Lime Mortar in Early 18th-Century Construction
Lime mortar has been an essential building material for centuries, valued for its workability, durability, and role in historic masonry. During the early 18th century, mortar was primarily composed of lime and sand, mixed with water to form a paste-like consistency. Builders adjusted the proportions of these ingredients based on the quality of lime available, ensuring optimal performance in masonry work.
Composition and Preparation of Lime Mortar
The fundamental components of lime mortar were:
- Lime (either stone-lime or chalk-lime)
- Sand (coarse and fine, in varying proportions)
- Water (added carefully to control consistency)
Builders recognized that higher-quality lime could accommodate more sand, making it more economical and structurally sound. For example, chalk-lime, though similar to stone-lime when first burned, quickly absorbed carbon dioxide from the air, reducing its effectiveness. This necessitated different mixing ratios:
- 150 pecks (37.5 struck bushels) of chalk-lime were typically mixed with two loads of sand (60 bushels total).
- 20 bushels of stone-lime could often bear two and a half loads of sand.
According to Dr. Higgins' experiments, the most effective mortar contained one peck of lime to seven pecks of sand. This proportion ensured a balance between workability and long-term durability.
Workability and Drying Process
Proper drying was crucial to mortar performance. Builders recommended using minimal water when applying mortar in fast-drying conditions. However, slow and gradual drying resulted in stronger mortar, as it allowed full carbonation of the lime. Historical sources suggest that keeping mortar sealed from air exposure could improve its quality, a practice recognized by the Romans. Pliny even noted that Roman law required mortar to be aged for three years before use, attributing the longevity of their structures to this method.
Challenges and Best Practices in Masonry
While well-mixed and properly cured lime mortar provided excellent bonding for masonry, several issues could compromise its performance:
- Improperly slaked lime led to poor workability and weak bonds.
- Excess air exposure before use diminished mortar strength.
- Dry bricks absorbed mortar moisture, reducing adhesion. To counter this, masons soaked bricks before laying them.
- Cracking occurred due to insufficient sand or rapid moisture evaporation.
- Freeze-thaw cycles affected mortar durability, particularly if the mix lacked proper sand proportions.
Dr. Higgins developed an alternative cement, blending:
- 56 pounds of coarse sand
- 42 pounds of fine sand
- 14 pounds of purified lime
- 14 pounds of bone ash
Though promising, this composition proved too costly and labor-intensive for widespread adoption.
Limitations of Lime Mortar in Water Resistance
Despite its advantages in general construction, lime mortar was unsuitable for water-resistant applications. Traditional lime-and-sand mortar failed under continuous water exposure, prompting the development of hydraulic lime and other cements for marine and subterranean structures. Builders and chemists sought improved formulations, leading to the eventual rise of Portland cement in the 19th century.
Conclusion
Lime mortar was a cornerstone of early 18th-century construction, carefully formulated to balance strength, workability, and longevity. Its role in masonry remains significant in historic preservation today, where traditional techniques are used to maintain the integrity of period structures. Understanding the properties and limitations of lime mortar provides valuable insight into early building practices and the evolution of construction materials.
