Hollow Clay Tile, Exposed!

I came across this textured wall the other day in my strolls around the city and I had to laugh.  It is pretty much the architectural equivalent of walking around with your slip showing.


What you see here is hollow clay tile, sometimes known as structural clay tile – the precursor to concrete block.  Hollow clay tile was developed in the late nineteenth century as a fireproof technology for building partitions or structural slabs.  It was a bit of a misnomer in that it was not a thin tile.  Instead, it was structural block made of porous terra cotta that was molded into squares or rectangles for walls and floors, or other shapes to encase steel columns or beams.  Each block has hollow cells inside that deaden sound, provide insulation and keep bugs and rodents out of the wall.  This sheet from the Henry Maurer & Sons catalog dates to about 1898 shows you common sizes and cell arrangements of hollow clay tiles.

HCT drawing - HenryMaurer

Because they were made of porous, moisture-absorbing clay, hollow clay tiles were always meant to be covered with plaster or stucco.  During manufacture, the exterior surfaces of the tiles were ribbed to increase adhesion of the plaster and stucco to the tile.  The ribs could be oriented horizontally or vertically, but were generally not mixed and matched.

Except on this building.  It looks like two masons built the wall from either end and met in the middle where they had a few awkward gaps.  A smart-thinking mason filled the holes with rectangular pieces of hollow clay tile and bits of brick.  Unfortunately, the ribs of those infill pieces were perpendicular to the ribs of the rest of the wall.  But no matter, thought the mason, it’s an interior wall.


Fast forward several decades: the adjacent building was demolished to create the parking lot, and the handiwork of the masons was uncovered for all to see.  The current building owner painted the wall to give it a more uniform appearance, but you can see the irregularity of the original interior construction.  Fortunately, the paint is protecting the porous tile from extensive moisture infiltration.  (Exposed walls of hollow clay tile normally deteriorate quite rapidly due to moisture and resulting freeze-thaw cycles.)  So next time you see irregularly-laid, ribbed masonry units on a side wall, you know that the wall was never meant to be seen.