The next operation is to give the plate a slight tooth so that the sensitive solution will flow evenly over the surface. Do this by removing all traces of grease.
This graining is done in a wooden tray, 24 inches by 18 by 6, lined with asphaltum or gutta percha and mounted upon rockers; it is quite necessary that the graining tray be of large size, as if only a little larger than the plate the returning wave of acid water will mark the edges of the zinc.
Into this tray pour a quart of clean water, and add one drachm of nitric acid and one ounce of a saturated solution of common alum. Place the plate in this, face up, and commence rocking it at once or else the acid will mark the surface, and it will require repolishing; rock slowly for five minutes, during which time the polished surface will give way to a fine matt, like fine frosted silver. Now remove the plate and rinse it under the tap, rubbing it gently with a fine sponge, or a pledget of cotton to remove the scum or deposit formed by the acid. The surface at this stage should be quite smooth; if it is at all rough, the acid is too strong, and the solution must be diluted with water; if the action of the acid is very slow, then a little more must be added; but it will only be necessary to add more alum when the plate is a bluish color instead of being a pearly gray.
A drop or two of acid will be required each time plates are to be grained.
In rubbing with sponge, or a pledget of cotton when under the tap, care {41} must be taken not to scratch the surface of the zinc, or else it will need repolishing.
COATING THE PLATE WITH SENSITIVE ALBUMEN.
If the solution of albumen was poured over the plate and drained and dried, the film would be too thick at the bottom and too thin at the top; it is, therefore, necessary, to insure an even film of sensitive albumen, to subject the plate to a quick circular motion, so as to spread the film of albumen evenly, and to get rid of the surplus solution. This is effected by placing the grained zinc in the jaws of an instrument called a whirler and made as follows:
Two pieces of wood, each half an inch thick, twelve inches long, nine inches wide at one end, and six inches at the other; upon the narrow end of one piece fix four screws, a piece of iron the shape of a
, the top end of which is shaped to fit into the socket of a carpenter’s brace (which must have the centre handle revolving, not fixed); the cross piece must be flat, and pierced with four holes, countersunk to admit of stout screws.