on transparent items.
Both these methods tend to leave bumps over the channel. So you can make a three layer piece. Cut the middle layer short enough to allow the element to keep the hole open (toothpick, cut piece of fibre paper, wire etc.) to be placed with enough overlap of the top layer to catch the bottom layer. In this kind of setup you need to make the top layer a bit longer than the bottom layer. Make sure you are generous in the length of the "hole keeper" so if the glass (now possibly 9mm) does expand you do not trap the material inside.
Of course on a three layer set up like this you could use thin glass which would give you about 6mm of thickness thus eliminating the spread due to volume. In this case you would need to use fibre paper or wire that is about 1.5mm high/thick. It is probably best to have a thin piece of glass on each side of the “hole keeper” to ensure the glass does not retreat due to lack of volume.
You can experiment with a layer of standard and two of thin in various combinations to find the one you like best.
Removing Bubbles
Inclusions and Bubbles
The inclusion of material between two or more sheets of
glass has the risk of creating bubbles.
The size of these often relate to the size of the inclusion. The inclusion can be glass (powders, frits,
cut pieces), mica, metals, foils, etc.
The important element in eliminating bubbles is to have a
long slow bubble squeeze from the bottom of the forming temperature to the top
slumping temperature. If this is
combined with supports at the edges or a fine film of clear powder, it will
help reduce the interior bubbles to a minimum.
The supports at the edges may be as small as fine frit (and some use
powder over the whole surface).
But, once you have bubbles in the piece, what can you do?
You can drill a hole in the bubbles, or break the bubbles and
fuse again, but there will be distortions visible in the resulting piece.
Another method to reduce the effect of bubbles, is to flip
the piece and fire upside down to drive the bubbles to the bottom of the
piece. Be careful to use low fusing
temperatures to avoid enlarging the bubble.
At the finish, the bubble will still be in the glass but will not be
protruding above the top surface.
It may also be possible to combine the two processes. Drill a small hole in the bubbles and fire
upside down. If you do this you need to
place the glass on porous fibre paper, not just Thinfire or Papyrus, to allow
the air to be compressed out of the bubbles.
You also need to allow a significant amount of time around the slumping
temperature for this to happen.
Once you have fired upside down, you will need to fire polish the surface again. Do not despair at multiple firings. A lot of people fire their pieces many times to achieve the effects desired.
Once you have fired upside down, you will need to fire polish the surface again. Do not despair at multiple firings. A lot of people fire their pieces many times to achieve the effects desired.
Wednesday 26 April 2017
Borax solutions
A borax solution can act as a devitrification spray. That is its usual application in kiln forming. But it can be used in other ways too.
Borax is a flux helping to reduce the firing temperature of glass. So, it can be used as a medium for powdered mica which can be painted or sprayed onto the glass. It also helps reduce the oxidisation of included metals.
Obtain borax that has no additives. Put a couple of teaspoons into water and bring to a simmer. Remove from the heat and cool. Decant the almost clear liquid off the sediment and you have a saturated solution of borax ready to use.
If you are really parsimonious, you can add water to the crystals remaining in the pot and heat to get another saturated solution. You could do this until there was no residue, but that would get tedious.
Add a couple of drops of washing up liquid to the solution. This is enough to break the solution's surface tension. It helps to give an even distribution of the solution across the clean glass by reducing the beading of the liquid that otherwise occurs.
You can paint the solution onto the material - glass or metal - with a soft brush such as a hake brush, or you can spray it on with a pump spray container. Be careful to clean the spray container immediately, as borax crystals form quickly.
Borax is a flux helping to reduce the firing temperature of glass. So, it can be used as a medium for powdered mica which can be painted or sprayed onto the glass. It also helps reduce the oxidisation of included metals.
Obtain borax that has no additives. Put a couple of teaspoons into water and bring to a simmer. Remove from the heat and cool. Decant the almost clear liquid off the sediment and you have a saturated solution of borax ready to use.
If you are really parsimonious, you can add water to the crystals remaining in the pot and heat to get another saturated solution. You could do this until there was no residue, but that would get tedious.
Add a couple of drops of washing up liquid to the solution. This is enough to break the solution's surface tension. It helps to give an even distribution of the solution across the clean glass by reducing the beading of the liquid that otherwise occurs.
You can paint the solution onto the material - glass or metal - with a soft brush such as a hake brush, or you can spray it on with a pump spray container. Be careful to clean the spray container immediately, as borax crystals form quickly.
Labels:
Borax,
Devitrification,
Kiln Supplies,
Painting,
Stephen Richard,
Verrier
Make Your Own Stopping Knife
“Stopping knife” is a traditional term for an oyster knife
with a weighted end. This makes it a
multi-purpose tool that manipulate glass, dress lead came, act as a fid, act as
a putty knife, and become a hammer. It
also stands up on its own. I find it the
single most useful too in leaded glass panel construction.
This note is how to get from here:
To here:
The process relies on the low melting temperature of
lead. This means that you can use stiff
paper wrapped around the handle of the knife to contain the molten lead until
it cools.
First you set the oyster knife into a vice and cut two
dovetail joints at right angles to each other into the end of the wood
handle. This will insure the lead is
firmly grasped by the wood and will not come loose during use.
I do this with a fine bladed saw such as a hacksaw, coping
saw or even a dovetail saw. There are
Japanese saws that work very well too, but are not so widely available.
The top of the dovetail joint should be just a millimetre or
two off centre.
The angle should be about 30 degrees from vertical. Saw down far enough to get a 6mm chisel into
the space between the two angled cuts.
Chisel out the wood between the cuts.
Repeat for the second dovetail at right angles to the first.
Now you are ready to prepare the oyster knife to become the
stopping knife.
Use paper of more than 90 grams per square metre, such as
cartridge paper to form the narrow cone.
Set the knife at a slight angle on the paper.
Secure the beginning edge to the knife handle with a bit of masking
tape. Mark the paper 5 mm – 10 mm above
the top of the handle. This will be the
fill indicator when pouring the lead. If
you over-fill the cone, the stopping knife will be heavy and uncomfortable to
use.
Roll the paper around the handle to form the cone. This cone should be as close to vertical as
possible. A wide based cone will, of
course, provide stability, but it will add so much weight as to be
uncomfortable to use. It will also be so
wide as be uncomfortable for the palm of your hand.
You can unwrap the paper and start over if the cone becomes
too wide. The key is to start the
wrapping just before the handle begins to taper toward the end of the
handle. The other way of looking at it
is to attach the paper just as the expanding taper stops.
Try to keep the paper cone as smooth as possible. This will form the shape of the lead end of
the handle. You want it to be as
circular as possible without dents or angles.
Now you are ready for the casting.
I use a small old cast iron pot to melt the lead. I place this over a camping gas burner to
provide the heat. I promise that I did
straighten the stabilising legs before lighting the camping burner.
Put some old lead came into the pot to be melted. While this is coming up to heat, place your
wrapped oyster knife in a vice with heat resisting materials around the site to
catch any spills.
Put sufficient lead into the pot, as there will be
impurities floating on top and the lead will cool quickly when taken off the
heat. The photo below shows the amount
of lead used. This 100mm diameter pot
has lead barely covering the bottom. You
do need enough lead to complete the pour at one go, as a second pouring will
not stick to the first adequately.
The photo shows the last piece of came just about to be
melted. This is the time to begin the
pour. If the lead is too hot, it burns
the wood creating gases and multiple bubbles splashing hot lead and leaving an
unpleasant surface for the tool. As the last piece of the came melts and leaves its
impression as the piece on the left, it is time to pour.
Pour at a steady rate into the paper cone until you reach
the height indicator you previously marked in the paper. When you stop pouring, set the pot on a heat
proof surface. You will notice some
smoke and browning of the paper. That is
normal. This picture shows the effect of
the hot lead on the paper once the smoking has finished.
This photo shows the inside of the cone while cooling. The cooling process will take about an
hour. You will be able to check, by
touching the paper, how hot the whole is.
This shows the roughness of the handle end. This is due to the bubbling from the
scorching of the wood and paper.
When the paper is removed and the lead is fully at room
temperature you can use a rough file to remove the bubbling and to round the
edge of the lead.
The oyster knife has been transformed into a stopping knife
and is ready to use.
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