Saturday, May 17, 2008

Carbon Printing By Artificial Light

           Although many photographers, both professional and amateur will be familiar with the fact that the printing of carbon tissue can be done conveniently with the stronger sources of artificial light, such as the electric arc and mercury vapour lamp, as well as by daylight, it does not appear to be so well known that by choosing a suitable source of light prints without the usual lateral inversion can be obtained from ordinary glass-plate negatives by the process of single transfer only. A few remarks on the method of obtaining such prints may therefore be of interest to those who have the necessary facilities in their studios or homes.
           To obtain the unreversed print the tissue most be placed in contact with the plain glass side of the negative, the latter being placed in the printing frame with the film towards the light. The negative moat then be illuminated by a very small but sufficiently actinic sources, and care must be taken that as little light as possible, other than the direct rays from the direct rays from the source, falls on the plate during the exposure.
           The most convenient and satisfactory illuminant is undoubtedly the comparatively new Ediswan "Pointolite" lamp. The actual source in this lamp is a small metal ball, the diameter of which is about two millimeters, supported in the centre of a glass bulb, some 10 centimeters in diameter. The ball is rendered incandescent by a small electric arc, which obtained by placing a second electrode immediately over the ball. The usual rating is 100 candle-power, and the current required about 1,5 amperes. This intense and practically “point” source of light is highly actinic, and forms an ideal illuminant for many optical purposes. In using the lamp for the purpose under consideration it is well to place the negative end of box coated inside with a dead black, and to the lamp outside the box at the other end. A small rectangular hole is to be provided in thia end, so that when the lamp u placed as close as possible to the hole the beam of light which enters the box will just cover the film of the negative. The distance between the centre of the bulb and the negative may be 20 centimeters, or even less. It is clear that a number of negatives may be printed at the same time, the bulb of the lamp being placed for this purpose in the middle of a circular or, say, octagonal box with suitable radial partitions; light reflected from the film of one negative must not be allowed to fall on the others. When six negatives are printed at once the cost of printing may be less than that of the final rapport used in the double transfer process, and there is, of course, also a considerable saving of time. With negatives of average density the time of exposure required is 40 to 50 minutes. The time may be shortened, if fine definition is not required, by reducing the distance between the negative and the lamp. It may be added that the "Pointolite" takes so little current that is may be connected to any lamp-holder, special wiring not being required.
           A second illuminant which will be found to give satisfactory results is the iron arc, but this can only be used where ordinary arc lights are installed or where the electrical fittings allow the use of a current of 5 or 6 amperes. The advantage of an arc with iron pole, instead of the usual carbon poles is that the iron burns away very slowly, so that no “feed” is required. In fact, the lamp in this case may consist simply of two iron rods, 1 or 1*(1/2) cm. in thickness supported in the same vertical line with a space of about 4 mm. between their nearer ends. The poles must be, of course, insulated and connected to the mains in the same manner as the ordinary arc lamp. The arc is most conveniently “struck” by drawing a third iron rod across the ends of the pole piece. Once the pole, become hot the lamp will run for long periods, sometimes hours, without requiring attention. If the power is supplied by direct current the upper pole should be made the negative one. It is well to place a tray containing .water below the lamp, as occasionally small pieces of molten iron may fall. As the light is very rich in ultraviolet rays it should not be used except when the eyes are protected by plain glass or ordinary spectacles. With this more powerful source a larger number of prints may be exposed at once, the frames being arranged in a circle, say 50 cm. from the light. At this distance no special screens are necessary and reflection from surrounding objects is of no account unless they are light-coloured. The exposure required at 50 cm. is about 20 minutes, with a current of amperes.
           With the distances quoted above the diameter of the circle of confusion representing the points of the image is about .005 mm, but there is a slight loss of definition which appears to be due to reflection between the front and back surfaces of the negative. The want of sharpness, however, is remarkably slight, and in most cases amounts to no more than a softening of the otherwise hard lines of the picture, an effect which in many subjects is quite pleasing.


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