There are at least three ways to achieve flat bottoms to bowls without the use of external supports.
Using drop out rings will enable you to get a flat bottom of whatever diameter you wish depending on how long you let the aperture drop run.
You can put some dry kiln wash into the bottom of the mould, then firmly press it flat with a round piece of glass. You will need to make sure it is horizontal, so the use of a small round levelling bubble can make this easier.
Grind a flat spot on the bottom of the otherwise finished bowl. It is a good idea to use a two way leveling bubble while grinding. The round bubble is easier to use, while the two way bubbles – two leveling bubbles placed at right angles – are more accurate.
Thursday, 17 February 2011
Sunday, 13 February 2011
Getting Water to the Mini Work Surface of a Glastar G8
Sometimes the water does not rise to the mini work surface. There are a number of things to check. These, in order, are usually the reasons the water does not get to the Mini Work Surface.
• Ensure there is enough water in reservoir, right up to the overflow
• Ensure channel from impeller to the up tube is clear
• Ensure the up tube is clear
• Ensure tap at the top is clear
• Flush the feed lines with a syringe or bulb instrument
• look at the position of the impeller on the shaft. It can move up or down. Repositioning it can improve the flow of water to the top story.
• Ensure there is enough water in reservoir, right up to the overflow
• Ensure channel from impeller to the up tube is clear
• Ensure the up tube is clear
• Ensure tap at the top is clear
• Flush the feed lines with a syringe or bulb instrument
• look at the position of the impeller on the shaft. It can move up or down. Repositioning it can improve the flow of water to the top story.
Wednesday, 9 February 2011
Supports for round bottomed bowls
A number of useful moulds for slumping do not have flat bottoms. There are a number of possibilities to have the bowl sit firmly without grinding the bottom flat. Remember that you do not need to surround the whole bottom to give the bowl stability.
Some of these include things like:
• A rubber “O” ring, although they usually come in black only.
• Thin slices of wide-diameter tubing.
• Wok support rings.
• Plastic tubing with a small joining dowel allows you to make any size. You can then paint it with the appropriate colour.
• Macramé, embroidery and curtain rings can be suitable.
• You can make them using hole saws. Cut out the big ring first so you can use the pilot hole to line up the smaller hole. Then bevel the inside to fit the bowl.
• Use three bumpons on the bottom. Be sure that the bottom of the bowl is perfectly clean, dry and free from oils. Then use some weight pressing on the bumpons for a day or more so that they stick permanently. You can do this by turning the bowl upright and fill it with some heavy objects.
Some of these include things like:
• A rubber “O” ring, although they usually come in black only.
• Thin slices of wide-diameter tubing.
• Wok support rings.
• Plastic tubing with a small joining dowel allows you to make any size. You can then paint it with the appropriate colour.
• Macramé, embroidery and curtain rings can be suitable.
• You can make them using hole saws. Cut out the big ring first so you can use the pilot hole to line up the smaller hole. Then bevel the inside to fit the bowl.
• Use three bumpons on the bottom. Be sure that the bottom of the bowl is perfectly clean, dry and free from oils. Then use some weight pressing on the bumpons for a day or more so that they stick permanently. You can do this by turning the bowl upright and fill it with some heavy objects.
Tuesday, 1 February 2011
Firing schedules – what are they for?
Firing schedules or programs are the means of controlling the temperature rises, soaks and falls to accommodate the needs of the glass. They consist of a number of segments –or steps - each of which includes: rate of temperature rise, target temperature, and soak time. They vary according to the thickness of the glass and the forming and annealing needs of the glass. Read and understand the Bullseye Technical Note on the way glass behaves at different temperatures. This will give you a good understanding of what happens to the glass at the different temperature ranges and will help you design a suitable schedule for what you want to achieve.
To assist in visualising what the numbers in a kiln programmer do, you can graph the temperature changes indicated by the numbers in the controller. Visualised from the start of the schedule, it appears as a mountain with a steep cliff on the left rising to a ledge. There is then a steeper rise to the top where there is a small plateau. The mountain then has a very steep face on the right, falling to a broad ledge a bit lower than the one on the left. There is a long shallow slope to the right of the ledge that leads to a much steeper drop to the level again. This is the shape – with variations - that you are attempting to achieve in each program/schedule.
The variations have to do with the type of glass being used and thickness of the glass. These variations determine the amount of heat and the speed with which it is put into the glass. It sets the points at which any soaks are introduced to allow the glass and associated moulds or kiln furniture to equalise in heat or to allow air to ease from between sheets of glass. It sets the top temperature and determines the length of soak at that temperature. It controls the temperature fall to the annealing soak - to equalize the temperature throughout the glass. It then controls the rate of fall to anneal the glass – removing the stress and follows up with the fall to room temperature.
A description of each of these stages includes the heat rises and any soaks required, the temperature fall, annealing soak and cool, and the cool to room temperature.
Initial heating rise
In the simplest form, the initial heating is a relatively slow rise to a point about 50C above the annealing point. This allows the glass to gain heat without thermal shock. The initial heating may be achieved in several segments, depending on what you are doing. A thick piece, or one fired many times, might be taken up in a number of stages - initially very slowly (with or without soaks - also known as holds), and then at more rapid increases. A 6mm piece being slumped into a simple curve mould would need only one segment to the top temperature.
Another example of variations required would be a 6mm piece suspended over a cylindrical mould for a drape. My experience has shown that there is a requirement for multiple segments. This starts with an initial rise of 50C/hr to 100C with a 10min soak, then 100C/hr to 250C, 10 mins, then 150C/hr to 500C, with 10mins and finally 200C/hr to forming temperature - in the region of 630C - 677C with an appropriate soak to achieve the effect desired - peeking is required to determine the length of this soak. The point being that some circumstances require much more complicated arrangements. Here it is because the mould drains the heat away from the centre of the glass while the edges heat up.
Final heating rise
Above the annealing plus 50C temperature is when the rise can be much faster up to the working/top temperature. This speed should not be as fast as possible, because it has a number of drawbacks. The speed of this rise is influenced by the amount of heat work you wish to put into the glass. This in turn will influence the top temperature and length of soak at that point.
You most often want to insert a bubble squeeze in this rise to avoid large bubbles due to trapped air.
Cooling phases
The cooling phases are several: fast drop to annealing soak, annealing cool, cool to room temperature.
Fast drop
Once the soak at top temperature is finished the requirement is to cool the glass and kiln as fast as the kiln will allow. This is to avoid the devitrification that can occur in the range of 650C to 760C.
Annealing soak
This soak at the annealing point is to allow the glass to reach the same temperature throughout from side to side and top to bottom. The length of this soak will depend on the thickness of the glass. More information on annealing is here.
Annealing phase
The slow steady cool from the annealing point to about 55C below the annealing point is where the annealing of the glass is done. What is required is a gradual, but steady decline in temperature to allow the glass to reduce in temperature evenly throughout its thickness. This even reduction in temperature should continue to the strain point and slightly below. So this phase must not be done quickly. For a 6mm piece 80C/hour is usually adequate. More on the annealing phase is available here.
Cooling to room temperature
Cooling to room temperature should be done at an even rate, although faster than the annealing cool. Too fast a cool below the strain point can cause thermal shock and therefore breakage. Typically the cool to room temperature from the strain point can be two to three times faster than the annealing cool. It is a good idea to control this cool to at least 100C. If your kiln cools more slowly than this, it will not be using any electricity, but it does protect against too rapid cooling if you open the lid or door.
To assist in visualising what the numbers in a kiln programmer do, you can graph the temperature changes indicated by the numbers in the controller. Visualised from the start of the schedule, it appears as a mountain with a steep cliff on the left rising to a ledge. There is then a steeper rise to the top where there is a small plateau. The mountain then has a very steep face on the right, falling to a broad ledge a bit lower than the one on the left. There is a long shallow slope to the right of the ledge that leads to a much steeper drop to the level again. This is the shape – with variations - that you are attempting to achieve in each program/schedule.
The variations have to do with the type of glass being used and thickness of the glass. These variations determine the amount of heat and the speed with which it is put into the glass. It sets the points at which any soaks are introduced to allow the glass and associated moulds or kiln furniture to equalise in heat or to allow air to ease from between sheets of glass. It sets the top temperature and determines the length of soak at that temperature. It controls the temperature fall to the annealing soak - to equalize the temperature throughout the glass. It then controls the rate of fall to anneal the glass – removing the stress and follows up with the fall to room temperature.
A description of each of these stages includes the heat rises and any soaks required, the temperature fall, annealing soak and cool, and the cool to room temperature.
Initial heating rise
In the simplest form, the initial heating is a relatively slow rise to a point about 50C above the annealing point. This allows the glass to gain heat without thermal shock. The initial heating may be achieved in several segments, depending on what you are doing. A thick piece, or one fired many times, might be taken up in a number of stages - initially very slowly (with or without soaks - also known as holds), and then at more rapid increases. A 6mm piece being slumped into a simple curve mould would need only one segment to the top temperature.
Another example of variations required would be a 6mm piece suspended over a cylindrical mould for a drape. My experience has shown that there is a requirement for multiple segments. This starts with an initial rise of 50C/hr to 100C with a 10min soak, then 100C/hr to 250C, 10 mins, then 150C/hr to 500C, with 10mins and finally 200C/hr to forming temperature - in the region of 630C - 677C with an appropriate soak to achieve the effect desired - peeking is required to determine the length of this soak. The point being that some circumstances require much more complicated arrangements. Here it is because the mould drains the heat away from the centre of the glass while the edges heat up.
Final heating rise
Above the annealing plus 50C temperature is when the rise can be much faster up to the working/top temperature. This speed should not be as fast as possible, because it has a number of drawbacks. The speed of this rise is influenced by the amount of heat work you wish to put into the glass. This in turn will influence the top temperature and length of soak at that point.
You most often want to insert a bubble squeeze in this rise to avoid large bubbles due to trapped air.
Cooling phases
The cooling phases are several: fast drop to annealing soak, annealing cool, cool to room temperature.
Fast drop
Once the soak at top temperature is finished the requirement is to cool the glass and kiln as fast as the kiln will allow. This is to avoid the devitrification that can occur in the range of 650C to 760C.
Annealing soak
This soak at the annealing point is to allow the glass to reach the same temperature throughout from side to side and top to bottom. The length of this soak will depend on the thickness of the glass. More information on annealing is here.
Annealing phase
The slow steady cool from the annealing point to about 55C below the annealing point is where the annealing of the glass is done. What is required is a gradual, but steady decline in temperature to allow the glass to reduce in temperature evenly throughout its thickness. This even reduction in temperature should continue to the strain point and slightly below. So this phase must not be done quickly. For a 6mm piece 80C/hour is usually adequate. More on the annealing phase is available here.
Cooling to room temperature
Cooling to room temperature should be done at an even rate, although faster than the annealing cool. Too fast a cool below the strain point can cause thermal shock and therefore breakage. Typically the cool to room temperature from the strain point can be two to three times faster than the annealing cool. It is a good idea to control this cool to at least 100C. If your kiln cools more slowly than this, it will not be using any electricity, but it does protect against too rapid cooling if you open the lid or door.
Friday, 28 January 2011
Ceramic Mould Repairs
Most moulds have a long but limited life due to cracks appearing and accidents. However the life of moulds can be extended with repairs. Most moulds can be repaired, unless shattered.
Cracks can often simply be ignored. If the glass is not getting marked by the crack, then you can keep using it until it widens or goes completely across the mould. If you feel the need to protect the mould before it completely fails, you can add a layer of cement on the back of the mould to support it.
The cement can be a high temperature product like “Sairset” or any other high temperature ceramic cement. The one I like is cement fondu. It comes as a powder – often from sculptural suppliers – which you mix with water to a paste. Wet the mould well to ensure it does not pull the water out of the cement, causing it to fail. Then apply the cement liberally to the back of the mould over the crack.
If you feel the need, you can fill the crack from the front also. Again insure the mould is wet and then press the cement into the crack. Wipe the excess cement off immediately or it will stick leaving blemishes on the mould. Use a wet cloth to do this. You can smooth the filler by using a wet finger to run along the filled crack. These notes apply to which ever kind of cement you use.
Divots or little chips from the surface of the mould can be ignored, if there is no effect on the glass at your operating temperatures. If they need to be filled, you can use a temporary patch by making a paste of batt/kiln wash and smoothing it over the divot. This will last a couple of firings probably. A more permanent repair is to use cements. Prepare as above and smooth into the depression. When cured, particular attention will need to be paid to getting a good coating of batt wash, because the cement surface will reject the water carrying the powder more than the ceramic surface does.
If the mould has broken you will need to stick it all back together. Do not attempt to smooth the edges, they are needed to make as close a match as possible to each other. The rough edges provide a key to location as well. Soak the mould pieces very well. Prepare the cement and apply a little to one edge of the matching pieces. Press together firmly and then apply a backing of the cement as for a crack. Clean off the face of the mould with a wet sponge or cloth until it is smooth and level with the working surface of the mould. Bind this as tightly as the shape permits and leave for several days.
Curing requirements
When using refractory cements, it is best if you can give it a wet cure for a day. This is often easiest to achieve by putting the cemented mould in a plastic bag. After the one day wet cure, it needs to dry for several days. Finally, it needs to have a permanent cure by firing to a temperature of about 25C above the operating temperature for the mould.
Cracks can often simply be ignored. If the glass is not getting marked by the crack, then you can keep using it until it widens or goes completely across the mould. If you feel the need to protect the mould before it completely fails, you can add a layer of cement on the back of the mould to support it.
The cement can be a high temperature product like “Sairset” or any other high temperature ceramic cement. The one I like is cement fondu. It comes as a powder – often from sculptural suppliers – which you mix with water to a paste. Wet the mould well to ensure it does not pull the water out of the cement, causing it to fail. Then apply the cement liberally to the back of the mould over the crack.
If you feel the need, you can fill the crack from the front also. Again insure the mould is wet and then press the cement into the crack. Wipe the excess cement off immediately or it will stick leaving blemishes on the mould. Use a wet cloth to do this. You can smooth the filler by using a wet finger to run along the filled crack. These notes apply to which ever kind of cement you use.
Divots or little chips from the surface of the mould can be ignored, if there is no effect on the glass at your operating temperatures. If they need to be filled, you can use a temporary patch by making a paste of batt/kiln wash and smoothing it over the divot. This will last a couple of firings probably. A more permanent repair is to use cements. Prepare as above and smooth into the depression. When cured, particular attention will need to be paid to getting a good coating of batt wash, because the cement surface will reject the water carrying the powder more than the ceramic surface does.
If the mould has broken you will need to stick it all back together. Do not attempt to smooth the edges, they are needed to make as close a match as possible to each other. The rough edges provide a key to location as well. Soak the mould pieces very well. Prepare the cement and apply a little to one edge of the matching pieces. Press together firmly and then apply a backing of the cement as for a crack. Clean off the face of the mould with a wet sponge or cloth until it is smooth and level with the working surface of the mould. Bind this as tightly as the shape permits and leave for several days.
Curing requirements
When using refractory cements, it is best if you can give it a wet cure for a day. This is often easiest to achieve by putting the cemented mould in a plastic bag. After the one day wet cure, it needs to dry for several days. Finally, it needs to have a permanent cure by firing to a temperature of about 25C above the operating temperature for the mould.
Monday, 24 January 2011
Making Powder Designs Crisp
Tidying up powder designs is often a time consuming process using brushes. One way of cleaning the edges of lines and the bottoms of furrows in the midst of the powder designs is to use a modified keyboard vacuum.
I use a Miele vacuum sweeper –it has a variable suction - with a keyboard cleaning attachment.
I have modified the finest nozzle by putting the end of a ball point pen in it and filling in the remainder of the rectangle with blutac or a similar material. Turn the suction on the vacuum down to minimum and you can be very accurate about the amount of powder you remove to achieve crisp lines.
Sunday, 16 January 2011
Creating your own Iridescence
Often iridised surface details are created by using iridised sheet glass and then masking and sandblasting off the unwanted portions. But you can make your own iridised surface detail much more cheaply by using pearlised mica powder.
One way to apply the mica in areas of detail is to make a stencil from stiff card and sift a smooth relatively thin layer of mica onto the area of glass you want to be iridised.
A second is to mix the mica and powdered clear glass in equal amounts and sift that onto the glass through the stencil. This can help more of the mica to stick to the surface.
A third is to sift clear powder on first and then a coat of mica. This works less well for me than the other two.
It does not matter if you put too much mica on, as the excess will not stick and can be brushed back into your container for future use. The firing should be at full fuse temperatures to allow the mica to sink into the surface of the glass. When you have poured the excess powder off you are left with an iridised surface where the mica has sunk into the glass. You can, of course, use any of the coloured micas for this purpose.
One way to apply the mica in areas of detail is to make a stencil from stiff card and sift a smooth relatively thin layer of mica onto the area of glass you want to be iridised.
A second is to mix the mica and powdered clear glass in equal amounts and sift that onto the glass through the stencil. This can help more of the mica to stick to the surface.
A third is to sift clear powder on first and then a coat of mica. This works less well for me than the other two.
It does not matter if you put too much mica on, as the excess will not stick and can be brushed back into your container for future use. The firing should be at full fuse temperatures to allow the mica to sink into the surface of the glass. When you have poured the excess powder off you are left with an iridised surface where the mica has sunk into the glass. You can, of course, use any of the coloured micas for this purpose.
Wednesday, 12 January 2011
Cutting Bottles
Cutting bottles seems to have a fascination for many people. There seem to be three methods – heat and cold, scoring, sawing.
There are various ways to apply heat and cold to assist with breaking the bottles.
- A string tied around the bottle and soaked in a flammable liquid is a common way to apply heat. As soon as the flame has gone out, you immerse the bottle in cold water; the temperature differential should crack the glass where the string was.
- Filling the bottle with water to the level where the break is wanted and then applying gentle heat with a torch flame at that level should promote a crack.
- Alternatively, the bottle can be scored and put into the freezer for a while and then into hot water.
Scoring is the common method to start a crack.
- This is followed by tapping from inside the bottle with tools from a purchased kit or home-made tappers – a metal ball on the end of a curved piece of metal.
- The score line can also be the preliminary step in the application of heat or cold.
These provide the cleanest edges to the cuts. However there is quite a high failure rate using these methods.
Sawing is method that provides a higher success rate, but is wet, and leaves rough edges to the cut, requiring further cold work.
- Band saws designed for glass can be used, but usually do not have a high enough throat to allow the thickness of the bottle to pass through.
- Most tile saws cut from underneath, so rotating the bottle can lead to a cut completely around. This requires a lot of skill to do free hand, so you need a jig to keep the bottle at right angles to the blade and the bottom the same distance from the blade while rotating the bottle all the way around.
There are various ways to apply heat and cold to assist with breaking the bottles.
- A string tied around the bottle and soaked in a flammable liquid is a common way to apply heat. As soon as the flame has gone out, you immerse the bottle in cold water; the temperature differential should crack the glass where the string was.
- Filling the bottle with water to the level where the break is wanted and then applying gentle heat with a torch flame at that level should promote a crack.
- Alternatively, the bottle can be scored and put into the freezer for a while and then into hot water.
Scoring is the common method to start a crack.
- This is followed by tapping from inside the bottle with tools from a purchased kit or home-made tappers – a metal ball on the end of a curved piece of metal.
- The score line can also be the preliminary step in the application of heat or cold.
These provide the cleanest edges to the cuts. However there is quite a high failure rate using these methods.
Sawing is method that provides a higher success rate, but is wet, and leaves rough edges to the cut, requiring further cold work.
- Band saws designed for glass can be used, but usually do not have a high enough throat to allow the thickness of the bottle to pass through.
- Most tile saws cut from underneath, so rotating the bottle can lead to a cut completely around. This requires a lot of skill to do free hand, so you need a jig to keep the bottle at right angles to the blade and the bottom the same distance from the blade while rotating the bottle all the way around.
Saturday, 8 January 2011
Float Glass Characteristics in Relation to Kiln Forming
A reported 90% of the world's flat glass is produced by the float glass process invented in the 1950's by Sir Alastair Pilkington of Pilkington Glass. Molten glass is “floated” onto one end of a molten tin bath. The glass is supported by the tin, and levels out as it spreads along the bath, giving a smooth face to both sides. The glass cools as it travels over the molten tin and leaves the tin bath in a continuous ribbon. The glass is then annealed by cooling in a lehr. The finished product has near-perfect parallel surfaces.
An important characteristic of the glass is that a very small amount of the tin is embedded into the glass on the side it touched. The tin side is easier to make into a mirror and is softer and easier to scratch. It also becomes apparent when compressed.
Float glass is produced in standard metric thicknesses of 2, 3, 4, 5, 6, 8, 10, 12, 15, 19 and 22 mm. Molten glass floating on tin in a nitrogen/hydrogen atmosphere will spread out to a thickness of about 6 mm and stop due to surface tension. Thinner glass is made by stretching the glass while it floats on the tin and cools. Similarly, thicker glass is pushed back and not permitted to expand as it cools on the tin.
The heat characteristics of Float glass depend in large part on which company manufactures the glass being used, so the temperature characteristics are given in ranges.
The softening point is around 760C
The annealing point is around 560—540C
The strain point is around 525-505C. The strain point being the temperature below which no further annealing can occur, but the glass can still be thermally shocked below this range.
The characteristic of float glass having a molecular level of tin left on the “tin side” but not the “air side” is important to distinguish. If any forming of the glass is planed after fusing, the tin side in compression will show a “tin bloom” similar to devitrification.
The fact that there are many manufacturers of float glass means that they are not all made to the same specifications. It is not advisable to fuse float glass from different suppliers in kiln forming, so the best advice is to fuse only from one sheet for each piece.
Due to the robustness of float glass, it can be fired with a quicker initial temperature rise than glasses formulated for kiln forming. The down side is that it devitrifies very easily and very badly. Rarely can you perform more than two firings before the devitrification begins to become troublesome.
An important characteristic of the glass is that a very small amount of the tin is embedded into the glass on the side it touched. The tin side is easier to make into a mirror and is softer and easier to scratch. It also becomes apparent when compressed.
Float glass is produced in standard metric thicknesses of 2, 3, 4, 5, 6, 8, 10, 12, 15, 19 and 22 mm. Molten glass floating on tin in a nitrogen/hydrogen atmosphere will spread out to a thickness of about 6 mm and stop due to surface tension. Thinner glass is made by stretching the glass while it floats on the tin and cools. Similarly, thicker glass is pushed back and not permitted to expand as it cools on the tin.
The heat characteristics of Float glass depend in large part on which company manufactures the glass being used, so the temperature characteristics are given in ranges.
The softening point is around 760C
The annealing point is around 560—540C
The strain point is around 525-505C. The strain point being the temperature below which no further annealing can occur, but the glass can still be thermally shocked below this range.
The characteristic of float glass having a molecular level of tin left on the “tin side” but not the “air side” is important to distinguish. If any forming of the glass is planed after fusing, the tin side in compression will show a “tin bloom” similar to devitrification.
The fact that there are many manufacturers of float glass means that they are not all made to the same specifications. It is not advisable to fuse float glass from different suppliers in kiln forming, so the best advice is to fuse only from one sheet for each piece.
Due to the robustness of float glass, it can be fired with a quicker initial temperature rise than glasses formulated for kiln forming. The down side is that it devitrifies very easily and very badly. Rarely can you perform more than two firings before the devitrification begins to become troublesome.
Thursday, 30 December 2010
Application of devitrification solutions
Smooth and complete coverage of the piece is the aim when applying devitrification solutions. A soft brush, an air brush, a mouth atomiser are some of the ways to apply the solution. Some even use a sponge - all these application methods will do the job.
It is a pretty simple process, but requires concentration to ensure the piece is evenly covered. If it isn't, there will be areas of devitrification left after firing.
It is a pretty simple process, but requires concentration to ensure the piece is evenly covered. If it isn't, there will be areas of devitrification left after firing.
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