Pigments from earth

Question

We're about to do lime wash on one of our heritage buildings. We decided to use ochre as pigment because it comes from earth. My boss says with any pigment that comes from earth, we don't need to worry about discoloration in lime mortar, which is high in pH value. But some of our guys are using laterite powder for brick repairs and I wonder if it is a good choice. Would it discolor in lime mortar? How would it react? Please, someone answer me!

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Maybe I need to mention more info in my question. I'm from myanmar and we've got a lot of sunshine and a lot of rain too during moonsoon season. We're not doing new buildings, just repairing a historic building built around 1900. This building is basically abandoned for decades, and it's quite big. A lot of bricks need to be replaced, but in some areas we intend to use lime mortar with brick dust to imitate a brick rather than raking out and replacing with a new one. But we're not achieving the same color as the original brick but when we add laterite powder to lime mortar, color is satisfactory. Our only concern is that we don't want the repaired areas faded away after three or four years.

Answer 1

Earth pigments generally don't discolor becasue they are already fully oxidized, unlike most artificial pigments. Laterite is commonly used to make bricks, so I doubt you will see any effects you would not already see on the bricks themselves. Just check to see if it is fired or not.

If you are repairing brickwork identifying the causes of stains will help:
Brick Stains: Identification & Prevention

Most staining is caused by improper cleaning not the mortar, lime run for instance is not actually caused by the lime. The only material you really have to worry about with lime is manganese, laterite is rich in manganese, but this is only an issue if it is fired first which mobilizes the manganese, acid washing further mobilizes it, the lime actually causes it to precipitate, if it is unfired however it is already stable and mixing it with the lime will not matter.

Answer 2

What the heck is an "earth pigment"? Dirt? Mud? The only naturally occurring "earth pigment" that I am aware of is used for adobe buildings of the American Southwest. Here, the "earth pigment" is taken from a riverbed and applied directly to adobe brick. It will not fade in sun.

However, it tends to run when it rains.

On the other hand...there are plenty of earth-colored dyes that are added to concrete mixes that will mimic mud and earth when applied as stucco. I can assure you that these, over time, will fade. UV radiation, ozone exposure, ground-level smog, and other general atmospheric crap will noticeably degrade these colors in a matter of years depending on where you live and weather exposure. In Arizona, for example, there is no shortage of stuccos colored with lousy "earth colored" dyes. Your "lime wash" application may respond very differently to your dye and water chemistry, and I'd strongly suggest a test surface before committing to an entire heritage building.

The fading, additionally, will be uneven where water, or sunlight, is unevenly distributed over a large surface. As for your "ochre" pigment, I suspect that it's not a powder ground from just straight up iron-oxide, but some kind of liquid ochre-colored emulsion specially formulated for mixture with concrete. Get an assurance that your boss's "earth color" dye, or additive, is lightfast, at a minimum.