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.
Saturday 8 January 2011
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. So, in the case of our work, annealing needs to be performed over a range.
The annealing point soak has the purpose of allowing all the glass to be the same temperature from top to bottom, and side to side. The annealing occurs during the slow cool past the lower strain point – usually about 50C below the annealing 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. So, in the case of our work, annealing needs to be performed over a range.
The annealing point soak has the purpose of allowing all the glass to be the same temperature from top to bottom, and side to side. The annealing occurs during the slow cool past the lower strain point – usually about 50C below the annealing 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.
Friday 10 December 2010
Firing for a Matte Finish
Glass can be fired to take on a satin appearance that is both appealing to the eye and pleasing to touch.
The first step toward the matte finish is to sandblast the piece after fusing, then fire to a temperature between 600C and 675C. A short soak - or no soak at all - is all that is needed.
The exact temperature needed depends on a number of factors, including:
• The specific glass being used. A soft glass such as black generally needs to be fired to a lower temperature than glasses that do not absorb the heat so easily. Every colour and type of glass will behave a bit differently, so experimentation and record keeping is critical.
• The grit and type of sandblasting medium. Generally, a grit from 120 to 200 is preferred, with aluminium oxide performing a bit better than silicon carbide – which can often lead toward some devitrification.
• The particular kiln being used. Your kiln is a bit different from any other one. Start with a temperature in the middle of the 600-650C range and adjust depending on the results you achieve.
• The finish you want will vary with only a few degrees difference. This means that you have to observe the firing. Make sure you keep good records of the specific firing schedule used so that you can make adjustments if needed for future firings.
Some variations can provide distinctive elements to the finished piece.
• Masking certain elements before sandblasting can provide contrasts of texture within the piece.
• Firing at a lower temperature for longer can give the results you want, without any additional marking on the bottom of the piece.
• To keep the matte texture, any subsequent slumping of the piece should be done at as low a temperature as possible.
The first step toward the matte finish is to sandblast the piece after fusing, then fire to a temperature between 600C and 675C. A short soak - or no soak at all - is all that is needed.
The exact temperature needed depends on a number of factors, including:
• The specific glass being used. A soft glass such as black generally needs to be fired to a lower temperature than glasses that do not absorb the heat so easily. Every colour and type of glass will behave a bit differently, so experimentation and record keeping is critical.
• The grit and type of sandblasting medium. Generally, a grit from 120 to 200 is preferred, with aluminium oxide performing a bit better than silicon carbide – which can often lead toward some devitrification.
• The particular kiln being used. Your kiln is a bit different from any other one. Start with a temperature in the middle of the 600-650C range and adjust depending on the results you achieve.
• The finish you want will vary with only a few degrees difference. This means that you have to observe the firing. Make sure you keep good records of the specific firing schedule used so that you can make adjustments if needed for future firings.
Some variations can provide distinctive elements to the finished piece.
• Masking certain elements before sandblasting can provide contrasts of texture within the piece.
• Firing at a lower temperature for longer can give the results you want, without any additional marking on the bottom of the piece.
• To keep the matte texture, any subsequent slumping of the piece should be done at as low a temperature as possible.
Monday 6 December 2010
Preventing Chipping When Using a Tile Saw to Cut Glass
One of the most common problems in using a tile saw to cut glass is the tendency for the saw to chip the edge of the glass as it completes the cut. This occurs when the blade of the saw has less glass to cut through. Excessive and uneven pressure and the lack of support cause this break-out.
It's possible to improve the quality of the cut by slowing down and pushing the glass through the blade more gently, but this seldom solves the problem completely. Pushing equally on both sides of the cut is also important to minimise the break-out.
One solution that does work is to provide support for the end of the bar. This adopts a woodworking method for preventing splintering at the ends of cuts.
Use a scrap length of pattern bar or other thick glass. Place it against the glass being cut. As the blade emerges from the glass being cut, hold the two pieces firmly together and continue cutting. The blade should immediately engage the second piece of glass. Once the saw blade entirely clears the first piece, you can turn off the saw and remove a chip-free slice from the pattern bar.
You'll need to trim off the ends of the scrap piece from time to time, but you can use the scrap over and over until it becomes too small to do the job.
This works best with a tile saw where the blade is below the cutting surface. When you use an overhead saw, the breakout is much rarer.
It's possible to improve the quality of the cut by slowing down and pushing the glass through the blade more gently, but this seldom solves the problem completely. Pushing equally on both sides of the cut is also important to minimise the break-out.
One solution that does work is to provide support for the end of the bar. This adopts a woodworking method for preventing splintering at the ends of cuts.
Use a scrap length of pattern bar or other thick glass. Place it against the glass being cut. As the blade emerges from the glass being cut, hold the two pieces firmly together and continue cutting. The blade should immediately engage the second piece of glass. Once the saw blade entirely clears the first piece, you can turn off the saw and remove a chip-free slice from the pattern bar.
You'll need to trim off the ends of the scrap piece from time to time, but you can use the scrap over and over until it becomes too small to do the job.
This works best with a tile saw where the blade is below the cutting surface. When you use an overhead saw, the breakout is much rarer.
Thursday 2 December 2010
Dams for Pattern Bars
Once you have cut and arranged the glass for your pattern bar, you need to dam the bars in the kiln to prevent the glass spreading.
The materials required for forming the sides of the dam can be made from anything that is rigid and can withstand the heat of the kiln, e.g., cut up kiln shelves, rigidised fibre board, vermiculite board. The material being used to dam must be over 13mm and preferably around 25mm thick. It should be capable of standing vertically on its edge without support. Cut the dam material into strips at least as long as the pattern bars you're damming, and at least as wide as the bars are tall.
You also need fibre paper for lining the edges of the dam and keeping the glass from sticking to the dam. Cut the strips of fibre paper to line the walls of the dam and keep the glass from sticking to the dam material when you fire the kiln. Three millimetre fibre paper works best.
The width of the fibre paper should be 3mm narrower than the pattern bars are high. By cutting the strips shorter than the pattern bars you allow the bars to round perfectly on top and help prevent needling.
The fibre paper should go around all sides without gaps. They should have straight edges so the glass of the pattern bar does not leak between or underneath the fibre paper. The use of iridised glass on bottom and sides will provide a smooth release from the fibre paper and is a second option. It is also possible to line the fibre paper with thinfire paper to provide a smooth release, although it is more time consuming than using the iridised side of glass against the fibre. But do not combine thinfire and iridised glass. There is a reaction that leaves holes and craters in the glass.
The materials required for forming the sides of the dam can be made from anything that is rigid and can withstand the heat of the kiln, e.g., cut up kiln shelves, rigidised fibre board, vermiculite board. The material being used to dam must be over 13mm and preferably around 25mm thick. It should be capable of standing vertically on its edge without support. Cut the dam material into strips at least as long as the pattern bars you're damming, and at least as wide as the bars are tall.
You also need fibre paper for lining the edges of the dam and keeping the glass from sticking to the dam. Cut the strips of fibre paper to line the walls of the dam and keep the glass from sticking to the dam material when you fire the kiln. Three millimetre fibre paper works best.
The width of the fibre paper should be 3mm narrower than the pattern bars are high. By cutting the strips shorter than the pattern bars you allow the bars to round perfectly on top and help prevent needling.
The fibre paper should go around all sides without gaps. They should have straight edges so the glass of the pattern bar does not leak between or underneath the fibre paper. The use of iridised glass on bottom and sides will provide a smooth release from the fibre paper and is a second option. It is also possible to line the fibre paper with thinfire paper to provide a smooth release, although it is more time consuming than using the iridised side of glass against the fibre. But do not combine thinfire and iridised glass. There is a reaction that leaves holes and craters in the glass.
Sunday 28 November 2010
Pattern Bar Box
Making a box for a pattern bar design that involves frit or lots of small pieces is necessary and simple.
Let's assume you want to make a pattern bar that's 25mm by 25mm by 200mm long. Start by cutting three strips of glass, each 25mm wide and 200mm long. Also cut two 25mm squares of glass. You can use any colour, but remember that the colour you choose will make up the outside of your pattern bar.
Assemble the three strips and two squares to make a small box, with one piece on the bottom and the others attached to form sides and ends. To do the attaching, use a hot melt glue gun.
The advantage of using hot melt glue to make a pattern bar box is that after the box is assembled, it can be filled with frit and other scrap outside the kiln, then easily carried into the kiln and dammed as usual. The glue will burn off during the firing. You can finish the box with a final strip of glass laid on top, but this isn't essential.
You then dam the box as for any other pattern bar.
Let's assume you want to make a pattern bar that's 25mm by 25mm by 200mm long. Start by cutting three strips of glass, each 25mm wide and 200mm long. Also cut two 25mm squares of glass. You can use any colour, but remember that the colour you choose will make up the outside of your pattern bar.
Assemble the three strips and two squares to make a small box, with one piece on the bottom and the others attached to form sides and ends. To do the attaching, use a hot melt glue gun.
The advantage of using hot melt glue to make a pattern bar box is that after the box is assembled, it can be filled with frit and other scrap outside the kiln, then easily carried into the kiln and dammed as usual. The glue will burn off during the firing. You can finish the box with a final strip of glass laid on top, but this isn't essential.
You then dam the box as for any other pattern bar.
Wednesday 24 November 2010
Designing a Pattern Bar
Assuming that you are not going to just dump your scrap glass in a random pattern to form a pattern bar, you need to spend some time designing it.
The simplest kind of bar is composed of strips of glass which are stacked or assembled in the kiln, but there are many other more elaborate configurations.
Because of the additional annealing time required for larger and thicker items, most pattern bars range from 1" by 1" to no larger than 2" by 2". The length of the pattern bar can be any length, up to the maximum that will fit in your kiln.
The design process begins by thinking about the cross section of the bar. This is what will appear when cut and assembled. As a simple exercise, assume you are making a diamond pattern in the bar. You can draw this out using 3mm as the thickness (or 1.5mm if you are using thin glass). Rough out the pattern and then begin using 3mm as the grid. Remember that you will need to cut your strips 4mm or wider to obtain a clean break. As you plan it out you will see that you need one length at the base one half of the space remaining after you have laid down the first, central piece for the diamond. The next layer will have two strips for the diamond, giving a requirement for one strip to fill the space between the two for the diamond shape and two strips each one half the remaining space. This process goes on until the area is filled.
The simplest kind of bar is composed of strips of glass which are stacked or assembled in the kiln, but there are many other more elaborate configurations.
Because of the additional annealing time required for larger and thicker items, most pattern bars range from 1" by 1" to no larger than 2" by 2". The length of the pattern bar can be any length, up to the maximum that will fit in your kiln.
The design process begins by thinking about the cross section of the bar. This is what will appear when cut and assembled. As a simple exercise, assume you are making a diamond pattern in the bar. You can draw this out using 3mm as the thickness (or 1.5mm if you are using thin glass). Rough out the pattern and then begin using 3mm as the grid. Remember that you will need to cut your strips 4mm or wider to obtain a clean break. As you plan it out you will see that you need one length at the base one half of the space remaining after you have laid down the first, central piece for the diamond. The next layer will have two strips for the diamond, giving a requirement for one strip to fill the space between the two for the diamond shape and two strips each one half the remaining space. This process goes on until the area is filled.
Saturday 20 November 2010
Pattern Bars
A pattern bar is a thick bundle of glass that has been fused together. These can be in the shape of a rectangle, or can be a thick pot melt – whether a disc or a rectangle. The length of the individual bars can be as long as your kiln allows, but needs to be practical to handle when cutting.
The basic steps involved in making a pattern bar include deciding on a design –whether controlled or random, cutting glass for the bar, assembling the cut glass into the desired bar shape, then firing to a full fuse. Once fired, pattern bars can be cut into slices with a saw - tile, glass, lapidary, or stone – which uses water for cooling and lubrication. The individual slices are then assembled and re-fused to make bowls, platters, and similar shapes. They can also be used as accents in any number of applications.
There is a caution about using pattern bar pieces. As the glass in the bars has been fired to a relatively high temperature, some of the characteristics may have changed. So you need to do a compatibility test before doing the main piece.
Designing Pattern Bars
Boxes for Pattern Bars
Dams for Pattern Bars
The basic steps involved in making a pattern bar include deciding on a design –whether controlled or random, cutting glass for the bar, assembling the cut glass into the desired bar shape, then firing to a full fuse. Once fired, pattern bars can be cut into slices with a saw - tile, glass, lapidary, or stone – which uses water for cooling and lubrication. The individual slices are then assembled and re-fused to make bowls, platters, and similar shapes. They can also be used as accents in any number of applications.
There is a caution about using pattern bar pieces. As the glass in the bars has been fired to a relatively high temperature, some of the characteristics may have changed. So you need to do a compatibility test before doing the main piece.
Designing Pattern Bars
Boxes for Pattern Bars
Dams for Pattern Bars
Tuesday 16 November 2010
Float Glass in the Kiln
An important characteristic of float 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 than the air side. 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. There are short wave UV light sources to help determine this. The tin side gives a whiter glow than the air side. If any forming of the glass is planed after fusing, the tin side needs to be on the side being stretched, as when in compression the tin side will show a “tin bloom” similar to devitrification.
If the tin side is down on both sheets, and it is slumped into a mould there will be no tin bloom because the tin layer is stretched. If the tin side is up on both sheets and it is slumped into a mould there will be tin bloom because the tin layer is compressed. If you have placed the tin sides together, or on both the top and bottom, one of the tin surfaces will be in compression and so will show tin bloom. This is often mistaken for devitrification, and no amount of any devitrification solution will help.
A borax solution can help with the devitrification on float glass in some circumstances. It is not a perfect solution. This is because tin bloom and devitrification are often not distinguished correctly. But a high level of cleanliness and polishing the glass until squeaky clean is the best start.
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 515-495C. The strain point being the temperature below which no further annealing occurs, although the glass can still be thermally shocked below this range.
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.
All window glass now seems to be referred to as float glass. However, the float glass process was invented in the 1950’s. Prior to that time, window glass was drawn. Float glass can use more iron in its composition, because it does not have to be drawn up out of a molten vat of glass as the drawn glass did and still does. Float glass is formulated to be stiffer at forming temperatures, whereas the drawn glass has to be flexible due to the mechanical stresses it is put under during the drawing. Except for low iron glass, the float glass has a distinct blue green colour when viewed through the edge. Drawn glass has a variation in thickness and is much paler when viewed through the edge. These visual differences can help distinguish the two kinds of glass, but are not foolproof.
More information on the general characteristics of float glass can be found here.
If the tin side is down on both sheets, and it is slumped into a mould there will be no tin bloom because the tin layer is stretched. If the tin side is up on both sheets and it is slumped into a mould there will be tin bloom because the tin layer is compressed. If you have placed the tin sides together, or on both the top and bottom, one of the tin surfaces will be in compression and so will show tin bloom. This is often mistaken for devitrification, and no amount of any devitrification solution will help.
A borax solution can help with the devitrification on float glass in some circumstances. It is not a perfect solution. This is because tin bloom and devitrification are often not distinguished correctly. But a high level of cleanliness and polishing the glass until squeaky clean is the best start.
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 515-495C. The strain point being the temperature below which no further annealing occurs, although the glass can still be thermally shocked below this range.
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.
All window glass now seems to be referred to as float glass. However, the float glass process was invented in the 1950’s. Prior to that time, window glass was drawn. Float glass can use more iron in its composition, because it does not have to be drawn up out of a molten vat of glass as the drawn glass did and still does. Float glass is formulated to be stiffer at forming temperatures, whereas the drawn glass has to be flexible due to the mechanical stresses it is put under during the drawing. Except for low iron glass, the float glass has a distinct blue green colour when viewed through the edge. Drawn glass has a variation in thickness and is much paler when viewed through the edge. These visual differences can help distinguish the two kinds of glass, but are not foolproof.
More information on the general characteristics of float glass can be found here.
Friday 12 November 2010
Stainless Steel Moulds
Stainless steel is sometimes called the almost the perfect mould material. It is lightweight, difficult to deform, and durable for a very many firings. Simple bowl forms are relatively inexpensive to buy — you can even use cheap stainless steel bowls. All you need to do is drill three or four small 1.5mm holes in the bottom for air to escape.
It often is a good idea to fire the mould to working temperature (say 650C) before attempting to kiln wash the form. This burns off the protective oils from the steel. Alternatively, sandblast the mould to clean it and give a small tooth for the kiln wash.
Stainless steel moulds do need to be covered with kiln wash. This is difficult to do when the mould is at room temperature, but it can easily be accomplished by heating the mould to around 150C, then brushing or spraying on the kiln wash while the mould is hot. The water in the wash will evaporate rapidly, leaving the protective elements behind. If you heat the mould too high the water will boil off, leaving gaps.
Also, it’s important to realize that steel contracts more than the glass. This is the opposite of ceramic, which contracts less than the glass. As a result, slumping on the outside of a steep stainless steel form generally works better than slumping on the inside.
Still, you can get away with slumping inside gentle bowl forms; just make certain it’s well covered in kiln wash. A sprinkle of a little kiln wash powder inside can also be considered. Be aware that slumping inside deeper forms may not work.
It often is a good idea to fire the mould to working temperature (say 650C) before attempting to kiln wash the form. This burns off the protective oils from the steel. Alternatively, sandblast the mould to clean it and give a small tooth for the kiln wash.
Stainless steel moulds do need to be covered with kiln wash. This is difficult to do when the mould is at room temperature, but it can easily be accomplished by heating the mould to around 150C, then brushing or spraying on the kiln wash while the mould is hot. The water in the wash will evaporate rapidly, leaving the protective elements behind. If you heat the mould too high the water will boil off, leaving gaps.
Also, it’s important to realize that steel contracts more than the glass. This is the opposite of ceramic, which contracts less than the glass. As a result, slumping on the outside of a steep stainless steel form generally works better than slumping on the inside.
Still, you can get away with slumping inside gentle bowl forms; just make certain it’s well covered in kiln wash. A sprinkle of a little kiln wash powder inside can also be considered. Be aware that slumping inside deeper forms may not work.
Monday 8 November 2010
Draping over steel
Steel absorbs heat much faster than glass, so the glass suspended on the steel is cooler than the suspended perimeter during the heating and cooling cycles of the firing. This does not apply to slumping when the glass is supported on the edges, as so little of the glass is touching the mould at the start.
The fact that the steel “bleeds” the supported glass of heat while the unsupported parts heat up, requires slow heating with or without periodic soaks on the way up to ensure the glass and steel are the same temperature up to about 540C or the upper strain point of the glass.
I tend to be very cautious, and for 6mm pieces heat up approximately like the following:
100C/hr to 100C, soak 20
150C/hr to 200C, soak 20
200/hr to process temperature
When cooling, the steel is in closer contact with the glass, no special considerations are needed, so the normal annealing soak and cool are used.
The fact that the steel “bleeds” the supported glass of heat while the unsupported parts heat up, requires slow heating with or without periodic soaks on the way up to ensure the glass and steel are the same temperature up to about 540C or the upper strain point of the glass.
I tend to be very cautious, and for 6mm pieces heat up approximately like the following:
100C/hr to 100C, soak 20
150C/hr to 200C, soak 20
200/hr to process temperature
When cooling, the steel is in closer contact with the glass, no special considerations are needed, so the normal annealing soak and cool are used.
Thursday 4 November 2010
Removing Glass from Kiln Shelf
Care is needed when removing glass that is stuck to the shelf. You need to protect your hands with thick gloves, as any slip will cut your hands deeply.
For mullite and other ceramic shelves you can use a variety of tools:
If there is a small amount of glass in one or more spots, you can use a scraper or lead knife. The wider the blade is, the less chance there is of creating a big divot beside the stuck glass.
If the stuck glass is large or thick, you can use a hammer and chisel. Care is needed to avoid creating a bigger hole in the shelf. Use very shallow angle, almost parallel to the surface of the shelf to chip out the glass.
Diamond hand pads are useful to get the last bits smoothed out. You need to be careful of creating a low spot by working only in a concentrated area. One way of avoiding that is to use a slurry of grit and grind with large sheet of float glass. The area being covered is large and so reduces the danger of creating low spots. Remember you can get away with smoothing the shelf, not all the glass has to come out of the shelf. If the bits of glass are only small, it will not reduce the life of the shelf much, although glass tends to be corrosive to kiln brick and ceramic that it is in contact with.
If removing the glass has taken a significant amount of the shelf surface off, you can repair it. A temporary repair is to fill the divot with dry kiln wash and smooth it with a plasterer’s float or a piece of float glass. A more permanent repair is to mix a small amount of cement fondue with or without a little vermiculite. Smooth this level with the rest of the shelf while wet, as it is very hard after curing, which occurs at about 600C. If the mix is of cement fondue only, it will tend to reject the kiln wash, as it is more dense than the shelf.
Removing glass from fibre shelves in some ways is much easier, as the shelf material comes away with the glass. This does mean that repairs are always necessary. This can be done with a temporary fill of dry kiln wash or more permanently with a mix of 1 part cement fondue to about 6-7 parts vermiculite. This makes a less dense filler than cement fondue on its own, which would be too hard for fitting with the fibre shelf.
For mullite and other ceramic shelves you can use a variety of tools:
If there is a small amount of glass in one or more spots, you can use a scraper or lead knife. The wider the blade is, the less chance there is of creating a big divot beside the stuck glass.
If the stuck glass is large or thick, you can use a hammer and chisel. Care is needed to avoid creating a bigger hole in the shelf. Use very shallow angle, almost parallel to the surface of the shelf to chip out the glass.
Diamond hand pads are useful to get the last bits smoothed out. You need to be careful of creating a low spot by working only in a concentrated area. One way of avoiding that is to use a slurry of grit and grind with large sheet of float glass. The area being covered is large and so reduces the danger of creating low spots. Remember you can get away with smoothing the shelf, not all the glass has to come out of the shelf. If the bits of glass are only small, it will not reduce the life of the shelf much, although glass tends to be corrosive to kiln brick and ceramic that it is in contact with.
If removing the glass has taken a significant amount of the shelf surface off, you can repair it. A temporary repair is to fill the divot with dry kiln wash and smooth it with a plasterer’s float or a piece of float glass. A more permanent repair is to mix a small amount of cement fondue with or without a little vermiculite. Smooth this level with the rest of the shelf while wet, as it is very hard after curing, which occurs at about 600C. If the mix is of cement fondue only, it will tend to reject the kiln wash, as it is more dense than the shelf.
Removing glass from fibre shelves in some ways is much easier, as the shelf material comes away with the glass. This does mean that repairs are always necessary. This can be done with a temporary fill of dry kiln wash or more permanently with a mix of 1 part cement fondue to about 6-7 parts vermiculite. This makes a less dense filler than cement fondue on its own, which would be too hard for fitting with the fibre shelf.
Tuesday 26 October 2010
Silver Foil Inclusions
Silver foil is better for inclusions than silver leaf as there is more substance and so less likelihood that it will burn away.
Sterling silver – not that common in foils – will darken easily on exposure to temperature and air.
Silver foil will also react with some glass colours. A good guide to this can be obtained from Bullseye’s chart on glass interactions. This shows which glasses react with silver. To minimize the reactions, minimize the amount of time spent above 600C.
Only a short bubble squeeze is required with foil as it is not so stiff that it will resist the weight of the glass on top, so creating bubbles. A short squeeze to allow the air out is still a good idea.
Sterling silver – not that common in foils – will darken easily on exposure to temperature and air.
Silver foil will also react with some glass colours. A good guide to this can be obtained from Bullseye’s chart on glass interactions. This shows which glasses react with silver. To minimize the reactions, minimize the amount of time spent above 600C.
Only a short bubble squeeze is required with foil as it is not so stiff that it will resist the weight of the glass on top, so creating bubbles. A short squeeze to allow the air out is still a good idea.
Friday 22 October 2010
Venting moulds
Raising the mould from the shelf to provide ways for the air to get out of the mould is as important as providing holes in the mould itself. Often people recommend placing the mould on kiln furniture, but it is very easy to have a number of pieces fibre paper to stack in three places under the edge of the mould to provide space for the air to be expelled from the kiln by the dropping glass. Only a little space is required for the air to escape.
Monday 18 October 2010
Vertical Kiln Formed Holes
For vertical holes in frit-cast reliefs you can fill a drinking straw with plaster, cut while still wet and build the frit or cullet around it.
Already fused pieces can have the holes made much neater and smoother by using the above method in pre drilled holes. Another method is to wrap a thin strip of fibre paper around a pencil or end of a paint brush. Then push this circle of fibre paper into the hole. If this is the same height or a little less than the glass, it provides a clean fire polished hole, if the glass is taken to the high end of fire polishing temperatures.
Already fused pieces can have the holes made much neater and smoother by using the above method in pre drilled holes. Another method is to wrap a thin strip of fibre paper around a pencil or end of a paint brush. Then push this circle of fibre paper into the hole. If this is the same height or a little less than the glass, it provides a clean fire polished hole, if the glass is taken to the high end of fire polishing temperatures.
Thursday 14 October 2010
Found Moulds
Found moulds are often ceramic bisque or greenware, sometimes glazed and fired for house or other final use. Many other materials, usually metals can also act as moulds. This article will address ceramic materials.
Shape
Pick out a mold that is not too complicated, detailed or deep. A shallow bowl or plate with a rim is ideal. When choosing bisque ware to use as a slumping mold, avoid complicated and deep shapes. Do not choose molds with intricate carvings or patterns for slumping. Those shapes would be better for frit casting. Instead, choose shapes with a rim or with gentle curves rather than steep slopes.
Slumping or Draping
Glass has a higher expansion and contraction rate than ceramics. This means that any draping has to be done over gently curved ceramic materials. So the general advice is to avoid draping over ceramics. If you do drape anyway, it is advisable to cover the ceramic with fibre paper in addition to the kiln wash.
Vent holes
You need to drill holes in the proposed mould to allow the air to escape as the glass slumps. You might want to see what holes are drilled on similar plates online. The holes should be small – about 1.5mm. Much smaller and they will get clogged up with kiln wash; much larger and they will mark the glass. The drilling should be from the inside to avoid any break out into the moulding surface.
Greenware is easy to drill, so don’t press hard; let the drill bit do the work.
Ceramic forms that have been glazed require more care to start the hole. The surface is so smooth the drill bit will tend to skitter around. You can place a bit of tape where you want to drill to reduce the movement of the drill. You can also start the hole by using a masonry drill bit and rotate it by hand at the point you want to drill. This will create a “divot” in the glaze to hold your drill. It is also easier to drill, if you sandblast the glazed surface first. This will give a bit of “tooth” for the bit to grip.
You should drill the hole(s) at the last place the glass will fall. In a completely round bottom you drill at the centre. If there is a right angle or steep part near the bottom of the form, that is the last place the glass will touch and so is where the holes should be drilled. Three, spaced equally apart, should be enough.
Preparation for use
Take the greenware and clean it with a mild abrasive pad or nylons to eliminate the mold marks and scratches on the piece. Have any greenware fired to at least bisque temperatures at a ceramics studio. Explain what you are doing and the working temperature. The ceramic does need to be fired high enough to be robust. The ceramics people can give you information on the performance of the ceramic when fired to various temperatures. The bisque mold must be kiln washed before use.
If you are using an already glazed form, you need to remove or roughen up the glaze enough to take the kiln wash. A sandblaster does a good and quick job, but it can be done by hand with wet and dry sandpapers. The process should be done wet to keep any dust from the glaze (a vitreous power) getting into your l
Test
Finally, you should test your mould with glass that has little value, before committing you best efforts to the mould.
Shape
Pick out a mold that is not too complicated, detailed or deep. A shallow bowl or plate with a rim is ideal. When choosing bisque ware to use as a slumping mold, avoid complicated and deep shapes. Do not choose molds with intricate carvings or patterns for slumping. Those shapes would be better for frit casting. Instead, choose shapes with a rim or with gentle curves rather than steep slopes.
Slumping or Draping
Glass has a higher expansion and contraction rate than ceramics. This means that any draping has to be done over gently curved ceramic materials. So the general advice is to avoid draping over ceramics. If you do drape anyway, it is advisable to cover the ceramic with fibre paper in addition to the kiln wash.
Vent holes
You need to drill holes in the proposed mould to allow the air to escape as the glass slumps. You might want to see what holes are drilled on similar plates online. The holes should be small – about 1.5mm. Much smaller and they will get clogged up with kiln wash; much larger and they will mark the glass. The drilling should be from the inside to avoid any break out into the moulding surface.
Greenware is easy to drill, so don’t press hard; let the drill bit do the work.
Ceramic forms that have been glazed require more care to start the hole. The surface is so smooth the drill bit will tend to skitter around. You can place a bit of tape where you want to drill to reduce the movement of the drill. You can also start the hole by using a masonry drill bit and rotate it by hand at the point you want to drill. This will create a “divot” in the glaze to hold your drill. It is also easier to drill, if you sandblast the glazed surface first. This will give a bit of “tooth” for the bit to grip.
You should drill the hole(s) at the last place the glass will fall. In a completely round bottom you drill at the centre. If there is a right angle or steep part near the bottom of the form, that is the last place the glass will touch and so is where the holes should be drilled. Three, spaced equally apart, should be enough.
Preparation for use
Take the greenware and clean it with a mild abrasive pad or nylons to eliminate the mold marks and scratches on the piece. Have any greenware fired to at least bisque temperatures at a ceramics studio. Explain what you are doing and the working temperature. The ceramic does need to be fired high enough to be robust. The ceramics people can give you information on the performance of the ceramic when fired to various temperatures. The bisque mold must be kiln washed before use.
If you are using an already glazed form, you need to remove or roughen up the glaze enough to take the kiln wash. A sandblaster does a good and quick job, but it can be done by hand with wet and dry sandpapers. The process should be done wet to keep any dust from the glaze (a vitreous power) getting into your l
Test
Finally, you should test your mould with glass that has little value, before committing you best efforts to the mould.
Sunday 10 October 2010
Thick Glass Firing
by Tony Roberts
My schedule for 50mm (2 inch) Pilkington’s Opticlear in a top-heated flatbed kiln is:
0 to 600C - 6C per hour rise - takes 6 days (if you start with a solid slab) (I start with smaller pieces, so can raise the temp much faster)
600C/hr to your soak temp - as fast as you like (I go to 840C and hold 4hours)
Soak temp to 565C - drop as fast as you can, then hold for 14hours
Anneal: drop at 0.75C per hour to 365C - this takes 11 days
Then drop at 1.5C per hour to 300C - another 2 days
Then drop to 60C at 4C per hour - 2 days and a half
A total of 16 and a half days
My schedule for 50mm (2 inch) Pilkington’s Opticlear in a top-heated flatbed kiln is:
0 to 600C - 6C per hour rise - takes 6 days (if you start with a solid slab) (I start with smaller pieces, so can raise the temp much faster)
600C/hr to your soak temp - as fast as you like (I go to 840C and hold 4hours)
Soak temp to 565C - drop as fast as you can, then hold for 14hours
Anneal: drop at 0.75C per hour to 365C - this takes 11 days
Then drop at 1.5C per hour to 300C - another 2 days
Then drop to 60C at 4C per hour - 2 days and a half
A total of 16 and a half days
Thursday 7 October 2010
Glass Selection
I have produced some notes on some of the elements in the selection of glass.
Here are the links:
Glass Density
Clarity
Advancing and Receeding Colours
Light and Dark
Colour Combinations
Here are the links:
Glass Density
Clarity
Advancing and Receeding Colours
Light and Dark
Colour Combinations
Wednesday 6 October 2010
Choosing Glass for a Harmonious Appearance, 3
Different colours, of course, have different appearances. The most commonly known one is the hot/cool colour combinations. This still applies when dealing with opalescent glasses, where reflection is the dominant experience of the colour.
But in glass where there is quite a bit of light transmission, the receding and advancing colours are not exactly the same as in painting and opalescent glass. The greatest separation comes with intense red (close) and intense blue (distant). In some circumstances these can be experienced as apparently being in different planes.
There are a few distinct advancing and receding colours, but most are much more subtle and are not all as expected from the experience of reflected colour. Clear, for example appears nearer than a strong blue. It is up to each person as to how far they wish to take these combinations.
Those who do want to investigate, should go to a place where they can view windows with small pieces and a variety of colour in strong light. They can then record which colours appear to “float” above others, or recede.
But in glass where there is quite a bit of light transmission, the receding and advancing colours are not exactly the same as in painting and opalescent glass. The greatest separation comes with intense red (close) and intense blue (distant). In some circumstances these can be experienced as apparently being in different planes.
There are a few distinct advancing and receding colours, but most are much more subtle and are not all as expected from the experience of reflected colour. Clear, for example appears nearer than a strong blue. It is up to each person as to how far they wish to take these combinations.
Those who do want to investigate, should go to a place where they can view windows with small pieces and a variety of colour in strong light. They can then record which colours appear to “float” above others, or recede.
Saturday 2 October 2010
Choosing Glass for a Harmonious Appearance, 2
Clarity of colour
When considering the representation of distance or depth you need to look for glass that is less pure. The colours that are muted or have a touch of white, blue or grey will provide a good representation of distance. The pure colours will appear more brilliant among the more muted colours.
This is where glass samples can be most useful. By holding them up to the light, you can see the effects one glass has on another and how one colour will appear among the others.
When considering the representation of distance or depth you need to look for glass that is less pure. The colours that are muted or have a touch of white, blue or grey will provide a good representation of distance. The pure colours will appear more brilliant among the more muted colours.
This is where glass samples can be most useful. By holding them up to the light, you can see the effects one glass has on another and how one colour will appear among the others.
Tuesday 28 September 2010
Choosing Glass for a Harmonious Appearance - 1
There are at least two major elements in choosing glass: density and clarity. A third is the “hot/cool” effect of colours. The appropriate combination of these elements leads to a panel with bright or hot spots where you want them. You can create a dramatic image or a more restrained one with more gradual gradations of light without obvious bright or dark areas.
Density
Density relates to the amount of light the glass allows through. Clearly black is the most dense glass – allowing no light through. In general, glass can be divided into opalescent and transparent.
Opalescent glasses range from the very dense opaque to less dense translucent glass.
Transparent glasses have a variety of densities too, although almost always less dense than opalescent glass. The density of transparent glasses relates to the intensity of the colour and the texture of the glass.
Colour intensity
The intensity of the colour is related to the amount of light allowed through. The intensity is directly related to the saturation of the colour. A further effect on colour intensity is the thickness of the glass. If you look at a handmade sheet of glass with different thickness on one end to another end, you can see the gradation of the colour and the amount of light that comes through.
Glass Textures
The texture of the glass affects the density of the glass. A smooth glass will have less density as the light passes through without dispersion. As the glass becomes more textured, the light is more dispersed and so appears more dense.
Density
Density relates to the amount of light the glass allows through. Clearly black is the most dense glass – allowing no light through. In general, glass can be divided into opalescent and transparent.
Opalescent glasses range from the very dense opaque to less dense translucent glass.
Transparent glasses have a variety of densities too, although almost always less dense than opalescent glass. The density of transparent glasses relates to the intensity of the colour and the texture of the glass.
Colour intensity
The intensity of the colour is related to the amount of light allowed through. The intensity is directly related to the saturation of the colour. A further effect on colour intensity is the thickness of the glass. If you look at a handmade sheet of glass with different thickness on one end to another end, you can see the gradation of the colour and the amount of light that comes through.
Glass Textures
The texture of the glass affects the density of the glass. A smooth glass will have less density as the light passes through without dispersion. As the glass becomes more textured, the light is more dispersed and so appears more dense.
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