Making Mars Colours — Synthetic Iron Oxide Pigments from Copperas

Dissolve 100 g of ferrous sulfate heptahydrate (FeSO₄·7H₂O, also known as green vitriol or copperas) in 500 ml of warm distilled water. The solution is a pale green colour — characteristic of ferrous (Fe²⁺) ions. Ferrous sulfate is one of the most common and cheapest iron compounds — in the medieval period it was produced as a byproduct of iron pyrite weathering or by dissolving iron in sulfuric acid. Today it is widely available as a garden supplement (lawn greener) or from chemical suppliers. It is mildly irritating to skin but essentially non-toxic.

Prepare a solution of 60 g of potassium carbonate (potash) in 300 ml of warm water. Slowly add the potash solution to the ferrous sulfate solution while stirring vigorously. A thick, greenish-brown precipitate of iron hydroxide (Fe(OH)₂ and Fe(OH)₃) forms immediately. The reaction is: FeSO₄ + K₂CO₃ + H₂O → Fe(OH)₂↓ + K₂SO₄ + CO₂↑ — the carbon dioxide fizzes off. Continue adding potash until no more precipitate forms. Let settle for 1-2 hours. Decant the clear supernatant (potassium sulfate solution — non-toxic, can be discarded). Wash the precipitate 3-4 times with clean water to remove all soluble salts.

Collect the washed iron hydroxide precipitate on fine cheesecloth and squeeze out excess water. The wet cake is a brownish-green, heavy mass. Divide it into five roughly equal portions — each will be fired to a different temperature to produce a different Mars colour. Spread each portion on a separate ceramic dish and let air-dry for 24 hours, or dry in a warm oven at 80-100°C. As the precipitate dries and oxidises in air, it gradually turns from greenish-brown to yellow-brown — the iron(II) hydroxide is converting to iron(III) oxyhydroxide (FeOOH, goethite), which is already Mars yellow in its raw form.

Place each dried portion in a separate crucible and fire in a kiln to different temperatures. MARS YELLOW (200°C, 2 hours): the iron oxyhydroxide retains its water and stays as goethite — a clean, warm yellow, equivalent to the best natural yellow ochre. MARS ORANGE (350°C, 2 hours): partial dehydration produces a warm orange. MARS RED (500°C, 2 hours): complete dehydration converts goethite to hematite (Fe₂O₃) — a strong, warm red, equivalent to the best Venetian red or Indian red. MARS VIOLET (650°C, 2 hours): hematite crystals grow larger, shifting the colour to a distinctive violet-red. MARS BLACK (800°C, 3 hours in a sealed/reducing crucible with charcoal): the iron oxide reduces partially to magnetite (Fe₃O₄) — a dense, warm black.

After cooling, grind each fired portion on a glass slab with a muller. The five Mars colours should form a clear progression: yellow → orange → red → violet → black — all from the same iron compound at different temperatures. This demonstrates beautifully how crystal structure and hydration state control colour. Mars colours are among the most permanent pigments known — completely lightfast, chemically inert, and compatible with all painting media (oil, watercolour, tempera, acrylic, fresco, lime wash). They are non-toxic and safe to handle. For painting, grind each colour separately with your chosen medium. Store dry pigments in labelled glass jars. The entire Mars colour family provides a complete warm palette for underpainting and earth-tone work.