In article ,
John Laird writes:

What makes you think the plaster is not bonded to the lath ? After all,

It might be for first few days, but after a few humidity changes,
it certainly won't be. Someone may have already fixed it by pouring
diluted PVA over the laths and plaster, which will bond them.
If not, that's something you might do, but I wouldn't risk it
without supporting the ceiling from underneath while it dries.
This is a common way of repairing a lath and plaster ceiling
once the plaster does come away from the laths, providing it
hasn't fallen down yet. A board supported by some slightly
over-long poles is used to push it back in place, and diluted
PVA poured over the back to soak in and set.

that's all there was to hold it up until the "nibs" dried.

Suction.

--
Andrew Gabriel