Molly:

This is a water barrier.  Without it, moisture would cause the plaster to
fall off.  The material is probably pitch, as you suspect.  Pitch is impure
coal tar; it contains carbon, coke, and other solid residues from coal gas
manufacture.  The material might also be asphalt, which is the residue from
petroleum distillation.  Asphalt was not the best material for this type of
coating, as the solvents (cutbacks) and oils in the product could bleed into
and discolor the plaster.  One can easily tell the difference between tar and
asphalt with gasoline.  Asphalt dissolves in gasoline, coal tar does not.

Steve Stokowski
Stone Products Consultants
http://members.aol.com/crushstone/petro.htm

In a message dated 10/12/99 11:10:30 AM Eastern Daylight Time,
[log in to unmask] writes:

<< I've been in a lot of old theatres lately, and am noticing a black
substance
 on the interior face of the bricks. It seems that bricks making up walls
 with an exterior face have had their interior side coated with some
 elastomeric substance, or maybe...pitch? We figure that it was some sort of
 barrier between the brick and the scratch coat of plaster applied directly
 onto it. Any takers? >>