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.
Since the 19th century machine-made paper and board material was generally sized with alum. This has been the main reason for the premature decay of paper. Panduran is an eriochrome dye which reacts chemically to the aluminium ions present in alum, and in the process changes from yellow to blue to violet, thus detecting the alum content in the paper being tested.» to the product