the illustrated apparatus comprises an inverform machine which manufactures a board consisting of four paper plies . the machine has an endless carrier web 11 driven continuously through a loop around support rollers 10 by drive means ( not shown ). the machine comprises four ply - forming stages 12 , 13 , 14 , 15 . each of these has the general construction which is illustrarted in more detail in fig2 . as seen in this figure a flow box 16 is located directly above the carrier web 11 to deposite pulp onto the carrier through slot outlet 17 . the deposited layer of pulp is held between carrier web 11 and a further web or &# 34 ; top wire &# 34 ; 18 as it passes beneath a forming roller 19 and between upper and lower vacuum boxes 21 , 22 by means of which it is dewatered to form a consolidated ply . the &# 34 ; top wire &# 34 ; 18 moves around a closed loop supported on rollers 20 . an autoslice 8 and a suction slice 9 are also provided in each stage . in each stage the top wire 18 is driven through an endless loop and it comes away from the ply at the end of this stage . thus , the upper surface of the ply formed in each of the first three stages is exposed briefly before entering the next ply - forming stage of the machine . layers of starch solution are deposited on the two exposed ply surfaces between the first and second forming stages and between second and third forming stages and a layer of starch solution with suspended clay particles is applied to the exposed ply surface between the third and fourth forming stages . the outermost ply formed by stage 15 would be made from bleached pulp but the other plies could be formed of pulp reclaimed from waste paper . the outermost ply may be very thin and serve merely to overlay and protect the piment material and prevent transfer of pigment and binder to the components of machine stage 15 . the three inter - ply layers are deposited by means of curtain coaters indicated in fig1 as 23 , 24 , 25 . they are of identical construction , this construction being illustrated in fig3 and 4 . each comprises a horizontal elongate hollow structure 26 , the interior of which serves as a liquid receiving pressure chamber 27 . structure 26 may be made from sheet metal as a long box of rectangular cross - section . it has top and bottom walls 28 , 29 , side walls 31 , 32 and end walls 33 , 34 . chamber 27 may be about 2 inches wide by 2 inches deep in a typical structure 26 would need to be about 15 feet long or even more . the top wall 28 of structure 26 is provided with liquid inlet tappings 36 to which liquid supply pipes 37 are connected . tappings 36 and pipes 37 together form liquid inlet ducts which extend downwardly into chamber 27 at locations adjacent side wall 31 and therefore remote from side wall 32 . inlet tappings 36 are spaced at regular intervals throughout the length of structure 26 . inlet pipes 37 are supplied with liquid starch under pressure . pipes 37 may , for example , be branches of a single supply manifold . side wall 32 of structure 26 is provided with a horizontal slot outlet 38 defined between upper and lower lips 39 , 41 formed on upper and lower plates 51 , 52 fastened to the outer - face of wall 32 . upper plate 51 is fastened to wall 32 by studs 53 and into 54 so that the position of upper lip 39 can be adjusted to vary the width of the slot outlet whereas lower plate 52 is permanently fixed to wall 32 by welding or a suitable adhesive . lips 39 , 41 are rounded being formed by suitable curving the respective margins of plates 51 , 52 . plate 52 defines an external upright surface 43 extending downwardly from lower lip 41 of slot outlet . it extends below the bottom wall 29 of structure 26 to terminate in a sharp blade edge 40 at the bottom of surface 43 . vertical baffle plates 44 , 45 are fitted within structure 26 to obstruct direct flow of liquid from inlet tappings 36 to the outlet slot 38 . more particularly baffle 44 extends upwardly from the bottom wall 29 adjacent tappings 36 to an upper edge which is above the bottom ends of the tappings . baffle 45 extends downwardly from top wall 28 adjacent slot 38 to a lower edge below the level of the slot . the starch pumped into the chamber via tappings 36 is therefore caused to flow in a sinuous path to reach slot 38 . vent pipes 46 are fitted to the upper wall 28 of structure 26 to vent the upper part of the chamber 27 to atmosphere . the liquid starch or starch and clay , as the case may be , is metered by pumping into the chamber 27 under pressure and fills the chamber to a level above slot outlet 38 . the liquid in the chamber thus forms a pressure head causing the liquid to wet both the upper and lower lips 39 , 41 of the slot and to be extruded horizontally outwardly through slot 38 and whereupon it falls down the vertical surface 43 to establish a thin falling curtain . it is found that surface tension effects due to the wetting of the upper slot lip 39 and the wall surface 43 produces a marked thinning of the extruded liquid and the curtain which is established on surface 43 is much thinner than the vertical width of the slot , i . e . the distance between the upper and lower slot lips 39 , 41 . this thin curtain or film falls away from the surface 43 at the sharp blade edge 40 to drop from the coater as a free falling curtain . the rounding of upper slot lip 39 assists in maintaining a film of constant thickness in that it minimises random variations in the wetted surface area at the lip which might otherwise upset the surface tension balance in the zone where the film is formed . the sharp blade edge 40 at the bottom of the surface 43 ensures that the film can fall away cleanly even though it is very thin and would tend to adhere to or run around a less sharp corner . in one particular apparatus constructed in accordance with the invention the slot outlet of each curtain coater is 3mm wide and its upper and lower lips are rounded to 5mm diameter curvature . the starch solution or starch and clay suspension , as the case may be , is applied at a rate of 0 . 25 liter / second per meter width of coating onto a paper web travelling at 250 meters / minute . coaters 23 and 24 receive an oxidised starch solution having 9 % solids concentration and a viscosity of 30 seconds steinhall cup at 18 ° c . coater 25 receives a liquid comprised of 3 % starch and 30 % clay by weight . it has been found that this liquid has much the same viscosity as the starch solution pumped through coaters 23 and 24 and it can be forced through coater 25 at the same flow rate to produce a stable curtain which provides a clay distribution of about 20 grams per square meter which will significantly improve whiteness and opacity for printing . generally in order to ensure satisfactory results the slot outlets of the coaters may have a width in the range 1mm to 15mm with a minimum flow rate of about 0 . 1 liters / second per meter width of coating . the viscosity of the starch solution or starch and clay suspension may be in the range 18 seconds to 45 seconds steinhall cup at 20 ° c . each coater is usually disposed with the outlet slot 38 facing in the direction of travel of the web and it may be tilted slightly so that surface 43 is not quite vertical but is inclined at an angle of approximately 10 ° to the vertical to minimise air entrainment and stress in the film and thereby promote smooth application of the film to the moving ply surface . the illustrated apparatus and the board - making process carried out on that apparatus have been described by way of example only and could be varied considerably . for example , the starch layers applied between the intermediate plies could be eliminated . moreover it is not essential that processes according to the invention be carried out on an inverform machine and it would be possible to use other types of formers where vertical access to the paper web can be achieved . it has already been mentioned that titanium dioxide could be used as pigment material either to supplement or replace the clay , and a wide range of liquid binders could also be used . it is accordingly to be understood that the invention is in no way limited to the illustrated apparatus or the details of the described process and that many variations will fall within the scope of the appended claims .