Instead of nitric acid, chromic acid may be used, but this is not absolutely necessary.

After one or two hours the plates are taken out of the acid bath, well washed under a strong water rose, and dried in a place free from dust. Then the plates are well rubbed with 1 : 10 tincture of iodine solution with a pad of clean filter paper and then polished with 50 per cent. alcohol. This preparation must be done with absolute accuracy, and thus they make all substrata of albumen, india-rubber solution, etc., for direct negatives unnecessary.

Plates which have been previously used are placed in pure water till the collodion film has become loose, and are then scrubbed with a stiff brush, treated for some hours to a bath of caustic soda, and then treated as above, when they may be used again. {37}

For reproduction of simple line drawings iodized collodion is used which consists of 2 per cent. raw collodion composed of—

Iodizing is done with

The last salt is dissolved in a few drops of distilled water, the other salts are dissolved in one part of the absolute alcohol. The iodizing solution is filtered into the raw collodion, which has already been allowed to settle and filtered, and allowed to stand for some days.

The sensitizing solution is nitrate of silver in the proportion of 1 : 10, with larger sizes 1 to 12–1 to 14, and the silver bath is faintly acidulated with chemically pure nitric acid.

For a monochrome subject the exposure lasts according to the size of the reproduction, and the rapidity of the objective, from three to ten minutes.

For development an iron developer is used, consisting of:—

The iron developer, with addition of sulphate of copper, is to be preferred for short exposures to iron and lead developer. If the plate has been correctly exposed the image gradually appears after a short time, and very soon gets vigorous. When the details appear in the deepest shadows the development must be immediately stopped, otherwise fogging will ensue. If the image does not gain vigour by long development it was under-exposed; with over-exposure the image appears quickly and remains thin. The developer, when fresh, works fairly quickly; older developers work slower, but give very clear negatives.

For very fine line drawings or reproductions of copper-plate engravings the developer is used somewhat weaker, and one-fifth of its volume of water added. The development takes somewhat longer; the fine lines remain, however, very clear.

After developing the plate is well washed and then intensified first with a solution of—

to which a few drops of a 1 : 12 silver nitrate solution have been added. Care should be taken that in the first intensification the glassy places remain clear. When, therefore, the desired opacity is attained the plate should be immediately washed. Too long intensification fogs the finer lines.

The plate should be fixed with 5 per cent. solution of potassium cyanide. For this, however, a concentrated solution of hyposulphite of soda can be used.

After well washing, the negative will show, when examined by transmitted light, whether the ground is opaque enough, or whether a further intensification is necessary. With correct manipulation this first intensification will be sufficient for fine, well-prepared drawings.

If, however, the opacity proves to be too little, after thoroughly well washing, the second intensification or blackening is effected with a concentrated solution of mercuric chloride in distilled water, which is allowed to act till there is a distinct precipitate and the film has become grey. After well washing the plate is flowed over with solution of thiosulphate of gold till the grey quicksilver precipitate has become absolutely black. The gold solution consists of—

Immediately before use solution A is added with constant stirring to solution B, and a few drops of ammonia added. If the desired blackening is not effected with one application the operation must be completed.

The solution of gold, when kept in the dark, will keep a long time.

For fixed negative the following intensifiers have been used with good results:—

1. Intensifier with pyrogallol.

To this solution is added—

2. The hydrochinone intensifier recommended by Baron Hübl:—

To this as much acid is added as will keep the solution clear for {39} five minutes, when it is mixed with a third of its quantity of 1 : 30 solution of silver nitrate. The addition of a few drops of sulphuric acid or 5 parts of citric acid is generally enough. This intensifier acts somewhat more delicately than the silver and iron solution, and has the advantage over the pyrogallol that it can be applied as frequently as desired in full daylight to the fixed negatives without any danger.

3. The metol intensifier.

The negative should be developed with the above-mentioned iron solution:—

The negative must be fixed before intensification with hypo and well washed.

A small quantity of solution A is poured over the negative to drive off the water, and then it is intensified in the ordinary way with 10 parts of solution A and 1 part of solution B.

Further intensification is effected, as described, with mercury, the blackening with thiosulphate of gold.

When the negative is finished and dried it is coated with a solution of gelatine 1 : 50, or a 1 : 15 solution of gum, in order to protect the film, and the negative should be levelled and allowed to dry.

Good opacity in the ground and clear glass in the lines of the drawing are the fundamental requirements of a negative for photo-lithography. But drawings are not always prepared in such a manner that this can be completely obtained. With flat drawings which are drawn in grey instead of black the first intensification is only continued till the finest lines appear clear. For further intensification the negative after drying is given a coating of a light sensitive solution of—

Ammonia is now added till a light colour is formed. After drying it is exposed from the glass side to 15° Vogel’s photometer, washed for about a quarter of an hour and then soaked in warm water till a relief is formed. By this operation a transparent relief is formed on the lines of the drawing, which absolutely protects the same from fogging. The intensification is then effected with mercuric chloride and chloride of gold, or an intensifier of {40} solutions of uranium and ferridcyanide is used, which is composed of two solutions—

The negative is first flooded with the uranium solution and then with the red prussiate.

Another process of intensification recommended by Prof. Eder and Toth is the so-called lead intensifier.

The somewhat under-exposed negative ought only to be developed with iron, fixed with hypo, and must then be well washed. It is finally well rinsed with distilled water, and then placed in a filtered solution of—

After a few minutes it becomes absolutely opaque in the covered parts. The negative is allowed to remain in the bath till it has attained the desired density. On the silver molecules of the image a pale yellow precipitate has formed by the action of the lead bath. The ferridcyanide of potash is reduced by the silver to yellow ferrocyanide of potash, and gives then, with the lead salt, an insoluble compound, ferrocyanide of lead. By the action of ammonium sulphide the unstable lead salt is converted into a stable salt.

As soon as the negative has been taken from the lead bath and well washed, till the washing water no longer gives a blue precipitate, it is flowed over with dilute ammonium sulphide 1 : 5. Sodium sulphydrate may also be used. The image becomes black instantaneously. The negative becomes generally vigorous; if, however, it was exposed too long the fine hair lines and points veil over very easily.

If the negative is not sufficiently intensified, which may happen with very much under-exposed negatives, it should after well washing, and before treating with ammonium sulphydrate, be laid in a bath of—

in which it becomes still whiter, and should then be treated with the ammonium sulphide. The lead, the cadmium, and the silver of the bath are converted by the ammonium sulphydrate into sulphide. The negatives treated with cadmium have a yellowish tinge, whilst those treated with lead are pure black. For {41} clearing up the white-lead image when it is too vigorous or is foggy hyposulphite of soda or a dilute solution of potassium cyanide can be used. This manipulation, however, requires great care. The negative treated with ammonium sulphide if foggy can be cleared up with hydrochloric acid, used weak and repeatedly poured on and off, with careful watching of the action. After this it must be well washed.

The lead solution will remain clear for weeks, and if it begins to work slowly can be freshened up with addition of lead nitrate and ferridcyanide of potash.

If stripping negatives are to be prepared, it is advisable, with the lead intensified negatives, to give the glass plate a preliminary rub with vaseline oil, to well wash, and then to coat with raw collodion.

Husnik’s graphite intensification is still to be mentioned, which, in some cases, may be of great service, but requires practice and care.

The expert photo-mechanical worker will adopt one or the other methods for his work, and as soon as he can work safely and without hesitation he will adhere to that.