Peter Woulfe (1771) "Experiments to shew the nature of aurum mosaicum,"Philosophical Transactions of the Royal Society of London, 61 : pp. 114-130. pp. 127-130: "A method of dying wool and silk, of a yellow colour, with indigo; and also with several other blue and red colouring substances." and "Receipt for making the yellow dye." — where Woulfe treats indigo with nitric acid ("acid of nitre").
