This reaction appears to be destroying the logs. when it is over only ash will remain, the law of conservation of mass tells us no mass is lost. how can you explain this?

The law of conservation tells us that no mass is lost. This is true because when wood burns or turns into ash, some of it is escaped as gases like carbon dioxide and water vapor into air . If we calculate the mass of these atmospheric gases that evolved when the wood turned into ash, we would find that the total mass of all these substances including the ash is equal to the log of wood.