PRESERVATION OF SENSITISED PAPER.
Paper sensitised in a neutral bath is extremely liable to deterioration by dis coloration, the prints produced by it, after even about two days, being very weak. To avoid this trouble, it is usual to " fume " the paper when more than a day old, or to impregnate it. with ammonia fumes. The object is to absorb the chlorine spon taneously liberated and prevent discolour ation or flatness. This operation is carried out by means of an ordinary wooden box (see Fig. 265) opening from the side as shown. It has rails at. A, over which run light wooden frames covered with clean white muslin, D. Beneath the lowest one is placed an evaporating basin c containing about 1 oz. of liquid ammonia. The sheets of paper arc laid face up on these frames, and the box is closed by means of the folding flap D, and fastened by the turn-button E ; the paper remains in the fumes for about ten min utes. Instead of using this method some printers place sheets of blotting-paper soaked in ammonium chloride solution and a similar quantity soaked in lime-water in contact, so that ammonia is liberated by double decomposition. The paper kept under pressure will keep much longer, and if kept between sheets of blotting-paper which have been soaked in a 5 per cent.
solution of citric acid it will remain un altered—say a fortnight or more. When an acid sensitising bath is used as described on p. 170 fuming is unnecessary, and the paper keeps well. This is the process em ployed by manufacturers of ready-sensi tised paper, which keeps for a consider able time. As a further preservative, it is usual to float the back of the paper on a bath of citric acid, the strength of the solu tion being about 20 grains per ounce ; or it may be floated, sensitive surface upper most, on a solution of 5 oz. of nitrite of potassium to WO oz. of water. When dry, the paper is rolled up, coated side out, and wrapped in blotting-paper soaked in the nitrite of potash solution, and dried. This method may be employed also with paper floated on a neutral bath. The objection to ready-sensitised paper is that its acid con dition limits the available tones to such as are not very pleasing ; with proper treat ment, however, this seldom proves a serious drawback. Instructions for dealing with it to the best, advantage will be given later. The toning, fixing, and washing of albumen paper are described in a sepa rate section,