It's not working. The tube, whether made of ceramic or quartz glass, is damaged beyond repair and must be replaced.

What's the tube material and at what temperature the glass molten? You can try remove the tube and heat it in vertical position at temperature above the glass melting point. Hope it helps...

The tube is supposed to be made of quartz and melted at a temperature of 800 degrees

Samah Dafalla For quartz based system, it will be difficult as it may have reacted chemically at 800 C. Try using HF acid to dissolve the glassy phase but take care as quartz is also soluble in HF. If it doesn't work, try the heating method above 800 C. Hope it works for you..