I was born in 1936 and my mother bought a small aluminum ( 6.5 inches in diameter ) cauldre, thus it is my own age, 75 years old. It had a beautifull black patina on the outside and to make a long story short, somebody in my family found it and removed all this patina and painted some ridicolous flowers on it. To do this the patina was removed ( maybe with sandpaper ). Now this item is again in my posesion and I removed the ridicolous flowers and now the small cauldre is shining aluminum color. I would like to ADD a black patina on it, on the OUTSIDE. All the information there is on the Internet is about seasoning the metal on the inside. Does somebody know how to do this ?
this is just my guess, but i would think that seasoning on the outside would be the same... like illustrated here at the Black Iron blog: http://blackirondude.blogspot.com/2009/01/easy-cast-iron-skillet-reconditioning.html
Most aluminum cookware today that is dark colored has been hard coat anodized. This provides a somewhat non-stick and protective surface. While the pot you refer to was quite old, it's still possible that it had an anodize coating on the outside. If true, then it will be difficult (expensive) to put back.
Old aluminum cookware I have from that same time period is just the basic silverish color further indicating to me that this is not some seasoning process that happened naturally.