482. Stripping Glass Negatives. Stripping, that is to say, detaching the film from the glass support, is used for certain methods of printing which require a reversed negative, or to save a negative which is cracked but not broken (f 436) or, on occasions, to reduce the space occupied by a collection of glass nega tives. The film can either be preserved as such, or on a fresh support.
To avoid the distension of the gelatine when freed from its support, it must be very well hardened. Preferably the plate should be kept at least one hour in a 3 per cent solution of chrome alum or ordinary alum, very summarily rinsed to avoid crystallization of the alum dur ing drying, squeezed and dried. In case of emergency, formaline may be used (§ 427). Whatever method of stripping is employed, the gelatine film is first cut through to the glass with a scalpel or a very sharp pen-knife, either about a quarter of an inch from the edge of the plate, or as far as may be done without encroaching on the required subject.
Commercially, a solution of hydrofluoric acid obtained by diluting the commercial product with about ion times its volume of water, is generally used (Bory, 1884) for the purpose, which frees the gelatine by dissolving the surface of the glass to a slight extent. If stripping is carried out only occasionally, the difficulties arising from the use of hydrofluoric acid can be avoided by employing less drastic methods. It is possible, for instance, to use a mixture, prepared at the moment of using, of equal volumes of a 5 per cent solution of sodium fluoride, and of a 5 per cent solution of hydro chloric acid or .1 per cent solution of sulphuric acid (measured in volumes).
After a few minutes the film becomes detached. To make sure, the margin beyond the cut is tried with the finger-tip. The whole film may then be lifted off. 1 For this purpose, a sheet of paper (preferably parchmentized, also called sulphurized paper, or, failing this, good-quality white paper) larger than the film, is previously put to soak in a dish of water. This sheet of paper is now placed on the negative, which is laid flat on the table, and, working from the centre outwards, the water is expelled by means of a squeegee or a soft, thick, rubber roller. A corner of the paper is then gently lifted until a corner of the film is visible. If the film has not come away with the paper, it should be lifted with a blunt point (such as a soft pencil) to make it adhere to the paper and, taking hold of both paper and film with the fingers, it is pulled off with a gentle and even movement. The film
thus taken off with the paper can then be trans ferred to its final support.
The above method, although quite suitable for small and medium sizes, is difficult for the treatment of very large negatives owing to the fragile nature of the gelatine film. In such cases the film should be strengthened, before making the marginal incision, by coating it with a 2 per cent solution of collodion to which a little glycerine is added, and then left to dry. 2 Prolonged immersion in the bath of fluoride solution is now necessary for the liquid to penetrate completely between the film and the glass.
483. A slower but very certain method (Liesegang, 1892, perfected by H. Drouillard, 1903) consists in placing the negative to be stripped, after it has been cut round the edges, in the following bath for at least half an hour (it can be left there all night). This bath can be used repeatedly— Water, to make 20 OZ. (I,000 c.c.) Soda carbonate, anhydrous 260-350 gr. (30 to 40 grm.) Formalinc oz. (so c.c.) Glycerine . . 9ornin. (Jo c.c.) On removal from this solution, the surface of the negative is blotted off, and, without any rinsing, it is then left to dry completely, hasten ing the latter process, if necessary, by gentle warming. 1 The dry negative is now placed in a solution containing about 5 per cent of hydro chloric acid. This causes an effervescence, some of the carbonic acid gas thus evolved being liberated between the film and the glass. In this way the film becomes detached, or at any rate its adhesion is so reduced as to allow of it being removed on to paper as described above.
Filially, attention should be called to the following rapid procedure, which is very suit able for negatives of small size and which can be used after washing but before drying them (A. Popovitsky, 1900). The negative is immersed for Jo to 15 minutes in a saturated solution of potassium carbonate (§ 429), in which about 5 per cent of caustic soda or potash has been dissolved. The negative is then dried between blotters, polished with a soft cloth, and a cut made round the part required. One of the corners is then lifted with a penknife and the film detached with a gentle pull.