FILM STRIPPING Films may easily be stripped from their glass supports and transferred as required, this course often being necessary when the glass of a nega tive is cracked and the film is undamaged. In reversing a negative for single transfer carbon or collotype work, the stripping method is also useful. If desired the film may be enlarged in course of transference, as described under the heading " Enlarging by Stripping." In cases where it is not desired to enlarge the film, the hydrofluoric acid used for stripping must be diluted with methylated spirit, which, to some extent, counteracts the tendency to expand. The following is suitable : Methylated spirit . 5 oz. i,000 ccs.
Water . thins. 37.5 „ Glycerine . . „ 37'5 „ Hydrofluoric acid „ „ Make this up without the acid and keep as a stock solution, adding the acid just before using. The negative to be stripped should not have been varnished, or, if it has been, the varnish must be removed before treatment. It should be noted that hydrofluoric acid attacks glass, and any mixture containing the acid must therefore be kept in an indiarubber or ebonite cup or dish, or in a glass vessel that has been coated internally with paraffin wax (the wax is melted, poured in and out again, leaving a coat ing on the sides). Some sheets of waxed paper are also needed before the work of stripping can be begun, also a glass plate coated with gum or gelatine on to which the film is to be transferred, for owing to the use of a large proportion of spirit the removed film will not adhere to plain glass. Lay the negative to be stripped on a perfectly level surface, and with the aid of a straightedge, and by means of a sharp penknife, cut through the film to the glass at + in. from the edge all round ; then pour enough of the stripping mixture on to the film and spread over evenly with a camel-hair brush, or a piece of paper or cotton-wool. In about five minutes the film will become loose, and the narrow bands of film at the edges may be stripped away, this being a good test as to how the stripping mixture is working on the film. If after five minutes the film refuses to move, a little more acid may be added to some more of the solution, and spread over the film. No attempt must be made to hasten the loosening of the film at the edges by pulling ; the acid must do all the work. When the margin comes away without the slightest resistance, it is a sign that the main film is in a state to be removed. A penknife may be in serted under one corner of the film just to see if this will come away easily from the glass. The film being still on the plate, drain off superfluous acid, and pour on more of the stripping mixture, this time without the acid. This in turn is poured off, and a sheet of the waxed paper brought down upon the loosened film, and lightly squeegeed down. The film will adhere per fectly to the waxed paper, and they can be removed together on to the new glass and squeegeed over lightly ; the paper is then pulled gently away, leaving the film upon the prepared glass.
When a reversal—as regards right and left— is wanted, the film on the original negative should be transferred to a plain piece of white paper covered with the spirit mixture minus the acid ; the waxed paper is then laid over the removed film, and the two papers, with the film in between, squeegeed into contact; the papers are then separated carefully, in such a manner that the loose film remains on the waxed paper ; it is then transferred to the prepared plate in the manner described above, care being taken to keep the film flat and in perfect contact with its support.
Owing to objections to the use of hydrofluoric acid, many prefer to do without it, and they employ some such method as the following. The film is liable to slight enlargement, by about one-thirtieth of its length, which matters little if it has been cut round the edges in the manner already described. The negative is placed in the following bath : Caustic soda . . 20 grs. 23 g.
Formaline . . 20 drops 21 ccs.
Water . . . 2 oz. 1,000 „ The formaline toughens the film, and in about ten minutes the film could, if it were desired, be rolled back with the finger. But do not do so ; instead, when it appears to be loose, transfer to the following bath : Glycerine . . 60-70 drops 63-73 ccs.
Hydrochloric acid . so „ 52 „ Water . . . 2 OZ. 1,000 in which it can be entirely detached, and as it floats in the liquid either side may be attached to the new glass. In order to ensure the film sticking to its new support, it is advisable that the following substratum should be applied to the glass : Formaline . . io drops i cc.
Gelatine . . . 4 grs. g.
Water . . . 2 OZ. 1,000 ccs.
Swell the gelatine in the water, dissolve by heating, and add the formaline ; coat the glass, allow to dry, transfer the wet film to it, press down, and allow to dry naturally. A fixed and washed unexposed dry plate also serves as a support.
Films are removed from celluloid supports by soaking in the caustic soda and formaline mix ture already given, and then in the hydrochloric acid and glycerine bath ; the films, slightly enlarged, are washed and transferred to glass or celluloid.
A convenient method of using hydrofluoric acid for stripping is to manufacture it as required, which can be very easily done by making a solution of sodium, ammonium or potassium fluoride, about fo grs. to the ounce of water (zo g. to i,000 ccs.), and acidulating with a few drops of some strong acid, such as sulphuric or nitric, applying the mixture to the nega tive. Hydrofluoric acid is generated and strips the film, and the solution may be thrown away when it has done its work. The dry fluorides keep well in ordinary glass bottles.