Do All Red Or Red-Brown Coins Turn Brown?

Slabs are not air tight. So tarnishing can continue inside them.

It is inevitable. Given enough time, a copper or bronze coin exposed on the earth's surface will turn back into the copper ore from whence it came. Earth simply isn't a very friendly planet for shiny copper and bronze coins.

Any coin container or holder is air resistant, but not airtight. And given enough time, even the best holders will age, crack or otherwise develop flaws and holes that allow air in.

Your only hope for "permanent" preservation is dropping the coin into a bath of molten plastic such as PMMA (acrylic) and letting the plastic set, entombing the coin like an ancient insect in amber. Most collectors would regard such treatment as sub-optimal.

Let's see 15 years and still red.....so...

"How long it takes" depends entirely on the environment the slab is kept in, and just how "leaky" that particular slab is. Under optimal conditions, it might last for centuries. Worst case natural conditions, a few weeks. Coin doctors deliberately trying to turn a red coin brown could make it change in seconds.

Determining environmental factors are temperature (cooler is better), humidity (drier is better), and levels of sulfur in the atmosphere (closer to zero is better). For temperature, it's generally regarded that keeping things at a constant temperature is better than keeping them cool; it is the constant increase and decrease in temperature that can cause condensation of moisture onto a coin's surface, accelerating the browning. All we're saying is, if you choose to keep your coins in the proverbial fridge, make it a walk-in fridge so you can go in there with your coins, rather than constantly taking the coins out of the cold and back in again - that's a very bad idea.

The good news is, it is entirely within the present owner's control - there is nothing that a coin can be exposed to now that can make a coin turn colour for a future owner in five or ten years time. So if you buy a red slabbed coin, you can take whatever precautions you wish to keep it that way, without having to worry about whether a prior owner might have done something bad to the coin that will adversely affect it in years to come. For copper coins, if exposure to darkening agents happens, the reaction is pretty quick.

We're not saying that a particular slabbed red coin will turn colour within your lifetime. We're saying it might turn, and unless the slab is stored carefully, whether it turns or not is effectively random. This is precisely why the TPGs refuse to guarantee their colour designations.

Many of the coins from the 1700's and early 1800's that have remained red were lacquered, which can be removed without damaging the coin with acetone

Acetone-cleaning is TPG-acceptable, so they'd slab them.