Casks are critical to the taste and appearance of the final whisky. The need is for casks wich will impart a characteristic taste to the whisky without dominaing it or imparting a 'woody' flavour. Principally two types of cask are used - Oloroso sherry casks and American oak Bourbon casks. Some distilleries use intact barrels, others remake barrels from selected staves from more than one source. The barrel may be charred before use, a process which apparently assists the release of vanillin from the wood. No two casks are the same - one may produce a fine whisky and may be refilled and used again whereas its neighbour may taste woody after one filling.