Ceramic backing strip

Ceramic backing is simple: it supports the weld from below so the weld pool doesn't fall through. The ceramic holds the heat and shields the root side from air, so you don't oxidize as much at the root. If the ceramic is damp or not fitted properly, you get porosity or even cracks in the weld. So: clean the plate, stick the ceramic on dry and gap-free, then the root burns through cleanly. After welding, knock off the remaining ceramic and briefly check the root for defects.