A method is described of determining the distribution of fibres in the thickness of a sheet of paper. It relies on observing the disposition of a small proportion of dyed fibres in a transparentised sheet. All the samples examined show a highly layered structure.
Density profiles of the boundary of a sheet being formed in a drainage apparatus have been measured and the movement of single fibres was observed in approaching the forming zone in a model of the wire part of a paper machine. Both experiments show that a diffuse zone exists at the boundary of the forming mat, in which thickening of the stock takes place. The length of this zone is only a few millimetres and most of the formation takes place by filtration rather than by thickening. This finding is confirmed by computations based on a theory of formation, which includes both thickening and filtration. An argument is advanced that the layered structure of paper is the inevitable result of this mechanism of formation, which takes place at the usual papermaking consistencies. Much higher consistencies would be needed in order to produce a more felted structure of the sheet.