Why is it that incense sticks emit smoke only after the flame is extinguished

Incense sticks are made up of a woody powder mixed with a binder and a scent. To light an incense stick, the tip is brought in contact to a flame which in turn raises the temperature locally to burn the material nearby.

Following this, the flame is put off but the heat generated earlier maintains the smouldering tip. The woody base material keeps the stick burning when the combustion products are emitted.

The scent is usually in the form of an organic solution consisting of a base organic liquid into which the good-smelling substance is dissolved. This organic material aids the prolonged burning process.

During burning, the organic material evaporates and is seen in the form of the white smoke which while diffusing in the air, emits a smell experienced by the people around. This happens at the temperature of the smouldering tip of the incense stick. This temperature is considerably lower than that of the flame.

But if a higher temperature is attained, the organic matter including the scent would burn to form the flame and would produce mainly ash, water, oxides of carbon and some carbon soot rather than the white smoke.

Thus, as long as the flame exists, the smoke forming vapour burns out and no smoke is seen. But after the flame is put off, the temperature of the hot tip happens to be just enough for vaporising the scent solution into a white smoke.