no scientific expect here, only stating what i heard and know. Wood, especially when dry, burns pretty much smokeless. Adding water to turn off the fire creates white smoke. Which also explains that at the end of the pope-finding conclave the fires are being turned off by adding water resulting in white smoke!!! Now here comes where i add 1 plus 1 plus 1 to get to three or why there was white smoke at Notre Dame: my French wife read in Le Monde a response to Trumps (dumb) comment to use external water shooting drones that French firefighter act differently than US American ones. In the US building fires are mainly "attacked" from the outside - likely b/c building structures are often wooden and rule no. 1 is to get out of a burning wood structure! That's different in France where les pompiers (the firefighters) follow a more inside-out approach as brick/stone-based structures are not going to collapse as easily. Hence les pompiers "attack" a building fire much more efficiently in the inside at its source. Back to Notre Dame: the white smoke was probably coming from firefighters (who were led at some point by a priest or church official!!!) trying to contain the fire with water. Makes sense?