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I have a concrete (stone) vertical wall. I want to put a picture on it.

I have several limitations:

  1. I want to make it in a security, so I want to place something on the wall, that would be transparent or very similar to the original color.
  2. I am not a scientist and have no access to industrial materials, so want to make it out with everyday materials.

The wall itself has properties:

  1. It looks on the south, so sun lights on it almost full day (as a consequence, it is certain temperature and low humidity)
  2. Overall direction is vertical; while it is not fully straight, it has relief.
  3. There is little overhand above the wall, so it would not get heavy water stream in case of straight rain

So I did think about colored bacteria or mold (or other unicellular), that i can put on the wall with nutrients, but in case of mold it requires warm but wet environment. In the case of bacteria, I have no enough expertise to figure out which are needed and how to make them grow.

So maybe there is some chemical reactives that I can put on the wall, and under sunlight it would become colored in some time. Or something that becomes colored after becoming wet after rain?

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    $\begingroup$ Do you know these mugs that reveal a picture when you fill in a hot liquid? $\endgroup$
    – Martin - マーチン ♦
    Aug 24, 2022 at 23:36
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    $\begingroup$ With such a security need, others must have the wall. ;-) $\endgroup$
    – Poutnik
    Aug 25, 2022 at 2:16

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