That picture is a terra cotta block that’s been removed from its location on the facade of an early-1900s building and temporarily stored on the roof. As is obvious, the block is broken, and will be replaced, but it’s been saved to serve as a mold for the new terra cotta to come. It’s worth noting that the upper white face was the outside face when the block was in its proper location, and except for some minor cracks in the glaze and some stray paint marks, it looks quite good. Certainly there are few natural stones used in construction that would have the tooling on the surface (the lightly scalloped three-dimensional relief) so sharp after more than 110 years.
Terra cotta blocks are hollow when fabricated and when they arrive on site. They are too big to be fired solid – they’d crack apart from thermal stress in the kiln – so the voids allow them to be made. The voids also serve, as can be seen on the right, as a convenient way to tie the terra cotta to the brick body of a wall by building brick partially inside the blocks.
What caught my attention with this block were the marks in the clay. Here:
Terra cotta units were originally all hand-made. Industrial production followed using presses and eventually extrusion in a process nearly identical to that used with Play-Doh. But complex shapes always required some hand-work, so you see finger marks and marks from small handheld tools on the inside of some blocks. It’s a nice visual reminder of the people working in the factories that churned out this material in an endless stream circa 1900 to 1930.