Age of Pot Stills - how long will they "hold up"?

[primate77]

I've been looking through some of the threads about history of rum, and saw the posts about Worthy Park Estate, and the comments Ed made about two pot stills in inventory dating back to the 1600's.

How long will a still last before it is no longer useful? Are there stills in current day use that are several hundreds of years old? Does the age of the still affect the the quality of the product (meaning, the older and more used, the better the product)? Or will a still affect an ongoing change in the taste of the rum produced as the still accumulates years under it's belt?

[Mr Fjeld]

Interesting question! I asked someone in the know (whisky) and it turns out that a pot still used in malt whisky production rarely last longer than 25 years. Even then the upper parts and the lower parts will have been replaced after between 10 to 15 years. Indirect firing or direct firing and also the use of rummager inside the still will affect the age but the oldest parts of the pot still will almost never become older than ca 25 years old before the whole still is replaced. I don't know how this applies to rum though?

[Mr Fjeld]

In case you're still interested I should perhaps mention that distillation "wears" the copper. The copper itself actually helps "clean" the spirit of volatile chemicals but while doing this the copper is also "sacrificed" . When it's 50% of its original thickness most distillers will replace the part in question - or the whole still if necessary. Hope this answers your question.

[primate77]

I do find that very interesting indeed.
In my profession, to "save" the guy anchors in guyed communication towers, "sacrificial" devices are placed in the ground next to the guy anchors. This is called a "cathodic protection system". The sacrificial anode pulls the decaying properties to itself and away from the guy anchor.

I know, sounds off the wall, but that is kind of how I relate to what you are saying. So how is it the copper is sacrificed? Is it high amounts of heat?

[Mr Fjeld]

Originally Posted by primate77

In my profession, to "save" the guy anchors in guyed communication towers, "sacrificial" devices are placed in the ground next to the guy anchors

I know they use zinc anodes with boats but I didn't know they use the same technology in building structures? I learned something today

Originally Posted by primate77

So how is it the copper is sacrificed? Is it high amounts of heat?

Indeed it is! During distillation the liquid & vapours will react with the copper; dissolving parts of it and transform it into copper salts. It's crucial to reduce the amount of sulphur in the new make. Ian Wisniewski, a whisky journalists says it so much better than I but the process of pot still distillation is the same with whisky and rum so here goes some of the most important factors in the process: (it's a very good article so I'm pasting a large part of it - it's from Whisky Magazine Issue 23)

..........During distillation copper absorbs sulphur compounds, converting them into other, less organoleptically active compounds (ie. less sulphur character), while also acting as a catalyst helping to manipulate the ester character. As sulphur compounds feature a distinctive line-up of notes, ranging from struck match, sulphurous, rubbery, meaty and sweaty socks, to cabbage and vegetal, they can easily dominate and ‘conceal’ other characteristics within the new make spirit. While a certain level of sulphur character can be highly desirable, depending on the house style, lowering the level of sulphur compounds allows the congeners, including esters, to show more readily.

The full extent of reactions taking place within the still is unknown, though it’s clear that any action only takes place when congeners touch the copper surface, either as vapour or liquid. (There again, as vapours are hotter than liquid, each reacts slightly differently, though being able to define that difference is another matter). The common denominator is that
components such as organic acid react with copper to create copper salts (verdigris is one example) on the surface of the still neck, and the condenser or worm.

Less than 50 sulphur-bearing compounds (ie. in which sulphur is only an element of the total) have currently been identified, and it remains to be seen how ongoing research affects this total. Some of these compounds have a surprising repertoire, bearing citrus and floral characteristics. As these were only identified a couple of years ago, exactly how they are affected by copper is not yet fully understood. However, it is clear that they only contribute to citrus, floral notes, which are primarily created by esters.

The desirability (or not) and the level of sulphur character required obviously varies considerably between distilleries, and various approaches can promote or discourage the level of copper influence.

The degree of reflux within the still seems like an initial consideration. A higher degree of reflux means more copper
interaction, and so the potential for more copper dissolving into the condensate. However, while vapours presented for a second or third time to the copper surface creates a greater separation of lighter and heavier flavour compounds, and promotes the passage of esters, the crucial question is how much of the dissolved copper actually carries over into the condenser. The answer is a minimal amount..........

[Edward Hamilton]

Thank you for that interesting article Mr Fjeld. The effects of sulfur on the still in the whisky and rum industries are similar. The biggest difference however is that sulfur content of the raw material. For rums made from molasses the effects of sulfur are much more dramatic and can often be tasted as a sharp note in the finish of the spirit.

During the sugar making process, sulfur, potassium and other compounds are concentrated in the molasses as the crystalline sugar is removed. Sugar cane is grown on volcanic soil often has the highest sulfur content so the effects on the still are the worst.

Unlike stainless steel, copper is sacrificed in the still, so even in column stills made of stainless steel, there are usually copper plates or a copper condenser. In some cases, chemicals are added to the wash that form compounds with the free sulfur which can more easily be removed during the distillation process.

Besides the sulfur content of the raw material, how much the still is in use obviously makes a big difference in its useful life.

[Michael]

Thanks for the informative excerpt and commentary.

Can one of our members be so good as to elucidate the phrase "... vapours presented for a second or third time to the copper surface creates a greater separation of lighter and heavier flavour compounds, and promotes the passage of esters ..." a bit? This novice would like to understand the role of refluxing in rum distillation, while avoiding conceptual mistakes at the outset. A couple of years of dimly-remembered college chemistry won't be much of an impediment.