The white colour indicates the onset of devitrification. Initially, devitrification will not degrade the performance, but eventually the devitrified area (with micro-fine cracks) will extend over most of the tube at which point it may be necessary to discard the tube. If devitrification is excessive, the tube will eventually crack spontaneously or fail through thermal shock. In some applications, where quantities of alkali elements are present in the samples, the devitrification process is accelerated. The alkali atoms migrate into the quartz tube and form silicates. These silicates then crystallize when the tube is cooled, causing devitrification. Be sure fingerprints are washed off with alcohol prior to use as contaminants will promote devitrification.