The most extensive examination of the changes which whisky undergoes during storage in casks is due to Crampton and Tolman, who carried out a series of experiments upon American whiskies over a period of eight years.1 The higher temperature of storage, however, and the practice of charring the interior of the casks, make the conditions in America very different from those obtaining in this country. For the present purpose, therefore, it will be useful to consider a smaller series of experiments carried out in the Government Laboratory, London, on whisky stored in cask and in bottle over periods of 2| and 7 1/4 years.2

Four casks of pot-still spirits were kept in distillery warehouses, and samples were periodically drawn for examination. Two of the whiskies were of special distillation, having undergone treatment for the removal of furfural; the other two were of the ordinary character. A summary of the analytical results, sufficient to exemplify the changes, is given in tabular form on p. 453.

The corresponding samples stored in glass bottles showed practically no changes.

It will be seen that in the casks the spirit showed a gradual and quite appreciable loss of alcohol. There was during the first six months' storage a fairly well-marked increase in every one of the secondary constituents, over the amounts found in the new spirits. After nine to twelve months' storage the higher alcohols and the furfural remained practically constant, and the increases in the other constituents were very slow.

On the whole, the changes shown by these experiments to occur in the secondary constituents of whisky, though well-marked, are of comparatively limited range, even where the storage is in wooden casks; and they are practically negligible in glass bottles. This fact is of some importance in enabling spirits to be traced and identified; or, on the other hand, in showing that they are different from what they are alleged to be.

It has already been mentioned that the chief analytical difference shown between new and old pot-still whiskies stored in casks consists

1 J. Amer. Chem. Soc, 1908, 30, 98.

2 Appendix Q, Report of Royal Commission.

Changes during storage in wooden casks.

in the increased amounts of acids, aldehydes, and esters. According to Thorpe,1 much of the increase is probably due to a process of extraction of secondary products absorbed in the wood of the casks from former spirits. This absorption is to some extent selective. The ethyl alcohol slowly diffuses through the wood of the cask and passes into the air, leaving some of the esters and higher alcohols behind in the wood, which thus becomes more or less charged with these secondary constituents, according to the length of time and other circumstances attending the storage. Probably also, especially when the casks are empty, there is some production of acids and aldehydes by the slow oxidation of the alcohols in and adherent to the wood of the casks, and the increased acidity would favour the production of esters also.

Crampton and Tolman2 found an increase in the alcoholic strength of spirits stored in wood, amounting to about 1 per cent. per annum. They explain this increase by the fact that water passes through the pores of the wood more readily than alcohol does. The increase in the proportion of higher alcohols is due, they consider, to the diffusion of water and ethyl alcohol through the pores, the wood being practically impervious to the higher alcohols. Furfural may be derived from the charred interior of the cask, and there is also an actual increase in the acids, aldehydes, and esters. The changes are comparatively rapid at first, but proceed very slowly after a period of three or four years.