Maturation process in casks I’m curious about the cleaning process of casks before a distillery transfers its whisky to mature/age. To use one of the popular names, just as an example, before Macallan transfers its whisky into a sherry [or bourbon or port or ale] cask, is the cask cleaned? If yes, by what method? Does it vary by distillery or country (à la, whiskey vs. whisky)? Is it all arbitrary or are there laws dictating cleaning process? 

I’ve researched (online only) but wasn’t able to find any (sourced) information. Hoping there are experts here who can enlighten us.
Liquor casks are not cleaned between transfers, just emptied and stored until they need them again. The liquor that soaked into the wood makes sure nothing will grow in them.

Wine barrels on the other hand, need to be cleaned between transfers. Typically they are rinsed with hot water and then maybe steam. Left to drip dry for a day or two. Then sulfur dioxide is put into the barrel either through a gas or burning sulfur in the barrel. This sterilizes the barrel. 

Many wine/sherry casks are reused as Whisky barrels. Minimal amount of cleaning is need before the strong alcohol is put in them (which sterilizes them).