Making Prussian Blue — The First Modern Synthetic Pigment (Berlin, 1704)

MANDATORY SAFETY: Work in a well-ventilated area — outdoors or under a fume hood. Put on chemical-resistant nitrile gloves, chemical splash goggles, and a P100/FFP3 respirator. The synthesis involves dilute hydrochloric acid (corrosive, produces HCl fumes) and potassium ferrocyanide (which releases hydrogen cyanide gas if overheated or exposed to strong acid). NEVER heat any solution above 60°C during this process. NEVER add concentrated acid to the ferrocyanide solution. Have clean water and baking soda available to neutralise any acid spills.

Dissolve 25 g of ferrous sulfate (iron(II) sulfate, FeSO₄·7H₂O — also known as green vitriol or copperas) in 250 ml of warm water (40-50°C) in a glass beaker. Stir until completely dissolved. The solution should be a clear, pale green. Ferrous sulfate is the iron source — it provides the iron(II) ions that will form one half of the Prussian blue crystal structure. Use laboratory-grade or technical-grade ferrous sulfate — garden-variety iron sulfate may contain impurities that affect the pigment colour.

In a separate clean glass beaker, dissolve 20 g of potassium ferrocyanide (K₄[Fe(CN)₆]·3H₂O — yellow prussiate of potash) in 250 ml of room-temperature water. Stir until completely dissolved. The solution should be a clear, bright yellow. SAFETY: Potassium ferrocyanide is of low toxicity in this form, but NEVER heat it above 300°C or mix it with strong acid — these conditions release hydrogen cyanide (HCN) gas, which is lethal. At room temperature in dilute solution, it is safe to handle with basic precautions (gloves, goggles).

Slowly pour the ferrous sulfate solution into the potassium ferrocyanide solution while stirring gently. A thick, dark blue-green precipitate forms immediately — this is 'Prussian white' (iron(II) ferrocyanide, also called Berlin white or Turnbull's blue precursor), which is actually a dark, greenish-blue at this stage. Stir thoroughly to ensure complete reaction. The mixture should be a thick, dark suspension with no clear yellow or green zones remaining. Let the precipitate settle for 15-20 minutes.

The initial precipitate must be oxidised to convert it from the greenish intermediate to the vivid blue Prussian blue. Add 50 ml of dilute hydrochloric acid (10% HCl) slowly, stirring continuously. The acid serves two purposes: it acidifies the solution (promoting oxidation of Fe²⁺ to Fe³⁺ by dissolved oxygen) and helps develop the characteristic deep blue colour. As you add the acid, the precipitate shifts dramatically from dark greenish-blue to the brilliant, deep Prussian blue. Let the mixture stand, stirring occasionally, for 30-60 minutes to complete the oxidation. The supernatant liquid should become nearly clear as all the colour concentrates in the precipitate.

Pour the blue precipitate through filter paper set in a funnel, collecting the clear filtrate below (dispose responsibly — it contains dissolved iron salts and traces of ferrocyanide). Wash the collected blue precipitate by pouring several portions of clean water through the filter — at least 500 ml total — to remove residual acid and salts. Unwashed Prussian blue tends to be unstable and can decompose over time. Once thoroughly washed, spread the wet pigment paste on a clean glass surface and let dry completely (1-3 days). The dried Prussian blue forms a hard, dark blue cake — grind to a fine powder with a glass muller. The finished pigment is an extraordinarily intense, transparent, deep blue. Store in a sealed glass jar away from strong light. Prussian blue is compatible with all painting binders and is especially valued in watercolour and oil for its intense tinting strength and transparency.