This information is reprinted from the Cranberry Corner column of Hand Papermaking Newsletter #55 (July, 2001).
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Wet Pressing and Consolidation
There are two reasons for wet pressing: water removal and sheet consolidation.
WATER REMOVAL. Excess water must be removed from the paper to reduce its moisture content to the point that the wet paper can be handled during the drying process. The “wet strength” of paper is highly dependent on its moisture content and increases dramatically as the sheet becomes drier.
SHEET CONSOLIDATION. The other important function of wet pressing is the consolidation of the sheet of paper. As the water is pressed out of the paper and flows out through the press felts, the fibres come into more intimate contact with each other and inter-fibre bonding starts to occur.
WET PRESSING PROCEDURES. In the last column, the stack of wet felts and freshly couched papers (the “post”) had been built on the bottom press board.
The centering jigs are unclamped and removed from the front edge of the press board, then the front edge of the press board (with the post on it) is lifted up and at the same time the press board and post are heaved into the press (simultaneously centering it!). With my press which has a maximum nine inch (23cm) opening, when making large sheets such as 22”x30”, the wet post of twenty seven sheets and felts can weigh up to two hundred pounds (over 90kg), so the papermaker must lift half of this when heaving the post into the press. This is not a sport for weaklings!
Once the post has been centered in the press, the top press board is placed on top of the post, the oil-hydraulic press pump (or manual jack as the case may be) is turned on and the wet pressing begins.
The wet sheets in the post are still very fragile and are just held together by the felts, so great care must be taken to apply the press load very gently at first. If the top press platen is lowered too fast, the wet paper will be squeezed out from between the felts and the paper will be crushed and ruined.
On the vertical side frame of my press I have marked the position of the top press platen in quarter-inch intervals. I watch both the platen’s downward movement and the rate at which the water is escaping as the pressure is gradually applied to the post. Once the flow of water has almost ceased, the pressing force can be raised to the maximum (in my case 30 tons) and held there for two minutes or so.
Just to make sure that all the possible water is removed by wet pressing, the top press platen is raised until it clears the post and then pressure is reapplied to the maximum for another minute. Then the top platen is raised completely and the post (which is, of course, much lighter now) can be eased out of the press onto a waiting dolly.
I should mention that the floor of the wet area of my paper mill is made of asphalt that has been sloped from each side of the room downwards to a drain which discharges through the exterior wall into a double filtration system which catches 99% of the fibres that escape. Thus, after papermaking and wet pressing the whole area can be hosed down using a pressure hose to keep the floor and equipment clean.