The function of backing wires in laid moulds
Following is my best effort at understanding why paper made on single faced laid and double faced laid moulds turns out so different. I think I’m on the right track but questions remain.
Single faced laid moulds make paper with distinctive characteristics. As paper is formed it becomes thin in the areas between ribs and thick in bands along and above the ribs. The cause would seem to be uneven drainage caused by the structure of the mould.
During the time taken to form a sheet of paper on the wire surface fewer fibers collect where drainage is poor and more fibers collect where drainage is good. The differences in good and bad drainage must be due to differences in the mould structure. I believe thicker paper along the ribs of a single faced laid is due to the vertical sides of the ribs having the effect of increasing flow there.
Surface tension prevents water from easily detaching from a horizontal surface (until enough accumulates to form drops). Water, as everyone knows, likes to run downhill and since the surface that is being drained (the mould) is basically horizontal there is little impetus for it to flow in any direction (there’s no downhill). Surface tension ‘sticks’ the water to horizontal surfaces until big enough drops are formed to be pulled off by their own weight. (I can’t explain why but I suspect this is not an efficient process.) But in the areas along the ribs a strong directional flow is created, as gravity encourages water to flow down the sides of the ribs thus drawing it away from adjoining horizontal areas. Since water flows strongly down the sides of a rib it builds up along its narrow bottom edge and streams off.
This seems to be an adequate explanation of the uneven nature of “antique laid” paper (paper made on a single faced laid mould). The structure of the mould, especially the interaction of horizontal and vertical surfaces creates areas of uneven drainage; poor between the ribs but improving with proximity to them.
I have a harder time convincing myself that I have an adequate explanation for the very even nature of paper made on a double faced laid mould.
Adding a second layer of wires to a laid mould (to make it a double faced laid mould) eliminates the problems of uneven drainage allowing sheets of even thickness to be formed. It is a little puzzling that the solution turns out to be so simple. Understanding why it works does not seem so simple. My first guess was that the extra wires simply isolate the upper facing by lifting it away from the ribs. I held this belief for a while, but now I think that the extra wires improve drainage and that the even formation of a double faced laid mould may be due to a combination of two factors; isolating (somewhat) the facing from the effect of the ribs, and improving drainage elsewhere. The extra wires might improve drainage simply by adding ‘pathways’, additional surfaces for water to flow along. In this scenario, the lower backing wires, being close to the underside of the laid facing are able to ‘catch’ and draw off water there. They would then function as additional routes for water to flow along. (But this isn’t completely convincing; there may be more ‘routes’ but they are all horizontal and still inefficient at creating directional flow.) Another possibility that occurs to me is that the two layers of wires (closely spaced laid facing wires and widely spaced laid backing wires) might create spaces narrow enough that surface tension could hold water between them, albeit briefly. If this were true (can it be possible?) the water could be drawn toward the ribs in a thick layer that would then turn and flow down their vertical sides.
Another possibility (that occurred to me while writing) is that the ‘vatman’s shake’ is not only instrumental in re-arranging the fibers in the paper being formed but also in helping drain the sheet by shaking the draining water sideways towards the ribs. But the shake is used for both single-faced and double faced moulds so clearly this can’t explain the different results from the two types of mould.
Fortunately it isn’t necessary to have a complete understanding of how moulds work in order to make (or use) them!
The function of backing wires in wove moulds
Backing wires serve a double function for wove moulds. The straight stiff wires provide a structure to support the woven wire facing while presumably working to improve drainage. ‘Wire cloth’ must be supported by backing wires; if sewn directly to the ribs it would soon sag between them from the pressure of couching.