304 



Popular Science Monthly 



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. 



