  We show how the morphological analysis of the maps of the secondary CMB
anisotropies can detect an extended period of ``smoldering'' reionization,
during which the universe remains partially ionized. Neither radio observations
of the redshifted 21cm line nor IR observations of the redshifted Lyman-alpha
forest will be able to detect such a period. The most sensitive to this kind of
non-gaussianity parameters are the number of regions in the excursion set, the
perimeter of the excursion set, and the genus of the largest (by area) region.
For example, if the universe reionized fully at z=6, but maintained about 1/3
mean ionized fraction since z=20, then a 2 arcmin map with 500x500 pixel
resolution and a signal-to-noise ratio S/N=1/2 allows to detect the
non-gaussianity due to reionization with better than 99% confidence level.
