Gene Newcomb schreef
Just to elucidate on PvR's clamp, the classic technique is the use of the pith from ...

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that would be Sambucus, elder
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It is sliced lengthwise and the material inserted into it. An available substitute that should work on lichen thalli is a piece of carrot. The little carrots sold in bags ought to work nicely.

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Or Styrofoam, in one of the denser grades
PvR