Why do we use only a very small flame to heat the thiele tube when determining the melting point of a crystalline solid?

The Thiele tube which is look like a triangle, used for the determination of melting and boiling point of the solid and liquid sample respectively. The tube consists of high viscous oil. The mouth of the triangle tube consists the thermometer and the sample through a rubber stopper. The tube is always heated in low flame as the heating mechanism of the oil is through convection current process, in which the molecules of the oil heated passes through out the bulk to make a uniform heat of the liquid. The uniform heat of the liquid is necessary to determine the exact melting point of the solid. Also the low heating rate is compulsory so that the rate of convection current be slow. The high heating rate may cause fast convection in the oil and the hot oil can spill out from the tube and may cause an accident.