II have never tried this experiment or even heard of the effect but, if the effect is true then the answer can only be that the sodium salt is acting as a thermal barrier. The molecular structure of the paraffin in the candle is dominated by weakly polar covalent bonds. Covalent molecules will aggregate into a molecular solid that is held together by weak interactions that include hydrogen bonding and VanDerwaal forces. This allows the paraffin to have a relatively low melting point and soft texture. Sodium Chloride, on the other hand, has an ionic structure and it aggregates into a very dense, high melting ionic crystal. Ionic crystals do not generally dissolve into molecular solids but it might be slightly soluble in the hot molten wax. For the wax to melt, the energy of the flame needs to excite the molecules of the wax into motion. The molecules start to rotate, and vibrate until the weak intermolecular bonds in the wax are broken and the wax flows. If the salt dissolves into the molten wax then the ionic compound will dissociate into the constituent ions. These large and massive ions of sodium and chlorine will absorb a certain amount of the radiant energy from the flame which reduces the amount of energy available for the wax. This reduces the number of collisions that occurs in the paraffin and slows down the rate of melting. If the salt does not dissolve then the effect is similar. The sodium chloride barrier will act as an insulation barrier and absorb some of the radiant energy from the flame before it gets to the wax. As an ionic solid, the salt will absorb a great deal of energy and still remain a solid. As the salt warms up to temperature the rate of molten wax production will be slower. However, once the salt heats up to the melting point of the wax, the effect would be nullified. .