Αn easy and convenient method that allows to distinguish the difference between safe (neutral or alkaline) paper and board & acidic materials. Easy to use; simply draw a small line on the material you wish to test. The Chlorophenol red indicator solution in the pen will turn purple on any paper with a pH of 6.8 and above. A clear or yellow colour indicates the material is probably unsuitable for conservation purposes. Paper can be respectably long-lived if its pH is as low as 6.0, especially if it is well made and carefully used and stored. In order to last for centuries in today's polluted air, it must have an alkaline reserve and this usually means a pH of 7.0 or greater. Please note unreliable results may occur on coloured or coated papers.