For some classes of work sensitized paper may be washed with advantage previous to drying, and there is much economy in this plan, particularly in hot weather, since it keeps of a purer white for a much longer period than where the silver nitrate is allowed to dry on the surface. It may not be out of place to call attention to the action of silver nitrate on the paper. If a stick of lunar caustic be applied to the skin when dried, there is a peculiar burning effect produced, and even in the dark the cuticle becomes discoloured, though not black. In the albumenized paper we have albumen and the gelatine sizing, and these substances behave somewhat like the skin. The gelatine particularly will become oxidized at the expense of the silver, a reddish organic oxide being formed; and again, if the silver nitrate be alkaline or strictly neutral, we have the same action occurring as when we precipitate metallic silver by means of an alkali, and an organic body such as sugar of milk. The gelatine takes the place of the latter. When the free silver nitrate is removed, the tendency for the spontaneous darkening of the paper is much diminished, since the chloride and albuminate of silver are much less readily reduced than the nitrate. The following plan is adopted for washing the paper:—The paper, after floating, is drawn twice rapidly through a dish of rain or distilled water, and, unless some other substance which can absorb chlorine be added to the last wash water, care should be taken not to soak out all the free nitrate, as then the paper would produce flat prints. It is then hung up to dry as before. Immediately before use it must be fumed with ammonia, in order that the prints may be "plucky," and free from that peculiar speckiness of surface which is known to the silver printer as "measles." We can readily trace the "measles" to their source. Suppose all free silver nitrate is washed away, and the paper be then exposed to light, the chloride is rapidly converted into subchloride, and chlorine is given off (see page 5); if there be nothing to absorb it at once it will attack the albuminate, which is blackened at the same time, and fresh chloride will be formed in little minute spots. These discolour, and are of different tint to the rest of the print, and give rise to the appearance of measles. This, of course, is not so marked when a little free silver nitrate is left in the paper; but as what is removed is principally removed from the surface, it may still be unpleasantly discernible. Fuming obviates it entirely if properly performed, for chlorine and ammonia combine to form finally ammonium chloride, a neutral and inactive salt.

Any other chlorine absorber may be substituted; thus citric acid, potassium nitrite, and many others are effective, and cause vigorous prints to be produced. Perhaps the easiest way of giving the paper the necessary amount of ammonia is that recommended by Colonel Wortley. This is to place overnight the pads of the printing-frame, if they be of felt, into a closed box in which is placed a saucer containing a couple of drachms of liquor ammoniÆ, and to withdraw them as required for the printing-frames. The pads will be thoroughly impregnated with the vapour of ammonia, and a couple or more prints, in succession, may be made before it is necessary to change them.

The ordinary method of fuming is that used in America. Hearn describes a box, which is very convenient and simple in construction. He says: "Take any common wooden box, large enough for the purpose, and make a door of suitable size for it, which, when shut, will totally exclude all light. Make a false bottom in this about six inches, or so, from the real one, and perforate it with holes of about the same size that a gimlet would make. These holes should be very numerous, and at the centre there should be, if anything, a smaller number of them, because the saucer containing the liquor ammonia is generally placed at the centre of the real bottom of the box."

For our own part we dislike the false bottom as constructed, and recommend one of fine gauze, and, instead of placing half-an-ounce of ammonia in the saucer as Hearn directs, we prefer to soak half-a-dozen sheets of blotting-paper in ammonium chloride solution, about 20 grains to the ounce, and the same number of sheets soaked in lime water; one sheet of each are placed together, and ammonia is liberated by double decomposition; calcium chloride being also formed.

This method is excellent in hot, dry weather, since it imparts a certain amount of moisture to the paper. In damp weather it is a good plan to dry the vapour by sprinkling on the gauze calcium chloride, which will rapidly absorb the aqueous vapour, and will allow the ammonia to pass on unimpeded. The sheets of paper are held at the top of the box by American clips, suspended from laths about three inches apart, and it is not a bad plan to fasten a lath on to their bottom edge by the same means, to do away with their curling. To fume a single piece of paper it may be pinned up to the inside of the top of the lid of a box, and a drachm of ammonia sprinkled on cotton wool distributed at the bottom. The point to be attended to is that the fuming shall be even, and it is evident that the ammonia should rise equally from any part of the bottom of the box. In the plan of the box given above, the bottom of the sheet is apt to get a little more ammonia than the top. The time of fuming depends on so many things that a rule can scarcely be given for it; twenty minutes may be considered about the extreme limit.

If this sensitizing bath be acid, the time must evidently be longer than when it is strictly neutral or slightly alkaline; and if the negative be hard, it will require to be less fumed than if it be of a weak nature, since the action of ammonia is to cause rapid darkening in the deep shadows. In hot weather the fuming should be shorter than in cold, since the ammonia volatilizes much more rapidly when the temperature is high. On the whole, we recommend Colonel Wortley's plan of fuming the pads in preference to fuming the paper.

Another mode of preserving the paper from discolouration is to add citric acid to the printing bath, which is effective owing to the fact given at page 32. The following formula is a good one, and has answered with the writer. It is—

The paper is floated for the ordinary length of time, when it is dried thoroughly and placed between sheets of pure blotting-paper. It will keep in its pristine state for months, if excluded from the air. It is better to fume this paper strongly before use, or the toning becomes a difficult matter.

Ordinary sensitized paper may be preserved for a considerable time if, when dry, it is placed between sheets of blotting-paper saturated with a solution of carbonate of soda, and dried.

Washed sensitized paper is also improved in sensitiveness by floating it for a few seconds on—

It can be fumed, when dried, in the usual manner. In the Year-Book of Photography for 1880 Mr. A. Borland recommends the following modification:—

He floats the paper on nitrate of silver, as usual, and after it has drained surface dry, blots off any drops that may remain at the edges, and then floats the back of the paper for about three minutes on the following bath:—

This is rendered slightly acid by a little solution of freshly prepared citric acid in water. The degree of acidity is regulated by litmus paper (the blue specimen), which should be slightly reddened by it. After this solution has been mixed about ten minutes, it is filtered, and the paper floated. Mr. Borland says the paper keeps well, and prints the same as ordinary paper, and any tone may be produced.