I don’t know a lot about ink. A lot of texts on papyrus from ancient Egypt have been found somewhat readable; but they were in very warm, dry areas.
Was there waterproof ink in 1926?
Yeah, it's called a pencil.
India Or Chinese black ink predates the Christian Era for thousands of years and is waterproof.
Not a high tech item.
Before the digital printing age it was the archival ink of choice, and mandated by government agencies and the corporate world for permanent originals and record copies.
Back in the day DC public school classes required all homework and tests required the use of waterproof ink. Early ball point ink was unacceptable because it smeared. In jr HS mechanical drawing class drawings were laid out light in pencil and final line work was done in India ink. Higgins was the cheap government contract stuff. Pelikan and Koh-i-noor were the choices of pros.
As an aside, the Mech Drawing classes REQUIRED that we had a pocket knife to sharpen wooden pencils and to have a supple of single edged razor blades. Razor was used to scrape off small errors such as small ink blots, over runs of line work. The ink was slowly and careful scraped off the paper which was then burnished with a glass marble to make the rough paper fibers smoothed and prevent bleeding of the ink.
If you had buddies in your class you carried a extra marble for the times they had “lost their marbles”.
I know, I know...more than you needed to know.
;>)