I wonder if you have undercooked bricks, rejects. Old bricks are soft,
but they shouldnt be that bad. Usually the rejects were used on the
inner layer of wall, so Vic houses tend to look a right eyesore if the
plaster's stripped off, with broken, misshapen, burnt and undercooked
bricks galore. Perhaps an inexperienced builder was involved. Who
knows.

Well, the bricks I've seen drilled have all been inside, mostly in the
fireplace, as elsewhere they are covered in plaster, so I can't see what's
going on. About a quarter are black (but probably because they formed the
flue, although there isn't much of a pattern), most are cracked.

Christian.