Recent developments in industrial practice are briefly reviewed and then improvements of our understanding of calendering are reviewed under the headings: the assessment of surface properties, the compressibility of paper, the effects of calendering variables on paper properties (for hard (iron) roll calendering, soft roll calendering and temperature or moisture gradient calendering, and the effect of calendering on paper strength) and rolling contact phenomena. The Concluding Comments list the implications of these industrial and scientific developments for future technical development and research into the calendering processes.
The last 8-9 years have seen further expansions and rationalisations within the paper industry so it is not surprising to read regular reports of new finishing lines on new or modernised machines. Particularly noticeable, however, have been the numerous installations of new supercalenders for LWC grades and the development of competing uncoated filled mechanical printing grades, also supercalendered. Also, there is a trend for makers of standard machine calendered newsprint to produce higher quality mechanical printing grades which require a supercalender finish.
These developments have increased the already considerable interest in the possibility of reducing the costs of supercalendering compared in detail by Muller & Schmidt with machine calendering and with on-line soft roll calendering, using an 8 roll machine (1) . Although the analysis needs to be updated in the light of experience with modern 2 x 2 roll tandem soft calenders, not then in service, the relative importance of running costs, capital investment and production losses through down time are carefully compared. Supercalendering of newsprint produced at 1000 m/min was estimated to cost DM16/ton compared with about DM4 for conventional machine calendering, before down time losses adjust them to DM25 and 27 respectively. (At that time, the on-line alternative was apparently far more expensive) . The higher selling price of the supercalendered paper was not allowed for.
Since 1980, there has been a rapid application of on-line soft roll calendering, in place of the conventional machine calendering with hard (iron) rolls. Excluding the gloss calenders developed about 1962, there are probably nearly two hundred installations, which, it may be said, give a supercalender type finish to papers which were usually not finished that way, or which were lightly supercalendered. Obviously, there has been considerable interest in seeing how far this on-line process can go to produce conventional supercalender grades.
Before reviewing the scientific developments in these operations, it is useful to review briefly the practical improvements which have been made since about the time of the review by Peel, Kerekes and Baumgarten (2) .