Whisky talk

Mortlach Whisky 2.81 Distilled Calculation – I was surprised when I couldn’t find a calculation online, so decided to have a crack at it myself…

Known as ‘The Beast of Dufftown’ due to its rather unique flavour profile, Mortlach whisky is often referred to as ‘meaty’ and ‘muscular’, a description which almost sounds more fitting of Frankenstein’s monster rather than a whisky, but to be fair, Mortlach is a bit of a monster.  Established in 1823, Mortlach like many of Scotland’s distilleries was built on the bones of a former illicit distillery, during the distillery’s early days it passed from owner to owner and at one point in time was used as a brewery, but it was in 1897 when Mortlach really became ‘The Beast of Dufftown’.

The majority of Scottish single malt whisky is produced using a double distillation process which is pretty simple to understand; your wash (fermented wort) enters the first copper pot still, this is called the wash still and will produce what is essentially an intermediate between wash and spirit, this is sometime referred to as low strength spirit but the industry term is low wines – these ‘low wines’ are usually around 20-30%ABV in strength.  Once the low wines have been produced they enter the second still, which is called the ‘spirit still’, it is in the spirit still that the low wines go in and the spirit comes out.  Aside from a few exceptions like Auchentoshan and Benrinnes (Pre 2007) which are triple distilled, the vast majority of Scottish single malt whisky is double distilled.  Irish whiskey tends to be triple distilled and the process is essentially the same as double distillation except you’ve got an additional spirit still added to the process, this was incorporated into the process of early distillation of Irish whisky due to the difficulties in obtaining a pure spirit with just two stills.  So you’ve got double and triple distillation, you couldn’t possibly have a distillation method that’s in between the two…or could you?

Well, thanks to the mind-boggling distillation process developed at Mortlach in 1897, they created something in between double distillation and triple distillation, and it’s called 2.81 distillation.  There are a number of sources online that have described the process, some of them better than others, but overall even after a few hours research you might still find your brains a bit scrambled.  When it comes to the magic number of 2.81 I failed to find a source that was able to provide the calculation, some people claim that the number is actually closer to 2.7 but I thought to myself, would Mortlach really be wrong about this? Would Diageo continue to put this number on their bottles if they thought the calculation was wrong? I don’t think so.

For a lot of people, understanding the calculation wouldn’t be important, but being a bit obsessive I couldn’t move on from it, so I have spent the past couple of days deciphering the process so that I could better understand it and hopefully help other whisky enthusiasts get a grip of it…if anyone happens to read this post that is.  I would also like to point out that what you see below is just my theory on how the calculation has been worked out, I am in no way claiming that this is the definitive way Mortlach came to the conclusion that their spirit is 2.81 times distilled.

First I will run you through the distillation process at Mortlach and hopefully the colour coded diagram I have created will prove to be a useful visual aid, God knows it helped me better understand the process.

Mortlach have six copper pot stills, three of which are wash stills, the No.1 and No.2 wash stills are the smallest at 7,000 litres, the No.3 wash still is much larger at 16,000 litres.  Now onto the remaining three stills, and I think it will be best to start with the most simple of the three.  Spirit still No.3 is fed by wash still No.3 in a conventional manner, i.e. the wash still produces low wines (low strength spirit usually between 20-30%ABV) and this spirit stream enters spirit still No.3 where cuts are made so as the foreshots and feints are recirculated into the still and the heart can enter the worm tubs. Now things start to get a bit more complex, but I’ll try to make it as clear as possible.

Wash stills No.1 and No.2 each have two spirit streams, 80% of the wash from both of these wash stills is channeled to the No.2 spirit still, this 80% is known as the heads, this is where the lighter chemical compounds and the majority of the ethanol is present. Once again cuts are made, the foreshots and feints are recycled back into the spirit still and the heart makes its way to the worm tubs.

The third of the spirit stills is the smallest of the three and is known as the ‘Wee Witchie’. The Wee Witchie is integral to the Mortlach distillery when it comes to producing the distinct weightiness of Mortlach, you could even say that it’s the Wee Witchie that makes Mortlach the ‘Beast of Dufftown’.  From the No.1 and No.2 wash stills 20% of the wash known as the tails are channeled into the No.1 spirit still (the Wee Witchie), these tails are low in strength, contain a lot of water and some of the heavier chemical compounds produced in the fermentation phase, one of these chemicals is sulphur and it’s the sulphur that gives Mortlach it’s weightiness.  Three distillations of the tails are carried out in the Wee Witchie, two of which are blank runs meaning that no cuts are made, and the all of the distillate reenters the still, it is only on the third run that cuts are made and the heart makes its way to the worm tubs.  What this triple distillation in the Wee Witchie does is limit the amount of sulphur that can be removed from the distillate so that more sulphur is present in the new make Mortlach than you would usually find in most whiskies.  When sulphur in the distillate reacts with the copper of the pot still it forms copper sulphate which essentially remains glued to the inner wall of the pot still meaning that there is less sulphur in the new make spirit as a result, but in the case of Mortlach the inner wall of the Wee Witchie becomes so saturated with copper sulphate that a lot of the sulphur can no longer bind to the copper and subsequently goes on its way to the worm tubs.

The worm tubs also play a crucial role in producing a more sulphurous spirit, there are only a handful of distilleries that continue to use worm tubs as their method of condensing.  The worm tubs are metal tubes, usually copper which are coiled around many times, these ‘worms’ sit inside ‘tubs’ of cold water, hence the name ‘worm tubs’.  Once the distillate vapour enters the worms it condenses and the liquid new make spirit comes out the other end.  Most distilleries today use ‘shell and tube’ condensers which are made up of many very thin copper tubes which are contained within a metal shell where they’re surrounded by circulating cold water.  The shell and tube condensers significantly increase the spirit’s contact with copper, thus removing even more sulphur.  By using worm tubs, the surface area of the copper that comes into contact with the spirit is greatly reduced meaning more sulphur remains in the spirit, this is another factor that contributes to Mortlach’s weightiness.

Mortlach Distillation Process.jpg

Mortlach 2.81 Distillation Calculation

Stage 1: Calculating the ratio of double and quadruple distilled spirit

Percentage of x4 distilled spirit from spirit still No.1 (Wee Witchie) = 33.9622641509434% (33.9623%)

Percentage of x2 distilled spirit from spirit still No.2 = 32.0754716981132% (32.0755%)

Percentage of x2 distilled spirit from spirit still No.3 = 33.9622641509434% (33.9623%)

Percentage of spirit that is x4 distilled = 33.9622641509% (33.9623%)

Percentage of spirit that is x2 distilled = 66.0377358491% (66.0377%)

2/100×33.96226 = 0.67925

2/100×32.07547 = 0.64151

4/100×33.96226 = 1.35849

Stage 2: Adding the ratios together to give the distillation figure (the simple way of doing this gives a figure of 2.7 which is often what some people claim as being Mortlach’s number of distillations)

0.67925+0.64151+1.35849 = 2.67925 (The 2.7 figure that some people often claim is the ‘true’ distillation ratio of Mortlach)

Stage 3: Incorporating what I see as a correction factor for the distillation process, which when added to the figure of 2.7 gives the stated figure of 2.81

1.35849-1.32076 x 3.396226 = 0.12814

2.67925+0.12814 = 2.80739 (2.81)

I hope you’ve enjoyed this post about my favourite distillery and I hope I have calculated this right, but either way, I haven’t seen a single source online have a crack at this, so if it’s wrong hopefully it will inspire a smarter person than me to give it a go.




4 thoughts on “Mortlach Whisky 2.81 Distilled Calculation – I was surprised when I couldn’t find a calculation online, so decided to have a crack at it myself…

  1. Tried to follow York calculation but don’t be see how you got there as oh just show those percentages without expanding how you calculated them. Be good if you could share these??


    1. Hi Graham, it’s been a long time since I did these calculations, but I will have a go at trying to explain. I actually haven’t looked at it in a while, mostly because the numbers sometimes confuse me even more. The percentage of 4x distilled spirit is 33.9623%, this is because only the hearts that come off the Wee Witchie are quadruple distilled (1 distillation in wash still number 1, 2 blank runs which circulate the wash through the Wee Witchie before the final distillation where the heart is cut, totalling 4), 1800L of hearts are collected off the Wee Witchie, this is 33.9623% of the total volume of hearts collected from the 3 stills. The remaining 66.0377% of hearts is collected from spirit stills 2 and 3, this is double distilled (1 distillation in the wash stills and 1 distillation in the spirit still).

      The ratio is calculated by the following formula = (number of distillations / 100 x percentage of hearts that have come off that particular still)

      If you add these ratios together you will get a figure of 2.7, which is often the figure some people claim is the true distillation figure of Mortlach.

      The correction factor is where it gets confusing, I had the maths down on paper but haven’t been able to find it since, but if my memory serves me it related to the swapping of heads and tails between stills 1 and 2.

      That’s the best I can do I am afraid, hope that helps you.



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