| Relative humidity of the air, percentage | Moisture contained in the paper, percentage |
| 100 | 21.5 |
| 90 | 13.5 |
| 80 | 8.9 |
| 70 | 8.4 |
| 60 | 6.5 |
| 50 | 5.6 |
| 40 | 3.4 |
| 30 | 2.3 |
In a sheet of paper, where thousands of fibers lie side by side, the combined expansion is distinctly noticeable in the changing dimensions of the sheet. This gives rise to difficulties in securing accurate register in color-printing, owing to atmospheric changes. The manufacturer may minimize this difficulty by a careful formation of the paper and the regulation of the drying, so as to turn out the paper as nearly as possible containing an average normal percentage of moisture.
The same conditions are responsible for wavy edges, which occur principally along the cross-grain dimension of the sheets. The ends of the fibers, being exposed, easily absorb moisture as paper lies in a pile, but the moisture seldom permeates more than a few inches into the pile. Therefore, the larger part of each sheet is unaffected, but the fibers exposed to the air expand when absorbing moisture increasing the area of the exposed end and, consequently, causing it to assume a wavy formation which is suggestive of a ruffle.
When feeding such sheets to a cylinder press, much trouble may arise if the waves occur along the “gripper edge,” which is usually on the longer dimension of the sheet. In some instances the difficulty may be avoided by ordering paper with the grain running the long way of the sheet, which also offers another advantage in relation to securing close register, namely this: the area of the sheet in square inches will increase least through atmospheric expansion which occurs across the grain if the cross-grain dimension is the lesser.
SUPERCALENDER STACKS, APPLETON COATED PAPER CO.
For a description of the Supercalendering process, see pages [55] and [56].