Clay wasn't "discovered" in a single year. It's a natural material that has been used by humans for thousands of years, dating back to the Stone Age.

Here's why it's difficult to pinpoint a specific year:

* Clay is ubiquitous: It's found all over the world.

* Early uses: Ancient civilizations used clay for pottery, bricks, and even early forms of writing.

* No specific "discovery": Clay is a naturally occurring material, not an invention.

Therefore, while we don't have a specific year, we know humans have been using clay for a very long time!