I've noticed that slurs occasionally appear over bar lines in the OF chant books, even full bar lines. Is that the right terminology? Are these curved brackets in fact slurs, or do they mean something else? Do they exist in the EF books? I don't remember ever having seen one. I believe the 1974 Gradual didn't change any bar lines from the 1908 and that asterisks were the only things that got moved or removed. Is that correct? What do you do, just treat it like a quarter bar with no breath? Still slow down when approaching the full bar but don't breathe?
In the Liber Usualis, the Introit for Purification, Suscepimus, has such a "tie" across a full bar. But your are correct, the 1974 Solesmes edition of the Graduale Romanum has a lot more of them. Applying the sure science of semiology, our choir tends to disregard the bar when this sign is present, creating a more deliberate connection between the preceding and following phrases.
The barring of chant is a relatively recent invention. IIRC they were first included, uniformly, in the Mediciaen books published after Trent. They were retained (partly as an indication of lenghtenings) by the editors of the Vatican Edition. Since the Solesmes books are based on the Vatican Edition, the barlines are included. The slurs indicate a barline that should be omitted, based on more recent scholarship -- it was simply easier to add a slur to the plate than to re-engrave the chant without the barline in question.
I've found that the tie/slur (don't stop!) markings don't always make the most of sense. If you are going to go over a chant that has such a marking, I think it would simply be best to go over it yourself, first, and make a decision. It's just like before an initial orchestra rehearsal for a piece, when a conductor will quickly go through various spots that he wants to make sure are performed to his specifications, rather than to those of whomever published that edition.
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