**1. Introduction**

Fired bricks have been used by people since 5000 BC in Mesopotamia and many types of this building material are still produced. Many historic buildings are built from classic, load-bearing, non-perforated and non-relieved bricks, which can be expected to be subjected to relatively large diffusion processes or easier biodegradation than lightweight perforated bricks. It can be assumed that the quality of brick firing is of great importance. The middle part of fired bricks may contain a black or grey reduction core, which distinguishes it from the red color of the rim or surface of the fired clay body. Such a black core can be found in many commercial bricks [1], its presence indicating insufficient firing and incomplete burn-out of organic components present in the used clay.

Diffusion plays a fundamental role in the distribution and behavior of substances in porous building materials. This is well described, especially for Cl− anions [2–5]. Cations have not been studied as much from this point of view [6]. The exception is radionuclides, but in this case the research is focused on the disposal of hazardous waste, in which less dangerous model elements [7–9] were used. At the same time, cations also play an important role in salinization of building materials, of which calcium is an example. This element occurs in free or bound form as Ca2+. It is among the elements that are important in the metabolic processes of humans and other organisms. Calcium cations are naturally present in water and soil, but they can also enter the environment excessively during liming of fields [10] and, to a lesser extent, during the salting of roads (CaCl2) as a less dangerous salt for the environment than NaCl or KCl [11]. The Ca2+ concentration is easy to assess via complexometric titration [12].

Biodegradation of fired bricks has been described in many studies performed in situ and also under laboratory conditions. The presence of algae, cyanobacteria and fungi [13–15] was studied. Their colonization depends on many factors, such as humidity, the amount of light, nutrients and also the structure of the surface of building materials on which organisms can create and maintain biofilm growth, e.g., Ref. [16]. So far, the influence of the degree of the firing of bricks on their ability to be bio-colonized has never been investigated.

In this work, the focus was placed on the influence of the quality of the firing of bricks from the surface to the depth on: (1) the ability of calcium diffusion; and (2) the effect of the firing on the success of the biological colonization of individual layers.
