One of the important adjuncts to mechanical drawing is that of "blue printing." To obtain a blue print you must have what is known as a tracing of the drawing. To get the tracing transparent tracing paper or tracing cloth is used. This tracing paper is placed over the original drawing and the tracing is made. There are two surfaces on tracing cloth, namely: the smooth or glazed side and the dull side. The dull side is the side which you place against the original drawing.
After the tracing is ready the blue print is made. Blue print paper can be obtained in any mechanical drawing supply store, but if you wish to make it yourself, it is made as follows: Dissolve 1 ounce of ammonia citrate of iron in 6 ounces of water, and in a separate bottle dissolve the same quantity of potassium ferri-cyanide in 6 ounces of water. Keep these solu-tions separate and in a dark place, or the solutions will be of no use. To prepare, mix the same amount of each solution and with a sponge or soft cloth spread it evenly over the surface. Let the paper remain in a horizontal position until the chemical has set on the surface, then hang the paper up to dry. When drying, see that no light strikes the paper or it will lose some of its value.
To make a blue print from the tracing, place the tracing with ink side out against the glass surface of the printing flame, then take the blue print paper and place the sensitized side down on the tracing. On the top of the paper place a felt cushion, which generally accompanies a good printing frame, and then put in place the hinged back of the printing frame. After this is done expose to the sunlight. To make good blue prints, being guided only by the exposed edge of the sensitized paper, take a small test piece of the same paper and a piece of tracing cloth with a few lines drawn on it and expose that to the sunlight the same time that you expose the large print.
By having a dish of water at your side you may tear off at different times pieces of the test blue print and wash it in the water. If the test piece shows up in a deep blue color and clear white lines, then it is time to take the big print out. After the print is taken out, it should be washed in cold water for ten minutes and then should be hung up to dry. Corrections can be made on the print with an ordinary writing or ruling pen and a solution of washing soda, caustic potash, strong ammonia, or any other alkali.
To obtain sharp lines on a blue print, all lines on the tracing should be heavier than on ordinary drawing paper, and a sharp inking pen should be used.
By using the following solutions, prints having blue lines on a white ground or just the opposite of a blue print may be obtained: 3 oz. common salts, 8 oz. ferric chloride, 3 1/4 oz. tartaric acid. 26 oz. accacia, 100 oz. water. Dissolve the accacia in half the water and dissolve the other acids in the rest of the water, then mix the two together. The solution is applied with a brush to a well-rolled paper in a subdued light. The paper should be dried as quickly as possible on account of the acid eating into the pores. When the paper is dry it is ready for use. One or two minutes are sufficient in strong sunlight, and a considerable longer time in a dull light.
To develop the print, it must be washed, after leaving the frame, in a very weak solution of potassium ferricyanide. None of this solution should tonch the back of the print. Developing takes but a minute or two. If the background of the print is of a blue color, the print was not exposed long enough, and if the background shows a pale blue color, then the print has been exposed too long.
When development is complete, the print is washed in clean water for two or three minutes, and then placed in the following solution for ten minutes: 3 oz. sulphuric acid, 3 oz. hydrochloric acid, and 100 oz. of water. In this solution all the iron salts not turned into blue compound, will disappear. After this is done the print is washed in water and then allowed to dry.
One of the important uses of blue prints is its use in the shop of the mechanic or engineer. The best advice to be given to them is to take the blue print and mount it on a pasteboard back, but if it is required to keep the prints in first class shape, mount them on sheet-iron or zinc backs and then apply a coat of varnish over each side to make it waterproof.
To make drawings from the prints, the blue prints may be inked over with waterproof ink and when thoroughly dry, washed with a solution of oxalate of potash; treated thus, the ink lines will remain and the blue ground will fade, leaving the background white and appear like an original drawing.
Erasing on tracing cloth, in case of mistakes or errors, should be done with an ink eraser or a sharp, round erasing knife. The surface of the tracing cloth must be made smooth in those places where erasing has been done. This is done by rubbing the cloth with soapstone or powdered pumice stone or talcum applied with the fingers - " The Practical Engineer. "