Calender for the treatment of a paper web and process for its operation

A calender for treating a paper web, said calender having a roller stack having a first end and a second end, said roller stack being loaded with a load on one end, said calender comprising:

• at least two hard rollers each having a substantially smooth outer surface, said at least two hard rollers each having means for heating a surface of said roller to a temperature of at least 100.degree. C.;
• and
• at least two soft rollers, wherein each of said at least two soft rollers is disposed adjacent to at least one of said at least two hard rollers to form a working nip therebetween, wherein at least one working nip has a dwell time of said paper web passing through said working nip of at least 0.1 ms, and said load on the rollers produces an average compressive stress in said at least one working nip of at least 42 N/mm.sup.2, an arithmetic mean of the numerical value of said surface temperature T, said dwell time t and said compressive stress p in all of said working nips satisfies the following relationship:
• a target value Zg=1.378-0.00356.multidot.T-(0.00825-5.12.multidot.10.sup.-5 T)p-›0.039+(0.188-0.00112T)p.multidot.e.sup.-0.093p !t.multidot.e.sup.-0.42lt =0.8 to 0.9.