An easy, convenient method for distinguishing between safe papers and boards, and unsafe acidic material. Easy to use; simply draw a line and read it as soon as it dries. The chloro-phenol red indicator solution turns a definite purple colour on any paper with a pH of 6.5 and above. Any other colour would indicate a material most probably unsuitable for conservation purposes. Please note: This testing method may produce unreliable results on coloured materials.