COLLODION (Fr., Collodion ; Ger., Kollodium) A solution of pyroxyline in a mixture of equal quantities of alcohol and ether ; it should be kept in a well-stoppered bottle. It is a colour less, syrupy liquid, being more or less fluid according to the quantity and nature of the pyroxline used. It will keep indefinitely if made with a good pyroxyline ; the pyroxyline should be first well saturated with the ether and then the alcohol added, and, on shaking, the cotton should completely dissolve. The solution should now be set aside in the dark and allowed to stand two or three days to allow any mechanical impurities to settle, this being preferable to filtration, as in this process some of the solvents are lost. The ether used may be the so-called methylated ether, and should have a specific gravity of •72o ; the alcohol may be the indus trial methylated spirit, but it is preferable to use the pure alcohol ; aqueous alcohol should be used when aqueous solutions of salts are to be added to the collodion, as is often the case in making collodio-chloride printing-out emulsion.
Collodion is used for enamelling prints (see " Collodion, Enamel ") and as the vehicle for the silver salts in the wet-plate process, dry collodion plates, collodion emulsion, and collodio-chloride paper.
It is important that collodion should always have that degree of viscosity which has been found the most satisfactory for the particular purpose. Viscosity may be defined as the thickness or syrupy nature of the collodion. A very thin collodion—that is, one with less viscosity—is apt to allow the silver salts to deposit at the bottom of the bottle ; on the other hand, for some purposes—such as enamel ling—a less viscosity is advisable. The simplest method of testing the viscosity is by means of Von Hfibl's viscosimeter, a glass tube 6 in. (15 cm.) long, 1.2 in. (3 cm.) internal diameter, with one end drawn into a fine aperture of about in. (i mm.). About in. (8 cm.) from the wide end, and on the outside of the tube, should be scratched a line. This tube should be filled up to the mark with distilled water, the small aperture being covered with the finger, and by means of a stop-watch the time taken for the water to flow out should be noted. The
mean of six tests should be taken. Then the same process should be gone through with the collodion to be tested ; the time taken by the collodion divided by that taken by the water gives the viscosity of the collodion. For in stance, assume the mean for six tests for dis tilled water at a certain temperature to be 84 seconds, and the time for a specially thick 4 per cent. collodion to be 187 seconds ; then 187+84=2.226, the viscosity of the collodion. The proportion of alcohol and ether is not a fixed quantity. In summer more alcohol should be used, and thus the loss from evaporation slightly checked. For coating large plates a collodion rich in ether is difficult to work, as the solvents evaporate before the plate is covered ; on the other hand, a film produced by a collodion rich in ether is tougher. In the wet collodion process excess of alcohol produces greater sensi tiveness, whilst in the dry collodion process the ratio of the solvents is of less importance, and certainly in those emulsions washed by precipi tation an excess of alcohol is an advantage. The solubility of the silver nitrate and the salts has also considerable bearing on this point, and it may be considered as a general axiom that all salts are more soluble in alcohol than in ether. More particular details will be found under the special headings.
In process work, collodion is an important factor on account of the facility and cheapness with which, by its help, negatives suitable for the various reproductive processes can be made. The comparative slowness of wet-plate exposures is no drawback where exposures are invariably made by electric light ; and the development, fixing, intensification, clearing, and drying are all executed much more quickly than on gelatine plates. The silver deposit being on the surface, the image is more susceptible to intensification and reduction than an emulsion film. Finally, on the ground of cheapness, wet collodion holds the field. It has been calculated that the average cost of making negatives in half-plate size is : Wet collodion, id. ; collodion emulsion, ; dry plate, 2fd. Collodion is also largely used in process work for stripping. (See also conclusion to article "Collodion Process (Wet).")