lichen on stone

[John Twilley <jtwilley@zzzzzzzzzzz>]

Norman,

Lichen are colonies of symbiotic organisms.  Each distinctive type is the
result of an algae that is capable of photosynthesis and a fungus that
provides some of the tough structure and chemistry necessary to gradually
penetrate the rock.  By living together they also manage to sop up water
to make it through dry periods.  Lichen are notoriously difficult to
cultivate and yet form easily in most unpolluted environments.  I suspect
that the dairy products involved here serve mostly to create a little
adhesion until the fragments of the lichen get a toe-hold on their new
substrate - the casein in milk forms a pretty insoluble calcium compound
when it encounters calcium from a rock.  (Hence the existence of
lime-casein paints in traditional finishes.)

John Twilley