The glass carboy is one of, if not the most important and useful item in home brewing your own beer, wine, or mead. It has many advantages over a plastic bucket which is the other common type of fermenting container. Here I'll discuss the pros and cons during each stage of fermentation and show why it's superior to other types.
What exactly is a glass carboy?
A glass carboy is the container that holds the water and ingredients that you are fermenting your homebrew in. It can be as small as 1 gallon on up to any size and I have seen them reach 15 gallons in size. It's almost always made from a clear see-through glass and the opening narrows to only a couple inches in diameter.
The other common type is a plastic bucket or pail that is cylindrical in shape including the opening. A plastic lid covers the entire opening which is about 1.5 feet wide. It's commonly called the "Ale Pail".
The primary fermenter
The purpose of a glass carboy or any type of fermentation container is to hold your water and ingredients as it ferments and transforms into your homebrewed beer or wine. The first stage is called the lag phase. It's when the yeast is building up their strength and getting attuned to their new environment. How can you tell when the lag phase is over and fermentation is beginning? Pressure in the fermenter will build as CO2 is released by the yeast and forced out of the airlock. In these early stages of fermenting additional nutrients can help out the yeast.
The obvious advantage here with the glass carboy is that one can see the activity level and bubbles on the top without opening up the carboy. With a plastic bucket it is guess work. Or you can open up the large 1' wide lid and allow oxygen to get in. Oxygen in the presence of alcohol will drastically change the taste and make it vinegary. In the first few days of fermenting the CO2 pressure is pushing out the oxygen. By breaking the seal and opening the plastic bucket you re-introduce oxygen back in and it must start that process over. You run the risk of altering your homebrew's taste.
The secondary fermenter
After activity has ceased or the alcohol % has reached the desired level the contents are siphoned over into another carboy. Here in the secondary fermenting stage we can have fermentation continue on a reduced level but the primary purpose is usually to let the majority of yeast settle in the case of beer and most wines or to allow for the adding of fruit and other ingredients that impart a taste but aren't needed during fermentation. Once again the glass carboy has a distinct advantage in that you can see what is happening. You can tell when the yeast has dropped and your beer or wine has gotten clear. If you need to make any changes or adjusts later on the opening of the glass carboy is very small and thus limits how much oxygen is sneaking in. The plastic pail will allow for adding in large pieces of fruit or other additives to the secondary fermenter and keep it tidy in a bag. However a filter over the siphon will keep these large pieces from being transferred later so a glass carboy can still easily be used here.
Glass carboys offer more choice for aging
Wine and mead needs to be aged and in bulk offers the best method of doing this. With a glass carboy you have a lot more choices in size. As your wine or mead is aging there will be no further fermentation to force out oxygen. The liquid should be at the very top to prevent off flavors from forming. Because glass carboys come in so many different sizes the home brewer has more freedom in storage and experimenting. Perhaps you can take a standard 6 gallon batch and split it into 2 3 gallons sizes with different flavors or conditions. Or you can start with a much smaller batch in the first place. While I am sure there are different sizes of plastic pails I have not seen them readily available beyond special order.
Cleaning and sanitation
The glass carboy has further advantages over a plastic pail in the area of sanitation. It's quite easy to look through the glass and see any deposits or dried spots that need further cleaning. Getting to those spots however may be tricky. Here the small opening works against the glass carboy. Homebrew supply stores though will sell long thing brushes for this specific purpose. Additionally there are sink faucet attachments which focus the water pressure strong enough to blast off most deposits from the inside of the carboy wall.
Plastic has a disadvantage that glass does not. If there are any scratches or breaks in the plastic surface then bacteria or germs can hide within the plastic fibers and be difficult to come out. Also there is a concern with plastic breaking down as it is scrubbed and filled with all sorts of cleaning agents. Most food grade material plastic buckets will help with this concern but sooner or later it might become an issue. Glass suffers no such problems with age and breakdown.
The glass carboy is the superior choice
A plastic pail will suffice in getting the job done with home brewing beers and wines. However the glass carboy offers many extra advantages of convenience that will make your home brewing hobby more enjoyable and giving you extra room to experiment with.