Sunday 17 December 2017
Figure Rolled Glass
The elaborate patterns found on figure rolled glass are produced by in a similar fashion to the rolled plate glass process except that the plate is cast between two moving rollers. The pattern is impressed upon the sheet by a printing roller which is brought down upon the glass as it leaves the main rolls while still soft. This glass shows a pattern in high relief. The glass is then annealed in a lehr.
Rolled Plate Glass
The glass is taken from the furnace in large iron ladles and poured on the cast-iron bed of a rolling-table. It is rolled into sheet by an iron roller. The rolled sheet is roughly trimmed while hot and soft and is pushed into the open mouth of a lehr, down which it is carried by a system of rollers. The method is similar to table glass, except in size and thickness.
Table Glass
This glass was produced by pouring the molten glass onto a metal table and sometimes rolling it. The glass thus produced was heavily textured by the reaction of the glass with the cold metal. Glass of this appearance is commercially produced and widely used today, under the name of cathedral glass, although it was not the type of glass favoured for stained glass in ancient cathedrals. It has been much used for lead lighting in churches in the 20th century.
Modern example of rolling glass. The operator is waiting to take the rolled sheet off the table |
Broad Sheet Glass
Broad sheet is a type of hand-blown glass. It is made by blowing molten glass into an elongated balloon shape with a blowpipe. Then, while the glass is still hot, the ends are cut off and the resulting cylinder is split with shears and flattened on an iron plate. (This is the forerunner of the cylinder process). The quality of broad sheet glass is not good, with many imperfections. Due to the relatively small sizes blown, broad sheet was typically made into leadlights.
According to the website of the London Crown Glass Company, broad sheet glass was first made in the UK in Sussex in 1226 C.E. This glass was of poor quality and fairly opaque. Manufacture slowly decreased and ceased by the early 16th Century. French glass makers and others were making broad sheet glass earlier than this.
According to the website of the London Crown Glass Company, broad sheet glass was first made in the UK in Sussex in 1226 C.E. This glass was of poor quality and fairly opaque. Manufacture slowly decreased and ceased by the early 16th Century. French glass makers and others were making broad sheet glass earlier than this.
Drawn Sheet Glass
Drawn sheet glass -sometimes called window glass or drawn glass – is made by dipping a leader into a vat of molten glass then pulling that leader straight up while a film of glass hardens just out of the vat. This film or ribbon is pulled up continuously and held by tractors on both edges while it cools. After 12 meters (40 feet) or so it is cut off the vertical ribbon and tipped down to be further cut.
This glass has thickness variations due to small temperature variations as it hardens. These variations cause slight distortions. You may still see this glass in older houses.
In more recent times, float glass replaced this process.
This glass has thickness variations due to small temperature variations as it hardens. These variations cause slight distortions. You may still see this glass in older houses.
In more recent times, float glass replaced this process.
Flashed Glass
Red pot metal glass is often undesirably dark in colour and very expensive. The method developed to produce red glass was called flashing. In this procedure, a semi-molten gather of coloured glass was dipped into a pot of clear glass. As the bubble became enlarged, the red glass formed a thin coating on the inside. The formed glass was cut, flattened and annealed as any other blown sheet.
There were a number of advantages to this technique. It allowed a variety in the depth of red – and other deep colours - ranging from very dark and almost opaque, and sometimes merely tinted. The other advantage was that the colour of double-layered glass could be engraved, abraded, or etched to show colourless glass underneath.
Other base colours are also used in making flashed glass, for example red flashed onto a pale green base. Also see this post on finding the flashed side of glass.
There still exist a number of glass factories, notably in Germany, USA, England, France, Poland and Russia which continue to produce high quality glass by traditional methods primarily for the restoration of ancient windows.
There were a number of advantages to this technique. It allowed a variety in the depth of red – and other deep colours - ranging from very dark and almost opaque, and sometimes merely tinted. The other advantage was that the colour of double-layered glass could be engraved, abraded, or etched to show colourless glass underneath.
Other base colours are also used in making flashed glass, for example red flashed onto a pale green base. Also see this post on finding the flashed side of glass.
There still exist a number of glass factories, notably in Germany, USA, England, France, Poland and Russia which continue to produce high quality glass by traditional methods primarily for the restoration of ancient windows.
Cylinder Glass
Cylinder blown sheet is a type of hand-blown window glass. Large cylinders are produced by swinging the cylinder in a trench or blown into a cylindrical iron mould. The glass is then allowed to cool before the cylinder is cut. The glass is then re-heated and flattened. The result is much larger panes and improved surface quality over broad sheet.
Cylinder blown sheet glass has been manufactured in France, Germany and Poland since the 18th Century, and continues today. It began to be manufactured in the UK in the mid 19th Century, although the only small remaining company has ceased manufacturing in the late 2010's.
Machine drawn cylinder sheet was the first mechanical method for "drawing" window glass. Cylinders of glass 12 m (40 feet) high are drawn vertically from a circular tank. The glass is then annealed and cut into 2 to 3 m (7 to 10 foot) cylinders. These are cut lengthways, reheated, and flattened. This process was invented in the USA in 1903. This type of glass was manufactured in the early 20th century (it was manufactured in the UK by Pilkington from 1910 to 1933).
Trench method |
Cylinder blown sheet glass has been manufactured in France, Germany and Poland since the 18th Century, and continues today. It began to be manufactured in the UK in the mid 19th Century, although the only small remaining company has ceased manufacturing in the late 2010's.
Mould method |
Machine drawn cylinder sheet was the first mechanical method for "drawing" window glass. Cylinders of glass 12 m (40 feet) high are drawn vertically from a circular tank. The glass is then annealed and cut into 2 to 3 m (7 to 10 foot) cylinders. These are cut lengthways, reheated, and flattened. This process was invented in the USA in 1903. This type of glass was manufactured in the early 20th century (it was manufactured in the UK by Pilkington from 1910 to 1933).
Crown Glass
Crown glass: The earliest style of glass window
The earliest method of glass window manufacture was the crown glass method. Hot blown glass was cut open opposite the pipe, then rapidly spun on a table before it could cool. Centrifugal force forced the hot globe of glass into a round, flat sheet. The sheet would then be broken off the pipe and cut into small sheets.
This glass could be made coloured and used for stained glass windows, but is typically associated with small paned windows of 16th and 17th century houses. The concentric, curving ripples are characteristic of this process.
At the center of a piece of crown glass, a thick remnant of the original blown bottle neck would remain. They are known as bull's eyes and are feature of late 19th century domestic lead lighting and are sometimes used with cathedral glass or quarry glass in church windows of that date. Optical distortions produced by the bullseye could be reduced by grinding the glass. The development of diamond pane windows was in part due to the fact that three regular diaper shaped panes could be conveniently cut from a piece of crown glass, with minimum waste and with minimum distortion.
This method for manufacturing flat glass panels was very expensive and could not be used to make large panes. It was replaced in the 19th century by the cylinder, sheet and rolled plate processes, but it is still used in traditional construction and restoration.
The earliest method of glass window manufacture was the crown glass method. Hot blown glass was cut open opposite the pipe, then rapidly spun on a table before it could cool. Centrifugal force forced the hot globe of glass into a round, flat sheet. The sheet would then be broken off the pipe and cut into small sheets.
This glass could be made coloured and used for stained glass windows, but is typically associated with small paned windows of 16th and 17th century houses. The concentric, curving ripples are characteristic of this process.
At the center of a piece of crown glass, a thick remnant of the original blown bottle neck would remain. They are known as bull's eyes and are feature of late 19th century domestic lead lighting and are sometimes used with cathedral glass or quarry glass in church windows of that date. Optical distortions produced by the bullseye could be reduced by grinding the glass. The development of diamond pane windows was in part due to the fact that three regular diaper shaped panes could be conveniently cut from a piece of crown glass, with minimum waste and with minimum distortion.
This method for manufacturing flat glass panels was very expensive and could not be used to make large panes. It was replaced in the 19th century by the cylinder, sheet and rolled plate processes, but it is still used in traditional construction and restoration.
Saturday 16 December 2017
Types of Glass
Glass
Types by manufacturing method
There are several ways of categorising glass and this overview of glass types looks at the way the glass is manufactured.
Crown
Glass
Crown
glass is the oldest method of producing sheet glass and continued to be used
until the 19th century. This method consisted of blowing a very
large bubble of glass. It was then spun
rapidly over a pit until the bubble collapsed into a disc that ranged from 1500mm
to 1800mm diameter.
This gave the thinnest
and least marked glass at the outer portion of the disc. The centre was the thickest and became known
as the bullseye. The glass was cut to
provide the best use of the disc. This
limited the size of panes to what could be cut from the disc. Diamond shapes were often cut from the
remainder and the central bullseye was used in less expensive glazing.
Corning Museum of Glass |
Cylinder Glass
Cylinder Glass is a handmade process that includes broad
sheet glass. It was widely used from the 17th to the 19th
century, and now is limited to a few manufacturers.
"Among the Glass Workers" Harry Fenn, 1871 |
An elongated bubble was blown. The top and bottom of the bubble are broken
off and annealed. Later the cylinder is
placed in the lehr for reheating. It is
scored and when it breaks open along the score, the glass is flattened. Characteristically,
it has a gradation of thickness with thicker edges where the top and bottom of
the cylinder were cut off.
From IdoStuff |
Flashed Glass
A
development in cylinder glass was to make the bubble of two colours, with the
dark colour gathered first and then encased in clear (or sometimes other pale
colours) and blown into a cylinder. This
made dense colours more transparent and enabled more detail through abrading
and etching.
Drawn Glass
Industrialisation
of glass production began with the development of drawn glass. This method of mass production of window
glass was invented and developed by Emile Fourcault in Belgium. Full scale
production began in the early 1900’s.
The
glass is drawn upwards from a vat of molten glass until it cools enough to be
cut into sheets at the top of the tower.
The process is subject to slight variations in thickness due to uneven
cooling and gravity. It enabled much larger panes of glass without the astragals
that are common in Georgian and later houses.
It was the most common method of producing window glass until the 1950’s.
Table Glass
Table glass is the process of putting molten glass onto a flat surface (the table) and rolling the glass flat. This has been used from the latter part of the 19th century to the present. It enables textures to be pressed into the glass from the rolling cylinder. It is easier to produce streaky and wispy glass by combining different colours on the table.
Table glass is the process of putting molten glass onto a flat surface (the table) and rolling the glass flat. This has been used from the latter part of the 19th century to the present. It enables textures to be pressed into the glass from the rolling cylinder. It is easier to produce streaky and wispy glass by combining different colours on the table.
Kokomo Glass Co. |
This can be done as single sheets or further
mechanised to roll out long ribbons of glass.
This is now mostly referred to as machine or hand rolled glass depending
on the amount of mechanisation.
Float Glass
The glass that we now rely on for large clear windows began with
the development of experiments by Alastair Pilkington and the company named after him. This
consisted of floating near molten glass on molten tin, hence the name, float
glass. This has been the standard method of glass for windows since the
1950’s.
Wednesday 13 December 2017
Annealing Large Pieces
A question was asked about how long to anneal a large piece
in relation to smaller pieces.
“Large” is in relation to the size of your kiln. A large piece for a 300mm square kiln would
be something 250mm square. For a kiln of
600mm square, 250mm would be a small piece.
It would contain a large piece of 500mm square as a large piece.
Large also relates to the distance from the edge of the
kiln. Although some kilns have much more
even heat than others, all have areas that are relatively cooler than
others. It is important to know where
those are, so that you can avoid those cool areas, by placing pieces to avoid
those spots or by altering the rate of cooling.
Bullseye has a tip on determining the relatively hot and cool temperatures are in your kiln.
In a rectangular kiln, there are usually cool spots in the
corners. Front opening kilns often have
cooler areas at the front of the kiln.
Knowing where these are will give you the information to know the area
of the kiln that has even heat. This
area tells you what the size of a large piece for your kiln is.
You can alleviate many of the differences in temperature in
your kiln by remembering that annealing is not simply a given temperature. It is a range.
The popular
perception is that the soak at the annealing temperature is all that needs to
be done to anneal. The soak at the annealing point
equalises the temperature throughout the glass. But it does not complete the
annealing. That continues through the gradual cooling of the glass down the
next 110°C.
Simply soaking longer at the annealing point, in the
circumstances where the temperature in not equal all over the glass, “locks”
the stresses of uneven temperatures into the glass. Instead, a gradual, slower than usual
annealing cool is required.
Of course, the rate of cooling is relative to the thickness
of the piece and the degree of temperature variation in your kiln. If you must utilise the area of the kiln with
slightly cooler temperatures, the minimum requirement would be to use a cooling
rate for a piece at least two times thicker than the thickness of the one you are annealing at
present.
But, to answer the original question - how long to anneal a large piece in relation to a small one of the same thickness? Given the precautions above, the size of the piece is not the major determining factor. The thickness of the piece is the important dimension when considering annealing.
But, to answer the original question - how long to anneal a large piece in relation to a small one of the same thickness? Given the precautions above, the size of the piece is not the major determining factor. The thickness of the piece is the important dimension when considering annealing.
Wednesday 6 December 2017
Flattening a Bubble
Sometimes a
large shallow bubble appears from under the glass. If it has not thinned there are some things
you can do.
First – do
not drill holes.
One
flattening method is to place the piece on 1mm to 3mm fibre paper and fire to a
slump temperature. The fibre paper of
these thicknesses will allow air out from under the glass. With sufficient time, the bubble will
flatten. It will take some time as the
weight of the bubble is slight.
Another
method is to fire upside down. It does
not matter whether the bubble is central or not. This will likely take less
time than the first method, but requires an additional firing. To use this method, place the glass upside
down on the shelf with an appropriate separator underneath. Take slowly to around 620C maximum for as
long as it takes to flatten. A low slumping temperature will reduce any marking
that later needs to be fire polished away.
When flat
and cool, clean and fire polish.
If the bubble has become large and thin,
this proposed process will not work. My suggestion for these is to avoid the
effort to do an unsatisfactory repair.
Instead use it for one of the many inventive process that use unsuccessful
projects.
Wednesday 29 November 2017
Glass Stuck to Element
First consideration you need to think about when you
discover glass stuck to an element is the nature of the metal of the
elements. Once fired, kiln elements
become brittle. This means that they are
likely to break if disturbed when cold.
So, you need to make sure you absolutely must do something to rescue the
kiln. It may be that you can just leave the
stuck glass alone. Where the glass is,
and how much of it, is stuck to the elements is important when considering what
to do.
Where
This brittleness of the elements means that the location of
the glass in relation to your firings needs to be considered. If the glass is on an element below your
normal firing position, you can think about just leaving it. This applies to glass stuck to the side
elements too, unless you are in the habit of firing very close to the side
elements. The heating elements of the kiln form an external layer of
oxidisation that protects the inner metal.
This means that small amounts of glass will not affect the operation of
the elements, nor your future pieces.
If the glass is stuck to top elements, you are likely to be
more concerned about future drips of the glass onto your future work. The glass is not likely to become hot enough
to detach or drip onto your work except at extended full fuse or casting
temperatures. This means that you can
observe the progress of any possible drip at each firing and only remove the
glass when it begins to begin to hang down from the element.
How Bad
How much glass is stuck to the element? Normally, if it is only a small amount, it
can be left. Ceramics kilns often have a
bit of glaze (a glass carrier of the colour) stuck to the elements and continue
to be fired for years without damage.
If there is a lot of glass stuck to the elements you will
need to remove most of it to avoid dripping onto future work.
Methods of Removing
In most cases where there are significant amounts of glass
stuck to the element, it is on the brick or fiber lining of the kiln too.
My recommendation is to heat the glass just a few
centimetres from where it is attached to the element. Use a hand-held blow
torch to do this. When the glass is red hot - enough to begin moving - you can
pull it away between the lining and the element with long handled tweezers. Do not attempt to pull it off the element
right away. You can later chip the glass
off the lining without damaging the element as the connection is separated.
As the element has begun to be warmed by the heat used to
separate the glass on the lining and the element, you can continue to warm the
element, moving the torch in a slow waving motion at least 10cm each side of
the stuck glass. When the glass and
element are red hot, you can begin to pull the glass off with long handled tweezers,
bit by bit. Keep re-heating the element
and glass as much as necessary so the temperature does not drop below cherry
red. This ensures the elements continue
to be flexible and will not break.
Of course, glass can be melted onto its kiln furniture and there are different considerations for those circumstances.
Wednesday 22 November 2017
Reducing a Bubble
A query
about reducing a bubble appeared on the internet recently. The bubble was from between the shelf and the
single layer glass. It was a relatively
shallow dome that did not seem to have thinned the glass much.
There is
quite a bit of information on reducing the incidence of bubbles. Among them are
my blog posts on the subject.
My view is
that large thin bubbles cannot be repaired successfully. As the bubble forms and grows, it pushes a
proportion of glass to the side. This
thickens the glass at the edge of the bubble.
Bursting the bubble and filling it with something (e.g., a piece of
glass, or frit) leaves marks at the thickened edge of the bubble, so it remains
a mark in the finished piece.
Method 1
However,
glass with a low uprising between the shelf and the glass can be successfully
repaired, if the uprising is low and the glass has not thinned. In the case
mentioned, the risk in simply re-firing right side up is that the bubble will
increase in size. The weight of the glass may not be sufficient to pull it down
except at higher temperatures – which is where the risk of increasing the size
of the bubble occurs.
Instead, flip
the piece over. Allow the weight of the glass to flatten the uprising.
You can use a much lower temperature to flatten the glass by taking advantage
of the weight of the whole piece. This
lower temperature means that you will not mark the surface so much as at higher
temperatures. Don’t worry if the uprising is not central, you do not have to balance
the glass on the point of the bubble for this process to work.
Take the
piece to 620°C maximum for as long as it takes to flatten. The rate of
advance should be slow – not more than 100°C per hour. This steady, slow input of temperature will
allow the glass to relax at lower temperatures than rapid increases.
You should
use the smoothest separator surface that you can – Thinfire or Papyros, or a
smoothed kiln wash. This together with a
low temperature will give minimum markings.
You must
observe the process from about 560°C to be able to stop the slump when the
piece is flat and advance to the annealing segment of the firing.
This post gives a further alternative. Use two shelves to compress the uprising
flat. Although the post is talking about thinning a pot melt, the principle is the same. Place fibre paper around the edge equal to the thickness of the glass piece and place another prepared kiln shelf on top. You do not need to invert the surface of the piece to do this. It may be that you will need a fire polish to remove any marks on top.
A plea
Do not
drill holes. Especially not in the case of a shallow bubble. The glass has not significantly thinned and
so can be rescued. Drilling a hole will
only leave an unwanted mark.
Wednesday 15 November 2017
Removing Fibre Paper Marks
We all at times take
short cuts or economies which lead to less than desirable outcomes. One of these is to piece together fibre
paper. Often the marks of the join –
which are always there – are just too obvious to leave. The question becomes whether the fibre paper
join marks can be eliminated.
Yes, there are at
least two ways to remove these marks.
One is to cold work the bottom with a flat lap or wet belt sander.
The other is to use
the kiln to re-fire the piece.
One method would be
to put fresh fibre paper or kiln wash on the shelf and fire. This will require temperatures near the full
fuse to achieve enough heat at the bottom of the glass to effect a significant
change in the markings.
My
suggestion for removing fibre paper marks - while it is still flat - is to fire upside down to fire polish
to get rid of the marks without much changing the desired final texture of what will be the top. This
is because the underside of the glass will not have the same heat effects as
the top side. This also has the advantage that you can observe when the marks
are eliminated.
When
fired, flip over, clean the piece well to remove any fibre or kiln wash, and
take to a quick fire polish to remove any marks - if necessary - caused by the upside-down
firing. This quick fire will be a slow
rise to ca. 600°C, and then quickly to the 740°C to 770°C range. This will cause the minimum change in the
surface of the piece.
You will
need to observe when both the evidence of the line disappears, and in a subsequent firing, when the final
top surface of the piece is fire polished.
Wednesday 8 November 2017
Kiln Wash Removal
There are a variety of ways to remove kiln wash. Many depend on whether the surface is flat,
smooth curves, angles or textured. Some
are applicable to both.
Flat surfaces are the easiest to deal with.
Abrasive methods
work well with a variety of tools.
They can range from large paint scrapers to smaller ones
with a Stanley blade inserted.
Coarse open mesh plaster board (dry wall) sanding sheets are
very useful. There are frames that you can fix them to, but sanding without the
frame works well too.
Using power tools to sand the shelf is not advisable. It is too easy to remove lots of material,
including the surface of the shelf – even the hard, ceramic ones. This leads to minor depressions in the shelf
and consequent bubble difficulties when firing.
Do not be tempted to sandblast as that will, almost
certainly, create small depressions in the surface of the shelf. Sand blasting is only possible on steel
moulds.
Wet
Wet methods are applicable if you are concerned about the
dustiness of the process. You can dampen
the kiln wash on the shelf and sand or scrape as above. You will create a paste or slurry in front of
the scraper which can be bagged and put in the waste.
You can also use a lot of water and the green scrubby
washing up pads. Unless you use a lot of
water, the kiln wash builds up in the scrubbing pads.
Some people use vinegar or chemicals such as lime away with
the water. Both are acids – lime away being much the strongest. I am sure these are used on the basis that
kiln wash is based on lime. However, the
material that makes the kiln wash stick to the shelf is china clay which is
barely affected by the chemicals. In addition,
the alumina hydrate is impervious to many chemicals available to kiln workers.
One drawback to using wet methods, is that the shelf is
wetted and needs drying before use. The
amount of water used in applying kiln wash is minor in relation to washing or
soaking the shelf to remove the kiln wash.
Do not be tempted to use pressure washers. Yes, they will
remove the kiln wash, but also leave little divots in the shelf which will
cause later problems.
Smooth curves
Kiln wash on moulds with smooth curves can be removed with
flexible sand papers or the plaster board sanding screens. Normally, the coating of kiln wash is thin
and does not require a lot of pressure or effort.
It is possible to dampen the kiln wash and take it off with
scrubbing pads. Make sure you do not use
excessive pressure. If you have wetted
your ceramic mould, you need to dry it very carefully, to avoid having the
mould break in the next firing. This is because
trapped water can turn to steam and the pressure will break the ceramic. It is
best to let the mould air dry for a week or so before putting it into the kiln
to thoroughly dry at about 90°C for a couple of hours.
Do not be tempted to use a pressure washer or water pick, as
both can erode the surface of a ceramic mould.
Curves with angles
Moulds with angled areas such as at the bottom or corners of
a rectangular mould need a flexible abrasive to clean out the angles. You can fold a piece of sand paper to use the
folded edge to do the final cleaning out of the angles.
The same can be done wet, but all the precautions about wet
removal of kiln wash need to be observed.
Textured
Textured moulds require much more care in cleaning the kiln
wash away, to avoid damaging the images and textures. The flat upper surfaces can be dealt with as
though it was a flat kiln shelf. The
indentations need to be more carefully treated.
Folded pieces of sand paper can be used to clean the delicate areas.
To ease cleaning of textured moulds it seems best to use
kiln washes without china clay as the binder.
These will brush out of the mould with a fibreglass bristled brush. It is now popular to use boron nitride - often sold as Zyp - as a coating for these moulds. This needs to be brushed out and renewed with each firing.
Removing kiln wash
from glass
Kiln wash stuck to the glass can present greater problems,
because you want to avoid marking the glass.
It is best to start with the least aggressive abrasive, such as a green
scrubby, and progress toward more aggressive and abrasive methods. When using the more aggressive methods, try
the finest grit first to see if that will work, as it makes for less work
cleaning up the grinding marks from the glass.
For flat glass, you can work with a succession of finer
loose grits, or a succession of finer diamond hand pads.
Flexible diamond impregnated sheets can be used for curved surfaces. Again, this requires a succession of finer grits to get to the polished stage.
You can use small hand held rotary tools with diamond and
felt pads to polish out stuck kiln wash.
This helps to remove some of the labour of polishing the glass.
Some people advocate the use of acids to remove the kiln
wash. However, you must remember that
glass is an alkaline material and acids will tend to mark the glass. Vinegar is a mild acid, but prolonged exposure
will etch the glass. Strong chemicals
such as lime away or etching cream or hydrofluoric are all strong acids and
will mark the glass after brief exposure to them.
Wednesday 1 November 2017
Holes vs. Elevation of Moulds
Drilling
holes and raising the mould are not the same. They achieve different things.
Drilling
holes allows air out from between the mould and the glass.
There are
some things you need to check about the vent holes in moulds.
Are the holes in the mould at last touchdown point(s)?
Sometimes
the vent holes in moulds are made at convenient points rather than at the
places where the glass will last touch the surface of the mould. On a
simple ball mould, a hole at the centre will be appropriate, as this is the
last place the glass will touch.
On a bowl with a square base, the last
places the glass will touch are the corners, so that is where the holes need to
be.
The vent holes in this could also be at the other two angles in addition to those at the top and bottom of the picture. |
Are there
holes in the side of mould to allow air out from under the mould?
If there
is one or more, there is no need to elevate the mould. The air will move
out from under the mould through the hole in the side. In general, moulds are
not so uniform on their base that they fit the shelf enough to seal the
displaced and expanding air underneath the mould. But you can be safe by
elevating the mould on pieces of 1mm or 3mm fibre paper.
This mould has side vents, although the holes at the base may be a little large. |
Are the
holes clear?
This is
more important. If the vent holes are not open due to kiln wash or other
things blocking the space, there will be no escape for the air. The vents
need to be checked on each firing to ensure they are open.
Does the
mould need holes at all?
There are shallow
slumpers and other simple moulds - such as a wave mould or any cylindrical mould
form - that do not need vent holes, either because they are so shallow, or
because the air can escape along the length of the mould.
More
information can be found in this and related blog posts.
Large thick
bubbles at the bottom of the glass
Not all
large bubbles at the bottom are the result of the lack of holes. Sometimes they are the results of too fast or
too high a firing. Some notes on this are given in this blog entry.
What does elevating the mould do?
The purpose of elevation is not allow air to escape from under the glass, although that may be a by-product. Elevating
the mould allows marginally more even cooling of the mould and glass if it is on a
thick kiln shelf. It will not create any problems, but you need to be careful
about how near the elements it will place the glass. The elevation does not need to be more than
25mm, just as for the shelf above the floor of the kiln.
Subscribe to:
Posts (Atom)