Monday, April 13, 2015
News of Yore 1916: A Primer on Line Photoengraving
A remarkably lucid explanation of the amazing amount of work that went into making a metal printing plate, via the method of line photoengraving. This explanation was originally published in The Students Art Magazine, April 1916.
By the term photoengraving is meant the production of printing plates having images formed in relief upon a metal surface, these images being obtained by a series of photographic and chemical operations.
There are two general classes of these engravings known respectively as line plates and half-tone plates-r-the former being reproductions of subjects formed only of lines or masses of solid black or white; the latter those of subjects, having intermediate tones such as photographs or wash drawings, ' This article is devoted to a description of the operations necessary in producing a line plate (commonly known as a zinc etching).
|Sample of a vintage print block (currently for sale on eBay)|
|Another from eBay; seller even found it being used in a 1916 newspaper, same year as this article|
The next operation is to saw a piece of zinc large enough to take the print from the negative, allowing about an inch margin all around. The zinc is polished with a piece of cotton, powdered pumice stone and water. While wet it is flowed over with the sensitizing solution, composed of albumen, fish glue, ammonium bichromate and water. In this condition the zinc plate is placed in a whirling device and kept in motion over a gas flame until dry. The negative is placed in a heavy printing frame and the sensitized zinc put in just as it is done for printing a post card from a kodak film. This printing frame is supplied with strong clamps that press the metal into absolute contact with the film. The exposure is made and the frame taken to the dark room where the zinc is rolled up completely with etching ink. The zinc with the ink covering is placed under a tap and water allowed to flow over it until all of the unexposed sensitizer is dissolved out, carrying the etching ink with it. Where the zinc has been exposed to light through the transparent lines of the negative, the sensitive coating becomes insolvent and remains in place, retaining the etching ink. When the print is properly dissolved out one has left a piece of polished zinc with the design upon it in lines of etching ink. Now the zinc plate is taken to the etching room, dried with a piece of chamois skin, warmed and powdered with etching powder or dragon's blood. When warm the ink lines are sticky and retain the powder, the surplus being brushed from the zinc with a soft brush. The plate is now placed over a gas flame and heated until the powder fuses with the ink. The back of the plate is painted with shellac to keep the acid from etching it. All the lines being protected with an acid resist we are ready for etching. The first bite, or etching, is given by placing the zinc into the etching tub containing nitric acid and water — about ten per cent solution. The tub is rocked until the metal in the exposed parts is etched away to a depth equal to the thickness of a sheet of heavy writing paper. The plate is taken out, dried, warmed and powdered again, this time in four different directions so that the powder will bank up against the sides of the lines and protect them from undercutting. Two bites are usually necessary and the same method of procedure for powdering must be gone through. When the design is etched deep enough the plate is taken out and the acid resist removed with hot lye water.
|Another one from eBay; this one is later vintage without wood backer.|
After this the etching is nailed to a block of birch or other hard lumber which has previously been planed flat in the surface planer. Then it is sawed up close and trimmed square on four sides. It is again placed in the surface planer, face down, and the back of the block planed down to type-high.
Several proofs are pulled on a hand press and the print examined for spots or flaws. These spots, etc., are removed by hand by the finished. The final proof is delivered to the customer with the engraving.
The foregoing will give some idea of the many operations necessary in producing even the smallest kind of a line etching. There are many schemes which the photoengraver must make use of in order to turn out the small engravings at a profit. Sometimes a number of exposures of different reductions can be made on the same plate. In this way all of them are developed in the same time it would take to develop one. A large number of negatives can be stripped onto the same plate glass so that the printing and etching can be done at one time.
In case an order comes for a large amount of engravings of the same kind and size, one or two engravings are first made from the copy. A goodly number of clean proofs are pulled from these engravings and the proofs pasted or arranged on a sheet of board so they can be photographed at one time.
The process of making the other kind of plate (halftone) from photographs or wash drawings is practically the same except in making the negative.