Progress,  page 108
I needed a collar of some sort to go around the last chimney section at the rooftop, and it needed
to be strong enough to support the decorative concrete surround that I intend to finsih it with.  It
also needed to be corrosion resistant.  I still had some sheets of heavy (about 1/16 inch thick)
copper left that I had used to fabricate the roof scuppers, so I cut a hole in the center of one and
gradually hammered it out to make this toilet seat shape.  It's 30 inches on a side and the hole
after hammering is just over 14 inches (the outside diameter of 12 inch insulated chimney).  It
turned out so nice I may try to make some other hammered copper items later just for the heck
of it.
Here's the actual upper side of the copper collar as it will be when installed.  I had to fasten the
copper sheet to a wooden frame before hammering it to prevent it from getting all deformed.
The top of the chimney box on the roof already had a good weatherproof membrane across it, so
I just cut a hole in it, passed the chimney section through it, and fastened the copper collar to the
chimney box with a good silicone caulk.  I'll apply a high temperature caulk to the joint between
the collar and the chimney before I cast a concrete shape over the top of the box.