More than 1000 birch bark manuscripts have been found in Novgorod and other Russian cities in last 50 years. Many of them stayed in the soil for more than 700 years. In the same time, no special chemicals or methods were used by Slavs to preserve these papers (as Egyptians, Indians and Jews done with their manuscripts). They are just random papers like letters, notes or even shopping lists.
And these texts, when found, look like this (this is a child drawing dated 1240–1260):
How can we establish the authenticity of these manuscripts? How confident can we be that these things are not fake, or there is always some degree of uncertainty?