Historic Charleston Foundation - Charleston, SC

istorically, bricks were fired in kilns heated by hardwood fires, and later by gas- and oil-fueled furnaces. Because the heat source was at the bottom of the kiln and controlled only by the fireman's experience, uneven firing caused hard burnt bricks (or "clinkers") near the heat source and soft, under-burned, salmon-colored bricks elsewhere. By 1880 beehive kilns were being widely used in the United States, allowing heat to be more evenly distributed. Both of these early methods of firing brick produced a somewhat soft porous brick.

Improper technique used in brick repointing

Early masonry mortar consisted of a lime and sand mix that worked well with the soft early bricks. As brick-making techniques progressed, so too did the mortar. Portland cement became available in the United States in 1871 and gray Portland masonry cement was introduced in the 1920s. By the early 20th century, both brick and mortars were more regulated. The American Standards Testing Material (A.S.T.M.) was formed to distinguish high strength (hard) mortars from soft flexible mortars and to determine their correct uses. The higher strength mortars containing Portland and no lime worked best with the harder 20th century brick.

High-strength modern mortars do not, however, work well when used with the early soft brick, such as Charleston gray brick, and can even be detrimental to the brick. This is because the softer material will crack and break under any movement within the building such that caused by high winds, the freeze / thaw cycle, and even excessive vibration. Gray Portland has no flexibility and can result in cracking and chipping of the brick, beginning a process of rapid decay and loss of brick.

Proper technique used in brick repointing
Note the way the new beaded mortar joint
matches the existing joints.

A higher lime content in mortars works well with old Charleston brick since it can expand and contract when and where necessary. These high-lime (soft) mortars also slowly gain strength over time and the masonry wall will see extra benefits as a result. For instance, if a hairline crack develops in the joint, a chemical reaction occurs when the mortar gets wet, resulting in the expansion of the lime mix that seals the crack.

If the proper steps are taken, a brick repointing job should last 30-50 years.

Guidelines For Repairs To Your Brickwork

Damaged bricks and mortar should be replaced with bricks and mortar to match the existing.
Take care to match the existing mortar in color, texture and strength. Call HCF for advice on an appropriate mortar mix for your brickwork.
The mortar joint should also match in style, i.e. grapevine joint, v-shaped joint, smooth rake, etc.

Additional Resources
Preservation Brief #1, "Assessing Cleaning and Water-Repellant Treatments for Historic Masonry Buildings" by Robert C. Mack, FAIA and Anne Grimmer

Preservation Brief #2, "Repointing Mortar Joints" by Robert C. Mack, FAIA and John P. Speweik

Preservation Brief #39, "Holding the Line: Controlling Unwanted Moisture in Historic Buildings" by Sharon C. Park, AIA