When too much iodizing solution is added to plain collodion, the film becomes thin and insensitive, because there is not enough pyroxyline and too much iodide and water in it, so that the sensitive iodide is washed off and transferred to the bath. When a film contains too little pyroxyline, and too much iodizing solution, the effect is first indicated by the formation of broad parallel lines or bands, like the edges of retreating waves upon a flat beach, all round the edges of the film ; and when this fault exists to a great extent in collodion it becomes hardly possible to obtain a trace of a picture, even after a very long exposure. The remedy is simply to add plain collodion until good results are obtained. This addi tion of plain collodion to iodized collodion which works badly will frequently remedy the evil. Another cause of the insensitiveness of collodion, and poverty of the negative, occurs when from any mis management or inaccuracy in mixing the ingredients the iodide of potassium is allowed partially to crystallize or solidify before the film is put into the nitrate bath. This produces a compact and insensi tive coating of iodide of silver, which is blue and not sufficiently transparent by transmittent light. The negative on such a film as this is weak and insensitive. It appears, therefore, that even when the materials are good, a variety of mishaps may occur through mis management in combining them.
Having now described the mode of preparing iodized collodion, ready for use, we proceed to the various operations of taking the negative.
To coat the plate with iodized Collodion. The plate must first be cleaned in the manner described in the article, " Cleaning," and then well polished with a cambric rag, or leather buff, immediately before pouring on the collodion, for unless the plate is wiped thoroughly dry and well polished before use, it will be covered with streaks or marks where the damp rag last touched it. The breath condenses upon a clean, dry, polished plate in an even sheet, without exhibiting marks or irregularities.
Hold the plate horizontally, by one corner, between the finger and thumb of the left hand, if a small plate, or place it on a plate holder (see Plate Holder) if too large to be conveniently held in this way, and pour upon the middle of it rather more collodion than is sufficient to cover it with a good thick layer. Then, tilt the plate so as to let the collodion flow towards the thumb, but without touching it, and afterwards to the other corners in succession, and pour off the surplus into the bottle from the corner opposite to )hat by which you hold it. This done, keeping the corner of the plate still resting upon the neck of the bottle, and holding it vertically, rock it three or four times quickly through a wide angle, in order, to prevent the formation of lines in the collodion ; then place it upon the dipper ready to be plunged into the nitrate bath.
Be careful to wipe the neck of the collodion bottle occasionally, as bits of dry collodion which are formed there are liable to become detached and deposited on the plate ; also, avoid dust in the dark room, and blow off any floating particles whith settle lipon the plate, before coating it.
To excite Vie Film. The ether evaporates Very quickly- from a coated plate, and the fluid collodion speedily gelatinizes. This is called " setting." When the collodion has sufficiently set, that is, as soon as it ceases dropping from the cornet of the plate, and is safely placed upon the dipper, immerse it, without pausing, in the nitrate bath; a pause during the immersion producing a line across the plate. Then, move it from side to side in the bath, for a few seconds, in order to prevent the formation of streaks in the direction of the dipper, and leave it immersed for a couple of minutes or so. Then raise it out of, and lower it into the bath half a dozen times; in order to wash off the ether and get rid of the greasy, streaky, appearance Of the film ; let it drain for a few seconds over the bath ; wipe the back of the plate with blotting paper, and place it in the slide, taking care never to invert the slide while the plate is in it, and thus allow the free nitrate which drains towards the bottom to flow back over the partially dry upper surface of the plate.
The strength and composition of the nitrate bath is a matter of the greatest importance. The formula for making it is simple enough, but the bath is liable to many kinds of irregularities in its mode of action, the principal of which will be described, and the remeclies pointed out.
The Nitrate Bath is made by dissolving 30 grains of nitrate of silver to the ounce of distilled water, and leaving an iodized collo dion plate in it until the iodide of silver is dissolved out. This may appear simple enough, but a solution of nitrate of silver may be either acid, neutral, or alkaline, and its condition in these respects materially affects the character of the negative.
Crystallized nitrate of silver is generally contkuninated with free nitric acid, and frequently adulterated with nitrate of potass. The latter impurity has no other bad effect than that of weakenino. the solution in which it is substituted for nitrate of silver, butt' free nitric acid in the bath has a very marked and injurious effect when it exists in too great excess.
Free nitric acid in the nitrate bath impairs the sensitiveness of the film, and produces in a greater or less degree, according to its excess, grey, metallic, and feeble negatives, which it is difficult to intensify sufficiently. A bath containing an exceedingly faint trace of free nitric acid is very good for all purposes where the light is strong, such as views, skies, &c., because it keeps the lights of the negative very clean, at the same time that it prevents the blacks from acquir ing an undesirable degree of opacity, but for in-door work it is better to neutralize completely the whole of the nitric acid, and acidify the bath slightly with an organic acid, such as the acetic.