I need to attach what would look like a small shelf (2"a x 3"b x 4"hyp): to a plaster wall. It is to support a magnifying lamp with a clamp-on pivot arm. The lamps weight is a bit of a beast. The shelf itself is not a problem, I can make it out of 3/4" plywood, but I do not have experience with this type of wall. It is not drywall, its the kind of very hard to knock on stuff, from 70 yrs ago. The small shelf is a , turned 90 degrees, for a beast of a clamp-on pivot arm to the || shelf. A small horizontal shelf dadoed into vertical pieces bolted into an inside corner wall will withstand the torque stresses. I have a 1/2" hammer drill and I will need to drill into the normal wall (inside corner) both above and below the shelf at right angles so the clamp-on pivot arm torque forces on the shelf will be pushing on the shear of the four fasteners. The distance from the adjacent wall will have to be the width of the 1/2 profile of the hammer drill (maybe 2" from the inside corners, about 27" up a 8' high wall). I have a feeling I will need to build a jig for the holes, probably with plywood with washers hammered in spaced spade drill holes, so the masonry bit doesn't jump all over the place. So I will be using masonry bits, but since I have never attempted such a hold, not drilling in this type of wall, I don't know what type of fasteners to use, or if I need to worry about drilling out with HSS-type regular-drill for lath or whatever might be in there from before I was born.
So the question is: What's in there & how do I drill it out (several drills, drill holes in wood shelf after?)? What kind of fasteners do I use through 3/4" plywood & into wall (lag bolts, expanding bolts)?