Secondary Fermentation: glass carboy only?

It's not just the "seal." The material itself is oxygen permeable.

 

Answered it yourself.

This is in reference to extended secondaries

 

Are you talking a plastic carboy, or a bucket?

 

Glass if typically recommended for extended secondaries and bulk aging since it is not oxygen permeable. For shorter secondaries, simply leave it in the primary if you can.

 

I wouldn't say that glass is recommended for extended secondaries and bulk aging. I mean, of course you will find people recommending that, but a lot of experienced homebrewers would recommend using BetterBottles (or similar competing products) instead.

I don't have time to dig up a source right now, but I think that, when it comes to oxygen permeability, the difference between a bucket and a carboy is vastly bigger than the difference between a plastic carboy and a glass carboy. This is because, while plastic may be very slightly more oxygen-permeable than glass, this is dwarfed by the amount of oxygen ingress through the stopper/airlock. And on that dimension, buckets (with their unreliable seals) are greatly outperformed by both glass and plastic carboys.

This is why, for instance, @OldSock lists glass carboys as a way to waste your beer money, writing: "I've had good luck with... kegs and plastic carboys for long term storage."

I would advise using BetterBottles for long-term aging. Whether there is any reason to do a "secondary fermentation," which most people on this forum will tell you is unnecessary and inadvisable, is another question.

 

Did you read the part where it says racking to a 2ndry is unnecessary in most cases?

 

I always thought the main reason you don't want to transfer to another bucket for secondary is all of the head space (oxygen) that would be in the bucket, compared to a 5 gal carboy.
I personally don't transfer to secondary unless it is for long term aging (1 year + sours) or to add fruit.

How long do you plan to be in secondary?

 

Let's start with why you want to do a secondary?

 

I'm going to go out on a limb and guess "apples."