Yikes, a patina thread!!!!
For those new to bonsai, patina is a general change in color of the pot, due to a layer of ????, who knows exactly what, oils from hands, sun, some "dirt", some minerals from the water inside the pot and outside the pot wicking up or through, minerals deposited by pollution in the rain, slow oxidation of pigments in the clay, and probably the most important, UV photo-degradation of of the pigments in the glaze and the surface of the pot. There are many ingredients that go into patina.
Key is the mineral deposits are cleaned off the pot every year, or at least regularly, and not allowed to get heavy, or caked on. The patina is an "all over" effect, it is more pronounced at edges, but it happens all over the pot. Dirt and grime and caked on minerals tend to happen on the feet, or base of the pot, and at the top rim or inside the rim of the pot depending on the average level of soil.
Dirt, grime and caked on minerals need to be cleaned regularly. The above trick looks great. The conventional, is to use a "scrubbie" that is soft enough to be safe for teflon, and vinegar, and scrub the caked on minerals off. If mineralization is bad, burying the pot in moist Canadian peat for a year can often remove the hard water minerals. While buried in peat, (due this outdoors) letting rain water run through the peat will help remove the hard water scale.
I don't know enough to go into all the details of patina, but I hope this helps distinguish patina from simple mineral build up from hard water. Hard water scale needs to be removed. Patina should be allowed to stay, as it adds value to the pot.
I don't have photos to demonstrate. But many when they come from the kiln are bright white, bright yellow, bright blue, bright red, all these sorts of bright colors, when you see photos of pots made with the same glazes, that have 100 years of age on them, the colors are no longer bright, often the colors have become quite subdued. That is the effect of patina.
