GLASS STRUCTURES

Technical Field of the Invention

The invention relates to glass structures, and in particular to a method and apparatus for forming glass structures that can be or are stress and strain free.

Background to the Invention

It is often necessary to form glass structures from two or more pieces of glass in which there is a consistent and airtight seal between the pieces of glass. Furthermore, the glass in the structure should not be subject to unnecessary internal stresses or strains introduced by the sealing process, which could lead to a subsequent failure in the glass.

Conventional methods of creating hermetic or airtight seals between glass plates generally involve the use of a solder glass or frit. This sealing method is currently used for commercial plasma display panels (PDPs) and field emission displays (FEDs), among others, and involves forming a peripheral seal using a solder glass or frit where a join is to be made.

The solder glass or frit has a lower melting point than the glass to be bonded and is tailored to exhibit a temperature coefficient of expansion that matches as closely as possible that of the glass plates and/or spacer. If, as is often the case, the expansion coefficient of the solder or frit does not exactly match that of the glass, the resulting glass package will have permanent residual stress in the sealed region of the package. This stress can, over time, result in package breakages and/or seal failure.

In addition, the sealing glass material is generally available in a powder form and is combined with organic carriers to form a paste, a preform or a tape. The presence of these organic additives is not desirable as it introduces contaminants that need to be removed. If these contaminants are not removed they are likely to affect the operation and performance of active display elements contained within the package. A common process used to remove these contaminants uses "getters".

Techniques have been developed that employ electrostatic or anodic techniques for bonding glass to silicon, and which create a diffusion seal between the glass and silicon. The diffusion seal is accomplished by pre-depositing a silicon-containing film

onto one of the glass surfaces. By applying a combination of heat, voltage and pressure, ionic diffusion takes place across the interface, causing ions from the silicon and from the glass to exchange positions.

These techniques are used for sealing protective glass onto silicon solar cells, and in the production of hermetic packages for micro-electro-mechanical systems (MEMS) and silicon electronic devices. These methods have been adapted for forming a glass- to-glass seal for the 'seal-in' operation performed at the completion of an emissive flat panel display vacuum packaging process. However, these techniques cannot easily be adapted to form a peripheral package seal because this seal normally has to accommodate electrical feed-throughs in the form of thin film metal or semiconducting address lines - a diffusion seal employing silicon would make the glass too conductive, compromising the insulating property of the glass surface needed between neighbouring electrical address lines. This is particularly problematic for high voltage connections. For this technique, extremely flat glass panels must be used, and the presence of contaminants will prevent a seal from being formed.

Similar problems to the above arise when creating vacuum ports in a glass package (the ports that are used to extract the air or other gas from the package to form a vacuum). These vacuum ports are formed in the glass package and allow the attachment of a glass tube in an airtight manner. Conventionally, this involves forming a hole in a glass panel (by drilling or otherwise cutting a hole in the glass) and attaching a tube with a flared end using solder glass or frit. Again, these materials are selected to have a lower melting point than the glass to be bonded and to have a temperature coefficient of expansion that matches as closely as possible that of the glass plate and tube. Again, however, there is usually a slight mismatch in the coefficients of expansion which results in permanent residual stress at the attachment point of the tube. There will also be contaminants present in the join, as described above.

Summary of the Invention

There is therefore provided a method of joining at least two pieces of glass, the method comprising arranging the at least two pieces of glass into a structure, there being a region in a first piece of glass that is to be joined with a region in a second piece of glass, the first and second pieces of glass being arranged such that the regions to be joined are adjacent to each other; heating the pieces of glass to a temperature that is

between the strain point and softening point of the glass; applying additional heat simultaneously to all parts of the regions that are to be joined, such that the temperature of said regions is taken to or above the softening point of the glass and the regions join together; and removing the additional heat to allow the regions of the pieces of glass to cool to a temperature that is between the strain point and softening point of the glass.

According to a second aspect of the invention, there is provided a vacuum package formed using the method as described above.

According to a third aspect of the invention, there is provided an apparatus for joining at least two pieces of glass to form a glass structure, there being a region in a first piece of glass that is to be joined with a region in a second piece of glass, the first and second pieces of glass being arranged such that the regions to be joined are adjacent to each other, the apparatus comprising a first heating means for heating the pieces of glass to a temperature that is between the strain point and softening point of the glass; and a second heating means that can be selectively activated for heating all parts of the regions that are to be joined simultaneously to a temperature that is at or above the softening point of the glass.

According to a fourth aspect of the invention, there is provided a method of creating a stub in a piece of glass, the method comprising heating the piece of glass to a temperature that is between the strain point and softening point of the glass; applying additional heat to the part of the glass that is to form the open-ended stub, such that the temperature of said part is taken to or above the softening point of the glass; forming a stub by manipulating said part; and removing the additional heat and allowing the part of the glass to cool to a temperature that is between the strain point and softening point of the glass.

According to a fifth aspect of the invention, there is provided an apparatus for creating a stub in a piece of glass, the apparatus comprising a first heating means for heating the piece of glass to a temperature that is between the strain point and softening point of the glass; a second heating means that can be selectively activated for heating a part of the glass that is to form the open-ended stub to a temperature that is at or above the softening point of the glass; and means for manipulating said part to form a stub.

A further aspect of the invention provides a method of forming a glass to glass diffusion seal between two or more pieces of glass made of a compatible glass material in which the glass assembly is heated to a temperature above the annealing point of the glass; selective heating is applied to the surfaces above and below where the seal is to be made, for a predetermined period of time; and the glass assembly is cooled to room temperature at a predetermined rate to ensure the glass seal remains stress-free.

Yet another aspect of the invention provides a method of forming a glass to glass diffusion seal between two pieces of glass comprising a face-plate and a base-plate of a field emission display, and a spacer frame made of a compatible glass material, in which the glass assembly is heated in air to a temperature above the annealing point of the glass; selective heating is applied to the plates above and below the spacer frame to create diffusion seals with the base-plate and face-plate where they makes contact with the spacer, for a predetermined period of time; and the glass assembly is cooled to room temperature at a predetermined rate to ensure the glass package remains stress-free.

Yet another aspect of the invention provides a method of forming a glass to glass diffusion seal between two pieces of glass comprising a face-plate and a base-plate of a field emission display, and a spacer frame made of a compatible glass material, in which the glass assembly is mechanically pressed and heated in a controlled gas atmosphere or dynamically pumped vacuum to a temperature above the annealing point of the glass; selective heating is applied to the plates above and below the spacer frame to create diffusion seals with the base-plate and face-plate where they make contact with the spacer, for a predetermined period of time; and the glass assembly is cooled to room temperature at a predetermined rate to ensure the glass package remains stress free.

In one embodiment, the spacer frame in the above methods contains an integral getter box and a pumping manifold.

A further aspect of the invention provides a method of attaching a glass or metal tube to a compatible glass surface by a glass forming process, in which a jig that contains the glass surface is heated in air to a temperature above the annealing point of the glass; selective heating is applied to the area of glass that is to be deformed and joined

to; pressure is applied to the locally heated glass surface; the heating and force applied to the glass is controlled to form a hollow glass protuberance extending from the face of the glass; and, subsequent to the cooling of the glass assembly, using standard glass blowing techniques to remove the end off the protuberance (if there is one) and attach a compatible glass or metal tube.

Yet another aspect of the invention provides a method of forming a glass pump port on a glass surface by a glass forming process, in which a jig containing the glass substrate is heated in air to a temperature above the annealing point of the glass; selective heating is applied to the area of glass that is to be deformed and joined to; pressure is applied to the locally heated glass surface; the heating and force applied to the glass is controlled to form a hollow glass protuberance extending from the face of the glass; and, subsequent to the cooling of the glass assembly, using standard glass blowing techniques to remove the end off the protuberance (if there is one) and attach a compatible glass or metal tube.

Yet another aspect of the invention provides a method of attaching a glass tube to a compatible glass surface by a glass forming process, using a carbon rod to apply the localised pressure required, in which a jig containing the glass substrate is heated in air to a temperature above the annealing point of the glass; selective heating is applied to the area of glass that is to be deformed and joined to; a carbon tool (which has the end dimensions the same as that required of the glass stub) is pressed into contact with the locally heated glass surface; the heating and force applied to the tool is controlled allowing it to deform the glass locally around the point of contact and form a hollow glass stub extending outwards from the face directly opposite the position of the tool; and, subsequent to the cooling of the glass assembly, using standard glass blowing techniques to remove the end off the protuberance and attach a compatible glass tube.

Brief Description of the Drawings The invention will now be described with reference to the following drawings, in which:

Figure 1 is a graph showing glass expansion versus temperature;

Figures 2(a) and (b) show an assembly for forming a sealed glass package in accordance with an aspect of the invention;

Figure 3 is a flow chart illustrating a method in accordance with an aspect of the invention;

Figure 4 is an illustration of a heater block in accordance with a specific embodiment of the invention;

Figure 5 is an illustration of a heating element in accordance with a specific embodiment of the invention;

Figure 6 is an illustration of an alternative assembly for forming a sealed glass package in accordance an aspect of the invention;

Figure 7 is a flow chart illustrating a method in accordance with a second aspect of the invention;

Figure 8 is a diagram illustrating the method of Figure 7;

Figure 9 is a flow chart illustrating an alternative method in accordance with the second aspect of the invention; and

Figure 10 is a diagram illustrating the method of Figure 9.

Detailed Description of the Preferred Embodiments

Although the invention will be described below with reference to the construction of a vacuum package with a pumping port, it will be appreciated that the invention is applicable to any glass structure in which a hermetic or airtight seal or join is required between two or more pieces of glass.

Furthermore, it will be appreciated that glass structures made in accordance with the invention do not have to include pumping ports and/or need to be used to create or hold a vacuum.

In particular, the invention will be described below with reference to a vacuum package for use in emissive flat panel displays, where the vacuum package is used to house some part of the display assembly that needs to operate in a controlled vacuum or gas

ambient environment. Examples of such displays include plasma display panels (PDPs), field emission displays (FEDs), and field emission backlights for LCDs.

The invention exploits the properties of glass at high temperatures, and particularly when glass is heated beyond its strain point, the temperature at which glass will relieve stresses over a period of hours. For soda-lime glass this temperature is around 500°

C. The annealing point of a glass is the temperature at which it will relieve stresses

(either compressive or tensile) in a matter of minutes (and is a temperature above the strain point). For soda-lime glass, the annealing point is approximately 520° C. The softening point of a glass is higher still, and is the temperature at which the glass softens and deforms easily.

Figure 1 is a simple graph illustrating how the length of a piece of glass changes with temperature. Thus, it can be seen that from an arbitrary temperature ti, the length of the glass increases substantially linearly as the temperature increases, up to the strain point t _s . At or beyond this strain point, the length of the glass does not substantially change, and the stresses in the glass can be relieved by maintaining the glass at this temperature for a period of time. The annealing point for the glass falls somewhere in the range of temperatures from the strain point t _s to the softening point t _m (where it loses its rigidity).

Essentially, the invention comprises heating a glass structure to a temperature that is above the strain point but below the softening point, and applying additional heat simultaneously to all parts of the glass structure that need to be joined or sealed together or to the part that is to be turned into a vacuum port, so that the temperature of these parts is taken to or beyond the softening point of the glass.

As the remainder of the glass structure is at a temperature that is above the strain point of the glass, no additional stresses or strains are introduced.

After a period of time (the length of which is dependent on the exact temperature and pressure applied to the join), the separate parts will become fused together and the sealed parts can be cooled at a steady rate to the temperature of the remainder of the glass structure (that is above the strain point but below the softening point of the glass). Any stresses or strains introduced into the glass structure by the additional heating will now be relieved in the glass structure. The structure can then be cooled to the ambient

temperature. Alternatively, when a vacuum port is to be created, a stub or hole can be created at the point where the additional heat is applied to the glass structure, and then the stub or hole can be cooled to the temperature of the remainder of the glass structure, so that the stresses can be relieved. The entire structure can then be cooled at a steady rate to the ambient temperature.

Thus, one aspect of the invention provides a method of joining or sealing two or more pieces of glass together without the use of a binding agent. The pieces of glass can be large flat panels joined only at their edges with or without a glass spacer between them. The sealing method will not affect the physical or optical properties of the glass panels. The seal formed will be stress and contamination free, and air tight, so is able to be used for a vacuum or gas ambient package. It will be appreciated, however, that all of the glass being joined together must have compatible expansion coefficients in the same manner as traditional sealing methods.

Figure 2 shows an apparatus for creating a seal between two glass panels and a spacer in accordance with a first aspect of the invention. Figure 2(a) is a perspective view of the apparatus including the glass panels to be sealed together, and Figure 2(b) is a cross-section through the middle of the apparatus and glass panels.

The two glass panels to be sealed to form the vacuum package comprise a top panel 4, a base panel 6, and a spacer 8. As described above, it will be appreciated that a vacuum package can just comprise two panels, or any other combination of spacers and panels.

In this illustration, the spacer 8 is in the form of a rectangular frame that has a slightly smaller circumferential dimension than the glass panels 4, 6. The spacer 8 provides a gap between the two panels 4, 6, in which a vacuum can subsequently be formed. In alternative embodiments, the spacer 8 can be in any suitable or desired shape. For example, the spacer 8 may comprise a rectangular frame as illustrated here, with additional portions that extend through the middle of the frame to create a cross- hatched pattern (see Figure 6 below).

The spacer 8 can be slightly smaller than the glass panels 4, 6 to provide a slight overlap which allows connections to be made to any metal tracks on the glass panels 4, 6.

Preferably, each of the panels 4, 6 and spacer 8 are made from the same type of glass, or at least different types of glass that exhibit the same coefficient of expansion with temperature and have similar strain and softening points. In one embodiment, one or more of the panels 4 or spacer 8 can also include a port or stub 9 for use in evacuating the glass structure once it has been sealed.

The panels 4, 6 and spacer 8 are arranged in the desired manner into the package, with the spacer 8 positioned between the top panel 4 and base panel 6. The apparatus comprises a top heater block 10 that is positioned over the top panel 4 and a base heater block 12 that is positioned under the base panel 6. These heater blocks 10, 12 provide the additional localised heat that is required to take the parts of the glass structure that are to form the seal or join to or beyond the softening point of the glass.

Thus, the shape of the heater blocks 10, 12 defines where the seals or joins will be formed in the glass structure.

Therefore, in this embodiment, the heater blocks 10, 12 correspond in shape to the spacer 8, as they provide the additional heat to soften the portions of the panels 4, 6 and spacer 8 where the joins or seals are to be formed. In alternative embodiments where the spacer 8 is a different shape or is omitted, the heater blocks 10, 12 have a shape that corresponds to the desired positions of the seals or joins for the package.

As shown in Figure 2(b), the width of the heater blocks 10, 12, indicated by w _h , is approximately the same as the width of the spacer 8, indicated by w _s . The width of the heater blocks 10, 12 roughly corresponds to the width of the seal or join formed in the package. Thus, it is possible to provide heater blocks with a width that is smaller than that of the spacer 8, although this will result in the seal not being formed across the full width of the spacer 8.

Once the glass structure has been arranged, the sealing process can begin. Figure 3 shows a method for creating seals or joins in accordance with the invention. In step 101 , the glass structure to be sealed is assembled, and the appropriate apparatus provided in the form of heater blocks 10, 12 that apply the additional heat necessary to create the seal or join.

In step 103, the entire glass structure is heated (preferably uniformly) to a temperature that is at or beyond the strain point, but below the softening point of the glass (the range of temperatures between these two points being further described herein as the "annealing range"). This heating stage can be carried out in an oven, furnace or a similar device.

Once the glass structure has been heated, the method moves to step 105 in which the additional heater blocks 10, 12 are activated or additional heat is otherwise applied to the parts of the glass structure that are to form the join or seal, while the remainder of the glass structure is maintained at the temperature that is in the annealing range. The additional heat is applied simultaneously to all parts of the glass structure that are to form the join or seal.

The additional heat acts to take the temperature of those parts or regions of the glass structure that are to form the join or seal to or beyond the softening point of the glass.

Preferably, the parts are not taken substantially beyond the softening point of the glass, or beyond the softening point of the glass for a substantial period of time, otherwise the heat will conduct through the glass into other parts of the structure that are not intended to be part of the seal or join, or deformation of the glass in the regions where the additional heat is applied will occur.

Pressure is also exerted on the glass structure to encourage a seal or join to form between the pieces in the glass structure. This pressure can be provided by the weight of the pieces in the glass structure or by additional weights or means to compress the structure (which can be integral to or separate from the heater blocks 10, 12) to provide the necessary compression.

The resulting seal or join conditions are a function of the specific temperature of the region to be sealed or joined, the time for which this temperature is maintained and the pressure exerted across the region to be sealed or joined.

After allowing an appropriate length of time for the seal or join to form, the method moves to step 107 in which the heater blocks or other additional heat source is switched off or removed, and the glass structure is allowed to cool to a temperature in the annealing range. The glass structure is held in this temperature range until the molecules in the glass realign themselves into a stable form, thus relieving the stresses

in the glass. The time required for this depends on the particular temperature in the annealing range that the glass structure is at. The higher the glass structure is in the annealing range, the less time that is required to relieve the stresses in the glass.

In step 109, the glass structure is then allowed to cool at a steady rate to the ambient temperature that is below the strain point (i.e. the oven or other heat source maintaining the glass structure in the annealing range is switched off or removed).

The above method of raising the temperature of the glass assembly to a temperature in its annealing range and then locally heating the area of glass to be bonded or joined can be applied to many different glass types and can be used to produce many different shaped packages or sealed regions of glass.

As an alternative to the method described above, the parts of the glass structure that are to form the join or seal can be divided into different sections, such that the additional heating step (step 105) and subsequent cooling step (step 107) are applied separately to each of these sections (although simultaneously to all parts of each section). For example, in the embodiment shown in Figure 2, a section could correspond to one of the straight edges of the glass panels 4, 6 and spacer 8. Once a section of the glass structure has been heated and sealed, a heater block can be moved to a different position relative to the glass structure to heat another section.

The heater blocks 10, 12 according to the invention provide a further advantage over much more focussed heating means (such as a laser), in that they allow seals or joins to be formed through the width of the heater block 10, 12, and in a central area of the glass structure, whereas with a focussed heating means such as a laser, it is only possible to fuse a very narrow edge portion.

A specific embodiment of the invention is described below with reference to Figures 4, 5 and 6. The top and base heater blocks 10, 12 are formed from a 10mm thick steel plate 14, which is milled out to be a hollow rectangle with a wall thickness of 8.5mm. Holes 16 with a diameter of 7.5mm are drilled into the arms of the plate 14, with 0.5mm between the end of a hole and the side of the next one.

Heating elements 18 (see Figure 5) with a resistance of approximately 10 ohms in total are placed in the holes 16. The heating elements 18 are designed to provide 1000

watts of heating at 100V. The heater block 10, 12 is sprayed with heat resistant paint to prevent corrosion.

The heating elements 18 are constructed from Ni wire 20 wound around a central quartz capillary tube 22, and placed inside another quartz tube 24, which has an outer diameter of 7mm. Quartz is needed to withstand the heat generated by the heating element 18. The heating elements 18 are sealed using a non-conductive ceramic paste and connected to a switchable power source with beaded wires.

The heating elements 18 are placed in the holes 16 in the steel block 14, and held in place with graphite tape, metal hooks, or any other suitable type of fastener or attachment means. By placing the heating elements 18 in the steel block 14, and not directly onto the surface of the glass panels 4, 6, good even thermal contact with the glass is ensured, and a flat heating surface is provided to minimize any deformation of the glass plate.

Referring now to Figure 2(b), the two glass plates 4, 6 are separated by a 2mm thick spacer 8 that is cut to the same size as the heater blocks 10, 12. The base heater block 12 is held off of the floor of the furnace with a metal frame to prevent heat build up under the centre of the base heater block 12. An identical metal frame is placed on top of the top heater block 10, with a 5kg steel weight resting on it to apply pressure to the glass to aid sealing.

The furnace temperature is raised to approximately 49O ⁰ C, which is in the annealing range for the glass in the panels 4, 6 and spacer 8. The heater blocks 10, 12 are activated and the temperature of the edge of the structure is elevated to 62O ⁰ C, just beyond the softening point of the soda glass used in the panels 4, 6 and spacer 8.

After maintaining that temperature for enough time for a seal to form (a minimum of 20 minutes based on the 5kg weight being applied to the structure) the heater blocks 10, 12 are switched off. The sealed glass structure is then held at approximately 49O ⁰ C until any stress in the structure has been relieved.

If the temperature of the heater blocks 10, 12 is substantially greater than 64O ⁰ C, then there is a risk of deformation of the glass plates 4, 6. In addition, any metal tracks that are required to power the resulting display device would be partially absorbed into the glass seal, potentially increasing their resistance.

The spacer 8 may be the sole means of supporting the glass walls of the structure.

Alternatively, where the structure is to form part of a display, the area within the structure may have additional distributed supports incorporated between pixels to enable a thinner glass package to be realised. This is shown in Figure 6. In this embodiment, the spacer 8 has an external perimeter 8a and an internal structure 8b.

In order to create the seal between the panels 4, 6 and all parts of the spacer 8, the whole of the spacer 8 is heated simultaneously to the temperature just beyond the melting point of the glass, by heater blocks 10, 12 that correspond in shape to the spacer 8.

Furthermore, the method according to the invention can be used to manufacture multiple vacuum packages at any one time. For example, in Figure 6, the structure of the spacer 8 can define the boundaries of each of the individual vacuum packages. Once the method according to the invention has been carried out on this structure, four separate enclosures will have been created. The glass structure can then be cut to produce the four separate vacuum packages.

The second aspect of the invention provides a method of attaching a tube to a piece of glass, such as a glass panel, that may or may not be a component in a glass structure. The method provides for the creation of an open-ended stub in the glass to which a tube can be attached. The stub can be a pumping port that is used to create a vacuum or hold an ambient gas in a package. The method provides the advantage that the need to use a glass sealing material is eliminated.

Figure 7 is a flowchart illustrating a method for creating an open-ended stub in a piece of glass in accordance with an aspect of the invention. Figure 8 illustrates the method of Figure 7. In step 201 and Figure 8(a), the piece of glass 20 is heated (preferably uniformly) to a temperature that is at or beyond the strain point, but below the softening point of the glass. This heating stage can be carried out in an oven, a furnace, or a similar device.

Once the piece of glass 20 has been heated, the method moves to step 203 and Figure

8(b) in which additional heat is applied to the part 22 (or parts if several stubs are to be formed) of the piece of glass 20 that is to form the stub, while the remainder of the piece of glass 20 is maintained at a temperature that is in the annealing range. By

applying the additional heat whilst the remainder of the piece of glass 20 is held in the annealing range, the build up of stresses in the glass is avoided. The additional heat can be provided by any suitable means, including a burner, or any other heat source that can heat a localised area of a piece of glass 20.

The additional heat acts to take the temperature of that part 22 of the piece of glass 20 to or beyond the softening point of the glass. Preferably, the part 22 is not taken substantially beyond the softening point of the glass, or beyond the softening point of the glass for a substantial period of time, to avoid distortion of the surrounding glass.

Once the part 22 of the piece of glass has been heated to or just beyond the softening point of the glass, a stub 24 can be formed in the glass (step 205 and Figure 8(c)) in any suitable manner. Furthermore, the stub 24 can take any desired shape. In Figure 8(c), a rod 26 is pushed into the softened area of the glass to create the stub 24. The rod 26 can be made from any suitable material, i.e. a material that can easily withstand temperatures in the region of the softening point of the glass, and which the softened glass does not adhere to. One suitable material is carbon.

Preferably, the additional heat is applied to an area that is slightly larger than the desired cross-section of the open-ended stub, in order to allow enough glass to be softened so that it can be drawn up to form the stub 24.

As an alternative to using a rod 26, the stub 24 could be formed by applying a vacuum or otherwise low pressure to the part 22 to draw the glass into a stub 24. Conversely, the stub 24 could be formed by applying positive pressure (for example in the form of a jet of gas) to the softened part 22.

Once the stub 24 has been formed, the method moves to step 207 (Figure 8(d)) in which the additional heat source is switched off or removed, and the piece of glass 20 is allowed to cool to a temperature in the annealing range. The piece of glass 20 is held in this temperature range until the molecules in the glass realign themselves into a stable form, thus relieving the stresses in the glass and stub 24. The time required for this depends on the particular temperature that the piece of glass 20 is at. The higher the piece of glass 20 is in the annealing range, the less time that is required to relieve the stresses in the glass.

In step 209, the piece of glass 20 is then allowed to cool at a steady rate to a temperature below the strain point (i.e. the oven or other heat source is switched off or removed).

Alternatively, after allowing an appropriate length of time for the stub to form, steps 207 and 209 can comprise switching off or removing all heat sources, so that the piece of glass 20 can cool down to the ambient temperature, or to a temperature that is suitable for a subsequent process. In this case, annealing of the glass can take place in this subsequent process.

Once the piece of glass 20 has cooled, the method moves to step 211 and Figure 8(e) in which the top of the stub 24 is removed using any suitable technique (for example by glass blowing or by piercing the top of the stub 24) to form an open-ended stub.

Thus, a stub has been formed in the piece of glass 20 to which a glass or metal tube can be affixed using conventional techniques.

Figures 9 and 10 illustrate an alternative method of forming an open-ended stub in accordance with the invention. In step 301 and Figure 10(a), the piece of glass 30 is heated (preferably uniformly) to a temperature that is at or beyond the strain point, but below the softening point of the glass. This heating stage can be carried out in an oven, a furnace, or a similar device.

Once the piece of glass 30 has been heated, the method moves to step 303 and Figure 10(b) in which additional heat is applied to the part 32 (or parts if several open-ended stubs are to be formed) of the piece of glass 30 that is to form the stub, while the remainder of the piece of glass 30 is maintained at a temperature that is in the annealing range. By applying the additional heat whilst the remainder of the piece of glass 30 is held in the annealing range, the build up of stresses in the glass is avoided. The additional heat can be provided by any suitable means, including a burner, or any other heat source that can heat a localised area of a piece of glass 30.

The additional heat acts to take the temperature of that part 32 of the piece of glass 30 to or beyond the softening point of the glass. Preferably, the part 32 is not taken substantially beyond the softening point of the glass, or beyond the softening point of the glass for a substantial period of time, to avoid distortion of the surrounding glass.

Once the part 32 of the glass has been heated to or just beyond the softening point of the glass, the glass can be manipulated to form the open-ended stub (step 305 and Figures 10(c) and (d)) in any suitable manner. In Figures 10(c) and (d), a rod 34 is pushed through the softened area of the glass until the rod 34 protrudes through the glass. The rod 34 can be made from any suitable material, i.e. a material that can easily withstand temperatures in the region of the softening point of the glass, and which the softened glass does not adhere to. One suitable material is carbon.

Once the opening in the glass has been formed, the method moves to step 307 (Figure 10(e)) in which the additional heat source is switched off or removed, and the piece of glass 30 is allowed to cool to a temperature in the annealing range. The piece of glass 30 is held in this temperature range until the molecules in the glass realign themselves into a stable form, thus relieving the stresses in the glass and stub. The time required for this depends on the particular temperature that the piece of glass 30 is at. The higher the piece of glass 30 is in the annealing range, the less time that is required to relieve the stresses in the glass.

In step 309, the piece of glass 30 is then allowed to cool at a steady rate to a temperature below the strain point (i.e. the oven or other heat source is switched off or removed).

Alternatively, after allowing an appropriate length of time for the open-ended stub to be formed in the piece of glass, steps 307 and 309 can comprise switching off or removing all heat sources, so that the piece of glass 30 can cool down to the ambient temperature, or to a temperature that is suitable for a subsequent process. In this case, annealing of the glass can take place in this subsequent process.

Thus, an open-ended stub has been formed in the piece of glass 30 to which a glass or metal tube can be affixed using conventional techniques.

A specific embodiment of the invention is described below with reference to Figure 8. A 4mm thick soda glass plate 20 that has been cut to the required size is placed in a furnace that is then heated to 49O ⁰ C. Above the plate 20 a weighted carbon rod 26 (approximately 30Og), is resting on the surface of the glass plate 20. The carbon rod 26 is 2mm at the extremity, increasing with a pitch of 1.5° to 4mm, 2cm from the end.

Once the furnace and glass plate 20 is at the required temperature (49O ⁰ C), a small flame is placed beneath the carbon rod 26. The flame is a mixture of butane and oxygen, with a medium heat.

The flame is then moved around the periphery of the part 22, 2cm beneath the glass 20, until the glass is soft enough to allow the carbon rod 26 to protrude through the plate 20, forming a 2cm long stub 24. After the plate 20 has cooled and been removed from the furnace, the top of the stub can be removed and a tube attached to the open- ended stub 24 using standard glass blowing techniques.

The angle of the point of the rod 26 and the positioning of the flame under the rod 26 determines whether an open or closed ended stub is formed. A sharper point on a rod and heat directly under the tip of the rod 26 will produce a thinner walled and open- ended stub, whereas a blunt rod 26 with heating around its periphery will cause a thick- walled and closed-ended stub.

There is therefore provided a glass-sealing technique and a technique for creating an open-ended stub in glass that does not use a glass sealing material.