## 3 Discussion  
Application of fluoride varnish involves the use of high concentrations of fluoride salts suspended in resin, which allows them to remain on the surface of the teeth for a period of time. The saliva-dissolved salts release fluoride ions, thus causing accumulation in higher concentrations (above 100 ppm) both on the enamel surface and in the enamel itself, as well as in the plaque. The presence of fluoride attracts calcium and phosphate ions from saliva or dissolved hydroxyapatite, forming small grains of calcium fluoride with integrated phosphates [3; 10; 11; 24; 25; 26; 28; 31; 32; 33]. Since pure CaF\({}_{2}\) crystal is more cubic than spherical, the resulting spherical deposits are described as calcium fluoride-like compounds, which, on the one hand, block the diffusion of acids, while on the other hand form a fluoride reservoir that is stable and insoluble in a pH-neutral environment. Phosphate ions increase its solubility compared with pure CaF\({}_{2}\)[5; 34; 35; 36]. In the event of a decrease in pH, the membrane coating is dissolved and fluoride ions are released and embedded in the enamel structure. A decrease in fluoride concentration below 100 ppm and an increase in pH result in saturation of the enamel in relation to the surrounding fluid and inhibition of the process described above [1; 3; 5; 7; 10; 24; 26; 33; 37; 38]. According to many researchers [27; 33; 35; 39; 40; 41; 42; 43], the CaF\({}_{2}\) formation process, in addition to a sufficiently high concentration of fluoride ions, is also affected by calcium and phosphate ions present in the oral environment, and the quantity of which is insufficient to bind the whole released fluoride [33; 44; 45; 46; 47]. The supply of these ions in second-generation varnishes, according to [PERSON] et al. [41], causes deposits of more calcium fluoride on the enamel surface, which in turn enhances the therapeutic effect [48; 49; 32; 44; 45; 44; 50; 46; 47; 48; 49; 51; 52; 53; 54; 55; 56; 57; 58; 59]. According to [PERSON] [35], these deposits can be observed in vitro at a high magnification up to 4 months after application. In vivo, due to chewing, swallowing, speaking and individual hygienic procedures, this time is much shorter and varies from several hours, according to [PERSON] et al. [5]; to 24 h, according to [PERSON] et al. [59] and [PERSON] et al. [60]; or up to 3 days, according to [PERSON] et al. [61]. It follows that in the oral environment, the release of ions from fluoride varnishes should take place in the shortest possible time before they are completely lost [45]. All three varnishes used in the study had the same fluoride ion content (2.26% F\({}^{-}\) in 5% NaF). In order to minimize the differences between specimens, uniform amounts of varnish were applied to the normalized area of exposed enamel, where possible--and quickly, so as to reduce the possible change in the weight of the varnish due to evaporation of volatile components. Duraphat, considered by many [61] as a so-called gold standard, is classified as the first-generation varnish, in which 5% NaF is the active ingredient. In addition to 5% NaF, the second-generation varnishes used in this experiment contain CPP-ACP (MI Varnish) and calcium phosphate coated with xylitol (CXP) (Embrace Varnish). Many researchers [25; 27; 44; 45; 46; 55; 57; 62; 63] claim that caseine phosphopeptide complex with amorphous calcium phosphate is the best technology, enabling both the stable combination of high concentrations of calcium, phosphate and fluoride ions with the tooth surface and plaque, as well as high bioavailability and the possibility of free movement into the enamel, resulting in active remineralization of subsurface lesions. According to [PERSON] et al. [22], [PERSON] et al. [44] and [PERSON] et al. [64], it also causes faster and easier release of fluoride ions in the first 24 h after application. In turn, according to [PERSON] et al.