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.
Wednesday, 24 November 2010
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 lungs
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 lungs
Test
Finally, you should test your mould with glass that has little value, before committing you best efforts to the mould.
Subscribe to:
Posts (Atom)