With a simple swipe of Abbey's pH testing Pen, you can determine whether the paper and board you are using is acidic. This easy, convenient method allows you to distinguish the difference between safe (neutral or alkaline) paper and board and 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 color indicates the material is probably unsuitable for conservation purposes. Paper can be respectably long lived if is pH is as low as 6.0, especially if 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 colored or coated papers.