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I was looking at a brick wall and noticed on the wall it had lots of small marks. They were black and some white patches.

What is likely to cause these marks? Is it an indication of a bigger problem or is it just aesthetic?

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Wall with lots of small marks

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    Please edit your second question into a second question. You have a good answer to your first question. One question per question works much better here.
    – Ecnerwal
    Mar 8, 2022 at 14:08
  • @Ecnerwal Do you mean 1 question for what the marks are and another question for asking if they indicate a bigger problem?
    – user1605665
    Mar 8, 2022 at 23:25
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    Yes. Feel free to link to each other for context, but they really are two different questions - see @Freeman's comment below the answer for why that's not ideal.
    – Ecnerwal
    Mar 9, 2022 at 0:14
  • I added to my answer at the end to at least provide a link to answers on the 2nd part.
    – Armand
    Mar 9, 2022 at 3:24
  • "If the substance in question is purple, pink or black, it is not efflorescence." nachi.org/efflorescence.htm - One of the chemicals mentioned here can be blue (I lost which one) sciencedirect.com/topics/chemistry/efflorescence - "Efflorescence is not normally damaging, but it is aesthetically undesirable." ... if they came like that, then that's the 'clinker' showing. Could be mold living in the efflo, or the dye is coming out of the brick with it.
    – Mazura
    Mar 10, 2022 at 1:09

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Looks like "efflorescence", basically just moisture moving through the masonry carrying dissolved salts, then those salts drying on the exposed surface as the moisture dries out there.

Here's a "Masonry Institute" paper on the subject with some practical suggestions

They note:

• First: There must be water-soluble salts present somewhere in the wall.

• Second: There must be sufficient moisture in the wall to render the salts into a soluble solution.

• Third: There must be a path for the soluble salts to migrate through to the surface where the moisture can evaporate, thus depositing the salts which then crystallize and cause efflorescence.

All three conditions must exist. If any one of these conditions is not present, then efflorescence cannot occur. Even though the efflorescence problem is complex, it is not difficult to prevent. Although no economically feasible way exists to totally eliminate any one of these three conditions, it is quite simple to reduce all three and make it nearly impossible for efflorescence to occur.

They suggest that in general the white salt efflorescence is more of an aesthetic problem than a structural problem, but caution:

Care must be taken not to trap the salts below the surface of the masonry. This condition is known as crytoflorescence. If the salts are stopped just below the surface, for instance by a silicon water repellent, the water will still evaporate, depositing the salts behind the surface, which then crystallize. The expanding salt crystals can exceed the tensile strength of the brick causing spalling or disintegration of the brick

The linked paper has voluminous details such as the chemistry of the efflorescence, various ways to remove it and prevent it, and so forth.

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    This does a great job of answering the "what is it" part of the question. Can you address the "is it a problem or just aesthetic" part, too? My guess is that it just looks bad and can probably be washed/scraped off with no other concern, but that's just my guess. Also, this is why 2-in-1 questions are considered too broad - if someone else provides a similarly excellent answer to only the second part, how the heck does the OP choose who to give a check mark to? :/
    – FreeMan
    Mar 8, 2022 at 12:56
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    The darker spots might also be lichen. The white is definitely efflorescence. I've certainly had little success in getting people to split their two, three, or X part questions into 2, 3, or X separate questions. I consider that a problem of the question, not the answer. +1
    – Ecnerwal
    Mar 8, 2022 at 14:02
  • Oh, I agree, @Ecnerwal. Unfortunately, "this is a problem with my bricks, therefore the questions are all related" is a difficult concept to break. Additionally, this answer now does a great job of addressing both parts of the question.
    – FreeMan
    Mar 9, 2022 at 13:18

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