A Compendium of Useful Information for the Practical Man/Vinegar

Cider Vinegar, Making
If you have a good barrel that has had vinegar in it before, it will always be desirable to keep it for the express purpose of containing it, and never to use it for anything else. The advantage of this will be, that your cask will be well seasoned, which will materially increase the acidity, promote the quality, and accelerate the progress of the vinegar, so as to bring it to perfection, and give it a superior strength and flavour.

Take any quantity of cider that is old, strong, harsh, or of an inferior quality, and add to it the same quantity of cider from the wring, or press; rouse it up well, and fix it in a warm place, or in the sun, which is certainly the best for its progress; stir it well till the acetous or second fermentation be brought on, after which it should remain in that state till it becomes quite sour; then remove it to the cellar for use, leaving open the bung at the time of making, and when settling after the fermentation. Stop it only with a slate or tile afterwards ; the vinegar will soon arrive at full flavour and perfection.

[http://books.google.com/books?printsec=frontcover&pg=PA215&id=w3AqcyOFlcIC#v=onepage&q=&f=false The vintner's, brewer's, spirit merchant's, and licensed victualler's guide ... By Vintner]