COVERING TYMPANS.
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.