removing plaster from ceiling

This is a bit of reverse Breaktime topic, but any and (most) comments will be appreciated.

I'm remodeling a bedroom in my house to be used as a library. The room's ceiling (dating to about the 1830's) is plaster. Sometime in the last 50 years, the ceiling was overcoated with a textured layer of plaster, which I'm trying to remove. 

I've tried a number of methods to remove the textured layer. What I'm learning is that if the textured plaster is thick (.030-.070 inch), a sharpened scraper easily pops off the textured plaster and leaves the original ceiling smooth and ready for paint.

If the plaster is thin, (<.030 inch), the only way to get the plaster off is to chisel it using the scraper and a hammer at a very steep angle, which is slow and chipping into the base plaster.

I've also tried my Fein oscillating tool, an orbital sander with 60 grit, and pre-wetting the plaster before scrapping with less than desireable results. 

Any suggestions? I'm getting a nice palm bruise from the scrapper butting into my hand.

Art