CLEANING THE PLATES.
Whilst the emulsion has been filtering, the plates are cleaned and prepared ready for coating. For this purpose, it is pos sible to use old negative glasses ; that is, those from which the film has been re moved ; but it is far better to procure some sheets of new glass, as besides the fact that the waste of time in cleaning the plates almost or quite equals the cost of new stuff, there is always the danger of chemicals still remaining on the glass, calico, washed in soda water, rinsed, and dried. Various methods have been sug gested for forcing the emulsion more rapidly through the filtering medium, by means of some pneumatic arrangement or by sucking it through by means of a filter pump (see Pg. 13S), but filtering is a tedious process. At Fig. 135 it will be noted that the emulsion filters into a yellow glass bottle, which is standing in a beaker of hot water. When the tubing arrangement is used, or the small tap as shown in Fig. 13G, this is supplied easily from the vessel ,tho•e. Filtering completed, the emulsion may at once be placed on the plates, regardless of the fact that it still contains the soluble nitrate, and is in an unwashed condition.
despite the fact that it may have been most scrupulously cleaned. There may also be scratches on the old plates. These would give rise to unaccountable spots and even more curious appearances on photo graphic plates. One photographer, who had exposed a plate on an open land scape in Switzerland, was alarmed to find, on development, a superimposed image of a lady in evening dress, whose costume and features were entirely unknown to him. The explanation was,
no doubt, quite simple. The plates were coated upon old negative glasses, and still preserved some reducing power from their former image. This is, of course, a thing which is hardly likely to occur in any large factory. In choosing glass for coat ing great care must be taken, first, that it is of correct thickness ; secondly, that this thickness is uniform and thirdly, that it is entirely free from striae, bubbles, etc., as these usually come on some im portant. part of the negative, and seriously affect the definition. The size of the plates is not important, so long as they plates being then stood in a rack to dry spontaneously. The plates should not be , rubbed dry, as friction seems to prevent ) the proper adhesion of the emulsion. When dry. the next operation is to edge them with india-rubber solution to prevent are not too large to handle conveniently. They may he cut up to suitable sizes after wards. The size most useful may be multiplied by a number leaving a slight margin to trim off, as the emulsion is sometimes not coated nicely at the edges. Odd sizes are inconvenient, as the amount of emulsion poured on the plate is in direct proportion to its area.