This information is reprinted from the Cranberry Corner column of Hand Papermaking Newsletter #53 (January, 2001).
To learn how to order Hand Papermaking bi-annual magazine and quarterly newsletter, click here.
Sheet Formation and Uniformity
This time we will talk about forming the sheet of paper and paper uniformity.
THE IMPORTANCE OF PROPER BEATING. As mentioned in a previous article, adequate beating of the pulp is critical to making good paper. When I worked in the papermills it was said that “newsprint is made in the goundwood mill!” By this it was meant that one cannot make a good quality paper from an inferior pulp.
If the pulp is not beaten enough, the paper surface will be coarse and porous with a non-uniform formation. If the pulp is beaten too much, the proportion of long fibrils will be reduced and too many “fines” or short, broken fibres will result. Thus the fibres must be “opened” into fibrils without cutting the fibres which would reduce their length and result in weak and brittle paper.
PAPER BASIS WEIGHT AND PULP CONSISTENCY. Paper is made from a water suspension, or slurry, of fibres normally ranging in concentration or consistency from 0.5 to 1.0% solids. The thicker the slurry, the heavier will be the basis weight, or grammage, of the sheet of paper.
FLOCS AND FORMATION. Also, the thicker the slurry and the longer the length of the fibres, the stronger is the tendency of the fibres to “floc,” or to form clumps. If these flocs are not dispersed before the sheet formation is set, they will result in a paper with a “floccy” or lumpy formation. This effect can be seen by looking through a sheet of paper with a strong light shining behind it, i.e., with transmitted light.
Thus, a floccy paper formation is caused by a non-uniform distribution of fibres in the paper, and in the extreme, this can result in poor, uneven printing and calligraphic reproduction.
AGITATION IN THE VAT. Just before the papermaker forms a sheet of paper, the pulp slurry in the vat must be agitated with a paddle or other stirring device (I use a child’s canoe paddle) to make sure that the fibres are well distributed. Then one must wait a few seconds until the eddies in the pulp have died down before the sheet is formed.
FORMING THE SHEET. To form handmade paper, the mould and deckle are held together at arms’ length vertically over the vat of pulp and then dipped down into the vat. The mould follows the radius of the arms’ length until just before the top edge of the mould enters the pulp. At this point the mould is scooped forward while at the same time raising it from the vat and immediately levelling it, thus capturing the pulp inside the deckle which acts as a dam. Right away the water starts to drain through the screen of the mould leaving the pulp to form the sheet of paper on the screen.
Raising the mould just before it is submerged in the pulp avoids the formation of a vacuum under the mould which would make it much more physically difficult to raise the mould from the vat. This, of course, is more important with large size moulds than with small ones.
THE PAPERMAKER’S SHAKE. As soon as the mould is levelled, the papermaker must observe the emerging paper formation as the water drains out, and start to “shake” the mould to break up the flocs. The shake consists of short, jerky movements of the mould alternating sideways and back and forth. With experience it is possible to observe how the flocs are broken up by these actions in order to obtain a uniform paper formation. This takes some practice to achieve!
Once the free water (i.e., the shiny water on the surface) disappears, it is not possible to improve the formation any more by shaking the mould, as the fibres have already set.
RECYCLING! At this stage, if the papermaker is not satisfied with the formation, or if the sheet just made is not level from front to back and from side to side, then, with the deckle removed, the mould can be inverted and slapped flat onto the surface of the pulp in the vat. The sheet of wet paper will then release from the mould and fall cleanly back into the vat. The papermaker then must start over again making sure that the pulp from the sheet is well dispersed.
MAINTAINING LEVEL IN THE VAT. While the formed sheet is setting, water continues to drain from the mould. In order to maintain the pulp slurry level in the vat, the water that filters through the screen is returned to the vat by suspending the mould above the vat by resting it on the front edge of the vat and on a sliding back board, which is suspended across the vat at the back of the mould.
REPLENISHING THE VAT. As each sheet of paper is formed, the concentration of pulp in the vat decreases by the amount of fibre removed. This would mean that each sheet of paper would be lighter basis weight (grammage) than the previous sheet.
Thus, ideally, after each sheet is formed, the vat should be replenished with fresh pulp. In practice this can be done after every three or four sheets have been made without experiencing a significant drop in sheet basis weight. This is, of course, dependent upon the volume of the vat.
To accomplish replenishing accurately one must test the consistency, or percent solids, of the pulp slurry in the pulp hold tank and, knowing the dry weight of a sheet of paper, calculate the volume of slurry required for replenishing the vat.