GB/T 1721-1979 "Determination of Appearance and transparency of varnish, clear oil and thinners" is applicable to the determination of Appearance and transparency of varnish, clear oil, paint and thinners. The standard requires the use of plugged Colorimetric tubes, Colorimetric racks and other equipment for determination, and the transparency of the sample is assessed by observing whether the sample contains mechanical impurities and the degree of turbidity. In the test, reagents such as direct yellow-brown new D3G solution and softener VS solution are used, and standard solutions are prepared according to regulations, so as to compare and measure with the sample. During the determination, if the sample is turbid due to temperature, it can be treated by water bath heating to 50-55 ° C and then chilled down to 25 +/- 1 ° C. This standard specifies the method of measurement of the sample to ensure the accuracy and comparability of the measurement results.