That sounds like your mess tin oxydized. Maybe that tin or the the water had acids in it. It isnīt directly related to the use of the esbit stove. Black residue on a cloth after cleaning aluminium without anodized surface is quite common.
Some people assume that there might probably be some possibility that the aluminium that is disolved in the food could possibly be related to a vague chance of developping Alzheimerīs. Their evidence however barely supports the firmness of this statement. <img src="images/graemlins/wink.gif" alt="" />
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If it isnīt broken, it doesnīt have enough features yet.