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According to the National Park Service Preservation Brief #2 “Repointing Mortar Joints in Historic Buildings” a compatible mortar is characterized as:
“…one that matches the historic mortar as closely as possible, so that the new material can coexist with the old in a sympathetic, supportive and, if necessary, sacrificial capacity. The exact physical and chemical properties of the historic mortar are not of major significance as long as the new mortar conforms to the following criteria:
Once the proper mortar has been determined based on permeability and strength, we focus on the aesthetic qualities of the mortar . Visually replicating historic mortar requires matching its sand (color, type and gradation), color, texture, and tooling. In many cases, historic mortar was made using local sand, sometimes from the jobsite itself. The quarries that supplied the original sand are often closed, so additional research will be necessary to locate an alternative. HFS provides aggregate matching services, and we will digest the original sample, analyze gradation, and develop a matching blend. We have access to sand from around the country, so if you run into a wall (pardon the pun) we can help.
Texture of the original mortar results from a combination of weathering patterns, the type of binder, and the sand type. While we do not recommend cleaning lime mortar with any strong chemicals or acids, the mortar can be “aged” by doing a light wash. This procedure takes off the surface layer of cream and exposes the aggregate. To achieve an aged texture, thoroughly wet the wall and wash with 1 part VanaTrol diluted to 10 parts water. Allow it to dwell for a few seconds before rinsing. As with any chemical application, always test first in an inconspicuous location and adjust dwell time accordingly.
When it comes to replicating texture, one of the most common mistakes installers make is using too much water. Aside from causing shrinkage cracking and smearing on the masonry units, excessive water produces a slicked appearance that appears inconsistent with the original. Lime mortar should be the texture of brown sugar during repointing, and should never be installed using a grout bag.
Color matching the mortar is dependent on the choice of aggregate and binder as well as pigments for colored mortars. We have over 150 powdered pigment colors in our lab and we can match nearly any color and we also keep an extensive onhand stock of pigments that can be purchased individually, both as stock items and by special order. We use Dynamic Color Solutions (DCS), which are pure mineral pigments designed specifically for concrete and mortar.
Although concave and convex joints are common today, they were not widely used in historic buildings. Lime mortar joints should be carved in a more aesthetic “V” struck, weather struck, or raked joint profile. Keep in mind that when a joint surface is slicked or compressed it will become less breathable.