A close-up of the upper lip shows a few of the techniques he used to make the lines:
You can see that he used the lack of tubing on the undersides of the 1x2 tiles to slide them different distances to create the slight angling of the lines. He also used hinged plates to angle lines in different directions.
Mark recently completed a second line art mosaic, this time of Charlie Brown.
I really like the striking images that can be made with this technique. I might have to try it out sometime myself.
EDIT: Mark let me know that he just finished his own blog write-up on the process of making these mosaics. You can find that link here. Thanks for sharing, Mark!