Historical American masonry mortars differ markedly from their modern counterparts in composition, performance, and durability. An accurate identification of materials, design, and performance history is critical for evaluating historical masonry assemblies. ASTM C1324 provides a framework for a thorough compositional analysis of existing mortar. However, it has received unjust criticism based on the misconception that it is only applicable to modern materials. In fact, the standard's method portion incorporates analytical techniques that have been used since the eighteenth century. It is only the exemplary calculations that are geared toward modern mortars. Often overlooked is the explicit requirement for the work to be performed collaboratively by experienced petrographers and analytical chemists. Moreover, the document indicates that historic mortar formulations will require individual treatment. This paper discusses methods the authors have used to reverse-engineer North American mortars. It emphasizes the value of careful and informed petrographic analysis. It also discusses alternative preparation methods, chemical digestions, and calculation approaches with a focus on how these address the broader combination of possible binders, varying material solubilities, and the secondary chemical alterations that can confound typical assumptions of original material chemistry. Finally, we stress the use of error analysis to evaluate uncertainty in any reported composition. Even where a sample yields larger error bars, constraints are often sufficiently narrow to allow for an informed repair strategy. Perhaps more importantly, the transparency allows future peer review because many analyses remain with landmarked properties.