Sue's spot on, real bark has been used in moulds by Michael Harris, Timo Sarpaneva, Geoffrey Baxter, Tim Harris (last year at the exceptional Studio Day), and many others! It is indeed dampened quite considerably, and certain bark is better than others. As the hot glass hits it, you get a belch of steam, and the surface is changed (burnt) each time, meaning each subsequent piece takes on a different (and lesser) texture. For 'production' pieces, such as Whitefriars' pieces, textured steel moulds were made to ensure consistency. Experiments were carried out with wood/bark/even nail lined moulds. It's also not just the texture of the mould that affects the texture of the final piece - the glass has to be at the right temperature and blown well into the mould to pick up the best level of texture. Furthermore, even metal moulds wear down after time....
Many points to consider!
Mark