Transferring the Wet Sheet onto the Press Felt, or
In the last column, we described how a cake of well formed pulp had been cast on the mould and the water was draining out of it back into the vat. After draining for a few minutes the deckle is removed, and the sheet is then ready to be transferred, or "couched" (pronounced "cooched", derived from the French verb 'coucher', to lay) onto a wet press felt. A number of press felts alternating with wet sheets of paper will be stacked up forming what is called a "post".
Wet Press Felts
Papermaking wet press felts were originally made from 100% wool, which became too expensive to use, so today's commercial press felts are made from a blend of synthetic yarns which are woven into a base fabric. Then multiple layers of batts of fiber are evenly cross-layered onto the surface of the base fabric and needled into it. This results in a soft, flat surface onto which the wet paper can be transferred. For handmade paper, the felts are cut uniformly to a size which is a couple of inches larger in both dimensions than the largest sheet of paper to be couched on them.
Critical Couching Factors
After draining away most of the free water, the wet pulp on the mould may consist of less than 20% solids and so has very little "wet strength". Thus couching can be a tricky operation, especially with the larger size moulds.
There are several factors which come into play simultaneously at this critical point in the papermaking process.
Firstly, the pulp must have been beaten and formed properly to attain the optimum fiber fibrillation, good formation and, hence, intimate contact between the fibers in the wet sheet.
Secondly, sufficient water must have been drained from the wet pulp on the mould to prevent it from falling off the mould when it is inverted during the couching operation.
Thirdly, just before couching, the surface of the press felt must be uniformly saturated with water. This can be readily accomplished by using an atomizing spray nozzle on the end of a wand fitted with a shut-off valve and connected to a garden hose.
The Bottom Press Board
The first press felt to be used will have been spread on a bottom press board cut larger than the press felt. It is made from L› urethaned marine plywood which has been permanently bent to bow upward from left to right, thus presenting a cylindrical surface to the mould.
The press board rests on a couching table, or is suspended between a portable bench and the front edge of the bottom press platen. Two flat wooden jigs are clamped vertically onto the front edge of the press board, (or are fastened to the couching table), against which the mould is held during the couching in order to stabilize it and to ensure that each sheet is cast exactly on top of the previous one.
Surface Tension Effect
At this point, under ideal conditions, there is sufficient surface tension between the wet pulp and the surface of the mould screen to hold the pulp on the mould when it is inverted during couching.
The Couching Action
In one operation, the mould is inverted, leveled, lined up against the jigs and the left side is slowly lowered into gentle contact with wet press felt. As soon as this contact is made, the mould is rocked from left to right in one continuous motion, making sure that the mould is in firm contact with the press felt throughout. The wet paper is thus couched, or transferred onto the press felt. As soon as the mould is clear of the press felt, it is swung away from it to the right to avoid drops of water falling on the wet sheet of paper.
More Surface Tension!
The sheet transfer occurs because the surface tension between the wet sheet and the saturated press felt is greater than that between the wet sheet and the mould screen.
Building the Post
A fresh press felt is then carefully centered and laid on top of the wet sheet of paper and the process is repeated, but alternating the couching action between left to right and right to left in order to build a uniform symmetrical post. This also makes couching easier!
The size of the post depends on the clearance of the maximum vertical opening of the press platens, and on the weight of the post that can be man-handled into the press.
Next time we will describe wet pressing, water removal and consolidation of the wet sheet of paper.
Edward H. Snider, Cranberry Mills Handmade Papers, RR#1, Seeleys Bay, ON, K0H 2N0, Canada (5/21/97)