Michael wrote:
   contractor in Buffalo, a firm that had been in business
since about 1906.  their spec included "iron filings" which I had never seen
before.  the owner swore by them, saying that when they rusted it expanded
the mortar a pressed it up against the bricks so the joints were tight.

The use of slag in mortars is very old; pre revolutionary Clinton Castle had
them (nr. Staten isle ferry); in March I was in a 400 yr. old stone farm
house in France that had a poured floor of slag ash and lime.mortar ( no
control joint)....one thing for certain; when you take your chisel to such a
joint it will be very resilant; yet no cracking.....on another note there are
limestones from Alabama that when quarried near the surface have have
occlusions of tar streaks within the stone ..but I suspect this is not on St.
Thomas.... Michael