The literature on the interactions between the coating colour and the base sheet during the pigment coating of paper is reviewed in the order to summarize the current knowledge within the area. The review is focused on the processes of forming and consolidation and on how the coating colour interacts with the base sheet during these processes, and how this interaction affects coating hold-out, roughness and coating mass distribution. A coating layer which stays on the surface of the base sheet and which has a uniform mass distribution is desired. The research world disagrees on whether coating hold-out is a relevant problem. The reason is that there is little direct evidence in the literature on coating penetration. However, there are numerous indications of an indirect character which suggest that coating penetration exists, both in blade coating and in coating with the metered size press. The pressure pulse to which the coating colour is subjected in the applicator nip during blade coating or in the transfer nip in metered size press coating and the permeability of the base sheet are factors which are said to control coating penetration. There is concordance in the opinion that the uptake of the aqueous phase of the coating layer is an extremely rapid process and that this uptake releases stresses in the sheet and plasticizes it. The release of stresses leads to roughening of the sheet whereas the plasticization makes it compressible and smooth in a compressed state beneath the blade tip during the forming of the coating layer. Much attention has been given to the roughening and a number of extensive studies have been published about that. The studies on plasticization and sheet compressibility and how they affect the mass distribution of the final coating layer are fewer in number but non-existent. The roughening and plasticization of the sheet are reported to be different for woodfree and wood containing sheets, due not only to the different types of fibres in the sheets, but also to their different densities. Woodfree sheets, which are generally the densest, are considered to be dimensionally the most stable. A number of researchers have reported that the pre-calendering has a great influence on the roughening. Studies have shown that the calendering builds in transverse stresses into the sheet and, in the case of wood- containing sheets, closes the lumen of thick-walled fibres. During the coating operation, when the sheet takes up the aqueous phase from the coating colour, these stresses are released and the lumen is opened. Several researchers have shown that all the smoothening effect of the pre-
calendering can be lost during the coating process.