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.
Friday, 28 January 2011
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.
Sunday, 26 December 2010
Longevity of Borax as a devitrification agent
It is true that Borax is water soluable. However, the borax has done its job by preventing the devitrification, so it does not matter whether it has or has not disolved, nor whether it is inside or outside.
Borax as a flux for paint in excessive quantities has the effect of corrosion on the paint or enamel it is mixed with. It is not actual corrosion, just that its effects are like that. The borax expands when wet. The expansion is very little, but over time "pops" off the paint - the time scale is 50-80 years. This happens on the inside of windows where the paint is. So it is not an inside/outside issue, just one of moisture.
But this irrelevant in kiln forming applications when attempting to prevent devitrification, or even to correct existing devitrification. The subsequent possible disappearance of the borax will not matter to the appearance of the piece. It has been reported that borax covered sushi dishes going through dishwasher cycles in a restaurant for years show no devitrification after the presumed disappearance of the borax. In fact, the proprietary devitrification solutions that contain lead would not be applicable in this food containing situation.
Other references to devitrification are:
Homemade devitrification solution
Description of devitrification
Temperature range
Borax as a flux for paint in excessive quantities has the effect of corrosion on the paint or enamel it is mixed with. It is not actual corrosion, just that its effects are like that. The borax expands when wet. The expansion is very little, but over time "pops" off the paint - the time scale is 50-80 years. This happens on the inside of windows where the paint is. So it is not an inside/outside issue, just one of moisture.
But this irrelevant in kiln forming applications when attempting to prevent devitrification, or even to correct existing devitrification. The subsequent possible disappearance of the borax will not matter to the appearance of the piece. It has been reported that borax covered sushi dishes going through dishwasher cycles in a restaurant for years show no devitrification after the presumed disappearance of the borax. In fact, the proprietary devitrification solutions that contain lead would not be applicable in this food containing situation.
Other references to devitrification are:
Homemade devitrification solution
Description of devitrification
Temperature range
Wednesday, 22 December 2010
Devitrification Prone Glasses
"Are there specific glasses that are more prone to devitrification, and knowing that, what steps can you take to try to avoid it?"
Glasses that are formulated and tested compatible for kiln forming are less likely to devitrify than other art glasses.
Opalescent glasses even if tested compatible for kiln forming are more likely to devitrify than their compatible transparent counterparts.
Yes, you can fuse some of the transparent glass made by a single manufacturer - Spectrum transparent and especially the water glasses are most often compatible within certain limits. But you will find that the edges show devitrification almost always. When using glass untested for compatibility, capping with clear glass often helps in reducing or preventing devitrification, as the clears seem less prone to devitrification than coloured glasses
You can clean very well and hope for the best, or you can clean and then use a devitrification agent - normally a flux or low firing glass in suspension - and spray or brush it on. If it is one of the low firing glasses in suspension, make sure you put it on before taking it to the kiln, as it will stick to other things when fired.
Another method is to avoid staying in the devitrification range of temperatures very long - both during temperature rise and cooling.
A description of what devitrification is
The temperature range in which devitrification occurs
A homemade devitrification solution
Glasses that are formulated and tested compatible for kiln forming are less likely to devitrify than other art glasses.
Opalescent glasses even if tested compatible for kiln forming are more likely to devitrify than their compatible transparent counterparts.
Yes, you can fuse some of the transparent glass made by a single manufacturer - Spectrum transparent and especially the water glasses are most often compatible within certain limits. But you will find that the edges show devitrification almost always. When using glass untested for compatibility, capping with clear glass often helps in reducing or preventing devitrification, as the clears seem less prone to devitrification than coloured glasses
You can clean very well and hope for the best, or you can clean and then use a devitrification agent - normally a flux or low firing glass in suspension - and spray or brush it on. If it is one of the low firing glasses in suspension, make sure you put it on before taking it to the kiln, as it will stick to other things when fired.
Another method is to avoid staying in the devitrification range of temperatures very long - both during temperature rise and cooling.
A description of what devitrification is
The temperature range in which devitrification occurs
A homemade devitrification solution
Saturday, 18 December 2010
Annealing
Stress is induced into glass during cooling through the outsides of the glass cooling more quickly than the interior. This contraction causes residual stress. Annealing is the process to relieve that stress. The annealing soak temperature is determined by a number of factors, of which coefficient of expansion, viscosity, exposed surface, and thickness are some. “The relief from stress happens because of a process of viscous flow. At the annealing point it can take place within a few minutes whilst at the lower annealing temperature…. It can take a few hours.” (Dictionary of Glass, Charles Bray, p.27)
The above statement is applicable to glass of a single colour from one manufacturer. When combining colours in kiln forming, the colours absorb and give off heat at different rates and so you need to allow more time for the annealing – relieving of heat induced stress – to occur.
The annealing soak has the purpose of allowing all the glass to be the same temperature (within 5°C) from top to bottom, and side to side. The annealing occurs during the slow cool past the lower strain point. The manufacturers give annealing and strain points for their glass. These should be observed, rather than anything pre-programmed into your kiln’s controller.
Note that the stress of incompatible glass cannot be relieved by annealing.
Also, each time the glass is taken to a temperature above the annealing point, it must be annealed again. There is no short cut to this.
There are more notes on annealing here.
The above statement is applicable to glass of a single colour from one manufacturer. When combining colours in kiln forming, the colours absorb and give off heat at different rates and so you need to allow more time for the annealing – relieving of heat induced stress – to occur.
The annealing soak has the purpose of allowing all the glass to be the same temperature (within 5°C) from top to bottom, and side to side. The annealing occurs during the slow cool past the lower strain point. The manufacturers give annealing and strain points for their glass. These should be observed, rather than anything pre-programmed into your kiln’s controller.
Note that the stress of incompatible glass cannot be relieved by annealing.
Also, each time the glass is taken to a temperature above the annealing point, it must be annealed again. There is no short cut to this.
There are more notes on annealing here.
Tuesday, 14 December 2010
Achieving a Matte Finish by Cold Working
Although sandblasting and then firing a piece can achieve a matte finish, there are several other ways to improve the quality of the final finish.
One of these involves the use of manual sanding after sandblasting in order to smooth out uneven spots and achieve a better final finish.
• Start with a 400 mesh diamond hand pad. It shouldn't be necessary to start out with a lower mesh (coarser) pad.
• Alternatively use wet/dry silicon carbide sandpaper. A combination of 400 mesh paper, followed by 600 mesh paper will work well.
• If you're using sandpaper, place a sponge between the paper and your hand for improved comfort and to improve the evenness of the final finish.
• An alternative to hand sanding is to use a electric sander or grinder, but be careful with the pressure you use, as it is possible to grind into the surface with a rapidly spinning surface. You also need to keep the surface wet to avoid heat build-ups.
You can also use a lathe with appropriately shaped wheels to give decorative effects to the object.
One of these involves the use of manual sanding after sandblasting in order to smooth out uneven spots and achieve a better final finish.
• Start with a 400 mesh diamond hand pad. It shouldn't be necessary to start out with a lower mesh (coarser) pad.
• Alternatively use wet/dry silicon carbide sandpaper. A combination of 400 mesh paper, followed by 600 mesh paper will work well.
• If you're using sandpaper, place a sponge between the paper and your hand for improved comfort and to improve the evenness of the final finish.
• An alternative to hand sanding is to use a electric sander or grinder, but be careful with the pressure you use, as it is possible to grind into the surface with a rapidly spinning surface. You also need to keep the surface wet to avoid heat build-ups.
You can also use a lathe with appropriately shaped wheels to give decorative effects to the object.
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