How I hang stuff on old plaster and lath walls

Have finally been hanging a few things on the wall, and this is a note-to-self about how best to do that in our current home. I’m more used to either plaster over masonry (UK) or more modern drywall and studs (US), but our apartment in an old Brooklyn brownstone is mainly plaster and wood lath over masonry. Very crumbly old plaster, surprisingly dense lath, and very little void space between the plaster+lath and the masonry. Hanging something is more art than science and requires a good understanding of what you’re working with. The first thing to consider is the “skin” of an old plaster-and-wood-lath-over-masonry wall which behaves very differently from modern drywall. It’s made up of horizontally-arranged long, thin wood* slats (a.k.a. lath) screwed to vertical studs with thin gaps (a.k.a. keys) between each lath. The plaster is applied on top of the lath and pushed into the keys to give a smooth surface. The older the plaster, the more crumbly and brittle it’s likely to be. Beneath…