Checking the progress of repair of a sandstone wall, I noticed, where 
delaminated sandstone had been removed but the setting bed was still 
partially in place,that the top surface of the mortar (a lime mortar) had a 
smooth black surface, almost shiny. Surprised, I dug around at an adjacent 
location and found the same thing. I couldn't find it in the collar joints.
Has anyone come across the same, and do you understand why? 
The sandstone has a considerable iron content, and the repair was a 
response to the damage caused by a failed gutter above, left unattended for 
a considerable time.

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