To the just-plain-folks of the day (as ignorant as I of the etymology of "crucible,") a "little cross" providing light seems to me a straightforward link.Fair enough. As a fellow seeker after knowledge, I'm sure we can have a fine discussion about the four elements, unless of course you are one of those heretical modernists who posit a quintessence.
Just-plain-folks don't seem to coin words like that. It seems a stretch that they would know the Latin word for cross, let alone know how to decline it (
crux > combining form
cruci-). And where di they get that -
bolum/-
bulum end-bit? There is a Middle English word
cresset used to gloss the Latin
Crucibolum in one of OE vocabularies. It is not a small cross, but from Old French
craisset 'grease' < Late Latin
crassa 'grease'. There are Latin words
crassipulum and
crassipularium, which are pots for holding grease or oil for use with lamps. French
gras 'fat', as in
pâté foie gras, is related, as is
grasset 'a kind of teal, a garganey'. I think that grease or oil has more to do with lamps (at least the wicked kind) than crosses and learned metaphors, but I'm just a simple guy.
[Addendum: just found a great folk-etymology that derives
crucible from
chrysoble from Greek χρυσός
khrusos 'gold'.]
