Which Limewash Lasts? NCPTT's 2005 Tests on Brick for Cane River Creole's Cabins

In December 2003, the National Park Service's National Center for Preservation Technology and Training (NCPTT), Cane River Creole National Historical Park (CARI) and the firm Quality Finish met to discuss limewashing the brick cabins at Magnolia Plantation to help them last longer. CARI wanted a durable, low-cost limewash that could go on in about three coats. Because CARI also hoped to limewash the wooden buildings at Oakland Plantation, the study grew to cover weathered wood, rough-sawn wood, handmade brick and modern brick. This summary covers the brick.

The recipes

Quality Finish interviewed community members to find limewash recipes used locally, but none could be identified, so the team turned to published recipes, including one from NPS contracting schedules. From November 2004, Quality Finish applied ten washes to handmade and modern brick samples, each over an Edison acrylic emulsion primer, with three samples per wash. Some of the washes:

The tests

• Artificial weathering: 800 hours in a QUV weatherometer, alternating four hours of UV light with four hours of condensation in the dark — a preview of how a limewash ages.
• Adhesion: an "X" cut through the limewash, tape pressed on and pulled off — how much limewash comes away shows how firmly it bonds.
• Abrasion: falling sand poured on until a 4 mm circle is worn through — a measure of how it stands up to wind- and rain-borne grit and people touching the walls.
• Appearance, photographs, color readings with a Minolta colorimeter, and a solids test of how much limewash went on.

Each wash was ranked from 10 (best) to 1 (worst) in each test, before and after weathering, and the ranks were totaled.

Results

Handmade brick — best overall: A, then E and K.

• A was best at abrasion both before and after weathering: unweathered, it took an average of 93 liters of sand, almost twice wash B and three times wash K; weathered, 8.25 liters. It was only middling at adhesion, and near the bottom after weathering.
• E tied for best in appearance and in adhesion after weathering, but was among the weakest at abrasion before it.
• K tied for best in appearance after weathering and sat in the middle in the other tests.

Modern brick — best overall: B, then D, then A and K equally.

• B did well at adhesion, abrasion and weathering, but poorly at adhesion after weathering.
• A again resisted abrasion best, needing three times as much sand as the next best, B.
• K did well at adhesion and weathering, poorly at adhesion and abrasion after weathering.

Color: every limewash darkened and yellowed somewhat with age, all by about the same amount.

Recommendation: no salt

Any field choice has to consider moisture moving through masonry and the salts it carries. Efflorescence is salt appearing on a surface; subflorescence is salt crystallizing inside a brick as the water carrying it evaporates. Soluble salts can cause spalling or flaking, slow drying, change porosity, and crack the pores; they come from rising damp, air pollution, mortar, neighboring materials, or the brick's own clay.

During weathering, modern brick with washes A, B and C showed significant efflorescence on the back of the samples; the handmade brick showed none, perhaps because of its porosity. So although A won on handmade brick and B on modern, NCPTT advised against adding more salt to brick buildings.

Its recommendation for Cane River Creole's brick: wash K — lime putty and water, with no additives that could harm the brick, and good results on both kinds. Alternatives: E (top group on handmade brick, middle on modern) and D (second on modern, middle on handmade).

NCPTT planned further tests — plain lime-and-water washes without the primer on brick and wood, and the three best washes from the wood tests on epoxy samples — with results expected in early 2006.

Sources

Based on "Study on the Durability of Traditional and Modified Limewash Recipes," National Park Service, National Center for Preservation Technology and Training, and its full report, Study on the Durability of Traditional and Modified Limewash Recipes: Results, Handmade and Modern Brick (NCPTT, November 1, 2005); a work of the United States government in the public domain. The article page carries only the report's introduction; the results come from the full report.