Never knew the powdered stuff would work. Glad to hear it.
Me, I had searched until I found a hardware that sold honest-to-gosh solid pieces of soft chalk.
No expert on this but it seems to help with my Ancient files.
By the way, years ago Nicholson File published a little pamphlet called File Philosohpy If I ever find mine again will scan it in for this site, unless it is already on tutorials or somesuch. I was in to Nicholson a couple times on business (heat resistant alloy for their lead, now bismuth, pots) & got to chat with guys. Think I got their Filosophical tutorial back when they were still in Philadelphia. And yes, Gentlemen, before the Rust Belt rusted we used to manufacture stuff up here.