Barrels intended for wine and beer needed to be strong enough to hold an expanding volume and higher pressure. The way to make wood strong enough is to put it under pressure of its own. (Is there a moral here?)
First step for wet coopers: use only oak. The oak must be thicker than the wood used for dry barrels or buckets. The expense of the wood alone was greater for these barrels, but on the other side, they were expected to last at least 50 years.
The oak staves were angled, but they were also carved to taper at both ends. The middle of the stave was wider, so the middle of the barrel was wider. This is all very well, if only thick oaken staves bent easily, like plastic or even metal. They don’t bend easily at all, of course. When the staves were forced to bend, they would always be under pressure to bend back straight. This pressure pushed back against the expanding alcohol that would fill each cask.
Wet coopers used large vats of boiling water to soften the pre-carved staves. Sometimes steam was enough, other times the wood itself had to be boiled. The barrel was assembled first at the bottom; an iron hoop held them in place without much difficulty. Then the cooper pounded a larger hoop into the center of the staves, at the widest point. So far, the wood hadn’t been put under too much stress.
The real trick of barrels came next. Anyone who’s ever dealt with wood furniture whose sides are exposed to different humidity knows what happens: one side expands, the other shrinks. The wood warps. Controlled warping was exactly what the cooper now wanted, so he put an iron fire basket into the bottom of the barrel. Its heat dried the inside of the staves, while the outsides were still wet. As this happened, the cooper and his assistants bent the staves inward until they could fit the end hoop. If the staves were not chosen, carved and softened correctly, at this point there was a loud snap and one of the staves broke.
When things went well, the staves gradually bent inward and the hoops were hammered into place. The cooper then used curved planes to make the inside of the barrel perfectly smooth. Rough wood harbored harmful bacteria that could spoil the wine or ale. The cooper also made the flat, round ends, called heads. Their edges were tapered to fit into the barrel perfectly, right where notches had been cut into the staves. By the time the barrel’s staves were perfectly dry, the wood could not completely straighten, so they could hammer the end hoops into a looser position, opening the barrel enough to fit each head into place.
Germany led the barrel industry. German coopers made watertight “dry barrels” for salted herring caught in the North Sea by the Hanseatic League. When hopped ale (beer) became an export to other parts of Europe, there was a huge need for wet barrels. Beginning in the 13th century, German coopers began formally competing to see who could make the largest barrel. The record was pushed upward and upward, to ridiculously unwieldy sizes, until the last record was set in Heidelberg, in 1751. This famous barrel is called the Heidelberg Tun. It used more than 100 oak trees and was never anything but a tourist attraction.
next: measuring liquids in barrels