Everything I read, such as http://www.nps.gov/tps/how-to-preserve/briefs/2- repoint-mortar-joints.htm , says it's very important to use a soft mortar w ith soft historic bricks. And that when masons use modern hard mortars wit h a high Portland cement content, it eventually damages the bricks. But I' ve had trouble finding out what exactly this soft mortar should be.
One mason told me he'd use type N mortar (750 psi) for this project, rather than the type S (1800 psi) he'd use on modern bricks. Is type N soft enou gh, or is it exactly what *shouldn't* be used?
http://www.finehomebuilding.com/how-to/articles/mortar-what-type-need.aspx says type N is 1 part Portland cement, 1 part lime and 6 parts sand. 1 par t white Portland cement, 2 1/2 parts hydrated lime, and 5 to 6 parts sand
http://www.sacredplaces.org/PSP-InfoClearingHouse/articles/Repointing%20Mas onry.htm says 1 part white Portland cement, 2 1/2 parts hydrated lime, and 5 to 6 parts sand is recommended for 19th century row houses in NYC.
Is there a commercially available mortar for historic bricks, or does it ha ve to be formulated by hand? Should I insist that the mason uses that 1 : 2.5 : 5 ratio?