Since the 19th century (approximately since 1840) alum Al2(SO4)3 was primarily used for sizing machine-made papers and board materials (acidic sizing). This has been the most important factor that has enhanced premature decay of alum-resin sized paper and board. Panduran is an eriochrom dye which dissolves in water and forms a yellow stained solution. In presence of aluminium Al+++ ions - as present in alum - it reacts to form a a blue violett complex. The reaction is not influenced by the pH level of the tested paper. In the absence of Al+++ the solution remains yellow in colour. Boards and papers with alum content are principally not suitable for long-term archival storage. After several usages of the felt pen it is important you clean the pen before it is again used for testing. All you need to do is to use the pen on normal neutral paper and then it can again be used for testing.