Sanchi paat (bark of the Agaru tree)
In ancient times Sanchi paat was used as the writing matterial in Assam. History says that the use of saanchi paat was only found in Assam. They were prepared from the bark of the Agaru tree (Aquilaria agallocha), a tree found in upper and middle Assam. A special type of water proof ink was used for writing on the sanchi paat prepared from Silikha, Kehraj ban and ashes, and instead of pen applied it with bird featers.
In 15th century, Mahapurush (Saint) Sankardev used Sanchi paat to write religious scripts on it. Most of these manuscripts were written either in Devanageri or in Sanskrit language. Those scripts are still being preserved in many places. Most of them are found in Satras and temples and some become the part of private collection also.
'Puthi' are used in Assamese rituals.Assamese people used to read these manuscripts in traditional rituals like Satyanarayan Puja and Graha puja.
These 'Puthi' or books are indeed treated with great respect and devotion. They are not allowed to read simply by general people. Brahmins and Bhakats after purifying rituals can only read them.
When these 'Puthi' are being read there must be some one to listen them. The reader usually describes the meaning and significance of the 'Slokas' written there such that they can be comprehensible by general people.