We do not "fill" the brick/tile with any type of filler material. Our Vitralase® and Vitrix® patented processes are created with the technology of lasers. By directing a continuous flow of intense laser energy, the image is vitrified into the brick or tile. The vitrification process actually converts the engraved portion of the clay product into a hardened glass. The indelible engraving is actually harder than the brick itself. Clay brick and glass are two substances that stand the test of time!