<P> This does not occur in steel barrels because steel is entirely non-porous and does not absorb any of the spirit . </P> <P> Water barrels are often used to collect the rainwater from dwellings (so that it may be used for irrigation or other purposes). This usage, known as rainwater harvesting, requires (besides a large rainwater barrel or water butt) adequate (waterproof) roof - covering and an adequate rain pipe . </P> <P> The standard barrel of crude oil or other petroleum product (abbreviated bbl) is 42 US gallons (34.972315754 imp gal; 158.987294928 L). This measurement originated in the early Pennsylvania oil fields, and permitted both British and American merchants to refer to the same unit, based on the old English wine measure, the tierce . </P> <P> Earlier, another size of whiskey barrel was the most common size; this was the 40 US gallons (33.3 imp gal; 151.4 L) barrel for proof spirits, which was of the same volume as 5 US bushels . However, by 1866, the oil barrel was standardized at 42 US gallons . </P>

Why are barrel staves dried prior to being assembled into barrels