The product ofthe distillation of alcoholic liquors, termed "low wine," does not usually contain alcohol in sufficient quantity to admit of its being employed for direct consumption. Besides this, it always contains substances which have the property of distilling over with the spirit, although their boiling-points, when in the pure state, are much higher than that of alcohol. These are all classed under the generic title of "fusel-oil": owing to their very disagreeable flavour and odour, their presence in spirits is extremely objectionable. In order to remove them, the rough products of distillation are submitted to a further process of concentration and purification. Besides fusel-oil, they contain other substances, such as aldehyde, various ethers, etc, the boiling-points of« which are lower than that of alcohol; these must also be removed, as they impart to the spirit a fiery flavour. The whole process is termed "rectification," and is carried on in a distillatory apparatus. Heat is first applied gradually, in order to remove the most volatile impurities, and to concentrate them in the first portion of the distillate. When the spirit coming over possesses no objectionable odour, it is caught separately as long as it is of sufficient strength.
The receiver is then changed again, and the remainder is collected apart, as weak spirit which contains much fusel-oil; the first and last runnings are then mixed together and re-distilled with the next charge. When a strong spirit is required, rectification may be repeated several times. It is customary, however, with the improved apparatus of modern times, to produce at the cutset spirit containing but little fusel-oil and at least 80 per cent, of alcohol; this is then purified and concentrated in the above manner, and afterwards reduced with water to the required strength.
Another cause of the offensive flavour of the products of distillation is the presence of various acids, which exist in all fermented liquors; they are chiefly tartaric, malic, acetic, and lactic acids. The excessive action of heat upon liquors which have been distilled by an open fire has also a particularly objectionable influence upon the flavour of the products.
The first operation in the process of rectification is to neutralize the above-mentioned acids; this is effected by means of milk of lime, which is added to the liquor in quantity depending upon its acidity; the point at which the neutralization is complete is determined by the use of litmus paper. In the subsequent process of distillation, the determination of the exact moments at which to begin and to cease collecting the pure spirit is very difficult to indicate. It must be regulated by the nature of the spirits; some may be pure 20 or 30 minutes after they have attained the desired strength; and some only run pure an hour, or even more, after this point. The product should be tasted frequently, after being diluted with water, or a few drops may be poured into the palm of the hand, and after striking the hands together, it will be known by the odour whether the spirit be of good quality or not; these two means may be applied simultaneously.
The process of rectification is usually carried on in the apparatus shown in Figs. 3 and 4. A is a still, 4/5 full of the spirit to be rectified. The condenser E and the cooler Q are filled with water.
After closing the cocks F and I, the con tents of the still are heated by steam, which is introduced at first slowly. The vapours of spirit given off pass above tho plates a of the column B, and escape through C and D into the condenser E, the lentils dd", and return in a liquid state through ff' and gg' to the upper plates of the column B. In these return pipes the liquid is volatilized, and constantly recharged with alcohol to be again condensed, until the water in the condenser is hot enough to permit the lighter alcoholic vapours to pass into the coil c, without being reduced to the liquid state. When this is the case, the vapours pass through F into the couler G, where they undergo complete enndensation. Great care mart be taken that the beat is not so great as to permit any of the vapours to pass over oncondensed, or to flow away in a hot state; and also to keep up a constant supply of water in the cooler without producing too low a temperature; the alcoholic products should run out just cold. The highly volatile constituents of the spirit come over first, that which follows becoming gradually purer until it consists of well-Haround alcohol; after this comes a product containing the essential oils.
The more impure products are kept apart from the test, and re-distilled with the next charge. Some hours generally elapse before alcohol begins to flow from the cooler. The purest alcohol is obtained while its strength is kept between 92° and 90° B , and the operatioo is complete when the liquid flowing through the vessel it is better, however, to stop the still when the backing or "faints" indicate 10° B., because the product after this point contains much fusel-oil, and is not worth collecting.
In order to cleanse the apparatus - which should be performed after each working - the still A is emptied of water by opening the cock 0. The contents of the condenser are then emptied in like manner by opening the cock J, through which they How upon the plates in the column B, and wash out essential oils which remain in them. These two cocks are then closed, and the door U is removed. The water in the cooler G is then run by means of a pipe into the still A, so as partially to cover the steam-coil in the latter. After again securing the door (J, a strong heat is applied, and the water in the still is well boiled, the steam evolved thoroughly cleansing all parts of the apparatus; this is continued for 15 or 20 minutes, when the heat is withdrawn and the still left to cool gradually.
The capacity of the rectifying apparatus has a good deal of influence upon both the quantity and the quality of the spirit obtained. Besides being much more difficult to manage, a small apparatus will not yield so large a proportion of spirit as a more capacious one, nor will its products be of equally good flavour. The proportion of alcohol which may be obtained from a successful rectification is very variable; it depends upon the nature of the spirit rectified, the method of extracting the sugar, and the manner of conducting the distillation; it will also be in inverse proportion to the quantity of fusel-oil contained in the raw spirit. The average loss of pure alcohol during the process of rectification is generally estimated at about 5 per cent.
Wood-alcohol [pyroligneous acid, or pyroxylic spirit] is one of the products of the dry distillation of woods, those chiefly used, stated in the order of merit, being birch, beech, elder, and oak. The seasoned and barked wood is placed in iron retorts, similar to, but larger than gas retorts, and heated to 400° to 500° F. (204 1/2° to 260° C.) for usually 6 to 8 hours. The slower the distillation can be conducted, the greater the yield of wood-alcohol, as a quick fire causes an evaporation of alcohol. The liquor from the distillation is run into pans, and left for the tarry matters, to float, when they are skimmed off. The acetic acid present is neutralized by lime, and forms commercial acetate of lime. The remaining crude liquor is re-distilled, and affords crude wood alcohol. It is further concentrated by a second distillation, and then rectified, to free it from tarry impurities, traces of acetic acid, and much of its characteristic odour. (For a full account of wood distillation the reader is referred to Spons' 'Encyclopaedia/ pp. 5-27.)