Russian has two words that are defined as ‘ash(es)’: зола [zolá] and пепел [pepel]. It recently occurred to me that I didn’t really know what the difference was, so I turned to Anatoly, saying in an e-mail: “I thought the difference might be that зола was in larger clumps, perhaps preserving a bit of the wood structure, but no, as far as I can tell through googling they’re both the gray, dustlike substance left after complete burning. Is зола used more for wood and пепел for cigarettes? Is there some other contextual difference in usage?” He wrote back that he had consulted with his wife and they had decided that “the meanings definitely overlap, but in most situations there is a clear choice of a term to use, thanks to idiomatic phrases”; he agreed that зола was “possibly (but, I would say, not necessarily) the more complex object of the two, retaining some structure, color or flakiness.” But he too felt uncertain, and said he’d ask the readers of his blog. Well, he’s done so, and the results are most interesting. It looks to me like there are basically two schools of thought among those who think there is a difference. One was expressed by dmpogo, who wrote “зола – это не совсем до конца прогоревшая древесина … А пепел – уже совсем до конца” ['зола is wood that hasn't completely burned, while пепел is burned to the end']; this was backed up by someone who agreed and added “зола еще ‘хранит’ огонь, а пепел уже нет; золу можно раздуть до огня, а пепел – нельзя” ['зола still 'holds' the fire, but пепел doesn't; you can blow зола into flame, but not пепел']. The other popular view was very different; in the words of cartesius, “пепел – то что сохраняет частично форму сгоревшего предмета: сигареты или бумаги, а зола -пылеобразная, тот же пепел от бумаги, если его растолочь в бесформеную массу” ['пепел is that which keeps in part the form of the burned substance: cigarettes or papers, while зола is like dust, that same пепел from paper, if you crush it into a formless mass'].
It’s very interesting to me that native speakers can disagree so completely about the meaning and use of such common words, and of course it was pleasing to learn that my own confusion was not due to my being a foreigner!
(Incidentally, зола is not a homonym of the name of the writer Emile Zola, because in Russian that gets palatalized to Эмиль Золя [emíl' zolyá].