Tympans are generally covered with parchment, which should be of an even
thickness, and about two inches and a half wider and three inches longer
than the tympans. Tympans have been sometimes covered with linen, which,
on account of its evenness, would answer the purpose; but it is so apt to
stretch, that the tympans become slack in a short time, and bag (as it is
termed), and thus slur the impression. Silk is excellent for fine work.
The pressman spreads as much good paste on the edges of the skin as will
cover the frame of the tympan, which is also well pasted. He then lays
the skin on the inner side of the frame, with the flesh side to face
the type, and draws it regularly, as tight as possible, on all sides.
The part of the skin that comes on the grooves of the tympan which
receive the point-screws, is cut and wrapped round the inside edge of
the grooves, which admits a free passage for the screws. After having
fastened the skin on the sides of the tympan, he draws it toward the
joints which receive the frisket, and with a knife cuts across these
joints to let them through the skin; he then puts the frisket-pins
through the parchment, and makes that end of the tympan fast. He next
proceeds to the lower joints, and brings the skin as tight as he can
round that part of the tympan. The point-screws and duck-bill are then
put on, which prevent the skin from starting. The inner tympan, or
drawer, is covered in the same manner. To prevent their warping when the
skin begins to draw, pieces of furniture or wood of any kind should be
placed across the centre till they are perfectly dry.
The skins are put on either wet or dry: if dry, they should be afterward
well wet, which will make them give somewhat; but when they dry they will
contract, and by this means will be rendered much tighter than they would
be if put on wet.