Wood-fired pieces are one of a kind. Unglazed when they go into the kiln, woodfired pieces get covered in ash in earlier stages of firing. This ash eventually melts at higher temperatures in the later stages of firing, which in turn solidifies into a glaze like coating when cooled. That's why it's also sometimes called "ash glaze".
The wood-firing process requires 24-hour involvement throughout the entire period of firing. Peak Temperatures achieved during the process is often in excess of 1250 degrees Celsius. Despite the laboriousness of the process, ceramists all over persist with wood-firing as it produces pieces that simply cannot be produced in electric or gas kilns.