engineering bricks

I Cleaned some tyrolean render off some engineering bricks and its left a residue on the brick, I have used brick acid, thinners, turps, wire brushes and nothing moves the residue, has anyone had any experiences like this.

Hi all, How can you tell the difference between a class A and a class B engineering brick? Are they marked somehow? I have a pile of bricks left from the previous house owner and am starting an extension. Would be nice to know if I could use these as steel beams require class A brick support...