At school I used to have a rubber (eraser), half of which was grey and rough. That half was claimed to be able to erase pen ink. It never seemed to work on biro or fountain pen ink. What sort of ink did these sort of rubbers erase and are there pen erasers which work?

Yes, gmack, I know what you mean, same happened to me, they were the most useless things ever produced. Here now in Oz we have Bic biros with a little black rubber on the end of the pen that actually DOES erase the ink from THAT pen. Very handy for crossword puzzles!

I always managed to rub a huge hole in the paper with those erasers. Beware of those self erasing biros. Don't write cheques out with them. We have heaps of cheque fraud over here with those.

A while back in the US we had pens called Erasamates, they were quite a fad because the ink the pens would use easily erased with a pencil eraser.

The problem was that this ink also would smear everywhere (being left handed I would always get the ink all over my hand), today these pens are no longer in stores.

The wonderful people of W.H. Smith's have got erasable pens made by Papermate, but I noticed that the ink in them tends to be a lot more flaky and dry when it comes out, so the ink doesn't actually sink into the paper.

The erasable pens use rubbery ink that does rest on the surface so has no problem being rubbed out. And how they get away with advertising ballpoint rubbers for the normal sort is a mystery as the ink is absorbed into the paper (and clothes, hands, etc.) and even most solvents can't remove it. All the rubbers did was be so abrasive as to remove successive layers of paper, in the hope enough ink would go with it before the paper became a hole.
This never happened in my experience.