Our house from the 1870s has some places in its brick foundation where the mortar has deteriorated.

Everything I read, such as http://www.nps.gov/tps/how-to-preser...tar-joints.htm , says it's very important to use a soft mortar with soft historic bricks. And that when masons use modern hard mortars with 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 enough, or is it exactly what *shouldn't* be used?

http://www.finehomebuilding.com/how-...type-need.aspx says type N is 1 part Portland cement, 1 part lime and 6 parts sand. 1 part white Portland cement, 2 1/2 parts hydrated lime, and 5 to 6 parts sand

http://www.sacredplaces.org/PSP-Info...%20Masonry.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 have to be formulated by hand? Should I insist that the mason uses that 1 : 2.5 : 5 ratio?

Jimmy