Thanks in advance for all of your helpful insights.
I have recently purchased some copper cookware that has scarcely been cleaned. It is a four piece set (not including the lids) made by Bourgeat. It is high quality (2.5 mm copper, lined with stainless steal, iron handles).
The original owner used them a lot, but never cleaned them. There are two forms of mire on these pots.
The first form is the typical oxidation byproducts from normal heating of copper, which has caused the pots to discolor. Some affectionately call this "patina," and many don't mind it. My preference it to keep my pots shiny, so I routinely clean this patina off of my other copper cookware with lots of paste formed out of powdered "Barkeeper's friend" and even more elbow grease. I don't mind this effort, and consider it the 'cost of doing business' when it comes to cooking with copper.
The second form of damage is the reason I am writing, and the likes of which I don't know how to effectively attack. What I am referring to are dark black stains of burned on oil and grease. These do not seem to be removed by Barkeeper's friend, no matter how hard I scrub.
The attached photos show the bottom of my frying pan, which has been cleaned with Barkeeper's friend. The "patina" has been removed, so the copper is bright and shiny, but the black stains remain. Another photo shows the side of the pan, which I have not cleaned yet, simply to demonstrate the state of the pan before my attempts to clean it. Near the handle, you can see the built-up black crusty deposits, though most of the black stains on the rest of the pan are flat, not raised or crusty.
When my enameled cast iron cookware (Le Creuset, Staub) gets these burned grease marks, I spray it with oven cleaner about once a year and let it sit for a day. Oven cleaner is essentially an extremely strong base (like high concentration sodium hydroxide). However, the oven cleaner specifically says "not for use on copper."
I am confident the oven cleaner will remove the burned-on grease from my copper cookware as it would from my enameled cast iron, but I don't want to ruin copper cookware that I can barely afford used, let alone new.
Will oven cleaner permanently damage the copper?
Will other chemical cleaners of copper be useful for this burned on grease? Some have recommended the following list of cleaners, but I don't know if any of them can tackle the burned-on grease marks:
Flitz Metal Polish
Am I stuck having to ship these to a copper cookware re-tinner to professionally buff the exterior of the cookware, or are there other metalworking shops that can easily and cheaply buff them?
Any and all suggestions are most welcome! Thanks again in advance for your efforts in helping me.