Eunuchs - How did the Romans do it ?

The Romans seem to have had many eunuch slaves as domestic servants entrusted with the care of their women. Somebody told me some time ago that they castrated their slaves by using a small round headed mallet and a bowl. I thought the person was joking, but I have since seen a couple of references which suggest this may it fact be true. Sounds extremely gruesome and painful. Eunuchs - How did the Romans do it ?