1920’s lathe & plaster

So, it's LATH and plaster.  Lathe is a tool used to make not-round parts into round parts.

If it's a 1920's era, high likelyhood that it's a 'kit' built house... like a four-square, or similar, from a Sears or Montgomery-Wards, as MOST of this period were built from mail-order kits.

The plaster mix used was somewhat variable, as the 'kits' were section-supply-job'd out to material providers all over the country, and there was frequent substitutes for 'or-equivalent' materials... this means that if a certain type of 'plaster' wasn't available, some other cementious product (like a high-strength portland cement) with sand might have gotten substituted... or the original builder used what was inexpensive and plentiful on the local market at that time.

My patio is an excellent example... I recently poured a slab patio, and priced out 5000psi mix, and they quoted me $192/yd.  Then they quoted me a 5000psi C3 highway mix at $12 LESS per yard... so I took that.  The C3 is slightly different, but in ways insignificant to my project, but it was lower-cost than the standard 5ksi simply because they were running large orders for the C3 mix, and all their machinery was set up and calibrated to run that mix... to stop that, reconfigure for the standard 5Ksi, would have taken lots of resources, so I ran with the C3, and it worked fine (it set up rather quick...)

All note regarding the walls not being 'flat'... are worthy of consideration, but take a string to the walls before jumping to conclusions.  Due to the hand finishing process, it was not necessary for the walls to be perfect, but builders were invested in being accurate, and the wood available at that time was really good.  I'm helping a buddy with a 1924 four-square right now, and we haven't found any deviations that will challenge drywall.

Horsehair... and silicosis... yes, plaster mixes for tha era had 'artisan's secrets', and horsehair for reinforcement was a common one.  A little later in history, asbestos fibers were added.  Sometimes, steel tailings from machine shops were used... and they're all kinda hard on the body.  There's concern for, and I don't know how warranted it IS, but just an awareness... is that anthrax spora were very common to horsehair of that era... and if one is busting up plaster where that might be the case, those spores being released into the dusty air would wind up in your lungs, so keep the masks on and the ventilation fans going.

As for getting through it for anchors... if it's hard, and doesn't blow out when you drill, use whatever anchors grab.

As for adhesive... I'd find a way to get a sample of the gak, and segment it out on test trays and experiment with different solvents to see what affects it positively.  Obviously, the harser petroleums will get into it, if it's a petroleum product... the citrus solvents will probably work it back, too.

After all my reading and getting caught up, I'll chime in on this one.....

It's LATH and plaster.  A lathe is a machine that you use to make metal or wood parts on.  LOL!!

There is NOTHING fun about tearing into the walls of a 100 year old house.  I also have one.  You would NOT believe that my bathroom floor had .... wait, lets go back.  The floor joists in my house are rough sawn 2" x 12" joists!!!  First time I've EVER seen that.  Anyway, putting in a new bathroom right after we bought the place,,,, tear out all the old lath and plaster, the sub floor and lo and behold,,,,,,,,there is ONE FOOT of a real dense concrete inbetween the floor joists.  I'm not chitting you!!!  Took my son and his buddy 2 days to chisel it all out.  I should have had an accident then and the place burned down....  Now onto the kitchen...  The walls have the same thing you are dealing with.  NONE of them are plumb,,,,they must have used the plumb-bob my Dad used...you hold up your thumb sky ward and when you think it's straight, you yell "NAIL IT".  LOTS of fun, but the history of the house kind of makes up for it...

Have fun and 'git er done'.