This can help confirm species identifications. Some lichens change colour when a drop of bleach or caustic soda is applied.
Watch for a pink to red colour change where you have applied bleach or caustic soda to the lichen medulla. This colour change may fade rapidly. If the colour changes this is called a + reaction.
Bleach. Put a small amount of the thin bleach (not the thick bleach which has other chemicals in) in a small labelled dropper bottle (old eye-dropper bottles are ideal) .
Caustic soda. Measure 100ml of water ( tap water will do) and add quarter teaspoon of household caustic soda ( use cook measuring set for a quarter of a teaspoon). The solution will heat up and when it has cooled put it in a separate labelled dropper bottle *** health and safety warning.
To do the tests: Scratch the surface of the lichen carefully to remove the upper layer (cortex) and expose the fungal layer (medulla) beneath. Note that you do not need to do this on thin crusts)
Put a drop of either bleach (C) or dilute caustic soda (K) on the scratched spot and watch for a colour change. C- or K- means no colour change.
Bleach response: fast reaction turning pink or red if positive but fading rapidly, signified by C+ in the text .
Caustic soda response: slower reaction (c.1min) and if positive may turn yellow, orange or blood red signified by K+ in the text.