CA1095247A - Temperature control of the counterflows of molten metal in the manufacture of float glass - Google Patents

Abstract

ABSTRACT
In the float process for flat glass manufacture, on a molten metal bath there is forward flow of molten metal entrained by accelerating glass over an upstream return flow of cooler molten metal which is received in a deepened region of the bath. Contact of the cooler molten metal with molten metal in the deepened region heats the cooler molten metal and local temperature variations in the molten metal are minimised.
Upstream molten metal flows are drawn from that deepened region to replenish the molten metal entrained by the accelerating ribbon of glass, so that the risk of local temperature variations in the molten metal supporting the accelerating ribbon is reduced and distortion in the undersurface of the ribbon is minimised.

Description

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This invention relates to a meth~d and appa~atus for the manufacture of flat glass. More particularly the invention relates to the manufacture of thin flat glass by the float process, for example float glass of thickness in the range 1.5 mm to 5 mm and more especially in the range 1.5 mm to 3 mm.
In the float process for flat glass manufacture, molten glass is delivered at a controlled rate on to one end, the hot end, of a molten metal bath contained in an elongated tank structure. Usually the molten metal bath is of molten tin or ~;
of a molten tin alloy in which tin predominates. The final ribbon of glass is discharged from the bath by traction means~
usually driven traction ro:Llers, disposed beyond the outlet end of the bath, which traction means applies tractive force to advance the ribbon along the bath.
In some ways of operating the float process, regulation of the applied tractive effort is effected along with regulation of the thermal conditions to which the advanci~g ribbon of glass is subjected so as to attenuate the ribbon to a desirea width and thickness.
When operating under high load conditions, for example at a rate of delivery of molten g]ass to the bath o~ 2,000 tonnes per week or more, a high speed oE discharge of the ultimate ribbon of glass from the bath, for example greater than 10 metres per minute, is necessary when attenuating the glass to thic]~nesses below 3 mm. When the advancing ribbon of glass is accelerating during attenuation to a uniform high speed for discharge from the bath, it entrains an appreciable quantity of the molten metal of the bath along the bath surface towards the outlet end of the bath, ` ~LQ~24~

which surface flow induces an upstream return flow of cooler molten metal from the outlet end of the ba-th along the bo-ttom of the bath towards the zone of the bath where the ribbon of glass is being attenuated. In this zone the glass is at a viscosity such that it is particularly susceptible to temperature variations across the surface of the molten me-tal bath, and it has been found that distortion introduced into the underface of the ribbon of glass in this attenuation zone is present in the ultimate ribbon.

Temperature variations across the surface of the bath can result from a temperature gradient through the depth of the bath and it is desirable to minimise such temperature gradients, particularly in the attenuation zone. However although a relatively small temperature gradient can be achieved by a relatively shallow bath depth at low ribbon speeds, a high ribbon speed over a shallow bath depth produces turbulence in the molten metal from which distortion in the ribbon can result.
greater bath depth will reduce turbulence at high ribbon speeds, but will inherently gi~e a grea-ter temperature gradient through the bath depth which can introduce distortion into the ribbon.

It has previously been proposed in Canadian patent No. 1~054~370 to combat the introduction of such distortion into the ribbon of glass in the attenuation zone by employing a first barrier at a first location in the region of the downstream end of said attenuation zone to constraiin molten metal flow at that location to forward flow of molten _ 3

2~7 metal entrained beneath the ribbon and counterflow of molten metal alongside the ribbon from downstream of that location, and employing a second barxier at a second location spaced upstream from said first loca-tion and in the region of maxlmum acceleration of the glass to constrain molten metal flow at that second location to forward flow of molten metal entrained beneath the accelerating glass and counterflow of molten metal alongside the ribbon ~rom downstream of the second location, there being established lateral access into the region of the bath supporting the ribbon between said first and second locations for said counter-flow of molten metal at said first location to ensure replenishment of the molten metal of the bath in the attenuation zone between the first and second locations.
Such an arrangement reduces the temperature difference between the surface molten metal and the molten metal beneath the surface in the a~tenuation zone, thereby reducing temperature variations in this zone which tend to introduce distortion into the ribbon.
The invention seeks to provide simplified control of the temperature of the counterflows oE molten metal replenish-ing the molten metal bath in the attenuation zone.
According to the invention there is provided a method of manufacturing flat glass comprising advancing a ribbon of glass along a molten metal bath, applying traction to the ultimate ribbon of glass to accelerate the glass to a final discharge speed thereby causing, as the glass accelerates, progressively increasing entrainment ~J ~
~,,,~f"~, of mol-ten metal of the ba-th over an upstream return flow of cooler molten metal from -the outlet end of the bath, and, in the region of the bath where the final discharge speed oE the ribbon is achieved, receiving the upstream return flow of cooler molten metal in a reserve region of greater bath dep-th than the bath depth adjacent that reserve region, from which reserve region of greater bath depth there are drawn upstream molten metal flows to replenish the molten metal entrained by the acceleratil1g ribbon.
The method of the invention may comprise containing the molten metal bath in a tank structure haviny a floor provided by abuttiny blocks of refractory material whose upper faces define the level of the bottom of the molten metal bath, and defining the reserve region of greater bath depth by blocks whose upper faces are at a lcwer level than the upper faces of the blocks defining the bath depth adjacent the reserve region.
Preferably the reserve region of greater bath depth exte~ds for a predetermined distance downstream sufficient to ensure mixing of the molten metal oE the re-turn flow with molten metal constituting the reserve region of greater bath dep-th.
Further the method of the invention may comprise constraining the upstream return flow of cooler molten metal to a depth less than the depth of the reserve region of greater bath depth, whereby the velocity of the return flow is reduced as the return flow enters the reserve region of greater bath depth and mixing bf the return flow with the molten metal in the reserve region is enhanced.
The invention may also comprise a method of manufacturing flat glass comprising advancing a ribbon of glass along a molt:en metal bath, applying traction to the ultimate ribbon of glass to ~L~95247 accelerate the glass to a final discharge speed thereby causing, as the glass accelerates, progressively increasing entrainment of molten metal of the bath over an upstream return flow of cooler molten metal from the outlet end of the bath, in the region of the bath where the inal discharge speed of the ribbon is achieved, receiving the upstream return flow of cooler molten ~ . , metal in a reserve region of greater bath depth than the bath depth adjacent that reserve region, constraining molten metal flow at a location immediately upstream of the reserve region of greater bath depth to forward flow entrained beneath the ribbon and counterflows alongside the ribbon from the reserve region of greater bath depth, and establishing lateral access to the region of the bath supporting the rlbbon upstream of the location for -the counterflows of molten metal which are drawn from the reserve region of greater bath depth to replenish the molten metal entrained by the accelerating ribbon.
Further the invention may provide regulating the applied traction to attenuate the ribbon to a desired width and thickness in an attenuation zone in which the glass accelerates along the bath, and enforcing the step of constraining molten metal flow at a location immediately upstream of the reserve region, which location is in the region of the downstream end of the attenuation zone.
Longitudin~l flow of molten metal along side regions of the bath may be obstructed at a position upstream of the location.
The longitudinal flow may be obstructed at a plurality of spaced positions upstream of the location.
The longitudinal flow may be obstructed at two spaced positions.
Still further the invention may provide electromagnetically inducing flows of molten metal through the lateral access to ~ S2~7 ^~ the region of the bath supporting the ribbon upstream of the location.
Flows of molten metal may be induced electromagnetically from beneath the ribbon upstream of -the locaticn to mix with the counterflow.
The counter~lows alongside the ribbon may be selectively ` heated.
Further the invention provides a method of manufacturing float glass of thickness in the range 1.5 mm to 3 mm, comprising applying marginal forces to the accelerating glass at a series of oppositely disposed positions spaced along the bath to control reduction in ribbon width and thickness, and enforcing the constraint of molten metal flow, a-t a loca-tion in the region of the downstream end of the at-tenua-tion zone and spaced downstream from the furthest downstream position at which marginal forces are applied to the ribbon.
Longitudinal flow of molten metal along side regions of the bath may be obstructed at least at one positi.on upstream from the location and spaced downstream from the furthest` downstream position of application of marginal forces to the glass.
The invention further provides a method of manufacturing flat glass comprising advancing a r.ibbon of glass along a molten me-tal bath, attenua-ting the ribbon to a desired width and thickness in an attenuation zone in which the glass accelerates along the bath, constraining molten metal flow in the region of the downstream end of the attenuation zone to forward flow entrained beneath the ribbon and counter-flows alongside the accelçrating glass from downs-tream of the attenuation zone, providing a deepened reserve zone of molten metal downstream of said attenuation zone in which return flows o~ molten metal from the outlet end of the bath '~ ' , . .

quiesce and are heated, supplying said coun-terflows by means of flows of molten metal from said reserve zone~ and from said counterflows feeding la-teral flows of molten metal drawn in-to -the molten metal flow entrained beneath the accelerating glass.
Still further -the invention provides a method of manufacturing flat glass comprising advancing a ribbon of glass along a molten metal bath contained in a tank structure having a floor of alumino~silicate refractory blocks whose upper faces are at different depths in different ~ :
regions of the bath~ attenuating the ribbon to a desired w~d-th and thickness in an attenuation zone in which the glass accelerates along a relatively shallow region of ~ ~.
the bath, providing a reserve of molten metal just down-stream of said attenuation zone in a relati~ely deep region of the bath defined in a recessed part of said refractory floor of the tank structure 3 receiving in said reserve return flows of cooled molten metal from -the ou-tlet end of the bath, which flows quiesce and are heated in said reserve, and directing flows of molten metal from that reserve alongside the accelerating glass to feed la-teral flows of molten me-tal drawn into the molten me-tal flow entrained in said relatively shallow region beneath -the accelerating glass.
The invention fur-ther provides a method of manufacturing ~lat glass comprising advancing a ribbon of glass along a molten metal bath contained in a tank structure having a floor of alumino-silicate refractory blocks whose upper .
faces are at different depths in different regions of ~ .
the ba-th, attenuating the ribbon to a desired width and.
thickness in an attenuation zone in which the glass ; . : '':; , ~

1~ ~ 5 ~ ~ ~
accelerates along a relatively shallow region of the bath, cons-training molten metal flow in the region of the down-stream end of the attenuation zone to forward flow entrained beneath the ribbon and counterflows alongside the accelerating glass from downstream of the a-ttenuation zone, providing a reserve o~ molten metal just downstream of said attenuation zone in a relatively deep region of the bath defined in a recessed par-t of said refractory :Eloor of the tank structure~ receiving in said reserve return flows of cooled molten metal from the outlet end of the bath, which flows quiesce and are heated in said reserve, and directing flows of molten metal from said reserve into said colmter~lows alongside -the accelerating glass to feed lateral - flows of molten metal drawn in-to the molten metal flow en-trained in said relatively shallow region beneath the accelerating glass.
The invention alsp comprehends apparatus for manufacturing M a-t glass comprising an elongated tank structure having end walls) side walls, and a floor for containing a bath of ~0 molten metal, means for delivering glass to the bath at a controlled rate ? and advancing the glass in ribbon form along the ba-th, and means for applying traction to -the ul-timate ribbon of glass -to accelerate the glass to a final discharge speed9 and wherein~ in the region of the tank structure where the ribbon achieves its final discharge speed, the floor of the tank struc-ture is deepened to define a reserve zone for receiving cooler molten metal ~low which is enforced in an upstream direction over the floor by -the entrainment of hot~
ter molten metal by the advancing ribbon of glass.
A preferrecl embodiment of the apparatus comprises a transverse barrier on -the floor of -the tank structure a-t a location immediately upstream of said deepened tank floor ?

_ 9 _ which barriex extends beyond the position of the edges of the ribbon with the top of the barrier positio~ed below ~ -the level o~ the bath surface hy a distance which is effective to constrain molten metal flow at that loca-tion substantially to forward flow of molten metal entrained beneath the ribbon and counterflow of molten metal alongside the ribbon.
The barrier may extend beyond the position of the -~
edges of the ribbon but stops short of the tank side walls.
The reserve zone defined in the floor of the tank structure just downstream of said barrier may be of greater depth than the bath upstream of said barrier.
The depth of the reserve zone may he approximately twice the bath depth adjacent said zone.
Preferably the reserve zone extends across the full width of the floor of the tank structure.
In a preferred embodiment the tank structure is encased in a metal casing, the floor of the tan]c structure comprises abutting blocks of refractory material which are secured to the metal casing, and said reserve zone of , greater bath depth is defined by blocks whose upper faces are at a lower level than the upper faces of the adjacent blocks.
The upper faces of the blocks upstream and downstream of the reserve zone may be at the same level. Preferably the blocks are of alumino-silicate refractory.
In one embodiment the floor of the tank structure downstream of said barrier is constructed to define, considered in the downstream direction, said reserve zone ., . : -. . , i of ~reater depth than the b~th depth upstxeam of said barrier, a reyion of lesser depth than the reserve zone, and a further region of yreater depth than the bath depth upstream o~ said barrier which further region extends to the outle-t end of the tank structure.
An abrupt step may be provided where the floor defines a change in bath depth.
In the preferred embodiment the elongated tank structure has a shoulder region which joins an upstream part of greater bath width to a downstream part of lesser bath width~ said reserve zone of greater bath depth is located at said shoulder region, and the barrier is located just upstream of said shoulder re~ion.
Top rol]s may be arranged to enga~e the upper surface of the ribbon margins at a series of oppositely disposed positions along the bath to control the reduction in width and thickness of the ribbon, the pair of top rolls furthest downstream being at a position spaced upstream from said barrier.
~t least one pair of baffles may be located adjacent `~
the bath side walls at oppositely disposed positions upstream frvm said barrier and spaced downstream from the furthest downstream pair of said top rolls to obstruct longitudinal flows of molten metal alon~ the bath side walls at those positions.
The apparatus may also include linear induction motors mounted over the bath surface in the region of the barrier to induce flows of molten metal electromag-netically.
Heaters may be mounted adjacent the tank side walls , ~
, .~.

,, .. . .

upstream of the barrier to apply local heating to the counterflows of molten metal.
The invention also comprehends flat glass produced by the method of the invention.
Some embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which:-Figure 1 is a plan view of an elongated tank structure containing a bath of molten metal for use in the float process for the manufacture of thin flat glass by the method of the invention, Figure 2 is a longitudinal section through the floor of the tank structure of Figure 1, Figure 3 is an enlarged view of part of Figure 2 , further showing a glass ribbon, Figure 4 is an enlarged view of part of Figure 1, Figure 5 is a view similar to Figure 4 illustrating a modification of the apparatus of Figures 1 to 4, Figure 6 is a view similar to Figure 5 illustrating a further modification of the apparatus o:E
Figures 1 to 4, Figure 7 is a view similar to Figure 5 illustrating further modifications to the apparatus of Figures 1 to 4, and Figure 8 is a longitudinal section through part of the floor of the tank structure illustra-ting another way of constructing the floor ~s~

Referring to the drawin~ Figure l illustrates in plan an elongated tank s-tructure for the manufacture of flat glass by the float process. The tank structure comprises an end wall 1 at its inlet end and an end wall 2 at its outlet end, and parallel side walls 3 extending from the inlet end to a shoulder region defined by inwardly inclined side wall portions ~
which connect the side walls 3 with further parallel side walls 5 extending to the outlet end. The -tank structure contains a bath of molten metal which is ~;
usually molten tin. The geometry of the tank structure is such that it will accommodate between the sicle walls

3 of its wide part upstream of the shoulder regi.on the maximum required glass layer on the bath surface, and between the side walls 5 of its narrow part downstream of the shoulder region the maximum required ultimate ribbon width.
Molten soda-lime-silica glass is clelivered on to the bath at the inlet end of the tan]~ structure by pouring from a spout 6 which extends over the inlet end wall 1. ~ regulating tweel 7 controls the rate of flow of molten glass over the spout on to the bath surface 8.
In manner well known in -the float process, temperature regulators are provided in a roof structure .
not shown which is mounted over the tank structure, and defines a headspace over the bath in which a protective atmosphere is maintained. Temperature conditions at the inlet end of the bath are such that the molten glass 9 arriving on the bath is permitted to flow freely,

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laterally unhinAered, during the firs-t part of its advance along the bath. The temperature of the glass is 990C when maximum spread is achieved and the glass -thickness is of the order o f 7 mm. The layer of mol-ten glass is advanced in ribbon form~ and the ribbon is initially constitu-ted by low viscosity glass, e.g. at a viscosity of 104-~ poises. This glass is gradually cooled during its initial advance along -the ba-th and ïts viscosi-ty slowly increases.
The temperature regulators in the roof s-tructure set a temperature regime to which -the advancing glass is subjec-ted, which regime maintains the glass in a deformable state over a longitudinally ex-tending region of the ribbon in which the glass is progressively atten-uated as its velocity increases under the influence o tractive effort applied to -the ultimate ribbon of glass 10 by driven rollers 11 located beyond the outlet end wall 2 of the tank structure. As -the viscosi-ty of the glass increases so does the influence of the longitu-.~ , dinally directed tractive force, originating from the rollers 11, in stretching the ribbon of glass. Gradual and progressive reduc-tion in width and thickness of the glass is controlled by the use of top rolls which engage the upper surfaces of the margins of the glass. `-. Initially while the glass is at a low viscosity the margins of the ribbon are engaged by a palr of inclined top rolls 12 mounted at oppositely disposed positions on shaf-ts 13 which extend through -the -tank side walls 3 and are driven by motors 14~ The top rolls 12 are knurled or too-thed graphite, s-tainless steel, or ~ .

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mild steel rol]s which are internally water cooled.
m e axes of the rolls are inclined at an angle to a line at right angles to the direction of advance of the ri~bon of glass along the bath~ Outwardly and longitudlnally directed forces are thereby applied to the margins of the nascent ribbon9 the outward force componen-ts providing restrain-t against undue loss in width. Slight attenuation o~ -the ribbon is begi~ning to occur in -this region.
~urther similar pairs of top rolls 15, 16, 17, 1 and 19 are provided spaced along the tan~ structure, being mounted on respective shafts 20, 2i, 22, 23 and 24 and driven by motors 257 26, 27, 28 and 29, the -top rolls o~
each pair being at oppositely disposed positions. With ?5 such pairs of top rolls a-t a series of spaced positions along -the bath control of -the progressive decrease in ribbon width and thickness is achieved. As the glass passes beyond the last pair of top rolls 19 its -tempera-ture is cooling below 880C cor~esponding to a viscosi-ty of 105'2 poises.
A~ter the glass leaves the furthes-t do~ns-tream pair of top rolls 19 its width and -thickness con-tinues to reduce un-til a position at or near the shoulder region of the tank struc-ture where its viscosity, under 2$ the applied temperature regime, is so high that the rîbbon a,ssumes i-ts ~inal width and thickness and achieves its ~inal discharge speed which is the effective surface speed of the rolls 11.
At or near the shoulder region the usual soda-lime-silica glass has a viscosity of 107 poises~

' ~ ',' . ~ ` '~ '' .' ' ~5~7 correspondi.ng to a temperature of 750C and is in a condition in which no further dimensional change can take place under the influence of the applied traction~
The rib~on co`ols further during its t~avel between the side walls 5 to the ou-tle-t end of the ba-th.
The g].ass is accelera-ted and the ribbon is attenuated in a zone upstream of the shoulder region of-the tank struc-tureO The downs-tream end of this atten- :
ua-tion zone ls generally at or near the shoulder region, and the position of maxim~ acceleration of the glass is generally up~tream towards the last pair of top rolls 19. As the ribbon is accelerating in the attenuation æone there is progressively increasing entrai~men-t of . molten me-tal of the bath in a forward surface flow which travels towards -the outlet end o~ the bath. This forward surface flow is over an upstream return flow of cooler molten me-tal from the outle-t end of the ba-th,and rnol-ten metal is continuously being drawn under the ribbon to compensate for that which is entrained. I-t is t~e generalised return flow of cooler molten ~etal along the bo-ttom of the bath ~hich produces -top to bot-tom temperature gradients -through the depth of the ba ~ ~Irhich have been .:
shown to be-particularly -troublesome in the region of the bath where -the rapidly accelerating ribbon is being attenuated, and particularly in the region between the las-t pair of top rolls 19 and the shoulder region. To combat this effect the up.stream return flow of cooler mol-ten me-tal is received in a reg~ion of greater bath depth than the bath depth adjacent that deeper region~
.

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~igure 2 shows the profile of -the floor FL of the -tank structure which provides different bath depths at different regions along the bath leng-th. At the inle-t end o~ the bath the floor defines an initial region 30 of greater depth than -the shallower region 31 following downstream, which latter region 31 provides the major leng-th of the bath ups-tream of -the shoulders and under-lies virtually -the entire attenuation zone~ The initial region 30 may have a depth which is approximately one and a half -times that of the downstream region 31. ~or exam~le the region 30 may have a depth of 83 mm and the region 31 a depth of 58 mm.
The region 31 extends downstream to a posi-tion close -to the shoulder region, for example to a position one or two metres upstream of a line joining the upstream ends of the shoulder side walls 4.
At this position there begins a region of greater bath dep-th than the bath dep-th adaacent that region. The deepened floor of the tank structure which defines the pocket region 32 is shaped as a recess in the floor extending across the full width of the bath. This pocket region 32 includes -the shoulder region and extends downstream a distance of about 3 rnetres beyond a line ~oining the downstream ends of the shoulder side walls 4 The pocket region 32 extends, for example, length-wise of the bath over a total distance of 7.5 metres, and provides a reserve zone for receiving cooler molten metal flow which is enforced in an upstream direction over the floor by the entrainment of hotter molten metal by the advancing rlbbon of glass. The depth of the .

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region 32 is approxima-tely twice the bath depth in -the adjacent upstream region ~1. For example, when the depth of the regi.on 31 is 58 mm the bath depth in the reserve zone 32 may be 108 ~n.
Downstream of the recess 32 the floor rises again9 for example o~er a len~th of 3 metres to provide a région adjacent the reserve zone the same bath dep-th as that of the region 31 upstream of the reserve zone. From the region 33 to t-he outlet end of the bath the floor level is such as to provide a region 34 having a bath depth the same as that at the inlet end region 30 of the bath, tha-t is, less than the depth of the reserve zone.
Where there is a change in floor level providing a change in bath depth the step in the floor may be chamfered as shown in Figure 2 or an abrupt step as shown in Figure 8 The provision of a deepened reserve zone alone 5 ~' such as the recessed pocket 32, in the region of the bath where thè final discharge speed of the ribbon is achieved has been found to be beneficial, since the pocket receives the upstream return flow o~` cooler molten me-tal, and mixes that cooler molten metal with molten metal hel.d in the reserve zone, so that the cooler molten metal is heated and there is minimal risk of the introduction of thermal inhomogeneities beneath the accelerating glass due to molten metal flows drawn upstream from the reserve zone to replenish molten metal entrained by the accelera-ting ribbonO
The ef~ect of the reserve zone is enhanced in the embodiments illustrated by the provision at a location .
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immediately upstream of the re~ion 32 o~ greater bath depth, of a transverse barrier 35 which projects upward-ly from the floor. The barrier 35 is a carbon bar of upstanding rectangular cross-section and has a dove-tail base 36 Figure 3, which is keyed into a matchingdove-tail groove 37 formed transversely of the bath tank structure in the floor at the downstream end of the region 31 -that is, in the re~ion of the downskream end of the attenuation ~one. The flat top of the bar is about 50 mm long in the direction of ribbon advance and is spaced from the level of the bath surface 8 by a sufficient distance to constrain molten meta:L flow at that location to forward flow 39 entrained beneath the ribbon and counter-flows alongside the ribbon from the region 32 of greater bath depth. The barrier 35 ensures that the lower layers of entrained molten metal o~ the forward flow are directed downwardly and then upstream as indicated by the arrow 38 in Figure 3. Usually the top surface of the barrier 35 is from 6 mm to 15 mm below the level of the bath surface, the optimum distance dependin~ on the speed and accelera-tion of the ribbon. In principle the top of the barrier 35 could be at a depth below the level of the bath surface 8 which is exactly such that all the entrained molten metal of the forward flow 39 travels over the barrier but no return flow o~ molten metal can pass over it. In practice, however, such an exact setting is difficult to achieve and the barrier height is therefore preferably set as described above to direct the lower layers of entrained molten metal of the forward flow as indicated at 38.

` ~L6i '~5~7 The molten metal flows ~ are ~ixected outwardly and have a beneficial effect on the temperature of the molten metal alongside the ribbon by intermingling or mixing with cooler upstream counterflows from the reserve zone as described below.
In the embodiments illustrated the barrier 35 extends transversely of the bath beyond the positions of the edges of the ribbon but stops short of the side walls 3. The ends of the barrier 35 are thus spaced 10 from the ~ide walls 3 to define channels for counter-flows of molten metal indicated by the arrows 40 in Figure 4, from the reserve zone 32 downstream of the barrier, round its ends, and into the region upstream ~;
of the barrier.
The barrier 35 obstructs direct return flow of molten metal along the bath bottom into the region upstream of the barrier location, but permits counter-flow round the ends of the barrier from the region of ;
greater bath depth thereby establishing lateral access 20 to the region of the bath supporting the ribbon as it is being attenuated by acceleration of the glass upstream of the barrier location.
The transverse barrier 35 is at a location immediately upstream of the upstream end of the deepened 25 region of the bath. For example the barrier 35 may be 150 mm from the ups-tream end of the pocket region 32.
Upstream flow, indicated by arrows 41 in Fi~ures 3 and 4 of cooler molten metal travelling along the bath bottom in an upstream direction towards the barrier location is 30 received in the pocket region 32 of greater depth just ., ~

downstream of the barrier 35. This reduces the velocity of the cooler return flow, thereby giving time for mixing the molten metal of said return flow with molten metal constituting said region of greater bath depth, so that there is time for heating of the molten metal of the return Elow to occur. The molten metal in the pocket region 32 effectively acting as a buEfer.
Replenishment of molten metal supporting the accelerating glass occurs by the counterflows 40 of molten metal from the pocket region 32 round the ends of the barrier 35 and into the region upstream thereof, the established lateral access enabling those counter-flows to be drawn under the ribbon.
The provision of the region 32 of greater bath depth in which the return cold flow of molten metal is received, ensures that the counterflows of mol-ten metal alongside the ribbon round the ends of the barrier 35 can have a relatively small temperature difference as between the surface molten metal and the molten metal below the surface. Such a small temperature difference as between the top and the bottom of the molten metal reduces the risk of local temperature variations in the molten metal on which the ribbon oE glass is carried as it is accelerated, thereby minimizing distortion in the undersurface of the ribbon.
Examples of measured top and bottom molten metal temperatures at positions just alongside the edge of the ribbon are given below, the temperatures being measured by thermocouples at a position A towards the downstream end of the pocket 32, that is 6 metres 2~ -~5~

downstream of the barrier 35; a posltion B around the middle of the pocket 32 3 metres downstream of ~he barrier 35; a position C just downstream of the barrier 35, that is at the ups-tream end of the pocket 32; a position D just upstream of the barrier 35; a position E 3 metres upstream of the harrier 35; and a position F 6 metres upstream of the barrier 35, that is 2 metres downstream of the last top rolls 19.
In one example of operation molten glass was del.ivered to the bath at a rate of 3326 tonnes per week to produce an ultimate ribbon 2.5 mm thickness having a gross width of 3.74 metres travelling at a ~-~
speed of 865 metres per hour. The pairs of top rolls 12, and 15 to 19 were spaced alony the bath at 3 metre intervals with the last top rolls 19 positioned 8.2 metres upstream of the barrier 35, and were disposed with their axes at angles of slew to an axis at right angles to the:direction of ribbon advance and were driven at peripheral speeds as follows:-20Top Rolls Slew Angle Speed ~, , 12 2 165 m/hr

5 181 m/hr 16 7 201 m/hr 17 go 232 m/hr 1% 9 291 m/hr 19 go 3~0 m/hr ;~
The top and bottom tin temperatures at the above ~:~
mentioned positions just alongside the edge of the ribbon ~:
were measured as follows:-~s~
Top Tin Temperature Bottom Tin PO F. 1 tion ~ _ Tem _ rc~-turc (C) . ~3 807 . 797 C . 818 812 It wil:l be seen that just ups-tream of the barrier 35, at position D, -the top -to bo-t-tom bath -teMperature difference was zero, and was less than 5C at a posi-tion 3 metres further upstream at posi-tion E.
It has been :Eound -that -the -temperature uniformi-ty can be fur-ther improved 9 i.e, -the top -to bo-ttom temperature diference in the molten me-tal reduced further upstream of the barrier, if longi-tudinal molten me-tal :Elo~s adja-cent the ba-th side ~ralls are obs-truc-ted at a posi-tion upstream ~rom the barrier location. To achieve this a pair of carbon baffles or flags 42 ma~T be mounted adjacent the side walls ~ respectively at opposed posi-tions spaced upstream from the barrier 35 as sho~m in Figure 5. The flags or baffles 42 ha~e a height greater than the bath dep-th, are seated on -the floor and abut agains-t -the side walls so as to obs-truc-t completely longitudinal mol-ten metal flo~.~s adjacent the side ~alls.
lt is believed tha-t such obstruction of longitudinal side lol~rs :improves in-termingling or mixing of out-rardly direc-ted flows, indicated by arro~s 43, of relatively hot surface molten meta]. from beneath the ribbon, ~ith the co~lterflows 40 of cooler molten metal coMing from - 2~ ~

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the de~eper bath xegion downs-tream o~ the barrier, such mixing~ or intermingling occurring alongside and not under the ribbon.
The flags or baffles 42 are believed to prevent the counterflows 40 from travelling along the bath side walls and -then under the ribbon at an ups-tream posi-tion without having mixed with the ~lows 43.
In an example o~ opera-tion carbon flags or baf.~les 42 were mounted adjacent the side walls 3 at opposed positions 3 metres upstream of -the barrier location, the fiags projecting inwardly from the side wall by a distance of 460 mm. Molten glass was delivered to the bath at a rate o~ 3400 tonnes per week to produce an ultimate ribbon o~ 2~5 mm thickness having a gross wid-th of 3.62 metres and travelling at a speed of 865 metres per hour. The top roll positions were the same as in -the previously desc~ibed example but the angles of slew of -the last three pairs were altered, and the speeds very slightly different as follows:- -~ 3~ peed : 12 2 165 m/hr . 5 182 m/hr 16 7 202 m/hr - 234 m/hr 18 ~o 292 m/hr 19 8 338 m/hr With -this arrangement the top and bottom bath temperatures at positions just alongside the ribbon edge upstrearn of the barrier were measured 7 the actual positions in th s case being posi^tion G 3 metres upstream ~:

-. - 24 - i from -lhe barrier and 1 metre do~ls-tream of t~le carbon flag or ba:`fle 42; posi-tion H 2,1 metres upstream of the carbon flag or baffle 42; and posi.-tion I approx-imately at -the po;sition of -the last top roll 19. The measured temperatures were:-Top Tin Tempera-ture Bo-ttom Tin Position (C) _ Tem~erature (C) It wil] be seen that at position G just downstrearn of the carbon flag or baffle 42 the top tO bottom temperature difference was only 2C, the bath bottom in ~act being hotter than the top and a-t posi-tion H ups-tream of the flag or baffle the difference was only 9C, which compares favourably with the 15 difference a-t roughly the same posi-tion F in the previous example. Even at the last top roll positlon I the top to bottom bath temperature difference was only 12C.
Longi-tudinal molten metal flows acljacen-t the bath side walls may be obstruc-ted a-t more than one position upstream from -the barrier location. For example, as ShOWIl in Figur~ 6, there may be provi.ded a further pair of carbon flags or baffles 44 mounted adjacent the side walls 3 at opposirtely disposed positions and spaced down~
stream from the flags or baffles 42 so as -to be located close to, but slightly upstream of, the barrier location.
The spaces between the end of the barrier 35 and the inner ends of the flags or baffles 44 iS sufficien-t to permit counterflows 40 of rnolten rnetal therethrough. The 5~47 dimensions of the flags or baf~les 42 and 44, i.e, the extent to which they project inwardly from the bath side walls 3, are selec-ted to suit the particular requirements of operation and -the upstream flags or baf~les 42 may project inwardly a different distance from that of the downstream flays or baffles 44. The effect of the addi-tional pair of flags or baffles 44 as shown in Figure 6 is similar to that of the .~irst pair 42 in that they are believed to cause outward flows 43 of hot molten metal from beneathi the ribbon better to mix or inter~
mingle with the counterflows 40 at a position alongside the ribhon, ancl to prevent the counterflows 40 :Erom travelling along the bath side wall and then under the r.ibbon at an upstream position without mix.ing.
Figure 6 also shows an additional pair of top rol].s 45, moun-ted on shafts 46 driven by motors 47, at oppositely disposed positions spaced downstream of the top rolls 19. This fur-thest downstream pair of top rolls 45 are useful when producing glass thinner than that of the previous examples.
In one example of operation with an arrangement as shown in Fi~ure 6 molten glass was delivered to the bath at a rate o:E 3380 tonnes per week to produce an ultimate ribbon of thickness 2.3 mm having a gross width of 3.65 metres and travelling at a speed of 940 metres per hour. The carbon flags or baffles 42 projected inwardly 610 mm ~rom the side walls 3 and the carbon ;
Elags or baffles 44 projected inwardly 460 mm from the side wails 3. The position of the top rolls 12 and 15 to 19 were as described in the previous examples and - ~6 -, ,.;. ~ . ~ .. , S2~
the addltional -top rolls 45 were at a position spaced 3 me-tres downstrearn from the top roll~s 19~ that is 5,2 me-tres upstream from -the barrier 35 and 2~2 me~res upst.ream from the fla~s or baffles 42. The sle~ an~les and peripheral speeds o~ the driven top rolls were as .~ollo~s:-olls Sle~ A~ S~eed 12 2 164 m/hr 3 182 m/hr 10. 16 5 202 m/hr ~7 7 234 m/hr 18 8 2g2 m/hr 19 8 338 m/hr 8 400 m/hr , The -top and bottom bath ternperatures were measured ju~st alongside the ribbon edge at a position .J 3 me,-tres downstrearn f`rom the barrier 35, that ls in the pocket 32; position K just do~rnstream of the barrier 35 a-t the upstream end of -the pocket 32; position L just upstream of the barrier 35 and flag 44; posi-tion M
approximately a-t -the position of the top roll 45; and at position N approximately at -the position of the top roll 19. The measured temperatures ~ere:-Top Tin Temperature Bottom Tin Po~sltion _ ~ _ T~m~:

- L ~42 842 3~ N 865 856 , -~9~
.
It will be seen that -the top to bo,ttom temperature difference JUSt upstream of the barrier 35 a-t position L was again zero, as at positi.on D in the example described above wi-th r'e,ference to Figure 4. The top to bottom temperature difference at the position of the top roll 19s posi-tion N, was only 9C. However, at -the position of the last pair of top rolls 45, the top to bottom bath -tempera-ture difference was somewhat higher being 17C a-t posi~
tion M.
The flags or baffles 42 and 44 were then changed -to increase their length by 150 mm so that the ~lags or baffles 42 had a length of inward projection from the ~ide walls 3 of 760 mm and the flags or baffles 44 had a length of inward projection of 610 mm. The inner ends 15, f -the flags or baffles 42 were then only 155 mm from the edges of -the ribbon.
In an example of operati.on wi-th this modified flag or baf~le arrangement9 molten glass was delivered to the bath at a ra-te of 3370 tonnes per week to produGe an ultimate ribbon of 2.3 mm thickness havirlg a gross wid-th o~ ~.58 metres travelling at a speed of 940 me-tres per hour. The positions of the top rolls 45 were moved 610 mm upstream so as to be about 2. 45 me-tres from -the top rolls 19. The top roll angles of slew and speeds 25, were:- .
Sle~ Ang~
12 2 162 m/hr ' ,~
3 180 m/hr , 16 - 5 201 m/hr 3 . 17 6 232 m/hr .

-- Z~3 --~ 7 18 7 284 m/hr 19 7 3~0 m/hr 45 7 493 m/hr With -this arrangement the top to bottom bath temperature difference at the position o~ the las-t top rolls 459 that is position M in Figure 67 was reduced to 12C.
In another example of operation with an arrange-ment as shown in Figure 6 a ribbon of thi~mer glass was produced at a considerably increased ultimate ribbon speed. Molten glass was delivered to the bath at a rate of 3410 tonnes per week -to produce an ultimate ribbon o~ thickness 1.8 mm having a gross width of ~.37 me-tres travelling at a speed of 1252 metres per hour. The flags or ba~fles 42 and 44 were posi-tioned as in the previous two examples, but the flags l~2 had an inwardly projecting leng-th of 510 mm and the flags 44 a length of 610 mm. That is, in this example the downstream flags 44 were slightly longer than the upstream flags 42, The top rolls were positioned as in the last previously described example and had slew angles and speeds as follows:-Top Rolls S:Le- A~ S~eed , 12 2 163 m/hr ~o 180 m/hr 16 5 201 m/hr 17 6 232 m/hr 18 10 284 m/hr 19 10 ~24 m/hr 11 402 m/hr - 2~ -. .

~os~

Top alîd bo-ttom bath tempera-ture measuremen-ts just alongside the edge of the ribbon ~/ere t~ken at the previously described positions J, K, L~ M and N as well as at further do~,ms-tream positi^.ns, narnely a position 0 just downstream OI -the do~mstream end OI -the pocket 32 and a position. P in the pocket 32 just upstream --of its d~mstrearn end. I~he measured temperatures ~/ere:-Top Tin Temperature Bottcm Tin _si-tlon ~

P 77~ 75 15 ` ~ 837 818 It will be seen that the top to bottom temperature difference, are 14C at position M at the last top rolls 45; and 19C at position N at the next to last top rolls '19. However, even at this high ribbon speed which is 45% faster than in the first two e~:a.np1es described above and 33~ faster than in the other examples, it will be seen that the top to bottom bath temperature difference juat ups-trearn of -the barrier at position L was only 1~
Further, the effectiveness OI the relatively deep pocket region 32 is particularly apparent froTn this example i.n that the top to bottom bath temperature difference at the do~nstream. end of the pocket 32l posi-tions 0 and P, was 20C, but was reduced to 10C at the upstream end of -the pocket, positions J and K.

. . .

.

.
. . .
; . . ;

~!915Z~7 It wa.s also foun~ that the pocket 32 and barrier 35 arran~emen-t had an advantageous effect in reducing lateral temperature variations across the bath and edge to centre temperature varia-tions in -the ribbonO
The barrier 35 need no-t stop short of the side .walls of -the -tank s-tructure as in -the embodimen-ts illustrated, but rnay extend right up to -the side ~alls 3 with recesses in the top of the barri.er alongside the ribbon to provide chalmels for the counterflows of mol-ten ~0 metal drawn ~rom the deepened region 32.
The region 33 of lesser b~-th dep-th immediately downs-tream of the region 32, has the same depth as the region 31 upstream of the barrier 35. The region 33 separa-tes the deepened region 32 ~rom the ou-tlet region 34 which has a bath depth less than -that in the deepened region 32 but grea-ter than the depth of the regions 31 and 33. The upstanding region 33 provides some obstruc-t.ion to return flow of cold molten me-tal along the very ~:
- bottom of the bath in the outlet region 34, whereby the veloci-ty o~ the re-turn flow is reduced as the ti return flow enters the region 32 o~ greater ba-th depth and mixin~ of the return flow with the molten metal in the region 32 is enhanced. The region 33 also provides a reglon of relatively shallow bath depth at which ~near mo-tors mounted over -the bath can be used particularly effec-tively -to control molten metal flowsO
The bath depth may ho~ever be constant from -the downstream end of the pocket region 32 to the ou-tlet end of -the bath~ The provision of an increased bath ~.depth along -the outlet regi.on 34, relative to that in J
. , ', ' _ 3~
,.
t , ~ O~i5 .

the region 31 upstream of the barrler 35j facilitates -the effec-tive location of coolers in the outlet end of the ... ' !:: .
bath. - .
If desired, linear induction motors may be employed to strengthen or control molten metal flows in the region of the barrier 35. Fi~ure 7 ~hows a pair of such motors 48 mounted above the bath surface upstr~m of -the barrier 35 to induce electromagnetically flows of molten metal from the counterflows 40 -to enter beneath the accelerating ribbon. Alternatively, the motors L8 may induce molten metal flow in an outward direction to streng-then the outward flows 38 and/or 43 and assist mixing or inter~
mingling of those ou-tward flows with the coun-terflows 40. Linear induction motors may also be positioned as indica-ted in broken line at 49 i~ Figure 7 to assist movement o~ molten metal in the pocket 32 into the counterflows 40. Further linear induction motors may be posi-tioned as-indicated at 50 and 51 to direct the counterflbws.
I~nersed or partially immersed heaters adap-ted to effect selective local heating of molten metal flowing under the heaters may also be employed to heat the counterflows. For example, a pair of such heaters 52 may be located one adjacent each end of the barrier 35 to heat the coun-terflows 40. If necessary, small extension pieces 5~ may be provided at each end of the barrier to ensure that all the molten metal flow past tha-t end of -the barriers travels under the respective heater 52. Heate~s may be employed in conjunction l~ith ~0 or in place of the linear induction motors at positions 50 and 51.

,.: ' - ~ . .. .

~ig52~47 As sho~n in Figure 8, -the floor FL.o~ -the tank structure may be formed by abutting blocks 54 of refractory ma-terial, preferably alum.ino-si.licate refractory, whlch are secured in kno~ln manner to a metal shell or casing 55 which encases the tank .structure. The upper faces of the blocks define -the bottom of the molten metal bath. The reserve zone 32 cf greater bath depth is defined by bloc7cs having a height dimension less -than that of -the blocks in ~h~ adjacent regions 31 and 33 so that the upper faces of the b1ocks in the zone 32 axe a-t a lower level -.
than -the upper .faces of the adjacent blocks.
However, as shown in Figures 2 and 3, in which the vertical dimension is grea-tly exaggerated relative to the horizon-tal dimension~ the blocks may be arranged to provide a stepped bot-tom to the tank s-tructure cO
tha-t a-t the inle-t end of the -tank struc-ture bath blocks of the same height dimension have -kheir upper f~ces at different levels to provide different. bath dep-ths in regions 30 and 31, and in the region of the ou-tlet end of the bath blocks of differen-t height di.mensions have -their upper surfaces at the same level to provide the same ba-th depth in the region 3l~.
The me-thod and appara-tus of the present inven-tion is especi.ally advantageolls for producing float glass of thiclcness in the range 1.5 m~ -to 3 mm.. The inven-tion can be used to advantage in producing float glass of greater -thicl{ness ~Yhen the load and ribbon speed are such that di.sa(lvantaOeolls molten me~al movemen-t occurs, for example glass of thickness up to 5 mm or more.
... .. " ' ' ,: ~
- 33 ~

..

. . : , ~ -. . . ..
' `"

~LO~9:2~7 .

The method and apparatus of the invention c~n be used when producln`g glass of even greater thicknesses~
Al-thouOh specifically described above .in relation . to a bath having a shoulder region? ~he invention can be : 5 applied to a tank struc~ure having parallel side ~allsextending at a constant spacing ~rom the inle~ end -to ^the ou~let end o~ the tank structureO
I~ desired an addltional barrier or addition barriers may be located on the floor of the tank structure . 10 ef~ec-tively to project upwardly into the bath at a posi-tion or positlon.s spaced upstream from the barrier 35, for example as described m the abovementioned patent I applica-tion.
.-i Further the barrier ~5, althouGh conveniently constructed and moun-ted in effectively ~ixed fashion in .the floor as described abo~e, could take a different - formg for example as described in the abovementioned paten-t applicationi. and could, in par-tioular, be cylindrical. Any addi.tional bar~ier or barriers may also take any of the ~orms described in the above mentioned patent application and may, if desired~
'~ I be mo~rable between different positions along the ..
\1 bath as therein descrlbed,.. ~

. . : ~ : . .
~.
.. ~: .
: . . , ,: , .
. : - .,. , , .
.

Claims (34)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A method of manufacturing flat glass comprising advancing a ribbon of glass along a molten metal bath, applying traction to the ultimate ribbon of glass to accelerate the glass to a final discharge speed thereby causing, as the glass accelerates, progressively increasing entrainment of molten metal of the bath over an upstream return flow of cooler molten metal from the outlet end of the bath, and, in the region of the bath where the final discharge speed of the ribbon is achieved, receiving the upstream return flow of cooler molten metal in a reserve region of greater bath depth than the bath depth adjacent that reserve region, from which reserve region of greater bath depth there are drawn upstream molten metal flows to replenish the molten metal entrained by the accelerating ribbon.

2. A method according to Claim 1, comprising containing the molten metal bath in a tank structure having a floor provided by abutting blocks of refractory material whose upper faces define the level of the bottom of the molten metal bath, and defining said reserve region of greater bath depth by blocks whose upper faces are at a lower level than the upper faces of the blocks defining the bath depth adjacent said reserve region.

3. A method according to Claim 1, wherein the reserve region of greater bath depth extends for a predetermined distance downstream sufficient to ensure mixing of the molten metal of said return flow with molten metal constituting said reserve region of greater bath depth.

4. A method according to Claim 3, comprising containing the molten metal bath in a tank structure having a floor provided by abutting blocks of refractory material whose upper faces define the level of the bottom of the molten metal bath, and defining said reserve region of greater bath depth by blocks whose upper faces are at a lower level than the upper faces of the blocks defining the bath depth adjacent said reserve region.

5. A method according to Claim 1, Claim 2 or Claim 3, comprising constraining said upstream return flow of cooler molten metal to a depth less than the depth of said reserve region of greater bath depth, whereby the velocity of the return flow is reduced as tile return flow enters said reserve region of greater bath depth and mixing of the return flow with the molten metal in said reserve region is enhanced.

6. A method of manufacturing flat glass comprising advancing a ribbon of glass along a molten metal bath, applying traction to the ultimate ribbon of glass to accelerate the glass to a final discharge speed thereby causing, as the glass accelerates, progressively increasing entrainment of molten metal of the bath over an upstream return flow of cooler molten metal from the outlet end of the bath, in the region of the bath where the final discharge speed of the ribbon is achieved, receiving the upstream return flow of cooler molten metal in a reserve region of greater bath depth than the bath depth adjacent that reserve region, constraining molten metal flow at a location immediately upstream of said reserve region of greater bath depth to forward flow entrained beneath the ribbon and counterflows alongside the ribbon from said reserve region of greater bath depth, and establishing lateral access to the region of the bath supporting the ribbon upstream of said location for said counterflows of molten metal which are drawn from said reserve region of greater bath depth to replenish the molten metal entrained by the accelerating ribbon.

7. A method according to Claim 6, comprising regulating the applied traction to attenuate the ribbon to a desired width and thickness in an attenuation zone in which the glass accelerates along the bath, and enforcing said step of constraining molten metal flow at said location immediately upstream of said reserve region, which said location is in the region of the downstream end of said attenuation zone.

8. A method according to Claim 6 or Claim 7, comprising obstructing longitudinal flow of molten metal along side regions of the bath at a position upstream of said location.

9. A method according to Claim 6 or Claim 7, comprising obstructing longitudinal flow of molten metal along side regions of the bath at a plurality of spaced positions upstream of said location.

10. A method according to Claim 6 or Claim 7, comprising obstructing longitudinal flow of molten metal along side regions of the bath at two spaced positions upstream from said location.

11. A method according to Claim 6 or Claim 7, comprising electromagnetically inducing flows of molten metal through said lateral access to the region of the bath supporting the ribbon upstream of said location.

12. A method according to Claim 6 or Claim 7, comprising electromagnetically inducing flows of molten metal from beneath the ribbon upstream of said location to mix with the counterflow.

13. A method according to Claim 6 or Claim 7, comprising selectively heating said counterflows alongside the ribbon.

14. A method according to Claim 7, of manufacturing float glass of thickness in the range 1.5 mm to 3 mm, comprising applying marginal forces to the accelerating glass at a series of oppositely disposed positions spaced along the bath to control reduction in ribbon width and thickness, and enforcing said constraint of molten metal flow, at said location in the region of the downstream end of said attenuation zone and spaced downstream from the furthest downstream position at which marginal forces are applied to the ribbon.

15. A method according to Claim 14, including obstructing longitudinal flow of molten metal along side regions of the bath at least at one position upstream from said location and spaced downstream from the furthest downstream position of application of marginal forces to the glass.

16. A method of manufacturing flat glass comprising advancing a ribbon of glass along a molten metal bath, attenuating the ribbon to a desired width and thickness in an attenuation zone in which the glass accelerates along the bath, constraining molten metal flow in the region of the downstream end of the attenuation zone to forward flow entrained beneath the ribbon and counterflows alongside the accelerating glass from downstream of the attenuation zone, providing a deepened reserve zone of molten metal downstream of said attenuation zone in which return flows of molten metal from the outlet end of the bath quiesce and are heated, supplying said counterflows by means of flows of molten metal from said reserve zone, and from said counterflows feeding lateral flows of molten metal drawn into the molten metal flow entrained beneath the accelerating glass.

17. A method of manufacturing flat glass comprising advancing a ribbon of glass along a molten metal bath contained in a tank structure having a floor of alumino-silicate refractory blocks whose upper faces are at different depths in different regions of the bath, attenuating the ribbon to a desired width and thickness in an attenuation zone in which the glass accelerates along a relatively shallow region of the bath, providing a reserve of molten metal just downstream of said attenuation zone in a relatively deep region of the bath defined in a recessed part of said refractory floor of the tank structure, receiving in said reserve return flows of cooled molten metal from the outlet end of the bath, which flows quiesce and are heated in said reserve, and directing flows of molten metal from that reserve along-side the accelerating glass to feed lateral flows of molten metal drawn into the molten metal flow entrained in said relatively shallow region beneath the accelerating glass.

18. A method of manufacturing flat glass comprising advancing a ribbon of glass along a molten metal bath contained in a tank structure having a floor of alumino-silicate refractory blocks whose upper faces are at different depths in different regions of the bath, attenuating the ribbon to a desired width and thickness in an attenuation zone in which the glass accelerates along a relatively shallow region of the bath, constrain-ing molten metal flow in the region of the downstream end of the attenuation zone to forward flow entrained beneath the ribbon and counterflows alongside the accelerating glass from downstream of the attenuation zone, providing a reserve of molten metal just downstream of said attenuation zone in a relatively deep region of the bath defined in a recessed part of said refractory floor of the tank structure, receiving in said reserve return flows of cooled molten metal from the outlet end of the bath, which flows quiesce and are heated in said reserve, and directing flows of molten metal from said reserve into said counterflows alongside the accelerating glass to feed lateral flows of molten metal drawn into the molten metal flow entrained in said relatively shallow region beneath the accelerating glass.

19. Apparatus for manufacturing flat glass comprising an elongated tank structure having end walls, side walls, and a floor for containing a bath of molten metal, means for delivering glass to the bath at a controlled rate and advancing the glass in ribbon form along the bath, and means for applying traction to the ultimate ribbon of glass to accelerate the glass to a final discharge speed, and wherein, in the region of the tank structure where the ribbon achieves its final discharge speed, the floor of the tank structure is deepened to define a reserve zone for receiving cooler molten metal flow which is enforced in an upstream direction over the floor by the entertainment of hotter molten metal by the advancing ribbon of glass.

20. Apparatus according to Claim 19, comprising a transverse barrier on the floor of the talk structure at a location immediately upstream of said deepened tank floor, which barrier extends beyond the position of the edges of the ribbon, with the top of the barrier positioned below the level of the bath surface by a distance which is effective to constrain molten metal flow at that location substantially to forward flow of molten metal entrained beneath the ribbon and counterflow of molten metal alongside the ribbon.

21. Apparatus according to Claim 20, wherein the barrier extends beyond the position of the edges of the ribbon but stops short of the tank side walls.

22. Apparatus according to Claim 20 or Claim 21, wherein the reserve zone defined in the floor of the tank structure just downstream of said barrier is of greater depth than the bath depth upstream of said barrier.

23. Apparatus according to any one of Claims 19 to 21, wherein the depth of the reserve zone is approximately twice the bath depth adjacent said zone.

24. Apparatus according to any one of Claims 19 to 21, wherein said reserve zone extends across the full width of the floor of the tank structure.

25. Apparatus according to Claim 19, wherein the tank structure is encased in a metal casing, the floor of the tank structure comprises abutting blocks of refractory material which are secured to the metal casing, and said reserve zone of greater bath depth is defined by blocks whose upper faces are at a lower level than the upper faces of the adjacent blocks.

26. Apparatus according to Claim 25, wherein the upper faces of the blocks upstream and downstream of the reserve zone are at the same level.

27. Apparatus according to Claim 25 or Claim 26, wherein the blocks are of alumino-silicate refractory.

28. Apparatus according to Claim 20 or Claim 21, wherein the floor of the tank structure downstream of said barrier is constructed to define, considered in the downstream direction, said reserve zone of greater depth than the bath depth upstream of said barrier. a region of lesser depth than the reserve zone, and a further region of greater depth than the bath depth upstream of said barrier which further region extends to the outlet end of the tank structure.

29. Apparatus according to Claim 19, wherein an abrupt step is provided where the floor defines a change in bath depth.

30. Apparatus according to Claim 20 or Claim 21, wherein the elongated tank structure has a shoulder region which joins an upstream part of greater bath width to a downstream part of lesser bath width, said reserve zone of greater bath depth is located at said shoulder region, and the barrier is located just upstream of said shoulder region.

31. Apparatus according to Claim 20, including top rolls arranged to engage the upper surface of the ribbon margins at a series of oppositely disposed positions along the bath to control the reduction in width and thickness of the ribbon, the pair of top rolls furthest downstream being at a position spaced upstream from said barrier.

32. Apparatus according to Claim 31, including at least one pair of baffles located adjacent the bath side walls at oppositely disposed positions upstream from said barrier and spaced downstream from the furthest downstream pair of said top rolls to obstruct longitudinal flows of molten metal along the bath side walls at those positions.

33. Apparatus according to Claim 20 or Claim 21, including linear induction motors mounted over the bath surface in the region of the barrier to induce flows of molten metal electromagnetically.

34. Apparatus according to Claim 20 or Claim 21, including heaters mounted adjacent the tank side walls upstream of the barrier to apply local heating to the counterflows of molten metal.