Good question! My take on the square and rectangular tins is that they are not 100% sealed, are slowly leaking from the get go, and over time will lose their seal. The round tins seem to do better and I'm thinking, son of an engineer that I am, that the pressure of the seal on a circular perimeter is more evenly distributed than on the square and rectangular tins and that the circular tins have a slight screw down thread, while the square and rectangular tins have only a gasket, which also helps. That said, I've bagged my older tins in heavy food grade Mylar, in the hope that the added barrier will be enough to keep the content from drying out. And there are other factors involved, such as the use of vinegar as a preservative, which can corrode a tin over time.
And cellarers who say they haven't had a tin fail are largely not dealing with 20+ year old tins. Cellaring didn't become widespread until the early 2000's. Most of us just bought what we needed for immediate use.
A number of my GL Pease canister tins from the early 2000's corroded from the inside out. Bad bets for aging include the original Renaissance, Bohemian Scandal, and Haddos. Renaissance did resuscitate fairly well. Haddo's becomes stale and quite nasty. I won't spend money on these, but will happily take them for free since that's all they are worth to me at this point.
Last year I bit the bullet and jarred all of my tins of Haddo's and am glad that I did, since some corrosion was forming at the solder join in the 8oz and 16 oz tins.
I interrupted their aging. Boo hoo. Still better than opening up a rust filled tin of mummy dust.
Of course, there will always be exceptions, but that does not disprove the trend. It's one reason that I've stopped buying vintage square and rectangular tins. Those 20 years old or older have largely been compromised and the contents have dried out.
And not all blends respond well to rehydration, one case being St Bruno and another being Haddos. But St Bruno's Ogden's base tobaccos can sometimes be slightly revived and provide an interesting smoke, though the topping is long gone. They're worth a couple of bucks as a gamble. I'm smoking some mid 90's St Bruno that I rehydrated from a failed tin, and it's not a bad smoke, just not Full St Bruno.
And it's not just the C&D tinned GL Pease tins from 2000 thru 2005 that have problems. I've had a couple of disappointments with Rattrays blends in the tall canisters from the 1990's.
After I started a thread in the spring of 2018 about the Pease tins:
With all of the crazy "collectible" tin buying action on eBay I've been following a lot of auctions. Several have caught my eye because they are offering something a bit different. Here's one that just completed...
pipesmagazine.com
I got PMs from several people about McClelland tins from the 1990's that had corroded from the inside out. Not many, but it does happen.
On the plus side, I've had some marvelous experiences with old blends, most recently tins of Edgeworth Slices, some '60's era Edgeworth Ready Rub from a can that opened with an audible whoosh, some '50's era Craven's Mixture that had faded a bit but still tasted fine, some '90's era Christmas Cheer that had developed a wonderful sweetness, some '80's era Balkan Sobranie that brought back some happy memories, and others that came from cutter tops.
So it's very much a crap shoot when dealing with really old tobaccos.
As for preserving tobaccos for long term aging, from my experience it's hard to beat a jar, or a cutter top.
You can put tins in jars and hope for the best, but some blends, like Haddo's are better just jarred to avoid corrosion from the inside out.
And keep in mind that all tobaccos, with the possible exception of burley, have a life span. They mature, peak, and fade. Flavorings fade, maybe not for a decade more or less, but eventually. Virginias last a long time, but they too peak and fade. They just take longer to do so than Latakia.
