Glass carboys are the best choice for secondary fermentation and some
primary fermentation. It has less chance of contamination than plastic
because it is harder to scratch. Scratches can harbor bacteria and
cause infections. Glass is easy to sanitize and will last a life time if
cared for. Never add hot wort to a glass carboy. The high temperature
may cause the glass to expand quickly and break. A 5 gallon carboy
makes a great secondary fermentation vessel for most home brew recipes
as most homebrewers brew 5 gallon batches of beer.