Inclusions of metals can be achieved with care. Copper is a very good metal, as it is soft, even though its expansion characteristics are very different from glass. This note provides some things you might consider when planning to include copper in your fused pieces.
The copper sheet should be stiff, but not thick. If the metal can be incised with a scribe and maintain that through gentle burnishing, it is suitably thick. The usual problem is that the copper is too thick rather than too thin. Copper leaf can be very faint if a single layer is used. Placing several layers of leaf improves the colour, but often provides wrinkles. In summary, the requirement is to get a thickness of copper that will retain its structure, but not be so thick and stiff as to hold the glass up during the fusing process.
Do not use the copper foil as used for stained glass applications. The adhesive backing produces a black colour from the adhesive and many bubbles - sometimes a single large one.
Copper can provide several colours.
Copper sheet normally turns burgundy colour when oxidised. This means that there is enough air reaching the copper to oxidise it to deep copper red. This most normally happens, because a lot of air can contact the metal during the extensive bubble squeeze usually given to inclusions.
To keep the copper colour, clean the metal well metal well with steel wool or a pot scrubber. If you use steel wool, wash and polish dry the metal before fusing. Reduction of air contact with the metal helps to retain the copper colour. There are two methods I have used. Addition of a glass flux like borax or other devitrification spray will help prevent the air getting to the surface. Another method of avoiding oxidisation, is to cover the copper with clear powdered frit, as well as the surrounding glass.
In certain circumstances you can get the blue green verdigris typical of copper in the environment. This is an extent of oxidisation that is between the clean copper coloured metal and the burgundy colour of extensive oxidisation. The key seems to be be a combination of restricted air supply, shorter bubble squeezes and lower temperatures. Experimentation is required to achieve this consistently.
The spaces under and over the copper give the opportunity for bubbles to form.
This means that the copper needs to be as flat as possible for one thing. Burnishing the copper can have a good effect on reducing the undulations in the copper. Thinner copper is easier to make flat than thicker. If you can stamp a shape from the copper with a stamper designed for card making, it is a good indication that it will burnish flat. Thicker copper sheet holds the glass up long enough in the temperature rise during the bubble squeeze to retain air around the metal. This remains the case even after burnishing to be as flat as possible.
The second element that can help to reduce bubbles around the copper is to sprinkle clear powder over the copper sheet once in place on the glass. The spread of the powder over the glass assists in giving places for the air between layers to escape.
These two things combined with a long slow squeeze can reduce the amount of bubbles you get. It cannot totally eliminate them.
Of course, a longer bubble squeeze allows air to be in contact with the copper and promotes the change to a blue green or burgundy colour.
Showing posts sorted by relevance for query borax. Sort by date Show all posts
Showing posts sorted by relevance for query borax. Sort by date Show all posts
Wednesday 11 April 2018
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.
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