gall nuts

Ingredients

gall nuts, water, pure copper (II) sulphate

Preparation Steps

Original Recipe Text

15 g of gall nuts cooked in 30 g of water for ten minutes after boiling. Add 0.3 g of pure copper (II) sulphate to the warm solution (p. 145).

Evidence

"I cooked 15 g of gall nuts in 30 g of water for ten minutes after boiling. I then progressively added pure copper (II) sulphate (0,3 g) to the still warm solution: it turned dark brown... I used a metallic pen, obtaining a very dark brown colour (Fig. 7.4, right side). This is due to the ionic exchange that happens when a solution with copper ions comes into contact with solid iron."

Remarks

The experiment demonstrates that copper (II) sulphate alone does not form a black complex with gallic acid, resulting in a light brown ink when used with a reed pen. However, using a metallic (iron) pen causes an ionic exchange where iron ions replace copper ions in the ink, creating a dark black/brown color (p. 145).