The symphony in brown above probably requires some explanation. It shows a steel column at the corner of a building downtown after the veneer brick has been removed. The more heavily damaged steel is the original column, the less heavily damaged steel appears to be an old repair that was installed without being properly painted or otherwise waterproofed. Here’s some annotation:
“A” is the newer reinforcing plate, “B” is the flange of the original column behind, “C” is terra cotta back-up masonry. I’ve also drawn three lines to emphasize the rust pattern, which is the reason I find this photo interesting. Those lines are where the mortar joints in the veneer brick were. The horizontal lines are where bed joints were and the vertical line is where a head joint was. Since the veneer brick was laid up in running bond, the head joints are staggered, which is why we don’t see the vertical line continuing up and down.
I know intellectually that mortar is more porous than brick, and I tell this to clients with some frequency to explain why repointing is necessary. But seeing this evidence of that difference in porosity is still amazing. Of course, we got lucky with timing: another few years and the rust would have spread from the areas behind the mortar to the areas behind the brick, leaving the “A” steel looking much like the “B” steel.