Found some kind of clay printed material around fire - victorian age

[newOwnerOldHouse]

Hi all,
I removed cement tiles that were boarding this fireplace up and have found this embossed (what I think is) clay?
Not sure!
Does anybody know what this is? I am hoping to be able to maybe clean / paint but not sure what to use.
House is from the 1890s.
Thanks!

 

[begreen]

It's tile setting mortar. This is the pattern created in the mortar by the backsides of the tiles that were there.

 

[stoveliker]

And the evidence for that is the mirror images of the letters