Matching a new mortar color formulation to an existing mortar is time-consuming and difficult. However, like matching paint or stain colors, time and care taken in the beginning pays off in the end.
Color matching is always a compromise as the remaining good mortar in a wall varies across a range of colors. It is impractical to attempt to blend different mortar colors in the same wall. If the wall is especially dirty, or has a distinctly dirty spot, washing the wall with a mild detergent and low water pressure may decrease the range of mortar color. If this is done, it must be done at least two weeks before work begins so the wall can fully dry. Washing a wall will initially darken the mortar, but will then lighten the mortar once it has dried. However, washing has also been know to expose a different color pattern and range, one that had been masked by the dirt.
To properly formulate a mortar color, the composition of the “binder” (Portland cement or lime) and the “aggregate” (sand size) has to be determined first. Once this is done, multiple samples have to be mixed across a range of possible options. For example, Sample A may have two units of a mortar color; Sample B may have three units of the same color, and so on. Depending on the color, from 3 to 8 samples must be mixed and installed in the wall in the same method as the actual finish work. Mortars having strong black and red hues are the most difficult to match and control. The finishing method (striking, raking and tooling) has a significant role in the final mortar color depending on how much of the aggregate is exposed.
The samples must sit for 10-14 days before they reach their final color. Lime mortars take longer than Portland mortars to reach their final color. During this time, lime mortars must be cycled through several wettings to fully “carbonate” the lime. The bricks will also be washed during this time to remove residual mortar.
Finally, the owner will choose the sample that is the best match and the mortar will scaled up to larger mixes for the actual repointing work.