In article , Andrew Gabriel
wrote:
Well, the front of my house is lime mortar with a black die.
It's nearly 100 years old and the die hasn't washed out.
When I've needed to repoint a few bits, I have been unable to
make my mortar dark enough, even by chucking in much more black
die than you're supposed to -- it still looks grey in comparison.
I do wonder what they used 100 years ago? (Soot maybe?)

"Lamp black" is a posh name for soot, made as a pigment. 100
years ago it was a major product for making inks, paints, etc. These
days people have long since realised that (1) it's a hand-made product,
therefore expensive compared to vat-made dyestuffs; and (2) it contains
non-carbon compounds like polycyclic aromatic hydrocarbon, which are
potentially carcinogenic. So, whatever is in your modern mortar dye
probably isn't "lamp black", which is pretty much the definitive "black
man looking for a black cat in a coal cellar at midnight" version of
"black". You might be able to still get lamp black if you specify it.
Also, your 100-year-old mortar will have lost appreciable
slightly-soluble salts over the century, which will still be around in
your new stuff. Look at the stalactites growing down out of an old
brick-and-mortar bridge - that "stal" material came out of the mortar,
and is more or less white, and it's still present in your new "black"
mortar.

--
Aidan
Aberdeen, Scotland
Written at Sun, 03 Dec 2006 14:05 GMT, but posted later.