Historic technologies provide enduring performance
In 1835, when the Pemaquid Point lighthouse was built in Bristol, Maine, the American paint industry did not yet exist. Portland cement technology was still more than three decades away from reaching the United States, and prepackaged paint production would start even later. Yet the stone lighthouse, built utilizing the best technology of its time, endured severe coastal exposures for 172 years with only minimal maintenance.
While modern materials offer significant benefits, including relatively low cost and ease of application, few are likely to duplicate this exceptional record of durability. The 19th century was a period of remarkable technological innovation, but the full benefits of those developments can only be fully appreciated in retrospect, with the benefit of more than 100 years of performance history.
The mortar used to assemble the 38-foot-tall stone tower in 1835 was based on natural cement. Natural cement was the engineering material of choice until the turn of the 20th century, when production of higher strength portland cement underwent explosive growth. Natural cement was produced by burning limestone with naturally occurring clay impurities, and then grinding it to a powder that could be used in much the same way we use portland cement today. Because natural cement remains more flexible than modern cements, however, it can endure long years of thermal cycling and structural shifting without cracking or losing adhesion.
Natural cement also has been a common ingredient in coatings and stucco. Simple recipes for durable whitewash were based on natural cement and water, producing a characteristic dark-brown finish. For lighter colors, lime would be added to the mix, and iron oxide pigments also would be used to produce red or yellow finishes.
Restoration of the Pemaquid Point lighthouse was undertaken in 2007. Modern paint, applied in 2002, was removed to reveal the structure’s bare stone, and partial repointing of the surface of the natural cement mortar was undertaken. Rosendale natural cement, produced from the recently re-opened natural cement mines in Rosendale, N.Y., was used for both the repointing and for the subsequent whitewash. The 2007 natural cement was visually indistinguishable from the original cement materials.
As the Pemaquid Point lighthouse – whose image is stamped on the Maine state quarter – still actively protects shipping lanes along the coast of Maine, Coast Guard requirements for a bright white finish coat had to be met. Because this requirement could not be fulfilled with a natural cement whitewash, an alternative, traditional technology was used for the final coating.
Mineral paints, based on potassium silicate binders, were first produced in Europe in the 1870’s. The silicate is capable of reacting with mineral substrates to form a strong, highly stable binder. Some of the earliest examples of mineral silicate coatings remain in good condition today, more than 125 years after their application.
The project’s historic preservation consultants, Building Conservation Associates of Dedham, Mass., selected EverKote 300 Patinar mineral paint for the final finish. The bright white, single component potassium silicate paint is produced by Edison Coatings, Inc., Plainville, Conn. Edison Coatings also produced the Rosendale natural cement mortar and whitewash used on the project. Work was performed by J. B. Leslie Masonry Co. of Berwick, Maine and was funded by the American Lighthouse Foundation (ALF), Lowe’s Charitable and Educational Foundation, and the National Trust for Historic Preservation.
Today, the Pemaquid Point Lighthouse stands restored to its original condition. Using 19th century technology with a proven ability to withstand more than a century of harsh coastal weather, members of the restoration team have openly speculated that this historic structure is ready to endure well into the 22nd century. “The minute you drive or walk into Pemaquid Point Park, the lighthouse commands your attention in the wake of its restoration – it’s like a shining exclamation point on a seascape of blue,” says ALF Executive Director, Bob Trapani, Jr.
As issues of sustainability and “greener” approaches to construction and restoration work have become high priorities, a fresh look at these 19th century technologies may provide some of the best solutions to 21st century challenges. Modern technology long ago pushed past these materials, but the benefits of extremely durable, Low-VOC/No-VOC coatings based on natural cement and potassium silicate have never been more compelling.
Michael P. Edison is a chemical engineer and president of Edison Coatings, Inc., in Plainville, Conn. The company produces a wide range of specialty coatings and repair systems for historic masonry and concrete structures. He is a past president of the Connecticut Chapter of the International Concrete Repair Institute, a current director of the Northeast Chapter of the Association for Preservation Technology International and chairman of ASTM Task Group C1.10.04 on Natural Cement.