A method comprising reacting glycerol with acetaldehyde or 1,1-diethoxyethane to produce 1,3-dihydroxyacetone, without using fermentation or direct oxidation of the glycerol is provided. 6 gcm3; a Shore D hardness of 5 to 40; an elongation to break of 100 to 450%; and, a cut rate of 25 to 150 \u03bcmhr; and, wherein the polishing layer has a polishing surface adapted for polishing the substrate. The method entails positioning the logging instrument within the borehole so that the transmitter system and receiver system are disposed in the vicinity of a formation boundary of interest and measuring the azimuthal orientation of the logging instrument. A size of a cross-section of the light guide portion far away from the light emitting body is less than a size of a cross-section of the light guide portion adjacent to the light emitting body.