COATI
CHAPTER
COATING
VIII
EMULSIONS
ON GLASS
Preparation and Cleaning of the Glass - Substratums - Drying Cupboards and Drying Problems - Coating Heads - Ventilation and Heating
THE
c.oating of negative and positive emulsions on glass, at one time the only support available, suffered a severe setback when rollfilm, and later cut film, came into such general use. Plate coating has nevertheless been continuously maintained on a large scale, particularly for the South American markets, and in recent years certain advantages over film base possessed by glass, especially in the photo-mechanical industries, have brought about a decided return to the dry plate. The study of silver halide emulsions necessarily involves coating and testing. them upon a suitable base, and for laboratory and investigation work glass is definitely, except where matters specifically relating to film technique are involved, the most satisfactory mediu,m, especially as it lends itself so well to hand coating. Carefully filtered emulsion of suitable viscosity is coated upon sheets of chemically cleaned glass. A known quantity is poured on and distributed evenly over the surface while warm, any excess over the desired amount having been previously poured off路 The coated glass is laid on a cold levelling table, where the emulsion sets to a firm jelly within thirty to sixty seconds. The set plates are placed in a dust-free dark cupboard to dry, where they may remain for from four to twelve hours, according to the temperature and humidity of the cupboard and efficiency of the ventilation. In commercial practice, the glass sheets are washed by a machine in which they are made to pass between reciprocating
G EMULSIONS
ON GLASS
139
brushes and rollers. A cleansing solution such as soda is sprayed on them during their passage through the first part of the machine. They next pass through water sprays, then between rubber rollers which squeeze off the superficial water, and are finally sprayed with a weak solution of chrome alum or sodium ilicate, which leaves the surface with a substratum or " tooth" which serves to anchor the emulsion to the glass surface and to prevent the danger of frilling or blistering during the processing of the plates. The chief commercial sizes of plates in use in the United States are 4 by' 5, 5 by 7, 8 by 10, II by 14 and 14 by 17 inches. Larger sizes are used in photo-engraving work and commercial photography. Four by five plates are coated as 8 by 10 and cut, when dry, into quarters; 5 by 7 would be coated as 7 by 10 and cut into two. The most popular English sizes are 3~ by 2!. (the higher figure is quoted first), 4% by 3t (quarter-plates), 6~ by 4* (half-plates), by 6~ (wholeplates), 10 by 8, 12 by 10 and 15 by 12 inches. The gauge of the glass increases -with the larger sizes; the sheets themselves run about one-sixteenth inch short of the nominal size in either direction to insure easy fitting in plateholders. The sheets of glass are delivered in crates, and require a certain amount' of sorting. The glass must be fairly fiat, uniform in thickness, and. as colorless as possible. It must also be free from bubbles and flaws. Drawn glass, which has come into use recently, may have slight channelings, the worst of which must be rejected, but" fiat" negative glass is ordinarily slightly curved, and the concave side is placed uppermost on the coating machine. The plates, after washing and drying, mu t be inspected to make sure they are all concave side to one direction so that no delay is caused in feeding them to the machine. It is never advisable to use old negative glass from which the
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