Well, you've given me some stuff to consider, and I'll have to back off the "that can't cause a transfer" position. Sometimes we learn something new from what we've had right there all along.
A number of things make offset transfers complicated.
One of those things is that the inks of the time were often proprietary trade secrets. Most were a pigment in a carrier, which could be petroleum based oil or some other oil like linseed oil, or a combination. Usually there was something added as a hardener.
1909-11 was right during a period where a lot was changing. Lithography was moving from stones to metal plates, inks were moving from pigments to dyes. And those that were dyes like red, were moving from natural to some new synthetics.
Some of those carriers dry/harden on their own, some only mostly dry.
It's possible that there are some inks that were made with a different carrier, because a different dye or pigment was used. I haven't done any experiments with cards in better condition, or with ones from different series. It's possible the back on the tested card picked up some dust making it less prone to transfer. "
Anyone with deeper pockets and no qualms about wrecking a nicer common care to repeat the test?
Thinking about whether I had a card that might have been exposed to the right sort of damage, I remembered one that probably came from a major early dealer who'd had a fire.
I don't look at the cards I've had for decades closely enough....
That one has obvious fire damage and a transfer, and it's got an EPGD back, which I don't recall seeing many transfers on. It doesn't show any obvious water damage, which is a bit surprising. Maybe it just didn't get all that wet.
So I would say it's also proven that heat plus pressure plus maybe water can also create a transfer, while merely water and pressure probably won't. Chemicals, especially solvents can probably also cause a transfer.
That may vary by color and brand, and maybe series, although I wouldn't expect it to differ by anything other than color. Whatever was done for the black ink, it seems more prone to offset transfers.