The standard size barrel is 53 gallons and is made with staves, hoops and heads. No screws, nails, bolts or glue are used. The barrel is held together with nothing more than pressure from the steel hoops.
White Oak trees are harvested, cut to length then aged according to the buyers specifications. After the wooden staves are fitted together and the steel hoops are added all by hand, then the barrel is sent to the charing department. Barrels are burnt on the inside for a certain amount of time. This charing process is given specifications ranging from 1 to 4, specifying the amount of time spent in the burner.
The burning process caramelizes the wood sugars along with creating other compounds that the distilled spirits will then dissolve. These sugars and compounds make up not only the color but also around 75% of the taste of the Bourbon.
Time in the barrel:
As the filled barrels are sitting in the rickhouse, the liquid inside is moving into and out of the wood based on temperature and pressure. Where a barrel is stored inside the rickhouses is more of an art than a science, however the higher up a barrel is stored the more extreme temperatures it experiences and that affects its taste. But time imparts the most characteristics.
During this time, the Bourbon takes on the majority of its amber color and the smoky/woody flavors along with the vanilla and overall sweetness.
During this time the color will continue to darken, and it will take on more of a fruity, candied and nutty flavor.
The Bourbon might get a little darker and the sweet, herbal and fruity notes start to fade away thanks to the liquid reacting to oxygen.
Warehousing, The Time In The Rickhouse
As a barrel ages in the rickhouse it will either gain or loose proof depending on the humidity and heat levels. Water and alcohol evaporate out of the barrel at different rates. A barrel on the top of the rickhouse, where the temperatures are higher and humidity is lower will loose more water thus increasing the proof. Conversely a barrel on the ground floor has milder temperatures and lower humidity and will loose alcohol and the proof will go down.