The cobbler’s children have no shoes. Early in my career I unknowingly vacuumed up some smoldering embers into my metal industrial vacuum cleaner. Three days later the fire department was spraying down the back of my truck while I was at the theatre watching a movie. The Rayon filter of the vacuum cleaner had caught fire. Embers can smolder in ash for a long time. Before man knew how to start fire, he would carry a pouch of ash with embers from the previous fire buried in the ash. He or she would revive the ember by blowing on it . This was then used to start the next fire. Here is a link to some good data on handling ashes (Thanks City of Portland):

 

https://www.portlandoregon.gov/fire/article/387766