Zinc Photography

Sun (Auckland), Volume 1, Issue 2, 24 March 1927, Page 11 (Supplement)

Zinc Photography

The Block-Maker’s Art

DHOTOGRAPHY, though simplified consider- •*" ably of recent years, is still a fairly complex process, but a finished photograph has Btill to pass through many hands and devices before it can be reproduced in SUN. On the roof of the building, the photographers and “Process” staff have the chambers in which they dabble In “dragons’ blood” and handle the queer apparatus of their allied arts. Mr. H. Banks, who is in command, is an artist and by profession, and he has worked in England and South Africa. From the time a photograph is received until the finished block is ready for placing in its reservation on the forme may be just an hour. Sureness of touch and accuracy are absolutely important. The advantage of the situation on the roof is obvious; the light is ideal. The very latest plant is used, and the speediest method of reproduction, known as “dry-plate,” is followed. The photograph Is pinned on a black-board in front of a large camera which slides along on steel rails, and may he used either for reducing or enlarging a picture. Made by the famous English firm of Hunters, Ltd., this camera has metallic continuity from copy-board to negative. This feature places it ahead of any machine yet invented for process work. The easy manner in which it is manipulated is surprising; saddle, turn-,table and camera travel along the cradle beam with scarcely any effort, while a gentle pull turns the camera into any position. The Cooke prismatic lens, manufactured by Taylor-Hobson, of London, prevents halation by a series of mirrors. A negative is then taken of the photograph as it is focused in the glare of two 5,000 candle power arc lamps made by the Macbeth Arc Lamp Company, of Philadelphia. The most important part of this operation is the placing of a valuable screen between the lens and the picture. The screen consists of a waxed sheet of glass, ruled Into squares and having 65 lines to each square inch, between two glass plates, and it is, of course, the delicate product of a photographic firm. On the resulting negative the squares come out in a regular series of dots. PRINTING ON METAL AND ETCHING. Meanwhile a plate of zinc T-16 of an inch thick has been covered to the thickness of a sheet of tissue paper with a sensitive solution of fish-glue and bichromate of ammonia. With a 15,000 candle power lamp, this plate is printed ffom the negative in the same way that the amateur uses contact P.O.P. to get his finished snapshot. The action of the light is to make the exposed parts of the sensitive film insoluble in water. When the plate is placed under water the film which has been under the darker parts of the negative Is washed

away, and the exposed portions remain. Heating over a gas flame bakes the film into a hard enamel and changes its colour from light brown to chocolate. Shellac is used to protect the back of the plate, and after “lining-up” and “spotting-out,” the plate is ready to be put in the Mark Smith paddle-revolving machine for etching. This measures 3£t. by 2ft. and is mostly of heavy porcelain. In It, nitric acid bites into the exposed metal, leaving a raised surface of dots covered v6y the enamel. When the process is as far complete as it can be carried with mechanical means, a skilful etcher retouches the block, brightening and obliterating as his knowledge dictates. Other electrically-driven machines are called into use to put a bevel on the plate, in order that it may be screwed on to a mount, and to rout out the waste metal. Then, after trial proofs are taken, the plate may be presented to the printer as a perfect “half-tone." REPRODUCTION OF SKETCHES. The reproduction of drawings*and sketches demands a longer process, known as the “line block.” In this the drawing is photographed with the Hunters camera, but w'ithout the lined screen. Printing on to the sensitive film of a zinc plate is carried out in the same way as before. The plate is then immersed in a bath of aniline dye, by which the lines of the drawing are coloured. The gas flames again hake the film Into an enamel. After slight etching lias been done by dilute acid, the plate is rolled with ink, and dusted, in four directions, with dragons’ blood; a red powder which reinforces the lines, forming acid-resisting walls. Several etchings must be carried out before the plate is sufficiently marked for printiug. This process takes about two and a-half hours.

The process department Is spacious. The Inside walls are finished with a "keen" cement, and the rooms are absolutely du%t-proof, an essential to the production of high-class work. The photographic or copying department is lighted by daylight as well as by electricity. When the staff is working at top speed, the electric power used is equal to 15 to 20 horse power, and large quantities of gas are also being burnt. Acid fumes are taken through chimneys into the open air, so that the health of the workers may not suffer. All the water used is supplied in copper piping which is tinlined to avoid taints, and higli-pressure filters extract the Impurities and give it the requisite softness.

In the laboratory almost every known chemical Is kept, from sulphuric acid to white vaseline. Baths and developing tanks are -lined with acid-resisting asphalt made by the Neuchatel Company of New Zealand, Ltd. Scales capable of weighing up to a millegram are used.