The art of matching mortar requires a patient process of trial and error
In short: When repairing historic brickwork, getting the mortar color wrong can ruin the aesthetic. To achieve an exact match, you must master the delicate balance between lime and Portland cement, accounting for the subtle ways these materials shift in hue as they dry.
Achieving a seamless repair during repointing requires more than just a quick mix. According to mason Mark McCullough, the process begins with identifying the base color. If the existing mortar is not gray, it likely contains a dye. In such cases, the most reliable method is to take a small sample to a local brickyard to request a professional color match. Most supplies needed for this task are readily available at local brickyards or home improvement centres.
If the mortar is gray, you can attempt a manual match using premixed bags. This involves a systematic experiment with lime and Portland cement. By mixing different proportions of these two ingredients in small buckets and then combining them with equal amounts of sand, you can create a spectrum of test samples. It is important to note that adding more lime results in a lighter mortar, while increasing the Portland cement makes the mixture darker.
Once your mixtures reach an oatmeal-like texture, you must test them carefully. By spreading a thin layer of each mixture onto scrap wood, you allow the samples to dry more quickly. This step is critical because mortar naturally lightens as it dries. Once the samples have dried, you can compare them to the existing joints. Once the perfect hue is identified, you must replicate those exact proportions for the final, larger batch to ensure consistency across the entire project. Carefully peel back the mortar blob to ensure only a thin layer remains on the wood. This technique facilitates a faster drying time for each test sample.
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- Summary by Intellect · source: How to Match Mortar | Ask This Old House
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