Panduran pen

Since about 1840, alum (aluminium sulphate, Al₂(SO₄)₃) has been used in the mechanical production of paper and board, particularly for sizing. This is one of the main causes for premature decay of paper. Panduran is an Eriochrome dye that reacts (metallisation) with Al⁺⁺⁺ (aluminium ions) which is indicated by a colour change from yellow to violet. Since the reaction is irrespective of the pH, panduran is not a pH indicator. Alum-containing papers or boards should not be used for long-term archiving.