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.
Thursday, 30 December 2010
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.
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.
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