Page "Paper machine" Paragraph 39

On lower speed machines at 700 feet per minute, gravity and the height of the stock in the headbox creates sufficient pressure to form the jet through the opening of the slice.

The height of the stock is the head, which gives the headbox its name.

The speed of the jet compared to the speed of the wire is known as the jet-to-wire ratio.

When the jet-to-wire ratio is less than unity, the fibers in the stock become drawn out in the machine direction.

On slower machines where sufficient liquid remains in the stock before draining out, the wire can be driven back and forth with a process known as shake.

This provides some measure of randomizing the direction of the fibers and gives the sheet more uniform strength in both the machine and cross-machine directions.

On fast machines, the stock does not remain on the wire in liquid form long enough and the long fibers line up with the machine.

When the jet-to-wire ratio exceeds unity, the fibers tend to pile up in lumps.

The resulting variation in paper density provides the antique or parchment paper look.