This article first appeared in Practical Engineering 1940 Vol1 No21. The information within the article is therefore valid as of 1940. This article discusses developments within the field of Engineering at the time.
Original drawings are never issued to the shops—they are retained by the drawing office; hence it becomes necessary to produce duplicates for the use of the shops. This can be done by hand, but that is a slow, laborious and costly process. It is far more satisfactory and economical to do it by other means, some of which are described.
The most widely-used duplicating method is that of making blue-prints. It is an extremely old process; Herschel read a paper on it to the Royal Society in 1842. The first step is to copy the original on cloth or linen paper. This translucent copy is placed in direct contact with a sheet of paper sensitised with ferro-prussiate and exposed to a bright light, usually an arc lamp. The effect of the light is to change the ferrous into a ferric salt. This, combining with the ferricyanide of potassium (which gives the characteristic blue) becomes insoluble in water.
Thus, after the exposure, that part of the sensitised paper which was not protected from the light by the ink lines on the tracing is left covered with an insoluble coating, the remainder being soluble. The print is now developed by washing thoroughly in water. The unaltered and soluble ferrous salt is removed and the insoluble ferricyanide turns the paper blue. The result is the well-known white outline on a blue background.
The advantages of this process are: ease and simplicity of operation; low first and running costs; permanent copies. The chief disadvantage is that the print must be washed thoroughly in water to remove the excess salts. On drying, the paper shrinks: thus the original must be fully dimensioned, and no measurements may be scaled from the prints.
This is very similar to blue-printing in that the tracing is placed in direct contact with paper sensitised with an almost colourless substance and exposed to an arc lamp. This time the light bleaches the parts unprotected by ink. The print is then developed in a developing solution, the unbleached parts turning black. The result is a black outline on a white base.
As the paper, is only damped in developing, the shrinkage is much less than in blue-printing, although shrinkage does occur. In addition to the other advantages possessed by blue printing it allows graphs, chart forms, etc., to be produced on a white base, thereby permitting figures to be inked in in any desired colour.
It suffers from two serious disadvantages. The first is that the prints fade in time with exposure to light. The second is that the tracings must be clean and in good condition—the more obstruction there being to the passage of light, the poorer the print.
True to Scale
Both blue- and dye-line prints suffer from the fact that the paper shrinks, due to washing. This is overcome in the true-to-scale method, and permits of measurements being taken direct from the prints. It is essentially a litho process, the copies being run off from an etching made on a special composition of gelatine and iron salts. This mixture is usually spread on an endless belt of linoleum running over and under a table.
An undeveloped ferroprussiate (blue) print is laid face down on the gelatine, lightly rubbed all over and then removed. The lines on the undeveloped blue-print etch the gelatine surface. Printing ink, coloured if desired, is applied to the surface, the etched parts only taking up the ink. A sheet of paper is next laid on the gelatine and so receives the inked from the etched parts.
Renewing the Coating
When the whole of the gelatine surface has been covered with etchings it is scraped off and a fresh coating applied by winding the belt slowly through a trough containing liquid gelatine compound.
The advantages of this process are: true-to-scale reproduction; permanent prints; any flexible material can be printed on, e.g., linen, leather or thin metal sheets. The disadvantages are: slowness in comparison with blue-printing; a maximum of only about 30 prints can be taken from each etching.
This method is chiefly used for the production of handbills, sketches for catalogues, etc. It is based on the principle that certain colloids are rendered insoluble under the action of light. A special sensitised film is placed against the tracing in a direct-contact photographic frame. After exposure the film is developed in a standard developing solution, washed in hot water and dried. This film can now be used as a stencil in a duplicator, and copies run off direct on to suitable paper.
The method is relatively expensive where only a few copies of each sketch are required, but where drawings have to be produced in large quantities the extra initial cost is well justified. Its chief attractions are: several thousand prints can be run off one stencil; very high speed of printing; the film can be filed for future use. Against these can be set: high installation costs; the maximum size of tracing and print is limited; tracings must be in good condition.
In all the previously described processes a tracing of the original had to be made on translucent paper; as this tracing is done by hand, careful checking must be carried out to ensure correctness of details, and even this does not obviate the possibility of costly errors creeping into workshop drawings.
Photographic reproduction does away with all that, and it introduces several other important advantages. Prints can be enlarged or reduced to any. desired size and still maintain perfect proportions, copies can be made of contracts and other legal documents and filed for future reference in the minimum of space.
A photograph is taken of the drawing, the size of the negative being about 2 in. by 5 in. This is developed, and an enlargement of the desired size made from this negative. Any number of prints can be run off from one negative, and the filing of the negatives is a simple matter due to their small size.
The initial installation cost is high, but the running economies justify the process, especially when the elimination of tracers and checkers is considered together with the saving in time and storage space.