So back in May of 2009 I had gotten a large poster tube from Dave Sim containing a bunch of mylar sheets. I blogged about issue #104′s Flaming Carrot cover then, but not the cover to Cerebus #103 which was also included (along with one sheet for the cover of #100):

The above picture is actually two smaller mylar pieces – one the size of a standard comic book cover, with all the text on it shown and the next the smaller one with the Cerebus art on it. The blue lines you see on it look like blue pencil, but I’m not going to try and erase it to validate that or not – I rather preserve the mylar overlays as is.  As I work with mylar on a daily basis at my job – in my experience mylar has emulsion on one side for plotting and that emulsion is one color: black. (edited to add: the red overlays I think are mylar too, but they don’t appear to be printed on they have cut outs in them to either show or hide the smaller mylar pieces.)

Looking at the large mylar overlays I got to wondering about the registration of each overlay to itself. I mean smaller overlays are just taped to the larger mylar overlay – no type of registration target system to align the two layers. But the larger overlays do have 1 hole and 4 slots on the side of them. Why one hole and 4 slots you ask. If you used all holes of the same size it wouldn’t allow for slop – mylar will expand and contract depending on the humidity of the environment it is located. So if it is really humid, the mylar will contract. If the air is really dry, the mylar will contract. Expect on average that mylar will expand / contract by ± .0005″ per inch (or .006″ per foot). So these large 4 foot mylar overlays could expand or contract by .024″. Yoinks!  That may not seem like a lot, but when you’re fixture a pin through a hole that means you’ll have to make the hole at least .024″ larger then the pin, just in case the hole “moved” due to the mylar expanding or contracting. But then there could still be fixturing issues. So just make the holes slots instead. Place the pin in the center of the sheet so it is as close to all 4 slots as possible, and use that to actually register the mylar overlays to each other – the slots are just there to make sure the mylar overlay stays in place on the fixture.

When you do this – I’d suggest using mylar that was made together (i.e. the same manufacturing lot) so it should expand / contract at the same rate, and you’ll have to have a special tool to punch the holes in the exact same spot on your mylar overlays – so they all align together. Though just taping the smaller overlay on top of the larger overlay just throws all registration out the window. . .

I’m not sure if these mylar overlays were just used to image onto a metal plate to use the metal plates as the printing plates or if they were actually used as the printing plates – I’ll have to ask Dave if he knows what Preney used, or if he has someone I can contact.

But I did my best to line up the different images and made a small animation of them:

I’m going to post all four of those pictures to my flickr account so you can ge a better look at them if you’d like.