During the years I have been traipsing through the woods, I have always been on the lookout for shelf fungi. When I find the right kind, I will write or draw on its white undersurface. A prime example is a small shelf fungus I used as a 50th wedding anniversary card for my wife, as shown in the photo.
There are about 500 different kinds of shelf fungi that grow throughout the United States, but the ideal species on which to write or draw is ganoderma applanatum - sometimes called "artist's conk."
You can find these fan-shaped shelf fungi growing on the side of tree trunks, stumps or on fallen logs. During each growing season, the undersurface of each fungus has a new layer of pores, from which thousands of spores are released to be blown about by the wind. Shelf fungi play a beneficial role in the ecosystem by helping in the decomposition of dead trees on the forest floor.
Artist's conks are woody and very difficult to break off from the tree or stump where they are growing. While you can find them every month of the year, the best time to take them for artist purposes is in late summer or early fall. In pulling off a shelf fungus, you should be careful not to accidentally bruise the undersurface, making a mark that can not be removed. You can use a nail or another sharp point to print words or create artistic etchings on its white underside exposing the brown tissue below. In craft fairs I have seen elaborate art work on some shelf fungi.
The secret is to write or draw on the shelf fungus within a day of picking it, because soon after being removed from the tree the the fungus' bottom surface will become very hard. Anything you write or draw on it will last forever, just like our 50-year-old marriage.