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Either Caloplaca or Candelariella but you need to test with K - Caloplaca's give K+ red/purple, Candelariella's are K-
Thanks for the comments Jenny
How do you test with potassium?
Small eyedropper bottles can be bought from a chemist and then follow the instructions here for mixing: http://www.thebls.org.uk/content/chemical.html
(I use approx level teaspoon of caustic soda to 100ml of water)
To test apply a small drop onto the lichen and watch for a reaction. If your species is a Caloplaca you will get a very obvious crimson/purple reaction where you've applied the K.
OPAL's 'Lichen Guide' (http://www.opalexplorenature.org/?q=LichenGuide) links to an interactive slideshow showing how to spot test on slide 21:
If you're seriously interested in lichens Frank Dobson's 'Lichens - an illustrated guide to British and Irish species' is a must-have. Bit daunting to begin with but he gives the chemical reactions.
Species details on http://www.irishlichens.ie/index.html include the chemical spot test reactions.
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