by Randy Ruth
Seven factors that can affect the color of lime mortar in no particular order of significance, Lime, aggregate, pigment, water content, rate of absorption, original surface texture and erosion. By its self, the color spectrum of lime can vary from bright white, light grey, slight pink or ochre colors. This color is dependent on a few factors such as the original stones chemical makeup and burning temperature. When an appropriate limestone is thoroughly burned (calcined) at a particular temperature to produce hydraulic or non-hydraulic quicklime and then hydrated to produce either a lime putty or dry lime hydrate, the result will be a white or off-white color. The first factor affecting the whiteness index of the lime will vary depending on the raw mineral impurities in the limestone. A limestone with a higher calcium content with all other factors aside will produce a whiter hydrated lime. If that same limestone is burned at a slightly higher temperature, the result will be a slightly grayer hydrated lime. Although, lime plays a role in the final color of a mortar, its significance today, when replicating a mortar joint is minimal, often due to the lack of availability or technical characteristics like Hydraulicity. Aggregate has a huge effect on the color of lime mortar. Historically aggregate for masonry mortar would be sourced from either local sand beds, found near creeks or rivers, or from the trimmings of stone on site and possibly brick pieces or dust as a pozzolanic additive.
The larger screenings of the aggregate play a role in the overall tone of the final mortar color but it is the fines that do most of the work. The smallest particles in the aggregate AKA fines will give the biggest impact on the final color. Brick dust, limekiln dust and clay impurities are pozzolanic fines that can be found accidentally and at times intentionally accompanying the aggregate. Today these impurities are almost never allowed into a replicate mortar mix, as the resulting technical data from such a mix design is often cost/time prohibitive for a project even if historically appropriate. As a result powdered pigments are often used today to achieve a particular mortar color. Just because pigments are predominantly used today in mortar mix designs, doesn’t mean that they weren’t used over 100 years ago. Colored mortar is an important design element in any building of today and yesteryear.
The types of pigments used in mortars have not changed all that much in past few hundred years. Iron oxide, carbon black, and natural ochre’s hold a solid footing in the industry today, each presenting their own limitations. It has been proven that carbon blacks can fade dramatically over a 30 year period in masonry mortar. Even though their tinting strength is very good, if not controlled carefully shades of grey can be very difficult to achieve. Natural ochre’s can produce wonderful colors and be very accurate when making accurate replicate mortars. The problem is in their tinting strength, and consistency in production on a large-scale. It may take above a 10% dosage of natural pigment to achieve the same color in a mortar using iron oxide pigments conforming to ASTM C979. Because of their durability, tinting strength and quality in production, iron oxides have been deemed the best pigment for coloring mortar on a large-scale. Even when using appropriate pigments at the correct concentration, water content in a mortar plays a big role in determining final color. Using the same exact mix in two batches and varying the water content by 10% will produce a significant change in color. From experience, I have noted that this problem is most evident when trying to achieve a red colored mortar. Light grey’s can also be problematic but are less evident and are usually deemed acceptable. That is why it important to measure all ingredients in a mix carefully to ensure consistency from batch to batch. The practice of mixing mortars consistently should carry over to pre-dampening of masonry units. By pre-dampening consistently as possible, the rate of absorption is controlled. This is a good practice just so mortar will not reach a flash set, and to control curing of the mortar which plays a role in the final color.
There is some debate on how a replicated mortar should look when not replacing all the mortar. Should it look new, with a smooth surface that stands out because of the way light reflects off two different surface textures? Alternatively, should it blend in with adjacent mortar joints? Personality, I believe in the latter. If a new mortar is inherently the same color at its core as the old historic mortar, than even though a slicked smooth replacement mortar will eventually blend in it can still distract the eye. A good repair for just about anything should be as seamless as possible. Besides, won’t a different texture erode differently, resulting in the continuation of a miss match over time? This brings me to erosion in mortar. As a mortar erodes, the color of the aggregate begins to come through. This color can sometimes throw off the human eyes perception of what is the color to be achieved when color matching. Someone explained this to me so well once that I must share. He had asked a room full of people what the color of foam on the head of a beer is. All replied white in color. We all got it wrong. The answer is amber like the color of the beer. This is because of the way light is reflected back to the human eye off a larger Surface area. Now that is in extreme case of a dark color turning lighter but the principle is still applicable to mortar. However, in most cases the rougher the color the darker a mortar is, and depending on who well the color of the sand is matched, you may just get a replacement mortar that will be seamless for generations.
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