Patent ID: 8871299

Claim:
A method for producing solid particles having a silica coating which are suitable as feature substances to increase the falsification security of documents of value, wherein the silica-coated solid particles have diameters in the range of 0.5 μm to 60 μm and the silica coating has a thickness in the range of 100 nm to 10 μm, the method comprising the steps: producing a buffered solid-particle dispersion by: dispersing acid-labile solid particles to be coated in an aqueous medium to produce a solid-particle dispersion, and setting the pH value of the solid-particle dispersion by a buffer system; or dissolving a buffer system in an aqueous medium to produce a buffer solution having a pH value, and dispersing the acid-labile solid particles to be coated in the buffer solution; and adding a basic silicate solution to the buffered solid-particle dispersion to form the silica coating on the solid particles during a coating time, wherein the buffer system is selected from the group consisting of potassium hydrogen carbonate/potassium carbonate, sodium hydrogen carbonate/sodium carbonate, and mixtures thereof, the composition and the quantities of the buffer system and of the basic silicate solution are selected such that the pH value of the solid-particle dispersion, before the addition of the basic silicate solution, is at least 8.5, and after the completion of the addition of the basic silicate solution is at most 10.5.