GB2415989A - Double glazed units with reproduction crown glass - Google Patents

Abstract

A double glazed unit comprises two sheets of glass <B>28</B>, <B>29</B>, a spacer 33, either side of which the glass sheets <B>28</B>, <B>29</B> are arranged so as to from a cavity therebetween and a peripheral sealant channel filled with sealant <B>32</B>. One of the sheets of glass <B>28</B> is reproduction crown glass formed by treating ordinary clear flat drawn glass sheet or flat float glass sheet. The flat glass sheet is treated by being kiln heat treated on a mould. The mould is shaped to produce bulges and blemishes in a central portion while leaving the peripheral edges flat.

Description

DOUBLE GLAZED UNITS WITH REPRODUCTION CROWN

GLASS

It is considered that the Romans made the first type of flat glass by casting methods originated by the Syrians. The glass was uneven and tinged with colour but generally flat. The largest piece known from these times was just under one metre square and probably the beginning of what is termed flat glass. In medieval times flat glass was manufactured by two methods, the cylinder method and the crown lo method. The cylinder method was developed by blowing molten glass into a cylindrical shape, cooling, cutting and heating to a suitable temperature, flattening and cooling and was used for general glazing.

The crown method provided a quite different visual product and consisted of drawing a quantity of molten glass on a metal rod and spinning the rod until a flat circular disc is formed then cooled and rectangular panes cut from the circular shape. This type of manufacture is generally credited as the first in England around Sixteen Eighty onwards at the Bear Garden Glasshouse. On each pane a crown was embossed and generally accepted as the origin of what was referred to as crown sheet. This type of manufacture of crown sheet gradually ceased when the method of drawing flat sheet became available. Some of the best old crown sheet with curvature and bulges can be seen at Hampton Cowt believed to have been glazed by Wren. The cylinder method is still used on a small scale today in Britain and Europe to produce mainly decorative coloured glass for the leaded glass trade, and some imported clear cylinder glass which is used in place of crown sheet in normally larger stock sizes or cut sizes from stock.

Until the Nineteen Fifties large scale production of clear sheet glass was by a method of drawing molten glass to a given height at which time the glass was sufficiently thin and cool and a section cut off, drawn upwards again and cut again in a continual process and was known as drawn sheet. Whilst the quality was good the optical quality varied. This type of glass is still available from overseas sources. The float process was to developed by Alistair Pilkington around Nineteen Fifty two, where molten glass flows from a furnace and floats across a liquid metal and eventually passes through a cooling zone and is perfectly flat and clear and is referred to as float glass.

This invention relates to a method of heat treating ordinary clear drawn Is sheet or clear float or any suitable glass to reproduce the look of old crown sheet with particular pronounced bulges and curvature in single panes to fit each individual sized pane in a window also in toughened reproduction crown glass, and used for single glazing purposes and in the construction and manufacture of particularly slim double glazed units with any combinations of suitable glasses with limited perimeter seal to suit mainly existing window glazing rebates.

Many local planning authorities and national heritage associations will not allow double glazing units in listed buildings and some conservation areas to ensure that the windows which are glazed with crown sheet are not replaced with ordinary clear glass or double glazed units which would completely change the visual appearance of the building.

However they are unable to prevent a glass breakage to be replaced with o ordinary clear glass. There is now considerable pressure to consider the question of carbon dioxide production in relation to single glazing in buildings. Government information states that when double glazing is installed on the average single glazed house, the energy saved will result in reduction of carbon dioxide emissions of one quarter tonne per six month season. This invention of reproduction crown glass will enable acceptable single glazing replacements to be made in heritage buildings and overcome the problem of double glazing units by being able to produce specially slim double glazing units incorporating one pane of acceptable reproduction crown sheet or ordinary glass with the double ho glazed unit specially designed to enable it to be glazed into existing mainly Georgian windows usually having twelve panes in a window.

A description of the invention will now follow by referring to the accompanying drawings which show the glass mainly after the heating process is completed.

S Figure I shows a cross section of an originally flat pane of glass kiln heat treated to produce a reasonably bulged pane with random convex undulations to provide distortions for single glazing purposes, with powder former shape.

Figure 2 shows a cross section of an originally flat pane of glass kiln heat 0 treated m a convex manner to produce a reasonably curved pane with convex undulation to provide distortions but with flat perimeter edges to enable double glazing unit to be manufactured and metal former shape overlaid with insulation heat resistant paper.

Figure 3 shows a cross section of an originally flat pane of glass being 1 kiln heated treated to produce a particularly pronounced concave curvature or bulge in a concave manner with heat resistant powder formed shape.

Figure 4 shows a cross section of an originally flat pane of glass being kiln heat treated to produce a particularly pronounced curvature or bulge l in a concave manner laid on a metal former overlaid with heat resistant paper.

Figure 5 shows a cross section of an originally flat pane of glass being toughened by passing over rollers in a heating chamber causing small undulations by temperature and time and rapid cooling to provide toughening or tempering stresses and distortions from the undulations.

Figure 5A shows a cross section of an originally kiln treated glass to 0 produce a pronounced convex bulge and by passing over rollers in a heating chamber to provide toughening or tempering stresses and distortions from the convex shape and resultant slumping to reduce the originally pronounced convex shape.

Figure 6 shows a plan of an originally flat pane of glass being kiln heat lo treated to produce convex undulations or curvature with flat perimeter edges in a flat plane in a raised or convex powder former by using heat resistant solid strips laid around the perimeter edges upon which the glass slumps on heating to ensure a margin of flatness in the same plane marginally around the perimeter to enable it to be used in the lo manufacture of a double glazed unit requiring a section of flatness around the perimeter.

Figure 6A shows a cross section of Figure 6 showing the convex undulations and the margin of flatness in the same plane around the perimeter of the glass.

Figure 7 shows a cross section of an originally flat pane of glass kiln heat treated in a powder former in an alternative concave manner shaped to provide random undulations or curvature.

Figure 8 shows a cross section of particularly slim double glazed unit constructed with one pane of reproduction crown glass with three millimetre centre cavity one pane of ordinary flat glass and five millimetre perimeter seal and periphery encased in weatherproof tape.

Figure 8A shows a cross section of a particularly slim double glazed unit constructed with one pane of ordinary flat float glass, with small centre cavity and one pane of ordinary flat float glass.

Figure 8B shows cross section comer of double glazed unit shown at Figure 8 or Figure 8A, to clarify the important position of the clear water proofing not generally visible.

Figure 9 shows a a cross section of particularly slim double glazed unit constructed with one pane of crown reproduction glass with small centre cavity and one pane of ordinary glass and small perimeter seal glazed into a standard astragal rebate in correct dimensional proportions.

A sheet of flat glass either Drawn Sheet, Float Glass or Rolled flat figured glass or any suitable glass of desired thickness is entered into a heating kiln of furnace which is cool. The glass is laid on a selected surface. The surface in the kiln or furnace on which the glass is laid can be any material sufficient to withstand temperatures in excess of six to hundred degrees centigrade and additionally of sufficient quality to ensure the clarity, transparency or finished surface of the glass is unaffected. The distortions, curvature or marks in the glass can be effected by two methods, one is to use a preformed mould Fig 2,1 former or shape made of any suitable material, but normally metal, with the s desired convex slight curvature fig 2,5 where required and suitably indented Fig 2.6 within that slight curvature to provide distortions and similarly if a reasonably flat surface is desired. When a metal former is used Fig 2,1 it has to be overlaid with a thin sheet of heat resisting material, such as Kaswool paper Fig 2,2 or suitable materials available for this purpose as generally used in the glass bending industry. This prevents the differential temperature between glass Fig 2,3 and metal Fig 2,1 in the heating process affecting the glass. Using this type of former very small marks can be made in the glass by including small specks of material Fig 2,4 able to withstand high temperatures laid between the glass Fig 2,3 and the insulating sheet Fig 2,2 of the former or shape, Fig 2,1.

The other method is to provide a bed of any powder material Fig 1,6 which is unaffected by high temperatures exceeding six hundred degrees centigrade such as powdered chalk. The medium selected can be shaped lo as desired Fig 1,7 raised in a convex manner mainly above the flat plane of the glass to provide distortions and similarly obtained by shaping the powder medium to a concave shape Fig 1,8 without much distortion or with many distortions as desired. Using the powder medium Fig 1,6 results in general in each pane having slight variations due to the soft resistance of the medium when the glass is carefully laid on the powder medium Fig 1, 6. The preferred medimn is powder which can easily provide many variations but when required for repetitive production the Conner Fig 2,1 or pre formed metal shaped is preferred. It is well known that firing or processing any glass through a kiln or furnace for any no purpose can cause slight optical distortions if using clear glass. The clear glass which can be any thickness or size desired but normally a thickness from two millimetres up to four millimetres is laid on the selected shaping medium in a cool kiln or furnace. The kiln which can be electric or gas fired, is closed, and the heating switched on. It is necessary, as is well known that glass in the kiln must be heated up gradually from about thirty minutes to forty five minutes and generally slightly longer to cool to ambient temperature. Timing is all dependent on the thickness of glass selected.

The glass can be any thickness desired and can be clear glass, obscure lo glass, tinted glass or emissivity glass or various special types of glass used in leaded windows or any glass currently available. Most of the glass currently used today is float glass because of its optical quality and to a much lesser extent drawn sheet which because of the method of manufacture cannot match the optical quality of glass manufactured in S the float process. Drawn sheet is mostly manufactured overseas with varying degrees of quality. The selected glass is cut to shape preferably two, three or four millimetres thick glass or as required either float or drawn sheet laid on a selected medium Fig 1,7 within a cool kiln, either in an uneven plane Fig 1,8 to provide the desired distortions Fig 1,9, or no shaped in a convex or concave manner Fig 1,9 again with a slightly uneven surface to provide a curvature or bulge with distortion Fig 1, often used to describe the shape of old crown glass. The heating kiln or furnace is at ambient room temperature when the glass is laid within the kiln. The kiln is closed and power switched on to commence the heating process. The heating process has a temperature control where time and temperature can be set as desired. The increase in temperature is gradual to prevent thermal shock which would cause the glass to crack Glass is a very poor conductive material and any significant differential temperature will cause cracking. The temperature within the kiln is he gradually raised from room temperature to over six hundred degrees centigrade over a selected period generally an hour or less. When the selected high temperature has been achieved the kiln can be held at that temperature for a selected period to enable more accurate slumping or merging on to the undulations Fig 1,10 or imperfections on the surface of Air the selected shaping medium below Fig 1,8 thus representing any imperfections or undulations on the lower surface. The glass is then allowed to cool very gradually. The gradual cooling will lessen the chance of breakage or fracture. The heating and cooling process can take up to two, three or four hours depending on the thickness of the glass selected. The preferred thickness of glass selected can be two millimetres, three millimetres, four millimetres or any thickness or if the glass panes are particularly large. Whilst the glass can be formed in a convex manner, it is also possible to form it in a concave manner. The selected shaping medium can be any suitable powder Fig 3, l l formed in a concave manner Fig 3,12, and processed in the heating kid, as described. The advantage of the concave method Fig 3,12 is that when a particularly deep bulge or curvature is required the glass will slump more accurately in a concave manner rather than convex as in Fig 1. Where repetitive concave deep curvature is required it is more economical to use a metal former Fig 4,13 overlaid with any heat resisting paper such as Kaoswool. Fig 4, 14 overlaid on the metal former Fig 4,13. The glass Fig 4, 15 which has been laid over the Kaoswool, Fig 4, 14 slumps during the heating process on to the Kaoswool and provides a fairly accurate shape on a repetitive basis. It should be noted that when producing bulges or curvature on the glass allowance has to be made for the slight decrease in the overall size of the glass. When the shaped or distorted glass is required for double glazing units and the amount required is fairly small and it would be uneconomical to manufacture a shaped metal former or mould. The power medium can be used to ensure marginally that the perimeter edges are in a flat plane. A preferred method to obtain flat edges is to use strips of heat resistant material such as sbestolux laid in rectangular flat form Fig 6, in a powder kiln, upon which the flat glass Fig 6,17 can be laid with the perimeter marginally over the heat resistant strips Fig 6,18 to provide sufficient perimeter flat margin in the same plane to enable it to be used for double glazing. The heat resistant powder is shaped Fig 6A, 19 within the rectangle formed by the asbstolux strips Fig 6,18 and Fig 6A, 16 and enable the glass Fig 6A, 20 to form over the powder Fig 6A,I9 during the heating process so described, and ensuring that the glass around the perimeter Fig 6A,21 remains reasonably flat in the same plane. The rectangular heat resistant strips Fig 6,18 can be adjusted to provide any size of perimeter flatness or simply use a flat heat resistant board for perimeter flatness. The surface material upon which the flat glass is laid can be any material lo sufficient to withstand the temperature in heating the glass in excess of six hundred degrees centigrade, and additionally of sufficient quality to ensure the clarity or finished surface of the glass is unaffected. The very old crown sheet mainly had a varying depths of curvature which was much preferred and was glazed in position with the convex side outwards which has a discernible effect on the light. This preferred curvature can be provided in the heating and cooling method described in single panes by varying the timing of the heating and cooling method described. If the desired curvature is not obtained in a single process of heating and cooling, a more pronounced curvature can be obtained by repeating the go heating and cooling. Care must be taken to ensure that the curvature is not excessive to prevent fitting or glazing to a flat opening. This method of providing curvature would normally be carried out on a pre determined size of pane to fit a given opening. When designing or forming a shape or mold for a single glass pane, it is necessary to take cognisance that the glass will be glazed into a single glazing rebate constructed for single glass, therefore any glass so treated to reproduce the curved effect found in some old crown glass the curvature must be minimal around the perimeter to enable the pane to be glazed. This can be achieved by using the method described above but it sometimes requires two heating and cooling processes to obtain the desired result of a concave or convex centre with sufficient flatness around the perimeter edge to fit correctly into a single glazing rebate. This particular effect would be on single fairly small pane size for glazing into a Georgian sash and case window for instance having approximately twelve panes, where the glass is glazed in a convex position, to produce the much desired visual effect. Current building regulations require low lying glazing on hazardous areas to be glazed in safety glass defined in British Standards.

To comply with these regulations this heat processed glass can be laminated to a standard flat glass by the resin interlayer method.

However this method has the effect of making the distortions slightly less visual. A preferred method is to toughen the glass which is a carefully designed method of heating and cooling to provide compressive and tensile stresses and has less effect on the desired distortions or imperfections in the glass. The preferred thickness of glass where it is required to be toughened is four millimetres on the basis that this is a standard toughening thickness, whereas thinner glass can be toughened but is substantially more expensive to toughen. The thickness of this glass when glazed is not visually apparent.

The toughening process for glass is well known where the glass is heated to a temperature under six hundred and fifty degrees centigrade depending on the type of glass and very quickly surface cooled to a temperature below five hundred and twenty degrees to create the compressive stresses. To provide a suitably distorted glass clear pane normally for a smaller pane of glass such as for a pane in a Georgian type window, a selected glass of either clear drawn sheet or clear fiat glass Fig 5,22 is entered into the furnace Fig 5,23 however the glass is heated to a temperature over six hundred and fifty degrees which is in excess of the necessary toughening temperature, on the oscillating rollers Fig 5,23 which oscillate at the required speed sufficient to ensure that at the given glass temperature the glass will remain reasonably flat. At a temperature in excess of six hundred and fifty degrees the glass Fig 5,22 becomes increasingly softer and the rollers Fig 5,23 oscillating at a set speed, are unable to keep the glass reasonably flat resulting in uneven surfaces to a greater or lesser extent depending on the temperature achieved. The oscillating rollers Fig 5,23a at a given temperature or time set propel the heated glass Fig 5,22 into the cooling area Fig 5,24, where the glass l -ls- surface is cooled within seconds to a temperature below five hundred and twenty degrees at which time any uneven distortions in the glass are permanently set and thereafter to ambient temperature. Where much more visually evident distortions bulges or curvature are required in the toughened glass it is necessary to first process the glass through the kiln process described and shown in Fig 2. The preformed glass with curvature Fig 5A, 25 is entered into the heating area Fig SA, 26, and heated to a temperature not exceeding six hundred and fifty degrees do centigrade on the Oscillating rollers Fig 5A,27 at a given temperature or time set the glass Fig 5A,25 is propelled into the cooling area SA,28 and cooled to below five hundred and twenty degrees in seconds at which time the curvature is set and thereafter to ambient temperature.

However the glass curvature shown in Fig 5A, 25, in the heating area will not be the same curvature shown in the cooling area Fig 5A, 26. Whilst the glass Fig 5A, 25 has been in the heating section the very pronounced curvature shown at Fig 5A,25, will subside to a lesser curvature shown at SA,26.

It is necessary for the kiln processed glass curvature glass to have a much To deeper or pronounced curvature when entering the toughening process, as during the toughening heating process the curvature in the glass will slump downwards to a certain extent but still result in a desired curvature or bulge to the glass. Care has to be taken not to drop the toughening temperature too low to maintain the glass curvature, as incorrect s toughening would result.

The construction of a double glazed unit specifically constructed to enable it to be glazed into the small rebate perimeters usually seven millimetre rebate depth and nineteen millimetre rebate width as evidenced in the astragals of Georgian type windows. The perimeter seal to depth of the double glazed unit will therefore be approximately five millimetres and the double glazed units with thickness not more than approximately twelve millimetres to allow sufficient face putty for glazing. The construction of this special double glazed unit will overcome the inability of standard double glazed units normally with a seal depth of at least ten millimetres to be accommodated in the glazing rebates of standard Georgian sash and case with astrals or similar. It is possible to construct a Georgian sash and case to accommodate standard double glazed units, but the astragals have to be manufactured much larger to accommodate the seal depth of the standard unit which destroys xo the visual appeal of a Georgian sash and case normally with slim astragals.

The construction of this special slim double glazed unit is as follows: The glass selected to form the double glazed unit is two panes of glass, either the reproduction clear crown glass Fig 8,28 described above, and an another sheet of clear float or drawn clear sheet Fig 8,29, or simply two panes of clear float Fig 8A,30 or any combination of glasses available. The glass thickness can be as required, but normally two millimetres, three millimetres or four millimetres. The preferred cavity is three millimetres, Fig 8,31 and Fig 8A 31 but can be up to six millimetres, provided the overall thickness of the double glazed unit does not preclude it from being glazed into an approximate nineteen millimetre rebate width Fig 9, 39.

The two selected panes are processed as normal through a washing machine to ensure that they are perfectly clean. The spacer selected is a solid tape type product Fig 8,32 and Fig 8A, 32 with adhesive on two sides which incorporates a drying agent or desiccant in the tape, which makes it unnecessary to use hollow spacers, fitted with a drying agent or desiccant to absorb any vapours between the glass. These hollow spacers are manufactured in various widths but the minimum rectangular size is approximately six millimetres by six millimetres and therefore not suitable for a minimum seal depth of five millimetres. The selected solid tape spacer Fig 8,32 and Fig 8A, 32 is a well known Canadian product, but the minimum manufactured size is three millimetres by six millimetres and for our purpose we process or cut the tape to a size of three millimetres by three millimetres. However whilst the preferred cavity width is three millimetres, the width can be four millimetres, five millimetres, six millimetres or six millimetres as the standard width of astragal Fig 9,39 is approximately nineteen millimetres. This size of adhesive spacer tape is applied to one pane of glass marginally about two millimetres inside the outside perimeter of the glass Fig 8,32. The other pane of glass is applied in a similar position against the prepared pane Fig 8, 32. The adhesive on the tape spacer hold the two panes together with a perimeter void of two millimetres by three millimetres which is then filled with a suitable type of sealant and the sealant Fig 8,33 allowed to cure or solidify and holds the two panes together. If the glass selected thickness was three millimetres, then the unit construction would be three millimetre glass, Fig 8, 29 three millimetre spacer tape Fig 8, 32 and three millimetre glass Fig 8,28 providing an overall thickness of nine millimetres and most importantly five millimetre seal depth, represented by three millimetres of solid tape Fig 8, 32 and Fig 8A, 32 and two millimetres of sealant Fig 8,33 and Fig 8A, 33. The sealant necessarily has a very low vapour transmission to prevent any later fogging between the glass. When an inert gas is to be used to fill the cavity between the glass as is well known in double glazing manufacture, there are either two or four small holes or entries left at each corner of the unit. This is done in the assembly of the work where the solid spacer tape is left short to provide a gap of about three millimetres, and similarly when the perimeter sealant is applied. The unit is then filled with gas under pressure through a nozzle inserted through the gap and when the correct amount of gas has been dispensed the holes are closed up by inserting a plug of any suitable material such as polyisobutelene with very low vapour transmission, followed by the perimeter sealant. Polyisobutylene lo can also be used by extruding it on to each side of the spacer to adhere to the glass, and is well known in the manufacture of double glazed units.

When this method is adopted it is referred to as the primary sealant and the perimeter sealant as secondary sealant. The double glazed unit which is left to cure is then encased around the perimeter with waterproof tape s Fig 8, 34 and Fig 8A, 34. A further application of a waterproof liquid Fig 8B, 35 is applied around the perimeter of the waterproof tape and particularly over the sight line of the tape marginally on to the glass and on curing provides a double seal and a product such as Humiseal used to protect electrical boards from moisture is suitable. To enable double it,' glazed units to be glazed into an existing or new Georgian type window it can only be glazed into the standard Georgian astragals Fig 9, 36 when the perimeter seal depth around the unit does not exceed five millimetres Fig 9, 37 as the astragal Fig 9, 36 has a rebate depth of approximately seven millimetres Fig 9,38 and the rebate width of the astragal is approximately nineteen millimetres Fig 9, 39 the overall thickness of the double glazed unit Fig 9, 41 is determined by the thickness of glass chosen Fig 9,42 either two millimetres, three millimetres or four millimetres but in this instance three millimetres, with a three millimetre cavity Fig 9,43, the central cavity may vary because of the curvature but around the perimeter of the double glazed unit Fig 9,43 the cavity is not more than three millimetres. This type of slim double glazed unit is lo fitted fairly tightly into the glazing rebates to ensure that the perimeter seal depth of the double glazed unit is not visible above the sight line of the astragal Fig 9, 44. It can readily be seen that this special type of double glazed unit constructed with a three millimetre cavity with any type of glass either ordinary annealed clear glass, toughened clear glass a- or the reproduction crown glass described, or any combinations can be glazed satisfactorily in single panes into a Georgian type window or any window having similar glazing rebate restrictions. l I'

Claims (14)

1. . . e e : . . . . . . . ë..

   PA method of manufacturing a double glazed product comprising two sheets of any suitable glasses or preferably one pane of any suitable flat glasses and one pane of reproduction crown glass bonded together in a spaced apart manner peripherally around and marginally on the inside faces of the spaced apart glass by way of any primary perimeter spacer and secondary hermetical sealant where the overall visual perimeter seal depth inside the edges of the spaced apart panes does not exceed seven millimetres but preferably five millimetres wherein the spacer is preferably a rectangular solid type containing a drying agent or dessicant formed integrally within the composition of the solid tape normally in a pre formed rectangular basis with or without the adhesive faces having a perimeter seal depth not exceeding five millimetres but preferably three millimetres by any width up to twelve millimetres but preferably three millimetres to six millimetres located between the inside perimeter faces of the spaced apart panes wherein the solid rectangular spacer located in a position not exceeding four millimetres but preferably two millimetres peripherally inside the outer dimensions of the spaced apart panes to provide a specified perimeter void or cavity depth between the spaced apart panes by any width up to twelve millimetres for application of secondary sealant to provide the hermetical seal between the spaced apart panes wherein any combinations of two any suitable flat glasses including toughened or e e e e . .. ::: laminated flat glasses or toughened or laminated or reproduction crown glass which can be produced by processing any suitable type of glass by heating the glass pane or panes which have been laid on or over a heat resistant shaped former convex or concave or any desired shape to distort the flat plane of the glass which can be subsequently laminated or toughened but necessarily for double glazed units with a reasonable marginal flat pane around the perimeter of the reproduction crown glass sufficient for the preferred perimeter seal depth of five millimetres wherein the spaced apart panes are in a reasonable marginal peripheral parallel for the preferred overall perimeter seal depth of five millimetres.
2. 2.A method according to claim 1 wherein the spaced apart completed double glazed product has the external periphery of the double glazed product or unit encompassed by a waterproof adhesive tape across the external face of the peripheral seal of the spaced apart glass and extended over each side of the glass unit on to the external faces of the glass sufficiently to visually cover the preferred peripheral seal depth of five millimetres followed by a waterproof clear coating such as Humiseal over the external applied waterproof tape whereby the coating extends marginally over the external peripheral tape sight line and on the external glass surface.
3. 3.A method according to Claim 1, characterized in that the perimeter overall seal depth of the double glazed product is five millimetres.
4. 4.A method according to Claim 1, characterized in that the perimeter overall seal depth is six to seven millimetres.
5. 5.A method according to Claim 1, characterized in that the perimeter width between the spaced apart panes is three millimetres.
6. 6.A method according to Claim 1, characterized in that the perimeter width between the spaced apart panes is from four to five millimetres.
7. 7.A method according to Claim 1, characterized in that the perimeter width between the spaced apart panes is six to twelve millimetres.
8. 8.A method according to Claim 1, characterized in that any suitable glass to manufacture a double glazed product can be used such as any annealed clear glass or obscure glass or tinted glass or coated glass or emmisivity glass or reproduction crown glass or any of these glasses in toughened or laminated form in any combination.
9. 9.A method according to Claim 1, wherein a reproduction crown glass can be manufactured by selecting any suitable pane of glass cut to the required size laying the glass on a suitable dimensioned former either powder form or metal former or ceramic former or any suitable heat resistant former either concave or convex or any desired shape to distort the flat plane of the glass with heat resisting ceramic paper type products such as Kaswool or similar laid between the former and glass to prevent thermal cracking of the glass during the process of heating the glass and former in a kiln from ambient temperature to a temperature of over six hundred degrees normally on a heating up timescale of one hour and held at this temperature for a specified period depending on the amount of slumping required on the glass and thereafter slowly cooled to ambient temperature within the kiln to prevent thermal breakage.
10. 10.A method according to Claim 1 or 9, characterized in that the farmers must be constructed with reasonably flat marginal dimensions around the perimeter bonding of the spaced apart glasses to be in a reasonable peripheral parallel for the preferred perimeter seal depth of five millimetres which can be achieved by the use of fire resistant powder with flat strips of heat resisting board such as asbestoslux or similar placed marginally under the perimeter edges of the glass whereby the glass can be slumped in a concave or convex manner or solid heat resisting farmers constructed with suitable flat margins around the perimeter in a concave or convex manner.
11. 11.A method according to Claim 9, characterized in that small specks of material able to withstand temperatures over six hundred degrees centigrade can be laid between the glass and heat resistant paper or similar to produce defects or marks during the heating process for reproduction crown glass similar to those evidenced in the old crown glass.
12. 12.A method according to Claim 1, wherein the reproduction crown glass can be laminated together with the well known resin laminating process with ordinary resin such as methacrylate or similar for safety or epoxy resin with additives for heat resisting qualities and safety.
13. 13.A method according to Claim 1, wherein the reproduction crown glass can be toughened for safety by the well known toughening process where the reproduction crown glass is heated to temperature normally not exceeding six hundred and fifty degrees centigrade then rapidly cooled within seconds to below five hundred and twenty degrees centigrade to provide the toughening stresses and IS crown glass although toughened will have less defined bulges or distortions but the toughening process can be altered in respect of timescale heating and cooling to have less effect on the desired bulges and distortions but not sufficiently to affect the toughening process.

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14. 14.A method wherein the selected perimeter width or cavity of the double glazed unit or spaced apart panes is constant the central cavity area will be variable where one pane selected is reproduction crown glass and filled with inert gas with either a single gas or combination of inert gases to provide the required insulation values.