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��of 60 lb. to the square inch. This is done by inserting a ^-in. iron pipe in the solu- tion, allowing the end to go within 6 in. of the bottom. In the course of 10 minutes this will bring the solution to the boiling point, when the steam must be turned off and the pipe removed. ! The next operation is the adding of 8 or 9 pints of nitric acid, specific gravity, 1.38 or 142, known as 38 or 42. This must be added slowly to the hot mixture while a second person keeps it stirred with a long
���The mixture is well stirred while heat is applied by steam through an iron pipe
strip of hardwood. During this operation dense red fumes of nitrous acid will be evolved and the mixture will boil vigorous- ly. As soon as this amount of acid has been added, the solution must be allowed time for cooling, which will require from 14 to 16 hours, when it will be found that the quantity has increased from 30 to 35 gal. This is due to the condensing steam and the addition of the acids.
Preparing the Cyanide Blue
Place 30 gal. of the cold iron mixture into a tub or stoneware vessel that will hold 50 or 60 gal. and add 10 gal. of cold water. Draw off 8 gal. of the cyanide concentrated solution, add 2 gal. of water and pour it all into the iron solution while stirring the mixture vigorously. Avoid breathing the fumes that may be given off, because they consist of diluted cyanogen gas. Fill the vessel with cold water, cover it and allow it to stand for 24 hours. Upon the addition of the cyanide solution a dense and volu- minous precipitation of Prussian blue will
��take place. As soon as the blue has become settled at the bottom, the clear liquid must be drawn off with a siphon made of plain iron pipe, care being taken not to permit the blue to be drawn off. This liquid, which still contains a fair proportion of a free salt of iron, may be placed in a suitable vessel for further precipitation. As soon as the liquid has been drawn off, the tub must be refilled with clear cold water, stirred well and allowed to stand for 12 hours, when the liquid may be drawn off as before and thrown away. About six such washings will be required to free the blue from the several soluble impurities. After drawing off the last washing water the precipitant is scooped up and poured into an unbleached muslin bag, which is suspended in a clean barrel in the same way as employed for the iron and cyanide solutions. The remaining blue is washed out of the tub or crock, placed in the muslin bag and allowed to drain for 24 hours. At the end of this time the contents of the bag must be spread out upon muslin in suitable trays. These should be 3 ft. long, 2 ft. wide and 4 in. deep. When filled they are placed in a drying room like drawers in a file where steam may be applied for heating it to a high temperature. The room should be well ventilated so that a current of air coming in at the bottom will pass out at the top after circulating about the trays. This will evaporate the moisture left in the blue. As soon as it has dried thoroughly it may be ground to a powder in any suitable mill and it is then ready for the market.
If tubs are used in the process of pre- cipitating the blue they should be well dried and the hoops tightened before they are used and the interior coated well with amyl acetate collodion by flooding and draining, or a solution of rubber cement that has been thinned with benzine or benzole may be used. This coating will preserve the interior against the action of the chemicals. The tubs must be well water-soaked previous to use until all leaks have ceased. The stoneware vessels need no preparation.
If it is desired to manufacture this blue on a large scale care must be taken to get rid of the fumes resulting from the addition of nitric acid to the hot iron solution, because of their poisonous properties. Proper ventilation and respirators will protect the workers against any danger in handling the mixture.
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