A dynamite book I just got my hands on is Whisky: Technology, Production, and Marketing from the Handbook of Alcoholic Beverages Series. I avoided the book for a while because it was so expensive but it is worth every penny and contains explanations I had been chasing after for years. Believe it or not, I somehow ended up with a few extra copies and if anyone is interested, I would be happy to gift a few to some worthy readers. Send an email.
An issue that had been perplexing me for a while was Germain-Robin’s use of unbleached toilet paper to cover his spirits receiver described in his text: Traditional Distillation Art & Passion. I did not enjoy this book and found it mystified distillation rather than opening it up to new distillers with articulate explanations. I could not find a reference to covering the spirits receiver to catch a copper salt anywhere else and I’ve read quite a bit so I was really skeptical but the guy must know what he is doing. In the text, the copper corrosion in question was attributed in unspecified fatty acids, but in Whisky: Technology, Production, and Marketing (p. 163), it is touched upon and explained differently. Variables I would have never considered are the culprit.
Sulphur compounds present in the distillate vapour are (as with wash stills) highly volatile and odorous substances that take their toll on the copper, forming sulphides; the carbon dioxide in the wash encourages the formation of copper carbonate, which manifests itself as verdigris. It has been customary to suspend a muslin gauze over the spirit bowl to collect any such offending solids that might otherwise find their way into the new spirit.
Attack by carbon dioxide and sulphur also thins the copper so that eventually areas subject to this attack (above the boiling line, the shoulder, the swan neck, lyne arm, and condenser tubes exposed to vapour or the start of the worm) wear away, needing constant patching and eventually complete replacement. A still affected by erosion emulates the breathing of a dog, with the shoulders rising and falling in a rhythmic pattern; such a condition is known as panting, and indicates the need for replacement of the pot.
The sulphides (insoluble in water?) & copper carbonate (insoluble in water?) in question here might ultimately be soluble in the alcoholic distillate but they are likely being blown out of the still dry before any vapor comes across, the mechanism being escaping CO2 dissolved in the wine or beer. The spirits receiver being setup in advance so no distillate drips on the floor. The busy distiller possibly hopes to catch the flakes of verdigris ending up on the toilet paper or muslin gauze but if opportunity is missed, distillate just pours right through. In theory, the distillate at this point is collected in the foreshots receiver and re-distilled anyhow.
The solubility of these particular copper salts is dubious so possibly you only catch them because you can. Otherwise they sink to the bottom of the collection vessel. The lack of necessity of catching these forms of exfoliating verdigris may be why it isn’t commonly practiced or better explained in the literature. The practice may just be the hallmark of a distiller, relying more on art than science, who put in his time and soaked up all his habits via good old oral tradition and diligent & studious observation.
Matt Rowley wrote an excellent blog post about replacing worn out sections of stills, which was my introduction to the worn out copper topic, but missed the amazing panting phenomenon described above in the text which must be quite the unnerving spectacle.