You can see the action of the brushstrokes on this piece of terracotta, which instantly takes my imagination right back to the person who originally painted it. It may be a white clay ‘slip’ that they have brushed on, known as terra sigillata, in which the particles have been ground to nano-size. When baked on, it forms a smooth and much less porous surface, useful for this purpose, the fragment is probably an old sewer-pipe.
Terra sigilata was used in ancient times. It’s only very recently we have figured out what’s happening at the nano-scale, and can call it nanotechnology. It has also been taken internally throughout history as an anti-poison medicine.