According to "Min":
"I called my friend who smokes cigars and who knows his stuff in chemistry this morning.
His explanation is that plumes are INORGANIC mineral salts crystals.
These salts are previously dissolved within the oils. They are non-volatile but are carried temporarily into the air when the oils evaporate. When they could be carried no further, they deposit onto the cigar as plumes.
Makes perfect sense to me..."
Quote from our knowledeable BOTL "Plume":
First let's clarify a few points made here:
1) Plume does not come from the “essential oils”, rather form from the “essentials” in the oils…in other words the oils carry the minerals that form the plume.
2) Plume is the crystallization of minerals just as it is with salt crystals, and other minerals forming crystals, sugar, gemstones, etc.
3) There are two ways to form crystals (scientifically speaking). The first way is when minerals in lava (or oils or other conducing materials) cools SLOWLY (as in lava what carbon in coal is to diamonds) This is the way that gemstones are formed, over long periods of time and typically involve pressure and heat and long cooling periods. The second way to form crystals is by evaporation, for example the way salt crystals are formed. Evaporation removes the water leaving the impurities (sodium chloride in this case) and the crystals form over a shorter time, by linking and chaining in repeating patterns at the molecular level.
So here’s my Junior Scientist opinion:
Since your cigars never get to the extreme temperatures of lava and the earth’s crust and since oils do not readily evaporate like water, plume is more likely formed by the addition and evaporation of water. In other words the presence and flow of humidity within the humidor carries the minerals out of the cigar’s plant oils to the surface and the slow evaporation of the water from the wrapper (closest to the air) causes crystalline growth of the minerals present on the outside of the cigar. Another factor, and in my experience that also supports Joe's conclusion, is that water evaporates more slowly at cooler temperatures. In other words cigars stored at warmer temperatures in humid conditions is more likely to give to plume when subjected to a cooler environment since evaporation occurs more slowly.
Hot temperatures makes water evaporate too quickly to allow minerals to truly crystallize.
Take two pots and put an equal amount of salt (or sugar) and water. Put one pot on the burner and the other at room temperature. The cooked salt will just leave salt residue on the bottom of the pot while the room temperature pot will create complex crystal formation.
So for me plume is merely the by-product of humidity and cool temperatures and occurs in cigars that have more oils and by virtue more essential minerals to crystallize.
Make sense?"
Plume is not related to the age of a cigar, see MRN page 216, the Hoyo EL Particulares (2000) is full of plume and that was back when the book was first written (or earlier depending on when the photo was actually taken).
Hope this helps.
~M