In article ,
Frank wrote:

Lousy MSDS. Tells nothing about chemistry and it would not take much of
a dye or pigment to set the stain. I might try some household shower
spray with bleach and if that does not work the suggestion to stain the
whole surface might not be a bad idea. One of my fireplace hearths is
white brick and I've found it impossible to clean as stuff soaks in.

Why not burn some wood which should discolor the whole thing evenly??

--
Searching is half the fun: life is much more manageable when thought
of as a scavenger hunt as opposed to a surprise party.
Jimmy Buffett