Select a repointing mortar that matches a historic masonry building's existing mortar in strength, composition, and appearance per NPS Preservation Brief 2's softer-than-masonry principle

Steps

1. Retrieve NPS Preservation Brief 2 (Repointing Mortar Joints in Historic Masonry Buildings) from nps.gov/orgs/1739 and review its core principle: new mortar must never be harder, less permeable, or less flexible than the historic masonry units or the original mortar
2. Identify the building's construction era and original masonry type (soft handmade brick, machine brick, stone) since pre-1930s buildings were typically laid with lime mortar, not portland cement
3. Have the existing historic mortar analyzed (composition and approximate strength) rather than guessing, and treat visual/era assumptions only as a starting hypothesis
4. Select a replacement mortar (lime-based or a lime-rich ASTM C270 mix) whose hardness and vapor permeability fall at or below that of the original mortar and the masonry units, not the highest-strength option available
5. Document the mortar type, sand source, and any pigments chosen so the specification is reproducible on future phases
6. Mock up and cure a sample panel on-site before full production to confirm color, texture, and joint profile match before committing