Prints which are made on development plates and papers, or carbon papers, where it is not necessary to judge the visible progress of printing, do not require the use of a frame with a divided back and separate fastenings ; the opening and closing involve unnecessary opera tions.
Various types of printing-frames with solid backs are obtainable with which several prints may be made from a part of a large plate with the certainty that the picture will be identical on each print (e.g. when printing on post cards or transparency plates). The same result can be obtained by using a printing-frame fitted with a piece of stout glass, and temporarily fixing a piece of thin cardboard to the negative, into the opening of which the sensitive plate or card fits closely.
For making prints of tracings on ferro prussiate or similar papers by artificial light, when the printing can be done with a constant exposure after preliminary trials if all the trac ings have been made on paper of equal trans parency, a frame is used consisting of two half-cylinders of glass fixed in a metal frame work. The tracing and paper are held against this curved surface with a strong linen blind or apron, which is secured by stretchers. The illumination is provided by a tubular lamp (mercury arc) fixed on the axis of the cylinder, or by an arc lamp which slowly moves up and down, with a correctly adjusted uniform motion, along a vertical line coinciding with the axis. In large studios, continuous machines are also used, the tracings and papers being carried along over a half-cylinder of glass by an endless belt, moving at a speed adjusted to the sensitivity of the paper.
Lastly, in photo-mechanical work (photo gravure, photozinco, etc.), a pneumatic printing frame is used the back of which takes the form of a woven rubbered cloth edged with soft rubber grooving, which is pressed against the glass of the printing frame by another frame with lever bars. By means of a vacuum or water-pump, the space between the glass and the apron is exhausted, the sensitive surface being then held against the negative by the pressure of the atmosphere. The pressure being thus balanced on both sides of the glass, the latter can be much thinner than in printing frames in which the glass receives a mechanical pressure on one of its sides only.
5o6. At the present day, in order to ensure a rapid and regular output in a professional studio, printing on development papers is done almost exclusively in printing boxes, the first models of which appeared about 1900.
A printing box consists essentially of a print ing frame placed horizontally at a height which is convenient for a standing operator, on a box light-tight but amply ventilated, enclosing the printing lamps and a red or yellow lamp (or an ordinary lamp in a light-tight lantern fitted with a red or yellow glass or flexible filter), allowing the paper to be placed correctly on the negative. The pressure-bar which is worked either by hand or by pedal, is locked auto matically at the end of the stroke.
The printer is often fitted with a switch which, when the back is in position and the paper in complete contact with the negative, switches the current from the red lamp to the white lamps—and back to the red lamp as soon as the back is released.' The use of such a switch is only advisable for current of low intensity. When using heavy current for print ing on slow papers We watts or over) a switch should never be used unless operated by a carefully constructed relay. In all cases, red sight-holes should be provided to allow of inspection of the white lamps.
Very often the lighting of the white lamps is controlled by a timing device which is released either by the closing of the back of the printer or by hand pressure on a lever. The exposure is then regulated over a wide range either by a clockwork mechanism or by the escape of air or glycerine from a small orifice (as in a gun brake).' Rather than attempt to regulate the exposure by altering the cross-section of the orifice from which the air or fluid escapes—a method which is always uncertain in its results— it is nearly always preferable to keep the latter fixed and to adjust the time by moving a contact on the travel of a solid plug or block on the piston of the pump. With some Printers the bolt which maintains the pressure on the back is automatically released as the white light is cut off.