Shawn,
I think your explanation is essentially correct. Here are the salient points as I see it based on your account. I would also like to hear what other folks have experienced as far as the thickness of the wrapper of cigars that have unravelled or cracked.
1. Moistened tobacco is more elastic than dry tobacco. More precisely, moisture in the wrapper will shift the yield/fracture point on the stress-strain curve. Mechanical engineers, please chime in. The effect can be illustrated by the following example: A dry piece of uncooked lasagna noodle is quite brittle, especially in tension (meaning pulling the noodle from its two ends). However, soak it overnight (you don't even need to cook it) and it will swell with water and become much more flexible. You will be able to grab the ends and stretch it slightly whereas doing so with the dry noodle would cause it to fracture well before the same degree of stretching is achieved. This is the key and leads to the second point...
2. A dry room will quickly drop the equilibrium moisture level in tobacco. It's commonly quoted that "wet" cigars contain about 14% humidity. This level is pretty low already and as the wrapper is so thin and light, very little needs to be driven from the leaf, (probably on the order of mere milligrams) to push it into the range where it will become brittle. Especially problematic is the fact that you were using a radiant heater. If the heater was directed toward the smokers, then the infrared radiation will directly warm and more rapidly dry off the wrapper, and unless you roll you cigar as you smoke it, one side will be preferentially affected. This effect spells the beginning of trouble and part two of the one-two punch is as follows...
3. Warm tobacco expands. More to the point, warm tobacco will also absorb more moisture as it expands. As is the case with most any material, making tobacco warmer will cause it to expand. However, in a cigar, the situation is slightly more complex with this complexity potentially resulting in a greater risk of splitting. The reason is this: Not only will tobacco expand slightly in thickness and across the flat dimension, the tobacco is stacked. The net result of this stacking is to add up stresses which result from expanding leaves that are physically in contact with each other. Try to visualize it this way:
You have a stack of 100 sheets of paper. Now, let's pretend that pushing the top sheet 1mm to the right hand side simulates the expansion of that piece of leaf. Ok, 1mm is not a lot. Now, push the second sheet 1mm. Then the third and so on. After 25 sheets have been pushed, the top sheet will be 1 full inch out from the stack. Of course, in the real case the additive effect is not always in the same direction nor are all filler leaves aligned and in perfect and intimate contact with each other. If this were true, the majority of all cigars would split under any condition.
Now, as for how your cigar split.
3. When hollow pipes fail, they split lengthwise. Hollow cylinders that are pressurized experience hoop stress. That is, because of the nature of stresses on a hollow shell, the greatest stress is in the circumferential direction. That's why when tanks and such fail, they will always appear to split from top to bottom. Rarely, if ever, do they split into a top half and a bottom half. Hoop stress failure is also the operative mechanism in the unfortunate medical condition, aortic dissection (such as in Marfan Syndrome).
So, the bottom line is as follows:
1. Taking a cigar from a humid atmosphere to a dry one will immediately begin the drying out process.
2. Drying out is exacerbated by forced air or radiant heating in the direction of the smoker.
3. Thin wrappers will lose humidity relatively more rapidly than thicker wrappers and thus lapse into brittleness.
4. When the wrapper fails, it will either crack and unravel, or in cases where the stresses have substantially built up, split the length of the cigar.
I hope that sort of paints the picture for you. I left out a few more engineering considerations but I think this covers the main effects.
Wilkey
