Plenty of smoke and black char, but no glowing ember. Moisture is driven from the wood. Very little wood is worn away. Temperature is probably less than 400 degrees Fahrenheit. 2. The wood starts to decompose and char.
Smoke starts coming from the wood.
The wood is being worn away. The temperature is beginning to rise from 400 to 700 degrees. The temperature of the fine wood particles being abraded off the junction between spindle and hearthboard rises to above 700 degrees. Ignition takes place if the particles are fine enough. Ignition will not happen until the temperature gets up to a critical level and the critical level depends on the fineness of the wood particles.