This is from Proust:
Ces charmes d’une tristesse intime, c’était eux qu’elle essayait d’imiter, de recréer, et jusqu’à leur essence qui est pourtant d’être incommunicables et de sembler frivoles à tout autre qu’à celui qui les éprouve, la petite phrase l’avait captée, rendue visible.
And for those who want more context, the sentence preceding is:
C’est que la petite phrase au contraire, quelque opinion qu’elle pût avoir sur la brève durée de ces états de l’âme, y voyait quelque chose, non pas comme faisaient tous ces gens, de moins sérieux que la vie positive, mais au contraire de si supérieur à elle que seul il valait la peine d’être exprimé.
There are three words that are giving me trouble in this passage "jusqu'à", 'pourtant' and "d'être". As for "jusqu'à" I just haven't been able to see how it's compatible with the other meanings of "jusqu'à" which I'm familiar with, such as "until" or "up to here". As for "pourtant" what's confusing me is whether it is equivalent to 'bien que'; if so the sentence would be:
leur essence qui bien qu'elle soit incommunicables et de sembler frivoles à tout autre qu’à celui qui les éprouve, la petite phrase quand même l’avait captée, rendue visible.
But that cannot be right because 'incommunicables' is plural and "essence" is singular. The Moncrief/Kilmartin translation is just as confusing:
It was the charms of an intimate sadness that it sought to imitate, to re-create, and their very essence, for all that it consists in being incommunicable and in appearing trivial to everyone except him who experiences them, had been captured and made visible by the little phrase.
I had to verify "for all that" just to be sure since it's such a rare expression but, yes, it means "although". So by replacing the archaic and rare 'for all that' with something more intelligible it would be:
their very essence, although it consists in being incommunicable and in appearing trivial to everyone except him who experiences them, it still had been captured and made visible by the little phrase.
So in the English it seems that 'incommunicable' modifies 'essence'. Another thing I don't get is in "qui est pourtant d’être incommunicables", if 'est' is singular then why is "incommunicables" plural? I also don't see the need for the word "d'être"; it seems that if you crossed it out the meaning would be the same.
One other point, it's always good to double check, "captée" is feminine because "l" in "l'avait captée" refers to "essence" which is also feminine, right?