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.
Sunday 26 January 2014
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.
Sunday 15 December 2013
Pot Melts – Weight of Glass Required
Circular pieces
This table assumes that a 150 mm diameter pot is being used, and assumes that 125 grams of glass will be left in the pot. Larger diameter pots or even pot trays can be used, but more glass will remain in the container. The following table gives the desired diameter of the melt and the weight of glass needed to achieve an average 6 mm thick result. To achieve a uniform six millimetre thick disk will require long soaks at both melting and fusing temperatures to allow the glass to even out in thickness.
50 mm diameter disk requires 154 grams of glass
100 mm diameter disk requires 243 grams of glass
150 mm diameter disk requires 390 grams of glass
200 mm diameter disk requires 596 grams of glass
250 mm diameter disk requires 861 grams of glass
300 mm diameter disk requires 1185 grams of glass
350 mm diameter disk requires 1568 grams of glass
400 mm diameter disk requires 2015 grams of glass
Thicker melts
Of course if you want a thicker pot melt — one that is confined so that it cannot grow larger, only thicker — you can use the following method to estimate the amount of glass required.
Choose the diameter wanted from the above table, and subtract 125 from the weight of glass required. Then multiply by thickness wanted divided by 6 mm. Add back 125 gms — the amount that will be retained in the pot — and you have the required amount.
For example: a 200 mm disk of 6 mm requires 596 gms. You want a 12 mm thick disk of 200 mm.
First subtract 125 from 596 to get 471 gms. 417 gms times 12 equals 5652. Divide this by 6 mm and you have 942 gms required. Add 125 gms — the amount left in the pot — and you have a requirement of 1067 gms for a 12 mm thick disk of 200 mm.
Rectangular pieces
These are easier to calculate than discs, as the calculation is length times height times depth (all measurements in centimetres).
If you are making a billet and using an empty margarine pot of 7 cm wide, 12 cm long and 7 cm high you will need enough glass to fill a volume of 588 cubic centimetres. As the specific gravity of glass is 2.5, you multiply the cubic centimetres to give the weight required in grams — in this case, 1470 gms.
If you wanted a 6 mm tile of 100 mm square you would need 150 grams of glass.
To make a 1 cm slab of the same size you need 250 grams of glass.
To make a billet of 5 cm by 10 cm square you need 1250 grams of glass (this is pretty close the the maximum that can be loaded in a 12 cm diameter Pot).
To make a small sample billet of 2 cm thick by 4 cm by 8 cm you need 160 grams of glass.
A billet or pattern bar of 5 cm by 10 cm by 5 cm needs 625 grams of glass.
This table assumes that a 150 mm diameter pot is being used, and assumes that 125 grams of glass will be left in the pot. Larger diameter pots or even pot trays can be used, but more glass will remain in the container. The following table gives the desired diameter of the melt and the weight of glass needed to achieve an average 6 mm thick result. To achieve a uniform six millimetre thick disk will require long soaks at both melting and fusing temperatures to allow the glass to even out in thickness.
50 mm diameter disk requires 154 grams of glass
100 mm diameter disk requires 243 grams of glass
150 mm diameter disk requires 390 grams of glass
200 mm diameter disk requires 596 grams of glass
250 mm diameter disk requires 861 grams of glass
300 mm diameter disk requires 1185 grams of glass
350 mm diameter disk requires 1568 grams of glass
400 mm diameter disk requires 2015 grams of glass
Thicker melts
Of course if you want a thicker pot melt — one that is confined so that it cannot grow larger, only thicker — you can use the following method to estimate the amount of glass required.
Choose the diameter wanted from the above table, and subtract 125 from the weight of glass required. Then multiply by thickness wanted divided by 6 mm. Add back 125 gms — the amount that will be retained in the pot — and you have the required amount.
For example: a 200 mm disk of 6 mm requires 596 gms. You want a 12 mm thick disk of 200 mm.
First subtract 125 from 596 to get 471 gms. 417 gms times 12 equals 5652. Divide this by 6 mm and you have 942 gms required. Add 125 gms — the amount left in the pot — and you have a requirement of 1067 gms for a 12 mm thick disk of 200 mm.
Rectangular pieces
These are easier to calculate than discs, as the calculation is length times height times depth (all measurements in centimetres).
If you are making a billet and using an empty margarine pot of 7 cm wide, 12 cm long and 7 cm high you will need enough glass to fill a volume of 588 cubic centimetres. As the specific gravity of glass is 2.5, you multiply the cubic centimetres to give the weight required in grams — in this case, 1470 gms.
If you wanted a 6 mm tile of 100 mm square you would need 150 grams of glass.
To make a 1 cm slab of the same size you need 250 grams of glass.
To make a billet of 5 cm by 10 cm square you need 1250 grams of glass (this is pretty close the the maximum that can be loaded in a 12 cm diameter Pot).
To make a small sample billet of 2 cm thick by 4 cm by 8 cm you need 160 grams of glass.
A billet or pattern bar of 5 cm by 10 cm by 5 cm needs 625 grams of glass.
Wednesday 11 December 2013
Supporting Overhangs on Moulds
In
general, the blank should be no larger than the thickness of the
glass over the mould. So a 6mm blank would have no more than 6mm
overhang.
In
the case of steep sided moulds, the glass should be entirely within
the mould to avoid any hangup on the edge, leading to uneven slumps
and needling on the edges.
But,
if you need the glass to be the size of the mould, you can make a
collar to go around the mould, which will support the glass while it
begins to slump into the mould.
Make
a donut shape that will fit around the mould (whether round, oval or
rectangular) and extend beyond. Support the collar on kiln furniture
to be as high or slightly higher than the top of the rim of the
mould. This makes a kind of drop out ring, allowing the glass to
drop into the mould.
Donut ring suitable for placing around a circular mould |
This arrangement is suitable for placing around a mould of the same diameter as the interior of the ring |
Make
sure that the collar is well covered with kiln wash to ensure the
glass can move along the fibre board. This includes both the surface
and vertical edges of the collar.
As
the glass softens and begins to fall into the mould, the glass at the
edge does not have the weight to bend down and so raises off the
collar and begins to slip into the mould.
And
finally, you need to ensure that the mould is not so steep as to trap
the glass inside. This is more of a concern on steel with its
greater expansion and contraction than ceramic.
A steel mould likely to trap the glass inside with its vertical sides |
Wednesday 4 December 2013
Super Glue
Super
Glue
There
are multiple cyanoacrylates (superglues) on the market, and they will
give very different results. Gel superglue formulations usually have
some type of rubber or fumed silica additive to make them thicker,
and the additive usually doesn't burn out. That's probably where the
"superglue leaves a mark" originates. Usually the cheapest
possible superglue is best for temporary glass holds because it'll
mostly be additive-free.
The
glue will burn out around 700F or so, so it shouldn't be used to
position the glass against gravity. I disagree, however, that it
should never be used. I buy cheap superglue by the carton and use it
in everything from temporary casting assemblages to making glass
boxes for frit panels to tack-fusing. It is the best way I know to
hold wobbly pieces in place until you can assemble the rest of the
glass around it.
Some
tips for using superglue:
- You are more likely to get whitish residues if you let moisture get to the superglue while it's drying, so keep the glass surfaces as dry as possible and don't put a superglue-assembled piece on a wet kiln shelf.
- Always try to put the glue under opaque or dark glasses, just in case something goes wrong.
- Use the smallest amount possible. Don't flood an area with glue and lay the glass on top - that will almost always leave too much glue on the glass. Instead, I assemble the glass and put a drop of glue right where the two glasses join. Capillary action sucks just the right amount of glue into the joint.
- If you wipe excess glue away with acetone, be careful about which acetone you're using. Some types (such as nail polish remover) can have additives that leave residues on the glass and make the problem worse. If the glue is in a readily accessible area, it is usually better to wait for it to dry, then peel it off the glass with a razor blade. Only use acetone where there's texture or something else that makes the glue difficult to remove. And in any case, don't worry much about removing superglue right on the surface--it will burn off.
- Superglue joints will NOT support the weight of your glass, i.e., never, ever lift your assemblage by a superglued-on piece of glass. Common superglue is actually a lousy glue for glass--which is why it works as a temporary hold.
Wednesday 27 November 2013
Disguising Joints in Fusing
One
advantage of fusing over leading or copper foiling is that shapes
impossible to cut as a single piece can be made from multiple pieces.
However these joints often show up in the finished work.
You
are always more likely to have the joints show when the cut coloured
glass is on the bottom. The infra-red heat of the kiln elements goes
through the clear glass to the coloured below, allowing it to soften
first. As the glass underneath softens and pulls in, it allows the
top glass to sink into the space. Upon cooling the seam is kept open
even sometimes showing a clear line at the joints.
Putting
the clear as the base and the jointed pieces on the top has a better
chance of having the joints fully fuse together. There is no glass
above to spread the pieces apart.
When
you need the joints to be concealed, you can put a line of powder the
same colour of glass over the joint. This line should be slightly
rounded above the surface along the joint to account for the
reduction in volume as it fuses. When it is two colours meeting,
using powder of the same colour as the darker glass is most
successful.
Fusing
to a contour fuse for 10 minutes is normally hot enough, but taking
the piece to a flat fuse – again for 10 mins - will certainly be
enough to fully melt the powder into the joint.
Sunday 24 November 2013
Installing Leaded Glass in Stone
Side rebates
One side of the rebate (or raggle) in stone should be deeper than the other. This allows the panel to be slotted in and then slid back into the shallower rebate. Which side the deep rebate is on is not important, but you must determine which is the deeper and its minimum depth all along the raggle.
Adjusting the placement of the panel
To help move the panel from side to side stiff oyster knives and lead knives are important. This allows you to get behind the edge and slide the panel to the side, especially when it is sitting on top of another panel to make the fine adjustments to get the lead lines flow correctly.
In some circumstances, especially when installing a single panel, it is necessary to bend the leaves of the lead toward the installation side. After placing the panel, you then fold the leaves out one at a time into the raggle slot.
Top and bottom rebates
For the top and bottom rebates it is important that the top is the deep one. You insert the panel up into the slot a the top and let it settle into the bottom rebate. The panel should be completely covered by the stone.
Extra came
In all installations into stone, you should carry extra came of at least 12mm (1/2”) to solder round the panel when the stone work is not as accurate as it should be, either through workmanship or weathering.
Wedges
Have some little blocks of wood and some whittling tool to hand to wedge the panel in till mortared. It is possible to use little scraps of lead for the purpose. These wedges don't need to be that robust, they are just there to hold the panel in place until the mortar is in.
Mortars
Mortars for stone should be of lime cement, or sand mastic. Don't use silicon, you'll never get it out again! Also don't use putty as this stains some types of stone and the oils leech in to the stone, causing the putty to dry and therefore the window ceases to be watertight.
One side of the rebate (or raggle) in stone should be deeper than the other. This allows the panel to be slotted in and then slid back into the shallower rebate. Which side the deep rebate is on is not important, but you must determine which is the deeper and its minimum depth all along the raggle.
Adjusting the placement of the panel
To help move the panel from side to side stiff oyster knives and lead knives are important. This allows you to get behind the edge and slide the panel to the side, especially when it is sitting on top of another panel to make the fine adjustments to get the lead lines flow correctly.
In some circumstances, especially when installing a single panel, it is necessary to bend the leaves of the lead toward the installation side. After placing the panel, you then fold the leaves out one at a time into the raggle slot.
Top and bottom rebates
For the top and bottom rebates it is important that the top is the deep one. You insert the panel up into the slot a the top and let it settle into the bottom rebate. The panel should be completely covered by the stone.
Extra came
In all installations into stone, you should carry extra came of at least 12mm (1/2”) to solder round the panel when the stone work is not as accurate as it should be, either through workmanship or weathering.
Wedges
Have some little blocks of wood and some whittling tool to hand to wedge the panel in till mortared. It is possible to use little scraps of lead for the purpose. These wedges don't need to be that robust, they are just there to hold the panel in place until the mortar is in.
Mortars
Mortars for stone should be of lime cement, or sand mastic. Don't use silicon, you'll never get it out again! Also don't use putty as this stains some types of stone and the oils leech in to the stone, causing the putty to dry and therefore the window ceases to be watertight.
Wednesday 20 November 2013
Brushes for Painting
A
quality paint brush will have hairs that form a point and have a good
spring to them - they bend while painting but return quickly to their
original shape. A good brush will also hold lots of paint and deliver
that paint evenly throughout the stroke. Brushes
usually have a number to indicate their size - the larger the number,
the larger the paintbrush. The larger the brush the wider the line
that can be produced, although with a light touch a fine long line
can be made because of the pointed nature of the brush.
The
best brushes are made from natural hairs, although there are brushes
made from a combination of natural and synthetic materials which are
adequate.
Sable
hair brushes are considered to be the best for painting. The hair
comes from a variety of pine martin and the Kolinsky sable from
Siberia is considered the best. These brushes are more expensive
than others, but are soft and flexible, hold their paint well and can
make an expressive thick to thin line.
Ox
hairs are normally used for making rigger brushes. This is a round
brush with long hairs, said to be used to paint the lines of ships'
rigging in the past. The hair is strong and springy making it useful
for long lines and thicker paints.
Squirrel
hair brushes are useful for applying paint in broad, thin layers for
matting.
Goat
hair brushes are normally known as hake brushes. These are a
traditional, oriental style brush. It lacks spring, but forms a good
point and so is useful to cover larger areas quickly with a gentle
touch.
Pony
hair is made into short round brushes used as soft stipplers.
Hog
hairs
are made into hard, very economical brushes. They come in flat and
round shapes. They are most used for stippling and can be trimmed,
shaped, used, and abused for years.
Badger
hairs
are thicker at the end and thinner at the root, creating a conical
shape. These soft brushes are used to blend paint once it has been
spread on the glass. The brush is swept across the surface of the
paint to blend or move paint and remove stroke lines.
Wednesday 13 November 2013
Glass Shifting on Mould
There
are a number of things to investigate if your blank is shifting on
the mould during firing.
Is
there a heat differential?
Glass
absorbs heat at different rates depending on colour and type meaning
that one part may begin to move before another. The solution to this
is to slow down the rate of advance to allow all the glass to gain
heat at the same speed. It may also be useful to slump at a lower
temperature.
There
also may be a heat differential within the kiln. You need to run a
check on the heat distribution of your kiln to be sure where the
(relatively) hot and cold areas of your kiln are. Bullseye published Tech Note no.1 on how to do this.
Not
perfectly balanced on the mould?
Glass
can be placed just off square or level and that can allow it to start
slumping unevenly. Measurements and observation can help to get the
glass placed squarely on the mould. Also a small spirit level placed
on the glass can tell you if the glass is level within the mould.
The
mould may not be level.
The
kiln, shelf and mould should each be checked for level in all
directions. The kiln level can be established and can be assumed to
be level until it is moved. The shelf level should be checked each
time it is moved. The mould level should be checked each time it is
used.
Is
the glass overhanging the mould?
Glass
overhanging the mould rim can hang up on some of the edges more than
others. Check the rim of the mould for any rough areas and smooth
them. If you do have glass overhanging, you should slow the rate of
advance to allow the edge of the glass to tip up and begin to slide
down into the mould. If the problem persists, make the glass blank
smaller, or support the overhanging glass with a collar.
Is
the glass heavier on one side?
The
glass may be uneven thickness and so heavier on one side. The
heavier area of the glass will begin to slump first and so promote
movement of the whole glass in an asymmetrical manner. The solution
to this is to fire slower and to a lower temperature.
Do
you have a wonky mould?
The
mould can be imperfect. So you need to check the mould for accuracy.
I have a slumper that has one side lower than the other three.
Being aware of this, I can place the glass so that it is still
useable. Measuring the mould in all directions will help determine
its symmetry.
If
all these things have been investigated and the solution not found,
it is possible to create a bevel on the bottom edge of the glass so
that the edge sits in the mould at the same angle as the mould. This
provides a larger contact point for the glass and mould than just a
thin edge. This appears to allow the glass to move evenly during the
slump.
Of course, a major solution is to observe the slump. Peeking into the kiln at the beginning of the slump soak and frequent intervals after that will show if the piece is slumping evenly or not. If it is uneven, you can put on the appropriate protective gear and with gloves on your hands, shift the glass to be set evenly in the mould.
The
major solutions to avoid uneven slumping are:
- Avoiding the hot and cool parts of the kiln
- Making everything level
- Careful placement on the mould
- Slower rates of advance
- Lower slumping temperatures
- Observation
Wednesday 6 November 2013
Using Space on Shelves
Often there is unused space on the kiln
shelves when you are firing a project. With a bit of planning, you
can make use of the spaces for a variety of things.
Frits fired on fibre paper |
Bowl made from frit balls |
You can place piece of frit in the
clear areas to make frit balls.
You can make colour tests on plaques of
glass to see the results of strikers, powder combinations or results
of various depths of colour.
Compatibility tests can be done with pieces of glass of which you are not certain.
simple stress testing set-up |
Strip of fired glass samples for testing |
Results - those with halo are stressed |
In the same way, annealing tests can be
conducted.
You can fire small pieces of jewellery
at the same time as your larger pieces.
You can also prepare elements for
incorporation into other fusing projects and lay them out in the open spaces on the shelf. Your use of the spare space is related both to your imagination and to your future projects.
Wednesday 30 October 2013
Cleaning Blending Brushes
Cleaning badger brushes just before use, is easy. Flick, gently and rapidly, the very ends of
the brush hairs against the side of your hand – but use respiratory
protection and be careful not to inhale any dust. If you notice
flecks of dust in your paint when you create a grisaille you’ll
know it’s time for a thorough and wet cleaning again.
After
each use, rinse out the brush tips in cool water. Gently rub the tips
of the brush hairs to loosen any extra paint. Grasp he hairs above
the tips to keep the water from the main part of the brush. Then wet
the exposed ends of the hairs and rub them gently until the water
runs clear.
If you use a blender for oil, you will need to use a small amount of natural soap, if
so, thoroughly rinse.
Flick
the brush to remove excess water, smooth the hairs into shape and
allow to completely dry by hanging the brush with the hairs pointing
downward – this avoids water flowing into the brush base where the
hairs are attached. If you have round-handled brushes, you can twirl
the brush between your hands to remove excess water.
Subscribe to:
Posts (Atom)