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.
Tuesday 9 May 2017
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.
Wednesday 19 April 2017
Light and Dark in Designs
Chiaroscuro – This word borrowed from Italian ("light and shade" or "dark") refers to the modelling of volume by boldly contrasting light and shade.
Glass artists need to be very cognisant of light and dark, both in terms of colour selection and in terms of density. A very thick dense glass of a dark shade of any colour will create a much more intense darkness than glass that is thinner and less dense.
In terms of colour, lighter hues go where the sun shines or where the eye is to be drawn. Pastel shades indicate brightness and light. Within some opalescent and art glasses it is possible to find a shade of colour graduating to white or light yellow.
Shading can be achieved by using the white areas to indicate where light is falling. A denser dark glass can be used to indicate where light does not fall, or where very little light can filter through. It can also play the part of negative space.
Sometimes, it is useful to use a monochrome scheme to assist in determining where the light and dark should be, as in this pear:
The contrast between light and dark can be used in several ways. Darkness can indicate depth of field or distance when used in a general landscape. Or, it can be used to bring a foreground out, making other elements more vivid.
The key thing to remember in using stained glass is to not be afraid of dark glasses. They can very useful, even if of very odd hues of colour.
Glass artists need to be very cognisant of light and dark, both in terms of colour selection and in terms of density. A very thick dense glass of a dark shade of any colour will create a much more intense darkness than glass that is thinner and less dense.
In terms of colour, lighter hues go where the sun shines or where the eye is to be drawn. Pastel shades indicate brightness and light. Within some opalescent and art glasses it is possible to find a shade of colour graduating to white or light yellow.
Shading can be achieved by using the white areas to indicate where light is falling. A denser dark glass can be used to indicate where light does not fall, or where very little light can filter through. It can also play the part of negative space.
The contrast between light and dark can be used in several ways. Darkness can indicate depth of field or distance when used in a general landscape. Or, it can be used to bring a foreground out, making other elements more vivid.
The key thing to remember in using stained glass is to not be afraid of dark glasses. They can very useful, even if of very odd hues of colour.
Wednesday 12 April 2017
Firing wire inclusions
Wire and other metal
inclusions often cause bubbles to occur around them. The standard solutions are to add frit to the
corners, or powder or fine frit around the inclusions. You can also flatten the wire or metal to reduce it height. These most often work well. Sometimes though they don’t eliminate big
bubbles around the metals.
In this case think
about firing upside down. This is not the whole piece; it is only the inclusion
and the bottom layer of glass. Place the
wire or other inclusion on the prepared shelf. It will be most successful if
placed on 1mm or thicker fibre paper to allow any trapped air to escape through
the fibre. Place the base glass on top and take to a tack fuse with a bubble squeeze
included. You might even want to
consider cutting the base larger than the final piece to be able to cut off the
thickened edges and make a more successful piece at the end.
After tack fusing
upside down, the inclusion will be imbedded in the glass with an almost flat
surface and little in the way of air pockets at the edges. Clean very well, especially any spalling from
the metal and of course, clean the glass thoroughly. Cap and fuse with a bubble squeeze
again. The bubbles around the inclusion
should be minimal if not eliminated.
This method will allow the glass to
sink around the glass making a much flatter piece for the capped full
fuse. It should also make for a flatter finished piece with many fewer
bubbles.
Wednesday 5 April 2017
Large Bowed Pieces
Occasionally,
large pieces in the kiln develop a bow at the end of
firing. The most obvious is when the bow
is upwards, but it also occurs that the piece is domed. This is much more likely to be observed when
there are complete sheets, rather than ones interrupted with other design
elements which break up the whole sheet.
This is a
result of a slight mismatch of compatibility.
One glass is expanding and contracting slightly more than the other. The bow is always toward the glass which
expands the most. When it contracts, it
also contracts more than the other glass, drawing the sheet with lower
expansion toward it to form a bow.
This is a
form of mild stress. It can sometimes be
seen in large sheets of streaky or flashed glass which are not completely flat.
It is not a
fatal flaw. A piece of this nature can
survive many years in that state. I once
had a large window to repaint, because of a football impact. When re-assembled, it showed that it had been
bowed from the outset, almost 90 years before.
It is not in a suitable state for wall pieces or other things that need
to be flat, of course.
Remedies
The remedies most often relate to reducing the stress in the piece.
This of course, relates to the firing schedule. Increasing the length of the soak at the
annealing point is one method. This
combined with reducing the rate of cooling can be effective.
Another method can be employed also.
This is to soak the glass just above the upper strain point of the
glass. This soak should be equal to the
one planned for the anneal. The upper
strain point temperature – that point above which no annealing can occur - is about 40C above the annealing point. Thus, this soak should occur about 55C above
the annealing point of the glass concerned.
Then proceed at a moderate pace to the annealing point. This rate may be the same as the second stage
of the anneal cool (as a starting point). Then anneal as usual for the
thickness of the piece. This method can,
of course, be combined with the extended soak and reduced cooling rate as first
suggested.
A third method can be employed, if the first two do not work. This assumes one of the sheets of glass is
clear. Place a sheet of clear on the
opposite side of the piece to form a glass sandwich with the two pieces of
clear. Then fire as for a three-layer
piece of glass. The assumption behind
this is the same as for toughened glass.
The outer layers will hold the inner layer in compression. But more importantly, will equalise the
slight stress, allowing the piece to remain flat when the firing is completed.
This can be used with any transparent glass, but the colour change may not be
acceptable.
A fourth method is possible. Turn
the fired piece over and fire, to allow the weight of the glass to overcome the
tension of the contraction of the more expansive glass. This can be successful, but it does retain
the stress within the resulting piece. As
such it is not a remedy for the stress, but is a way of flattening.
Placement
The place of
the glass in the kiln can have an effect too.
If the sheet is near the side of the kiln, there can be a stress
inducing effect. All kilns are a bit
cooler at the perimeter than at the interior.
This applies to circular, oval and rectangular kilns. Rectangular kilns have additional cool spots
at the corners. If the glass is near the
capacity of the kiln, the cooler corners can induce this bowing stress to
otherwise compatible glass. The thing to
do is to stay about 50mm away from the edges of the kiln when firing large
sheets into one piece.
Testing
The ideal is to know before firing the large piece whether there will be a problem to overcome. This requires a simple test of the glass to be used.
The ideal is to know before firing the large piece whether there will be a problem to overcome. This requires a simple test of the glass to be used.
Assuming the final piece is to be two layers thick of
different glass colours, cut a strip of each colour about 50mm wide and as long
as the final piece. Assemble them in the
same order as you plan for the final piece.
Add an annealing test square of the two glasses
stacked on top of one another. If one is
opalescent and the other is transparent. Make the transparent larger than the
other. If both are opalescent, you will
need to run a compatibility test at the same time as this test. In simple
terms, it is to put each of the opalescents on a strip of clear or transparent
with the gaps between the opals filled with the transparent. This test will tell you whether you have
fired so fast as to induce stress and so invalidate the test.
Fire as though for a 50mm piece of jewellery – about
200C to bubble squeeze - but without a soak - and then at 400C to top
temperature. Cool to annealing
temperature for 15 minutes and cool at 120C per hour to 370C and turn off.
If the long strip is bowed, and the anneal test piece
shows no stress, there is enough compatibility mismatch to require the use of
one of the remedy methods outlined above for the main piece. It may of course,
cause a reconsideration of the glasses to be used or the size of the piece.
Wednesday 29 March 2017
Colourising Metals for Glass
There are many minerals and chemicals that are used to give
glass its variety of colour. This note
attempts to give information on the most common elements and combinations used
to impart the colours to the glass.
Antimony oxides produce white glass as do tin oxides. Together with lead, antimony results in yellow.
Chromium is a very powerful colourizing agent, yielding dark green or in higher concentrations even black colour. Together with tin oxide and arsenic it gives emerald green glass. Chromium aventurine, in which aventurescence was achieved by growth of large parallel chromium(III)
oxide plates during cooling, was also made from glass
with added chromium oxide in amount above its solubility limit in glass.
The material can be
introduced into glass either in the form of chromic oxide or potassium
dichromate, the latter being a more convenient form. Potassium chromate is yellow and this colour can be imparted
to certain glasses. To produce emerald green glass in which a yellowish cast must
be avoided, the addition of tin oxide and arsenic is necessary.
Chromium is
associated mainly with the production of green glass, but other colours from
yellow through bluish-red, red to dark green or even black can be achieved in
combination with other oxides.
Cobalt is the most powerful blue colorant used in glassmaking
producing rich blues when used in potash containing mixes, but it can also give
shades of green when used with
iodides. The deepest of blues are
produced when used in glass containing potash.
Small concentrations of cobalt (0.025 to 0.1%) yield blue glass. The best
results are achieved when using glass containing potash. Very small amounts can be used for decolourizing. Addition of 2% to 3%
of copper
oxide produces a turquoise colour.
Copper is a very powerful
and versatile colouring agent when used in colouring glass. Copper greens
and blues are not difficult to
produce, although the behaviour of copper in a silicate melt can be
complicated. Copper was used most
profusely to produce green glass.
Pure metallic copper produces a very dark red, opaque
glass, which is sometimes used as a substitute for gold in the production of ruby-coloured glass. The art of using copper for ruby glass goes far back to ancient times
but even so using copper oxide to make ruby glass can be very difficult. Today
we find copper being used to produce turquoise
blue tones.
Didymium gives green colour (used in
UV filters) or lilac red.
Metallic gold, in very small concentrations (around 0.001%, or 10 ppm), produces a rich ruby coloured glass (gold ruby),
while lower concentrations produces a less intense red, often marketed as cranberry. The gold is used as gold chloride. The colour is caused by the size
and dispersion of gold particles.
Iron is a very useful and powerful colouring
agent even though it can be an undesirable impurity in making glass. Iron in
its metallic forms cannot
remain in equilibrium with glass and can be disregarded. But its ferrous and
ferric forms are of a great help in producing coloured glass. Iron(II) oxide may be added to glass resulting in bluish-green
glass.
Together with chromium it gives a richer green colour. In a reduced condition, it can be combined with chromium to produce a deep green glass.
Used with the
combination of carbon or other reducing agents, sulphur and iron sulphides give
a dark amber colour.
Lead compounds produce a range of yellows.
Manganese can be added in small amounts to remove the green tint given by
iron, or in higher concentrations to give glass an amethyst colour.
Manganese dioxide, which is black, is used to remove the green colour from the glass. This results in a
very slow chemical process where it is converted to sodium permanganate, a dark purple compound. Windows made with manganese dioxide solarise to change to a
colour which is lightly tinted violet because of this chemical change.
Manganese in its low
state of oxidation is colourless, but it is a powerful oxidising agent and can
be used for decolourising purposes to oxidise the iron content. Manganese is mainly used in the production of
purple glass resembling the colour of potassium permanganate crystals. The
purple colour is achieved by the trivalent manganese however in its divalent
state it only imparts a weak yellow or brown colour.
Nickel, depending on the concentration,
produces blue, or violet, or even black glass. It
is used in the production of smoky coloured glass
Lead crystal with added nickel acquires purplish colour. Nickel together with a
small amount of cobalt can be used for decolourizing of lead glass. When it is introduced into lead crystal it
gives a purplish colour, which compensates for a yellow tint produced by other
constituents.
Selenium, like manganese, can be used in small concentrations to decolourize
glass, or in higher concentrations to impart a reddish colour, caused by selenium nanoparticles dispersed in glass. It is a
very important agent to make pink
and red glass. When used together
with cadmium sulphide, it yields a brilliant red.
Silver compounds such as silver nitrate and silver
halides can produce a range of colours from orange-red to yellow. The way the
glass is heated and cooled can significantly affect the colours produced by
these compounds.
Sulphur, together with carbon and iron salts, is used to form iron polysulphides and produce amber glass ranging from yellowish to
almost black. With calcium it yields a deep yellow
colour.
In borosilicate
glasses rich in boron, sulphur imparts a blue colour. Cadmium sulphides,
which have a deep yellow colour, are often used in the production of glazes and
enamels.
Tin oxide with antimony and arsenic oxides produce an opaque white glass (milk glass), first used in Venice to produce an imitation porcelain.
Adding titanium produces yellowish-brown glass. Titanium, rarely used on its own, is more often employed to
intensify and brighten other colourizing additives.
Uranium (0.1% to 2%) can be added to give glass a fluorescent yellow or green colour. Uranium glass is typically not radioactive enough to be dangerous. It is
often referred to as Vaseline glass by USA collectors. When used with lead glass with very high
proportion of lead, produces a deep red colour.
Chart
A good visual chart of minerals and resulting colours is
here.
Influence of the glass-making process on colour.
It is not only the minerals that give the glass the colour; it is
combined with the way in which the materials are treated. The physical conditions under which the glass
is made also have an influence on the colour.
The main ones are:
1. The
temperature of the melt/batch
2. Temperature
of reheat during the working of the glass
3. The
temperature of the 'Lehr' (Annealing Oven)
4. Duration
of the melt/batch
5. Time
and temperature relationship at different stages in production
6. The
type of colorant being used
7. Concentration
of the colorant
8. Atmosphere
of the furnace
9. The
composition of the colorant within the glass composition, as is the case when
iron is added to glass. The type of iron oxide formed decides if the glass will
be blue or yellow
10.
The number of times the same glass is
melted. Repeated melting of the cullet will usually give a darker tone to the
finished piece
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