Such repointing should be done with pure lime mortar. Portland cement should not be used in any way. Cement is not compatible with the fieldstones because it's too hard and traps moisture.
Bagged hydrated lime found at masonry yards is intended for mixing with portland cement and is not the best material for making pure lime mortar. Instead, look for slaked lime putty (which you mix with your own sand) or a hydraulic lime mortar mix, which is premixed with sand.
Slaked lime putty produces a nonhydraulic mortar that may be too soft for locations exposed to severe weather.
Hydraulic lime mortar becomes harder and may be better for exposed locations. It may be too hard, however, for old soft bricks.
The following publications are useful and reliable:http://www.doeni.gov.uk/niea/tn37_repoi ... rick-2.pdfhttp://www.dumgal.gov.uk/CHttpHandler.ashx?id=1492&p=0http://www.heritage.nsw.gov.au/docs/rep ... update.pdf