After several failures/learning experiences, I think I've figured out what to do with the ceiling.
The first failure was not pulling down the old ceiling right away, and instead supposing it could just be covered over with another layer.
Next failures were various attempts at scratching old ceiling and seeing how a new lime plaster would stick. These were abandoned because it was hard to get it to stick well.
Final learning experiences were this week, putting up reed lathing different ways-- first over 30mm thick wood laths, realizing it was too much space, cutting down wood lathing to make 10mm with 30cm between laths. Then realizing that it needs more support, hence the mess in the photo.
Since that "test" square meter was already plastered, but had plaster falling from where the reeds sagged, I tried first stapling chicken wire, and then screwing down lathing over all of it. It holds and doesn't seem to fall. The rest of the ceiling, I can slip the lathing underneath the reeds first.
The first layer plaster is a mix of: 40% clay from our field (according to shake test, its pure clay), 40% coarse river sand, 20% lime putty (non-hydraulic), 5 liters of chopped straw to one wheelbarrow. It seems to apply best by hand, and its something like 15mm thick, pushed through the reeds and touching the old ceiling.
I'll have to smooth it over with another layer of plaster, and make it white with white-wash to meet the expectations of my boss (wife).
If anyone sees anything obviously bad that would result in it all following off later (my wife's fear) or other problems, please tell me!
There's also a photo of the wall. On the right side it is more finished-- coarse plaster over earth bricks, then finer plaster (2 parts fine sand, 1 part clay, 1 part lime), then rubbed down with sponge trowel and lime putty, and just needed limewashing with pigments.
The other two rooms I have to do, the ceiling is getting stripped of all old plaster first...
Thanks tiny ad, for helping me escape the terrible comfort of this chair.