Tea + paper = goo. Huh?

Factual Questions
1

My GF has been experimenting with a new pad of heavy watercolour paper she got recently. One of the things she tried the other day was submerging a piece of the paper in tea (technically speaking, once-used leaves in hot water) to give it a nice brown wash.

The paper soaked for a couple of days. When she took it out she was surprised to discover a layer of mucilaginous goo coating the surface of the paper. This was a bit of a shock, and we wondered: what, chemically, could have happened to produce this goo? What was the goo made of? Would this have happened if we’d been soaking in coffee or plain old water? Did we have some kind of nasty bacterial colony feeding off the paper, or is it relatively harmless?

2

It could be almost anything, but my guess is that it was some kind of coating that got soaked and became slimy - perhaps a mixture of china clay and a binder. It could be a bacterial slime though, I suppose. Try it with another piece of paper in boiled plain water in a sterilised container, in the fridge - if it still gets goo after two days, then it’s probably not anything alive.

3

High quality watercolour paper is often sized with gelatin.