Leonardo da Vinci wrote most of his personal notes in mirror, only using standard writing if he intended his texts to be read by others. The purpose of this practice by Leonardo remains unknown.
Mirror Makng
In ancient times, obsidian stones were used to make mirrors, as these stones when highly polished were able to reflect with an incredible clarity.
The process of making mirrors is simple. It involves coating a glass sheet (which needs to be cut in the requisite shape prior to coating) with silver or aluminum with the help of a chemical process. It is very important that the glass is polished to perfection as any dip, well or impurity left on it would cause waves in the mirror, which would cause distortion of the image reflected.
Mirror writing is formed by writing in the direction that is the reverse of the natural way for a given language, such that the result is the mirror image of normal writing: it appears normal when it is reflected in a mirror. It is sometimes used as an extremely primitive form of cipher.
Mirror writing calligraphy was popular in the Ottoman Empire during the 18th and 19th centuries among the Bektashi order, where it often carried mystical associations. The origins of this mirror writing tradition may date to the pre-Islamic period in rock inscriptions of the western Arabian peninsula.