Tubes of fused quartz or fused silica classify under 700231 regardless of expansion coefficient. For other glass tubes, those with a linear thermal expansion coefficient not exceeding 5×10⁻⁶ per kelvin (measured at 0–300 °C) classify under 700232 — this captures borosilicate and similar low-expansion glasses used in laboratory and pharmaceutical applications. All other glass tubes fall under 700239.

This heading covers only unworked tubes — cut to length but otherwise not further processed. Once a tube is bent, fire-polished at the ends, coated, or assembled into a device, it typically moves to a more specific heading (for example, laboratory glassware or lighting equipment). The 'unworked' condition is the key classification criterion.