The washback is where all the alcohol that finds its way into a distillery's whisky is produced. Later stages may concentrate this alcohol as it passes through the wash still and then the spirit still, but its existence in the first place is entirely due to the action of the yeast in the washback.
If the mash tun can be thought of as the tea pot in the distillery, then the washback is (or, more usually, the washbacks are) the brewery. The sugary liquid, or wort, from the mash tun has been collected in the underback. From there it passes through a cooler to ensure it doesn't kill the yeast and is pumped into the washback at a temperature of about 23°C.
The washback is a large container usually around 12ft across and often 20ft or more deep. The depth is not obvious on most distillery tours as the washbacks are usually seen from a working floor placed about 4ft from their top. But find a vantage point from which you can see beneath the floor and the true size of the washbacks becomes apparent. Those at Highland Park were used as communal baths by servicemen from the naval base at Scapa Flow during WWII. The effect of this on the taste of immediate post-war production has gone unrecorded. (Continues below image...)
After the washback has been two-thirds filled with wort at 23°C, yeast is added, and the brewing process begins. This differs from beer production in one important respect. At no point is the wort boiled (unlike in the production of beer) so as the yeast is busily converting sugars into alcohol in the washback a range of other complex reactions are also going on, carried over from earlier stages in the process: in particular any remaining starch in the wort is still being converted to sugar during the process.
Fermentation continues for between 2 and 4 days. Little seems to happen at first, but then the yeast really kicks into action and the effect can be tumultuous, rattling and shaking the washback and its surrounds and threatening to overspill the washback with active froth. This is often countered by the switcher, a rotating arm designed to skim the froth before it reaches the top of the washback. Elsewhere the overspill problem is sometimes tackled by closing the lids and placing large bricks on them.
Taking too close a look inside a washback at this point in the process can be a literally breathtaking experience. The action takes place below a blanket of carbon dioxide that has displaced all the oxygen in the washback, producing a sensation in the top of the nose like every fizzy drink you've ever consumed, all at once. Most distilleries use carbon dioxide extractors to prevent too much escaping into the distillery itself. After fermentation is complete the vaguely beer-like wash, which has an alcohol content of about 7-8% by volume, is pumped from the washback and into the wash still for its first distillation.
As the images on this page make clear, there are two very different types of washbacks in use, those made of wood (often larch or Oregon pine) and those made of stainless steel. The choice appears to be partly down to tradition. Once a distillery has established its process using a particular approach, it is reluctant to change anything in case that also changes the character of the end product, and some distilleries that have changed from wooden washbacks to stailess steel have reported changes in the end product as a result. Beyond that, it appears to be a choice between the much easier to clean qualities of stainless steel on the one hand, or the better heat insulation properties of wood on the other, which tends to make it easier to maintain a constant temperature in a wooden washback.