Cleaning reclaimed bricks.

Hello.

I have a project in the garden for which i'm using reclaimed bricks from different sources.
i have 200 or so London style bricks (with frogs) and these all cleaned up very easily with a bolster and hammer.

I've just picked up 100 or so engineer bricks ?
these don't have frogs but rather holes running down the middle of the brick.
the style of the brick has no impact on the project, but they are being an absolute bugger to try and clean up. the mortar seems to be stuck like glue and just won't come off cleanly.

is there a "nack" that i missing ?