In The Children of Húrin, there is the following interesting passage:

They led the old Dwarf away to their dismal camp, and as he went he muttered in a strange tongue that seemed harsh with ancient hatred; but when they put bonds on his legs he went suddenly quiet. And those who were on the watch saw him sitting on through the night silent and still as a stone, save for his sleepless eyes that glinted as they roved in the dark.

[...]

Mim opened his eyes and pointed to his bonds; and when he was released he spoke fiercely. 'Learn this, fools!' he said. 'Do not put bonds on a Dwarf! He will not forgive it. I do not wish to die, but for what you have done my heart is hot. I repent my promise.'

Later, Mîm says the following:

'This is my son. An arrow was in his breast. Now he is beyond speech. He died at sunset. Your bonds held me from healing him.'

As well as:

'[...]Do I not remember that your hands were among those that put bonds upon me, and so held me that I did not speak again with my son? [...]'

It is possible that Mîm simply means that, because of his bonds, he couldn't have escaped and gone home to heal his son. However, to me, it seems like it is suggested that Mîm was trying to perform some sort of magic, in order to communicate with his son or heal him (or both), which his bonds prevented him from empolying (since it required the use of his hands).

So, which is it? Is there any more evidence on this anywhere?