But if this method of working be not available, it is quite possible to form a stripping skin by coating a collodionised glass with the bichromatised gelatine; expose, wash, decolorise, and then, after freeing from all chromic salts, squeegee the "stripper" negative on to it, and after a short time immerse in hot water to remove the paper; then dry and strip in the ordinary manner. - (W. H. Bolton.)

(c) The stripping of a negative film when it is in collodion is a comparatively easy matter, but, in the present state of photography, it is quite necessary for one who takes up certain photomechanical processes to have a ready means of stripping gelatine films. The descriptions of such processes are meagre. Wilkinson, for example, in his excellent little book on photoengraving and photo-lithography, describes methods of stripping collodion negatives only. Such descriptions are generally insufficient in detail. Any one taking them up and trying to work from them will find that, he is not at once able to strip negatives and get satisfactory films from them, but that he has to work out all the details himself.

The process generally advocated is that of Plener, in which hydrofluoric acid is used, to separate the film from the glass. Plener's method is so ingenious that one is sorry to pour any cold water on it, but apart from the use of hydrofluoric acid, which every one will avoid who possibly can, it would appear that the etching action of the acid on the glass has nothing to do with the stripping, inasmuch as almost any other acid will act as well as hydrofluoric acid.

This observation was made by W. B. Bolton. The action, whatever it is, is really the same as the "frilling" of the plate. Although almost any acid that is strong enough or that has a strong enough action on gelatine will serve to strip, it is, perhaps, scarcely correct to say that any other acid will do as well as hydrofluoric acid, inasmuch as there are differences in the certainty in the action of different acids. Sulphuric acid is the most certain in its action of all acids, hydrofluoric included.

The difficulties that will generally be found in stripping are as follows: - (1) The films refuse to strip at all. A process which will be quite successful with a negative freshly developed and fixed will often be found not to work at all with a negative that has been treated with alum, and is, say, a year old. (2) The stripping takes place between the new support and the film, instead of between the film and the glass. (3) The film after stripping stretches so that not only is the negative to a larger scale than before - and with a corresponding increase of density - but probably there is likewise distortion, as it is highly improbable that a film will stretch exactly to the same extent in all directions. (4) The film is so hard and brittle when dry that it cannot be handled without danger of cracking it. (5) The film twists or curls in drying, so that it is impossible to get perfect contact between it and a flat surface, such as a collotype film.

If the following directions be precisely carried out, it will be found that any negative, however old, may be stripped with certainty and without encountering any of the difficulties just enumerated. It need scarcely be mentioned that, if the negative has been varnished, the varnish must first be removed by the application of hot methylated spirits.

The following is made up: - 2 oz. hard gelatine and 10 oz. water. The gelatine is, of course, soaked in the water till it is soft, when heat is applied. The solution should be heated to not very far short of the boiling point, and should then be filtered through flannel.

A negative to be stripped is warmed, and accurately levelled, and a certain amount of the gelatine solution is poured on the surface and guided with a glass rod till it covers the surface evenly. It is of great importance to measure the quantity, as the ultimate thickness of the film depends on this, and it has to be borne in mind that printing has, if the film is to be used as a reversed negative, to go on through, the gelatine.

A quantity of 4 oz. per square foot will be found suitable for average work. This gives a film, when dry, of about 1/100 in. thick. This sounds very thin, but, in fact, it is a good stoat film. The film of an average gelatine dry plate, when dry, is only about 1/2000 in thick. Still 1/100 in. will not interfere with ordinary work. For very delicate work, the amount of the gelatine solution may be reduced to 2 oz. per square foot. It is not advisable to use less than this, as otherwise the films will be too delicate for free handling. It is quite possible to flow as much as 10 oz. per square foot on glass without any special arrangement for preventing it from overflowing the edges, but a film of the thickness so got is far too thick for any practical use.

If, however, such a film be wanted for anything, it will be necessary to give four times as long in each solution as is hereinafter directed, as it takes a long time for any liquid to penetrate a very thick film.

A saturated solution of chrome alum is taken, and there is added to it, drop by drop, liquid ammonia, till there is the slightest possible permanent floc-culent precipitate. This is merely to neutralise any free acid that there may be, as any such free acid greatly impairs the setting powers of the alum solution.

The gelatine solution having set on the plates, the latter are placed for 5 minutes in the chrome alum solution. They are then removed and are washed till all the blue colour of the chrome alum has disappeared. They are then placed in a bath of methylated spirit for 1/2 hour. They are taken from this, the surface is blotted with blotting paper, and they go into a bath of 1 oz. sulphuric acid, and 2 pints water. Use ordinary commercial sulphuric acid; the exact specific gravity is not of much importance, as a considerable variation in the strength of the bath is permissible.

The following solution is now needed: - 1 oz. liquid ammonia, 1 oz. glycerine, 2 pints water. The films are left in the sulphuric acid bath till the greasiness caused by the alcohol disappears; this will take about 1/2 hour. At the end of that time, it will be found that the corner of a film may be lifted, and that the whole will leave the glass with the greatest ease. The films go direct from the sulphuric acid bath to that of ammonia and glycerine. The object of the glycerine is to secure flexibility of the films, the object of the ammonia is to neutralise the sulphuric acid. It takes a long time to wash sulphuric acid out of a thick film, and if a long time be given in this case, the film will stretch. The small amount of sulphate of ammonia produced in the film by the acid and the ammonia is quite harmless.

Glass is prepared by rubbing the surface with a solution of a few grains of beeswax in each ounce of benzine, and by polishing this away with powdered talc. A film is taken from the last solution as soon as it tastes of acid, probably in about 5 minutes; it is placed on a piece of glass, a pad of blotting paper is placed on the back of it, and the squeegee is applied with great vigour. The plate supporting the film is then placed in a fairly warm place, so that the latter may dry; but it is necessary, after a few hours have elapsed, to paste down the edges with narrow strips of paper and gum solution, otherwise they are sure to rise from the glass before the whole film is dry, and it will be found impossible to get flatness. An attempt must not be made to strip the film from the temporary class support until it is perfectly dry. Then it is only necessary to insert the thin blade of a knife, when the film will come away in a form in which it is beautifully flexible, and may be printed from either as a direct or a reversed negative.

It is to be observed that if the film, before it goes into the sulphuric acid, be dried spontaneously instead of by the aid of alcohol, the gelatine support will probably part from the film instead of the film parting from the glass.

The acid bath may, of course, be used for a long time; in fact, as long as it acts. The ammonia and glycerine bath may be used as long as it continues to smell distinctly of ammonia.

On trying to dry the stripped films on vulcanite, it was found impossible to prevent them from leaving the surface before they were dry, with the effect that they were hopelessly far from flat.

The process as described reads as if it were very complicated. It is not really so. A negative may easily be stripped and le drying within 2 hours of the time of beginning operations, and quite a number may be done at the same time. The drying is the thing that takes longest. With films of average thickness it takes 24 hours. - (W. K. Burton.) (See also iv. 366, 395.)