Would you recommend immediately opening all new tins purchased and transferring them to mason jars if they are going to sit for more than a couple years? 4 years? 6? 10?
What do you do?
And also, as far as buying "estate" tobaccco on ebay, etc. How old is too old to be sure?
Good question!
It really kind of depends on the type of tin, the blend, and the blender. Not all of the early C&D tinned Pease blends have had problems, but Bohemian Scandal, Raven's Wing, Renaissance, Haddo's, Samarra, and several others have become very "iffy". If you are going to pay a few bucks for a tin it might be worth the gamble, but not the prices usually being asked unless you like taking a gamble and accept the odds.
McClelland used food grade tins, but even these have failed. Shortly after I made that post, several people contacted me to tell me that some of their 1990's McClelland tins had failed.
Square and rectangular tins are slowly leaking from the get go, and there's no surprise there, as there's no way for them to maintain an even amount of pressure all around the perimeter. Round tins tend to hold up better in my experience, as do most canisters.
But none of these tins were ever intended for long term storage. They're not heavy duty cutter tops. GL Pease wrote in his FAQ that tobacco should be left in the tin to age, but last year he changed his thoughts after finding that tobaccos in his cellar had been ruined by failing tins. Now he recommends jarring them as soon as possible for long term storage.
I have a lot of Esoterica Bags, and a member here discovered that some of his bags had pinholes in the creases and that the tobacco had dried in those bags. So I bought a stack of heavy duty, .7 mil metalized Mylar food grade gusseted storage bags and heat sealed the Esoterica bags inside of them, one Esoterica bag to one heavy duty Mylar bag.
The rectangular and square tins are sealed inside other heavy duty Mylar bags as well. I haven't gotten around to the round tins as of yet. And I jarred a lot of tobacco, small 4 oz jars for more immediate use, larger jars for the longer haul. None of my McClelland tins are old enough for me to be worried, yet, but I'll eventually seal them up as well, or jar them. Just don't have the time at the moment.
As for how long should one trust a rectangular or square tin? Personally, I'd say not more than 5 to 7 years, tops. Sure, some will last longer, but others won't last even 5 years. If you think you are going to age something longer, jar it or bag the tins.
But also be aware that not all blends age well, and some varieties of tobacco, like Latakia, have a shorter shelf life.