Hi! I was reading your blog piece on patinas of Feb.10 2009 but silly me I cannot figure out how to find the next day (you said it would be continued tomorrow.)
I impulse bought a hammered fired huge bathtub on Craigslist for a song, and am designing my bathroom. Due to space limitations, I will use the tub also as a shower, behind a wall of heavy glass doors. I read today on another site that the dark fired finish will turn as it is a living finish unless I treat monthly with beeswax. I'm trying to decide if I want to just let it go into its living finish and enjoy that evolution or treat it now. It's been in the garage for some months and I notice that where a drip of water got on it it is green patina'd. Do you know if it's possible to bring it back to a dark fired finish after I let it go green and beyond? I am chemically sensitive so I wouldn't want to use caustic chemicals to do so.
The problem with buying things for a song on Craig's List is that you have no idea where something's been. Seriously, unless you can find out the name of the manufacturer and contact them, you're shooting in the dark. That tub appears to be copper from that photo and based on the fact that it developed a green color in reaction to a water drip. But beyond my assumption that you're dealing with a copper tub, I haven't a clue whether or not your tub's been patina-d or spray painted to look the way it does.
So here are a few things I know about patinas and living finishes. I wrote a five-part series on this topic in February of '09 and those posts are here:
So What the Devil's a "Living Finish" Anyway?
No Really, What's a Living Finish?
Patinas on a Budget
The Peoples' Faucetry
All Good Things Must End: My Last Post on Faucets for a While
In a nutshell, metals like copper are reactive. That means they react to chemicals in their environments. Copper usually reacts to acids and alkilis to form a variety of chlorides, sulphides and carbonates known collectively as verdigris. Verdigris is composed of copper carbonate or copper chloride primarily and those chemicals make up the green patina most people associate with copper. However, not all patinas on copper are green and not all patinas are the result of natural, chemical processes.
That's a lot of chemistry for somebody who says she's chemically sensitive, whatever that means, but reality has a way of intruding on the best-laid plans.
Pure copper develops a patina in what's essentially a process of decay. Patinas are the symptom of that decay but they also serve as a protective surface. A patina-d surface layer on copper seals off the copper from the outside world and the decay stops. Remove the patina and the decay starts again until it forms a new patina then it stops again. Repeat this process for a thousand years or so and your copper object will disappear.
The confusion with all of this comes from the imprecise language used to describe patinas. In the commercial sense, a patina is a surface treatment of a metal. The same word gets used to describe a patina that occurs naturally and a patina that's applied. Copper left out in the rain and elements will eventually turn green in reaction to oxygen in air and water. The brown color on the tub in the photo got there as a result of some chemical or several chemicals and pigments being applied to its surface.
Whether or not its a living finish is a function of it either having or not having a clear coat on its surface. Applying a clear coat stops the chemical reaction driving the patina. Not applying a clear coat allows the patina to evolve.
Don't waste your time applying beeswax. Use polyurethane instead. It lasts forever (more or less) and beeswax will have its own effect on copper over time. Just because one material comes from a bee and another material was developed in a lab doesn't make beeswax a better sealer, nor is it automatically benign. Whatever you end up doing, never scrub your copper tub because you'll disturb either the patina or the clear coat preserving the patina.
So to answer your question, once your tub starts to change color, it won't ever go back to what it was when you bought it.