This invention is concerned with improvements in or relating to the manufacture of rocf tiles and is particularly concerned with a novel process and a novel kiln for use in the manufacture of clay roof tiles.

By "clay", where used herein, it is meant "clay" as won from a clay-pit which clay generally comprises a "clay mineral" fraction and a "non-clay" fraction, the definition also extend¬ ing to modified clays, that is, "clay" as won to which or from which proportions of "clay minerals" and/or non-clay fractions have been added or removed.

By "kiln", where used herein, it is meant a one-high, counter¬ flow, continuous kiln comprised of heating, firing and cooling zones into which kiln products to be fired, i.e. "green state" clay roof tiles are introduced one at a time to be successively subjected to the heating, firing and cooling effects thereof wherein the air-flow is through the length of the kiln, or sub¬ stantially so, from an outlet end to an inlet end thereof and the kiln is operated continuously while products are fed there¬ through.

By "products", where used herein, it is meant "green state" roof tiles, "partly fired" roof tiles, and "fired" roof tiles during their transit through the kiln of the present invention.

By "green state", where used herein, in reference to clay tiles it is meant freshly moulded tiles which are still wet and such tiles which have been dried, or substantially so, wherein the temperatures of the dried tiles are at ambient temperature or at temperatures up to 200 ^C C.

By "partly fired" roof tiles, where used herein, it is meant tiles which have been raised to and are being maintained at their "peak firing temperature".

By "fired" roof tiles, where used herein, it is meant tiles which have been subjected to their "peak firing" temperature and are in the process of being cooled to ambient temperature or have been so cooled.

By "peak firing temperature", where used herein, it is meant that temperature to which a product must be raised for "vitrification" thereof to be effected.

By "vitrification", where used herein, it is meant the develop¬ ment of a microstructure necessary for the end product, i.e. a fired clay roof tile, to possess adequate properties, e.g. weather resistance, for purpose.

By "rock flour", where used herein, it is meant any hard rock which can be comminuted to an acceptable size, viz., substan¬ tially all of which is less than 200 microns and having a median particle size of between 20 and 60 microns and which, when fired, will not fail due to bloating, swelling, cracking or delamination as a result of, amongst other things, the evolution of gases which cannot escape from the matrix during firing.

Clay roof tiles are produced by very many methods in a multi¬ plicity of kiln structures from innumerable clay or clay-like compositions. However, whilst in the past and despite the rudimentary nature of some methods and kilns, merchantable products were produced from locally won clays at acceptable energy consumption rates, it is now essential that in order to conserve clay deposits and to restrict the depletion of fuel reserves, clay roof tile production methods and kilns therefor have to be energy efficient. To this end kilns have become more sophisticated leading to a uniformity of product from virgin clays or clays comprising, in admixture, clays from various sources.

The improvements in manufacturing processes and kiln technology now available to the producers of clay roof tiles is reflected in analogous steps forward in the manufacturing processes and kilns available for use in other areas of ceramics, e.g. those processes and kilns for use in the manufacture of clay floor and wall tiles.

One such improvement in the production of clay floor and wall tiles is to be found in United Kingdom Patent No. 1489021 in which the fast firing of floor and wall tiles is disclosed. From an untutored appraisal of GB 1489021 it would seem that in order to fast fire clay roof tiles one simply subjected them to the production method as taught in GB 1489021. However, our experiences in this area reveal that the unsophisticated nature of clay composites normally utilised for roof tiles, when subjected to fast firing regimes, results in total destruction of the products.

This phenomenon is not a mere happenstance; it is an occurrence which results from the inability of the unsophisticated clay compositions normally used for conventionally fired clay roof tiles to stand up to rapid increase of the kiln temperature from ambient temperature or substantially so to the firing tempera¬ ture of 800 to 1200°C. In GB 1489021, it is disclosed that, for a floor or wall tile of some 4-5 mm in thickness manufac¬ tured from a triaxial composition of clay, talcum and wollas- tonite, heating the products would be effected in 4 to 6 minutes, firing the products would be effected in 11 to 15 minutes and cooling down of the products to ambient temperature would take some 8 to 10 minutes.

The disclosure goes on to instruct that tiles of up to 10 mm would require longer heating and firing times of some 6 to 8 and 17 to 20 minutes respectively and if cooling times are extra¬ polated from those given for a 4-5 mm product, it is obvious

that a total process time exceeds 36 minutes for the 10 mm product.

The clay roof tiles provided by the present invention typically have thickness in section in excess of 14 mm for plain tiles, i.e. a beaver-tail tile or similar product, and from 15 to 30 mm in section for contoured interlocking tiles, i.e. a double roman tile or similar product.

To utilise the teachings of GB 1489012 for producing utile clay roof tiles would be a very expensive business since, to with¬ stand the harsh firing regime, the tiles would have to be made from high grade triaxial compositions similar to those taught by GB 1489012, viz. compositions made up from clay, talcum and wollastonite. In addition, because of the requisite heating, firing and cooling times of some 50 to 85 minutes required to produce products in excess of 14 mm in section for plain tiles and from 15 to 30 mm in section for contoured tiles, the energy consumption rates would also be prohibitive.

Among the further drawbacks of known processes and kilns and particularly those of roller hearth kilns are the difficulties in progressing non-flat tiles, that is plain tiles with hanging nibs and contoured interlocking clay tiles through such a kiln. The only device disclosed in GB 1489012 is a roller hearth kiln which operates as a "counterflow" vessel in which cooling and heating gases are caused to flow as between outlet and inlet ends respectively, i.e. in the opposite direction to that of the products, i.e. the wall and floor tiles, passing through the kiln. It will readily be appreciated that there are no diffi¬ culties in conveying planar wall and floor tiles through a roller hearth kiln. On the other hand, plain tiles with hanging nibs and contoured interlocking tiles each require setters or cassettes to assist in their passage through a roller hearth kiln and to ensure that the upper surface, that is the

surface of the tile which is uppermost in use, is not damaged by engagement with the rollers. This requirement for setters may double the energy requirements of such kilns.

The novel process and kiln provided by the present invention seek to mitigate the difficulties met hitherto in the manufac¬ ture of clay roof tiles. Thus, the present invention provides a process for the manufacture of clay roof tiles in a one-high, counterflow, continuous kiln as hereinbefore defined wherein the process comprises the steps of :

a) subjecting the "green state" tiles successively to a temperature gradient sufficient to raise the temperature of said "green state" tiles to their "peak firing tempera¬ ture" in 6 to 20 minutes to produce "partly fired" tiles; b) maintaining the "partly fired" tiles successively at their "peak firing temperature" for between 2 and 10 minutes to produce "fired" tiles; c) subjecting the "fired" tiles successively to a diminishing temperature gradient over a period not exceeding 25 minutes; and, d) removing or allowing the "fired" tiles to be removed from said kiln.

The invention still further provides a process for the firing of clay roof tiles comprising the steps of :

a) introducing "green state" tiles successively into a first zone along which there is created a rising temperature gradient; b) causing the "green state" tiles to successively traverse said first zone in 6 to 20 minutes whereby said "green state" tiles are successively heated to their "peak firing temperature" to produce "partly fired" tiles;

c) causing the "partly fired" tiles to successively travers a second zone wherein said "partly fired" tiles are main tained at their "peak firing temperature" for between and 10 minutes to produce "fired" tiles; d) causing the "fired" tiles to successively traverse a thir zone wherein said "fired" tiles are subjected to a dimin ishing temperature gradient for not more than 25 minutes and, e) successively removing or allowing the "fired" tiles to b successively removed from the third zone.

The invention also provides a process for the manufacture o clay roof tiles in a one-high, continuous, substantiall counterflow kiln as hereinbefore described wherein the composi tion for the tiles comprises :

i) between 50 and 100 parts by weight of clay, and ii) between 0 and 50 parts by weight of rock flou wherein all, or substantially all of the compositio has a particle size of less than 200 microns with median particle size of between 20 to 60 microns;

wherein said process comprises the steps of:

a) moulding "green state" tiles from said composition; b) drying said "green state" tiles; c) introducing the "green state" tiles successively into a first zone of the kiln along which first zone there is created a rising temperature gradient; d) causing the "green state" tiles to successively traverse through said first zone in 6 to 20 minutes whereby said "green state" tiles are successively heated to their "peak firing temperature" to produce "partly fired" tiles; e) causing the "partly fired" tiles to successively traverse a second zone of the kiln wherein said "partly fired"

tiles are maintained at their "peak firing temperature" for between 2 and 10 minutes to produce "fired" tiles; f) causing the "fired" tiles to successively traverse a third zone of the kiln whereby said "fired" tiles are subjected to a diminishing temperature gradient for not more than 25 minutes; and, g) successively removing or allowing the "fired" tiles to be successively removed from the third zone of the kiln.

Conveniently the composition for the tiles may comprise:

i) between 60 and 100 parts by wt. of clay; and, ii) between 0 and 40 parts by wt. of rock flour;

the particle size of at least 95% of the composition being less than 150 microns with a median particle size of 40 to 50 microns.

Preferably the "green state" tiles are raised, i.e. heated, to their "peak firing temperature" in 7 to 12 minutes and more especially in less than 8 minutes.

The "partly fired" tiles may be conveniently maintained at their "peak firing temperature" for between 4 and 8 minutes and more especially for less than 6 minutes.

The invention also provides a process wherein the "fired" tiles are subjected to a diminishing temperature gradient for not more than 22 minutes and more especially not more than 18 minutes.

Conveniently the invention further provides a process for the manufacture of clay roof tiles in a one-high, continuous, substantially counterflow kiln as hereinbefore described wherein the composition for the tiles comprises :

i) between 60 and 100 parts by weight of clay, and ii) between 0 and 40% by weight of rock flour wherein the composition has a particle size of less than 150 microns, or substantially so, with a median particle size of between 40 to 50 microns; wherein said process comprises the steps of:

a) moulding "green state" tiles from said composition; b) drying said "green state" tiles; c) introducing the "green state" tiles successively into a first zone of the kiln along which first zone there is created a rising temperature gradient; d) causing the "green state" tiles to successively traverse through said first zone in less than 8 minutes whereby said "green state" tiles are success¬ ively heated to their "peak firing temperature" to produce "partly fired" tiles; e) causing the "partly fired" tiles to successively traverse a second zone of the kiln wherein said "partly fired" tiles are maintained at their "peak firing temperature" for less than 6 minutes to produce "fired" tiles; f) causing the "fired" tiles to successively traverse a third zone of the kiln whereby said "fired" tiles are subjected to a diminishing temperature gradient for not more than 22 minutes; and, g) successively removing or allowing the "fired" tiles to be successively removed from the third zone of the kiln.

The "peak firing temperature" of the composition from which the tiles are to be made may conveniently be between 900°C to 1250°C and preferably the "peak firing temperature" is between 1050°C and 1150°C and the energy required to fire a tile is between 1.5

and 3.5 MJ/kg of tile weight and more especially less than 2.5 MJ/kg of tile weight.

The invention also conveniently provides a counterflow, continu¬ ous kiln as hereinbefore defined comprising :

i) a first zone along which there is created, when the kiln is in use, a rising temperature gradient for heating products fed along and through said first zone to their "peak firing temperature"; ii) a second zone in which, when the kiln is in use, the products are maintained at or near their "peak firing temperature" as they pass along and through said second zone; and, iii) a third zone along which there is created, when the kiln is in use a diminishing temperature gradient for reducing the temperature of the products as they pass along and through said zone;

said kiln being a sliding batt kiln wherein, when the kiln is in use, sliding batts are transported along and through the three zones of the kiln to convey products carried thereby through the kiln to effect the firing of said products, charac¬ terised in that the kiln comprises guide rails on which the sliding batts are transported as aforesaid, the kiln also comprising an underflow channel over which, when the kiln is in use, the products are transported on the sliding batts as aforesaid thereby ensuring the even firing of the products.

Preferably baffle means are provided in said underflow channel of the first and third zones to further ensure the even firing of the products as aforesaid wherein said baffle means comprise wall portions provided across a floor of the underflow channel whereby, when the kiln is in use and products are being carried on said sliding batts as aforesaid, the flow rates of heating

and cooling gases over and under the product and batt respec¬ tively in said first and third zones respectively are such that the rate of heat transfer to and from each side of the product and batt is equal or substantially so.

In a preferred embodiment the wall portions are provided at spaced intervals along said floors of the underflow channel in the first and third zones of the kiln wherein said wall portions conveniently create, when the kiln is in use, a weir effect to control the flow rates of the heating and cooling gases as aforesaid and the guide rails are conveniently provided by lower inwardly projecting stub walls of an inverted "U" shaped channel section which extends along and through the three zones of the kiln.

Conveniently the underflow channel in the second zone of the kiln is deeper than the underflow channels in the first and third zones and in the second zone the inverted "U" shaped channel section is at least twice as deep as the inverted "U" shaped channel sections of zones one and three of the kiln and wherein the channel sections are conveniently made from silicon carbide.

The present invention also conveniently provides a kiln wherein first and second air-locks are provided at upstream and down¬ stream ends respectively of the kiln and propulsion means are provided for causing, when the kiln is in use, the sliding batts to be propelled seriatim through the kiln from the upstream side of said first air-lock to the downstream side of said second air-lock.

The invention also conveniently provides a kiln according to the last preceding paragraph wherein the propulsion means includes transfer means provided at the downstream end of the kiln whereby, when the kiln is in use and after it has received its

full complement of sliding batts, and while an upstream end said second air-lock is open to connect the kiln therewith a a downstream end of said second air-lock is closed, a slidi batt is engaged by said transfer means and conveyed thereby in said second air-lock, said transfer means also being operati when the kiln is in use and an upstream end of the seco air-lock is closed to isolate the kiln therefrom and t downstream end of the second air-lock is open, to convey sa sliding batt out of the second air-lock.

The invention still further provides a kiln wherein t propulsion means for propelling the sliding batts seriati through the kiln is provided by an indexing mechanism provid upstream of said first zone thereof, the forward end of o sliding batt engaging with the trailing end of the sliding bat immediately downstream thereof to transfer the stroke of th indexing mechanism to all of the batts in the kiln, and wherei the sliding batts are provided by "setters" or "cassettes" whic may conveniently be made from cordierite.

Preferably the kiln operates at an energy consumption rate o less than 2.5 MJ/kg weight of the products fired therein, th products preferably being profiled clay roofing tiles as herein before defined.

When the kiln of the present invention is in use fired clay roo tiles are produced in less than 40 minutes at an energy consump tion rate of less than 2.5 MJ/kg of tile weight.

A preferred composition for use in the manufacture of a fire clay roof tile said composition comprises:

i) 60 to 100 parts by wt of clay; and, ii) 0 to 40 parts by wt of rock flour;

wherein the composition has a particle size of less than 150 microns with a median particle size of between 40 to 50 microns, said tile being produced by the process and in the kiln as hereinbefore defined.

There now follows by way of example of the invention a detailed description of a novel process and kiln for use in manufacturing clay roof tiles according to the process, which description is to be read with reference to the accompanying drawings in which:

Figure 1 is a schematic plan view of a kiln assembly;

Figure 2 is a schematic section view of a heating zone of the kiln along the line II-II of Figure 1;

Figure 3 is a schematic section view of a firing zone of the kiln along the line III-III of Figure 1;

Figure 4 is a schematic section view of an input end of the kiln along the line IV-IV of Figure 1;

Figure 5 is a schematic section view of an output end of the kiln along the line V-V of Figure 1;

Figures 6a to 6h are foreshortened schematic side views of the kiln illustrating operational sequences thereof; and,

Figure 7 is a composite view of a temperature/time graph over a block schematic of the kiln of Figure 1.

The present invention provides a novel process which is effected in a kiln 2 of a kiln assembly 4, see Figure 1, which process enables the production of clay roof tiles from a composition comprising a clay/rock flow blend, said roof tiles being

produced in said kiln in less than 40 minutes and at an energy consumption rate of less than 2.5 MJ/kg of tile weight.

The kiln 2, as shown in Figures 1 to 7, is of one-high configu¬ ration, that is, it is only capable of receiving products one at a time in a continuous stream, which products, when the kiln is in use, are carried therethrough on sliding batts 5 supported for this purpose as hereinafter described on a pair of spaced apart rails 1 which extend in a contiguous channel 3 provided throughout the length of the kiln 2.

The kiln 2 is made up of thirteen sections 6 to 18, see Figure 1. Sections 6, 7 and 8 provide a first zone 20 of the kiln 2 in which first zone a rising temperature gradient is created when the kiln 2 is in use. Sections 9 and 10 provide a second zone 22 of the kiln 2 and are the plenum chambers of the kiln 2 where arrays of burners 24 and 25 are provided to create an even, or substantially even, "peak-firing" temperature there¬ through when the kiln is in use. Sections 10 to 18 provide a third zone 26 of the kiln 2 in which third zone 26 a cooling temperature gradient is created when the kiln 2 is in use.

The sections 6, 7 and 8 and Sections 11 to 18 of the kiln are identical in construction, or substantially so; therefore, only section 7 will now be described with particular reference to Figure 2. Thus, section 7 comprises a fabricated metal kiln casing 30 of generally rectangular section supported on a floor mounted fabricated metal frame 32. Enclosed in the casing 30, section 7 comprises support brickwork, that is insulating brick¬ work, 34, in a lower half thereof with a channel portion 36 of the channel 3 being formed in an upper mid-section 38 of said brickwork 34, see Figure 2. The channel portion 36 is substan¬ tially "T" shaped in longitudinal cross-section. The brickwork 34 defines upper left and right hand horizontal arms 40 and 42 of the channel portion 36 to provide support for left and right

hand rail sections 44 and 46 respectively of the rails 1, see Figure 2. The rail section 44 is of "L" shape in longitudinal cross-section and the rail section 46 is of reverse "L" shape in longitudinal cross-section, see also Figure 2.

The channel portion 36 of the section 7 is provided with wall portions 48 on a floor 50 of the channel portion 36, the purpose of which wall portions 48 will be made clear hereinafter. In Figure 2, the wall portion 48 fills approximately half of a vertical stem of the "T" shaped channel portion 36 with left and right hand gaps 52 and 54 being evident at either end of the wall portion 48 between itself and the brickwork 34.

Section 7 is completed by fibre insulation 58 which is provided in an upper half thereof, as shown in Figure 2.

To complete the first zone 20, the sections 6, 7 and 8 are bolted together in known manner with the channel portions 36 and rail sections 44 and 46 thereof in alignment.

To complete the third zone 26, the sections 11 to 18 are bolted together in known manner with the channel portions 36 and rail sections 44 and 46 thereof in alignment.

Sections 9 and 10 of the second zone 22 are also identical in construction one with the other, or substantially so; there¬ fore, only section 9 will now be described with particular reference to Figure 3. Thus, section 9 comprises a fabricated metal kiln casing 60 of generally rectangular section supported on a floor mounted fabricated metal frame 62. Enclosed in the casing 60, section 9 comprises support brickwork, that is insulating brickwork, 64, in a lower half thereof with a channel portion 66 of the channel 3 being formed in an upper mid-section 68 of said brickwork 64, see Figure 3. The channel portion 66 is of exaggerated "T" shape in longitudinal cross-section with

the depths of the limbs and stem of the "T" section being at least twice the depth of the corresponding limbs and stems in the sections 6, 7 and 8 of the first zone. The brickwork, defining upper horizontal arms 70 and 72 of the channel portion 66, provides support for left and right hand rail sections 74 and 76 respectively of the rails 1.

The rail section 74 is of "L" shape in longitudinal cross- section and the rail section 76 is of reverse "L" shape in longitudinal cross-section, see Figure 3.

Section 9 also comprises fibre insulation 78 which is provided in an upper half thereof, as shown in Figure 3.

Section 9 comprises the arrays of burners 24 and 25 arranged on opposite sides respectively of the section 9 which burners 24 and 25 may be of any suitable configuration as illustrated schematically in Figure 3.

To complete the second zone 22, the sections 9 and 10 are bolted together in known manner. The kiln 2 is completed by bolting the three zones 20, 22 and 24 together in known manner with the channel portions 36 of sections 6 to 8 and 11 to 18 arranged in substantial alignment with the channel portions 66 to provide the channel 3 in which the rail sections 44 and 46 and 74 and 76 respectively are also arranged in substantial alignment to provide the pair of rails 1.

The kiln assembly 4 includes propulsion means 90 whereby, when the assembly is in use, products are propelled through the length of the kiln 2. The propulsion means 90 comprises an indexing mechanism 92 of conventional configuration at an upstream end 94 of the kiln 2. Thus, the mechanism 92 com¬ prises a piston and cylinder arrangement 96 which is connected to a pusher rod 98 which in turn carries a pusher plate 100 at

a leading right hand end 102 thereof, see Figures 6a to 6h, the purpose of which plate 100 will be made clear hereinafter.

The propulsion means 90 also comprises a transfer means 104 provided by a short belt conveyor arrangement 106 at a down¬ stream and 108 of the kiln 2, the purpose of which arrangement 106 will also be made clear hereinafter with reference to Figures 6a to 6h.

The kiln assembly also comprises a first air-lock 110 at said upstream end 94 of the kiln 2 and a second air-lock 112 at said downstream end 108 thereof, the purpose of which air-locks 110 and 112 will be made clear hereinafter.

The first air-lock 110 is located at and fixedly secured to the upstream end 94 of the kiln 2 as aforesaid between the kiln 2 and the indexing mechanism 92 as shown in Figures 6a to 6h.

The first air-lock 110 comprises an inner door 114 mounted on a rod 116 of an associated piston and cylinder arrangement 118 which inner door 114, in an operative sequence with respect to the pusher plate 100, which also acts as an outer door of the air-lock, when the kiln is in use, ensures that the upstream end 94 of the kiln 2 is isolated in use.

The second air-lock 112 is located at and fixedly secured to the downstream end 108 of the kiln 2 as aforesaid and comprises an inner door 120 mounted on a rod 122 of an associated piston and cylinder arrangement 124 and a pivotally mounted outer door 125 pivotally connected to a piston rod 126 of an associated piston and cylinder arrangement 128, see Figures 6a to 6h. The doors 120 and 122 are operated in sequence to isolate the kiln 2 when it is in use.

The kiln assembly 4 further comprises a product feed mechanism 130 which straddles said indexing mechanism 92 at the upstream end 94 of the kiln 2.

The mechanism 130 consists essentially of conventional elevator means for presenting products P to the pusher plate 100 when the kiln 2 is in use as hereinafter described.

The kiln assembly 4 comprises a product despatching mechanism 132 downstream of the transfer means 104, see Figures 6a to 6h. The mechanism 132 comprises a short longitudinal conveyor assembly 134 arranged in line with said transfer means 104, the assembly 134 being carried by a lateral transfer mechanism 136 of the mechanism 132.

The kiln assembly 4 also comprises a return loop conveyor 138 connected between the mechanism 132 and the product feed mecha¬ nism 130 for a purpose to be made clear hereinafter.

A full operating sequence of the kiln 2 and kiln assembly 4 will now be described with particular reference to Figures 6a to 6h and Figure 7.

"Green state" roof tiles, i.e. the products, are produced from a composition comprising:

i) between 50 and 100 parts by weight of clay, and ii) between 0 and 50 parts by weight of rock flour wherein the composition has a particle size of less than 200 microns with a median particle size of between 20 and 60 microns.

More especially, the composition comprises between 40 and 60 parts by weight of clay and 0 to 40 parts by weight of rock

flour and the particle size is less than 150 microns with a median particle size of between 40 and 50 microns.

The composition with added water is thoroughly mixed to give a homogenous mixture which is introduced into a conventional tile moulding press; thereafter the "green state" products are dried before being fed seriatim to the kiln assembly 4.

On approaching the kiln assembly 4 the products P are individu¬ ally mounted on sliding batts 5 provided by tailored setters or cassettes made from cordierite.

The cassettes 5 together with the products P mounted thereon are fed one at a time on to a support frame 131 of the product feed mechanism 130 when in the position as shown in Figure 6a. With the piston rod 98 retracted, i.e. moved to the left viewing Figure 6a, to the position shown in Figure 6c, the support frame 131 is lowered into alignment with extension portions la of the rails 1.

At this time the inner door 114 is activated by the piston and cylinder arrangement 118 to isolate the kiln 2 from atmosphere.

The piston and cylinder arrangement 96 is then operated to cause movement of the cassette 5 to the right viewing Figure 6a to introduce the product P into the kiln 2, the door 100 acting as a pusher plate for this purpose.

When the cassette 5 has fully entered the first air-lock 110 and the plate 100 becomes effective as a door thereof, the inner door 114 is opened to allow the cassette 5 and product P to be moved into the kiln 2 as shown in Figure 6a.

This sequence is repeated until there are sufficient cassettes 5 and products P to fill the kiln 2.

Figure 6b shows the pusher-plate door 100 during a fast return stroke of the piston and cylinder 96 in preparation for the feeding of a subsequent cassette 5 and product P supported at this time on the support frame 131 of the mechanism 130. It is to be noted that the doors 114, 120 and 125 are all closed at this time.

Figure 6c shows the support frame 131 lowered into alignment with the extension rails 1, the doors 114, 120 and 125 still being closed.

Figure 6d shows the pusher-plate door 100 during a fast forward stroke of the piston and cylinder arrangement 96, the cassette' 5 and product P being located wholly within the air-lock 110 which is closed by the door 100 enabling the sequential operation of the piston and cylinder arrangement 118 to open the door 114 thereby facilitating the entry of the cassette 5 and product P to the kiln 2. At this time, the arrangement 96 is operated slowly to inch the cassette forward, i.e. to the right in Figure 6d to bring a forward end thereof into pushing engage¬ ment with a trailing end of a preceding cassette 5.

The slow pushing stroke of the arrangement 96 continues, where¬ upon the line of cassettes 5 and products P is slowly moved towards the right, viewing Figure 6e, to bring a leading cassette 5 into juxtaposition with the inner doors 120 which is opened upon sequential operation of the piston and cylinder arrangement 124 to allow the leading cassette to enter the air-lock 112 as shown in Figure 6f. At this time, the doors 100 and 125 are closed and the doors 114 and 120 are open.

The slow pushing stroke of the arrangement 96 continues as the transfer means 104 is operated sequentially to firstly transfer the leading cassette 5 wholly into the air-lock 112 whereupon the door 120 closes and the door 125 opens, see Figure 6g, to

allow the leading cassette 5 to be transferred partially onto an endless conveyor 132 of the conveyor assembly 134. Operation of the conveyor 132 positions the leading cassette 5 for subsequent transfer to a splitting arrangement 140 shown in block form in Figure 1. At this time, the arrangement 96 completes its forward stroke to once again position the next in line cassette 5 adjacent the door 120, see Figure 6h, and the door 125 closes once again to isolate the kiln 2. It will be obvious that the above sequence of operation is repeated while cassettes 5 and products P are fed to the kiln assembly 4.

Thus, when the kiln 2 and kiln assembly 4 are in use, each product P is subjected to a process provided by the invention and comprising the steps of:

a) introducing the products P seriatim into the first zone 20 along which there is created a rising temperature gradient as shown in Figure 7; b) causing the products P to traverse said first zone 20 in 6 to 20 minutes whereby said products are successively heated to their "peak firing temperature" of 1150°C; c) causing the products P to successively traverse the second zone wherein the products are maintained at their "peak-firing temperature", i.e. 1150°C, for between 2 and 10 minutes; d) causing the products P to successively traverse the third zone 26 wherein the products P are subjected to a dimin¬ ishing temperature gradient for not more than 25 minutes during which the temperature of the products is reduced to ambient temperature or substantially so; and, e) successively removing the fired products from the third zone.

As set out hereinbefore the products P are subjected to a rising temperature gradient along zone 20, a constant temperature

profile in zone 22 and a diminishing temperature gradient in zone 26. These conditions are provided by the size of burners in zone 22, the mass of the products P and cassettes 5 propelled through the kiln, the speed at which the products P and cassettes 5 are propelled through the kiln 2, the counterflow current of air which is introduced at the downstream end 108 of the kiln 2 and exited at the upstream end 94 thereof and the quantity of air provided in counterflow.

As depicted in Figure 7 the three zones of the kiln act effec¬ tively as heat exchangers with the air giving up its heat to the product and cassettes in zones 20 and 22 and with the products and cassettes giving up their heat to the incoming air in zone 26 of the kiln.

There is a temperature differential of some 200 ^β C along the kiln between the respective temperatures of the air and the cassettes/products.

The heat exchange as between the air and the products and vice versa is also assisted in no small measure by the wall portions 48 which are provided at intervals through the length of the zones 20 and 26. It will be appreciated that the geometry of a product being propelled through the kiln 2 presents an uneven upper surface to the air passing thereover in counterflow and thus, without the wall portions 48, the air in the lower portion of the channel 3 would pass through said lower portion unimpeded leading to a differential heating and cooling as between the upper and lower surfaces of the products P.

The effect created by the wall portions 48 is to locally pressurise the air in the lower channel portion to eliminate or at least to reduce the pressure differential as between the air-flow over and under the products P thereby enhancing even

heat exchange as between the products P and the air and vice versa in the zones 20 and 26.

In the process just described the cassettes 5 utilised measure 0.28 m wide and 0.457 m long and when fully loaded there are 53 such cassettes 5 in the kiln 2.

The fast forward stroke of the pusher-plate door 100 is effected at 0.5 m/s and lasts for 1.25 seconds, while the slow forward stroke of the pusher-plate door 100 is effected at 0.0175 m/s and lasts for 29 seconds and the return stroke of the pusher plate is effected at 0.5 m/s and lasts for 2 seconds approximately.

Between each forward and return stroke of the pusher arm there is a dwell time to allow for the loading of a next in line cassette 5 and product P onto the rail extensions la.

Products subjected to the novel process when carried out in the improved kiln of this invention consumed 2.5 MJ/kg of product weight and gave an end product, i.e. a clay roof tile having a cross-bending strength of between 10 and 30 MPa and with acceptable frost resistance and permeability.

Modifications envisaged within the scope of the invention include a multi-channel kiln with appropriate buffers between each, channel if required; however, it is envisaged that judicial tailoring of any wall between each channel will enable the present side wall mounted configuration of burners to be used without excessive modification thereto.

It is also envisaged within the scope of the present invention that the walls between each channel may be omitted in the second zone 22 of the kiln 2 and that the arrays of burners 24 and 25

may be complemented by or replaced by burners located in the roof and floor of the second zone 22.

Further modifications may include a tailored cassette which would incorporate depending fins to complement or replace the wall portions 48.

In further modifications within the scope of the present inven¬ tion, each zone 20, 22 and 26 may be of unitary construction instead of the modular construction described herein. Also the rail sections 44, 46, 74 and 76 may be provided by continuous rails extending the length of the kiln 2 and may, in fact, be provided by lower inwardly projecting stub walls of an inverted "U" shape channel section which is arranged to extend along and through the three zones of the kiln.