Syntheses of Prussian Blue Pigment Following 18th-Century Methodologies: Factors Influencing Product Purity and Syntheses Yields

In the present work, Prussian blue syntheses were performed based on the methodologies described in "The Handmaid to the Arts Teaching", v.1 and 2 (1758). Since calcination is considered the crucial step in the synthesis, the reagents and parameters of this step, such as time, temperature, and calcination vessel, were varied. Attempting to reproduce the calcination as closely as possible to the original description, in addition to commercial reagents (potassium carbonate, blood meal, alum, and iron(II) sulfate), reagents such as <i>in natura</i> ox blood and ash alkali were employed. The ox blood was dried in a muffle furnace and then macerated. The ash alkali was obtained by Candeia wood (<i>Eremanthus erythropappus</i>) ash lye, and its composition was characterized by the compounds K₂CO₃, KHCO₃, K₂SO₄, KCl, K₂O, and SiO₂. In the calcination step, alumina or iron vessels were used. It was observed that the iron vessel enhanced the reaction yield, as did the amount of HCl added in the last step of the synthesis. The results of the calcination time and temperature parameters are related to the material of the vessel used; however, a minimum temperature of 400 °C was necessary for the formation of Prussian blue. Impurities of amorphous carbon, KCl, alum, sulfates/sulfides, and aluminum hydroxide were also identified.