The study is aimed to examine peculiarities of applied seals of the 16th - 19th centuries made from black sealing wax, which were relatively rarely studied. 56 seals dated by the 16th - early 19th c. were selected for research. The objects were examined under a microscope and also observed in different light (VIS, UV, IR). Some samples were reddish when viewed under UV radiation, while others did not change their color. This allowed the hypothesis that there was a significant difference in the materials used to make these seals. The analysis of sealing wax recipes showed that in Europe since the middle of the XVI century there were two recipes of sealing wax: in one recipe the main component was resin and in the other one - shellac. To test the hypothesis, experimental samples were made in accordance with historical recipes. They showed similar behavior: in ultraviolet, the sample from shellac took a rich dark red tone. Another conclusion of the experiment was that the shellac sample had significantly better adhesion than the resin sample. This explains the comments made by the authors of the recipes about the incomparably higher quality of shellac seals. In addition, it was found experimentally that shellac is much more comfortable to work with: it is easier to melt and mix with the pigment than resin, and almost does not smell or irritate the mucous membrane. Further observations revealed that the black sealing wax seal has a heterogeneous structure that is invisible when examined in visible light even at significant magnification. Although the main surface area of some seals is dark red under UV light, some segments retain their black colour. Infrared examination allows to suggest that this heterogeneity is due to the main seal material and not to the distribution of soot dye. This heterogeneity should be taken into account for further research.