Investigations on Self-extinction of Incense Sticks

A low-probability but serious self-extinction of smouldering incense sticks is an undesirable characteristic. This paper reports the investigations on its cause and remedy, conducted for an incense stick manufacturer. Measurement of smouldering rate and surface temperature of glowing incense stick tip of several known compositions were made by recording the time required for the propagation of smoldering front and using thermal camera respectively. Several possible explanations like presence of (a) phosphorous, potassium, and sodium related compounds, (b) inadvertent inclusion of inorganic compounds, and others ruled out the role of phosphorous and related compounds, and a simple role of inorganic compounds despite the fact that defective incense sticks invariably contained large fraction of silica (SiO₂) as revealed by scanning electron microscopy/ energy dispersive X-ray diffraction (SEM/EDX). Ultimately the issue was traced to the presence of antigorite that must have got infused into the incense sticks through termite mud into the raw materials. X-ray diffraction (XRD) of incense stick samples confirmed silica is in the form of α-quartz associated with antigorite in all defective samples and also in termite mud infested sample. Thermal studies of the incense sticks using differential thermal analysis (DTA) show endothermic decomposition of defective samples at 550 to 580°C and is also confirmed through differential scanning calorimetry (DSC) which shows an endothermic peak around 576°C corresponding to the endothermic phase transformation temperature of antigorite. It is therefore inferred that the presence of materials like antigorite in combination with α-quartz in incense sticks produce significant endothermal decomposition leading to self-extinction. The primary practical cause has been traced to termite mud infusion into the raw materials used for making the incense sticks.