The wood assembly is made of stained pine, which mimics the other trim in the house. Three metal hooks are screwed to the wood, and the whole assembly is screwed to the wall. This is not particularly noteworthy, but if offers an opportunity to talk about various procedures to hang things on the walls in a natural house.
Reminder: the interior walls have about 1/2" of clay plaster on top of 3.5" of Light Straw Clay, which is packed between conventional 2x4 studs. This is somewhat analogous to drywall in that if you want to hang something heavy, you'd better find a stud. Clay plaster by itself, like drywall, is limited in the weight it can safely support.
The coat rack may have to carry 15-20 pounds, so it is screwed into two studs. Fortunately, I was thinking ahead and marked all the stud locations before plastering over them. In my house, you just need to push up the ceiling panels, and my marks are visible in the plywood that lines the perimeter of the cavity between the first floor ceiling and attic floor. Use a plumb bob to project the marks down to the portion of the wall where you are looking to drive a screw. (Fortunately, I hired expert framers and this house is amazingly square, level and plumb!)
I used 2-inch long #8 screws that are counter-bored into the pine, meaning they bite 1" into the studs (1" bite + 1/2" of plaster + 1/2" of wood = a 2" screw). 1" of bury is more than really necessary, but 1 1/2" screws would have only had 1/2" of bury, and that would have left me a little nervous. I drilled a pilot hole for the screw and didn't bother with drilling a clearance bit through the plaster--it is soft enough that the screw drives right through the plaster with minimal effort.
I've only hung a few pictures and paintings where the weight is carried by the plaster. The super-light one is just hung on a nail. The medium one is on a standard hook nailed into the plaster, and the biggest, heavy painting has twin hooks nailed into 2 studs. My intuition says that the clay plaster has maybe 1/2 to 2/3 the carrying capacity of drywall, which means you should choose a bigger nail/hook than you would for a conventional wall.
There's nothing significant about any of that, except that I was a little nervous about nailing into the plaster, worrying that it might be brittle and crumble. Point #1 is that I did the nailing in the summer when the air is more humid and perhaps the clay is a softer and less brittle. Point #2 is that I used a small paintbrush to wet the wall where I was going to drive the nail. And I kept re-wetting it every 5 minutes or so for 20-30 minutes to pre-soften the clay. At this point ,the nails drove in quite easily, and I let the clay dry completely before hanging the art on the hook.
With this methodology, it took a little longer to hang the art than it would have with a conventional drywall wall, but there have been no oh-crap moments. If I had mock-up Light Straw Clay and drywal wall sections, I would do some destructive testing where I installed different size nails and hooks and hung weights on them until the fittings pulled out. Alas, I wasn't thinking ahead on that, so that will have to wait until the next project.
