Consider the two pictures below.
Black gunk buildup on atomizer builds is evidence that something is being reduced to carbon. In other words dangerous by-products were being produced. Even though the number of and amount of by-products is lower than what is in analog cigarette smoke, black gunk is not good. Black gunk equates to risk. I do not accept this as normal and have endeavored to eliminate it. The objective is to minimize or even eliminate black gunk buildup. I have done this by applying several techniques described in Temperature Control.
In these two examples it is something in the eLiquid. Neither has a burnt wick. The one on the left was a chocolate flavoring. The one of the right was used on a VW i.e. non-TC mod. Something in chocolate is decomposing even at 360F on a TC mod and was absorbed by the wick. While the one on the right did not taste bad, the coil is gunked up. Both are a sign that a rebuild (new coil and wick) is necessary and signs of something decomposing.
The eLiquid in the wick is protecting the cellulose from decomposing. Here is a cellulose test that I performed.
Just one drop of eLiquid on the wick so it is damp (right picture above), not even saturated, elevates the temperature at which the cellulose wicking starts decomposing. Yes 10s is an abnormally long puff duration. The objective of the test is to find the temperature not the duration. Longer run times affect more of the wicking for visually identifying the presence of the decomposition process at a given temperature. In other words I don't need a microscope to see the decomposition with a short run time.
All the information contained in these pages are only the opinions of the author and the author is not an expert at anything.