It's my understanding that copper darkens because of oxygen.
Yes, but also because of other copper salts or compounds formed by
reacting with things other than oxygen. Sulphur is the main one I’m
thinking of. Just as sterling silver tarnishes from forming black
silver and copper sulphides (not, by the way, oxides, at least not
for the black tarnished colors), so do copper and it’s alloys. You
can verify this by darkening your copper with a liver of sulphur
solution, just as with sterling silver. What is formed is sulphides,
not oxides. Copper oxides come in two versions. Only one of them is
actually black, the other is red. That’s why heat colored copper can
show those lovely reddish tones in addition to just blackening.
However, in terms of your question regarding sealing, the issues are
So, if we can get a good seal it shouldn't darken. I've tried
several different sealants with varying results.
Some so-called sealing agents are sufficiently gas permiable as to
slowly allow oxygen to penetrate (even gold electroplate will allow
this if it’s thin). And since oxygen isn’t the only issue, it also
matters whether the sealing agent itself has some sort of sulphur
compounds in it, as these can directly react with the copper.
.... Sometimes my copper begins to darken immediately, but if I
bring it to a high shine, it takes much longer for any darkening to
start. I can't figure out why. Does the polishing cause the surface
to become less permeable by oxygen?
The reactions with either oxygen or sulphur or other chemicals that
will cause patinas to form happen on the surface. Not under or within
it. The metal does not need to be permiable to oxygen, gasses, or
chemicals in order to tarnish. What is happening, however, has to do
simply with surface area. The smooth polished surface has less
surface area than a rougher unpolished surface, so less area
available to react. Even if the same percentage of a surface forms
oxides or sulpides, the surface area of the polished surface is so
much less that visually, the amount of visible tarnish shown by that
same percentage, will be much lighter. Also, the rough unpolished
surface will have pits and pores and a texture at the microscopic
level that can trap reactants more than a smooth surface will, so it
can be more reactive. As well, in polishing, the wax or grease based
polishing compounds will tend to fill microscopic surface pores,
acting a bit like a sealant already, unless you use fairly
aggressive cleaning methods (like an ultrasonic) that will remove
that invisible stuff from any surface pores.
make it impermeable?
Inpermiable coatings, electroplatings, modifications to the alloy,
or the like. If a given metal or material is gas permiable, changing
the level of polish won’t affect this.