Boric acid
Pyrolysis

When heated, orthoboric acid undergoes a three step dehydration. The reported transition temperatures vary substantially from source to source.

When heated above 140 °C, orthoboric acid yields metaboric acid (HBO) with loss of one water molecule:

Heating metaboric acid above about 180 °C eliminates another water molecule forming tetraboric acid, also called pyroboric acid (H B O):

Further heating (to about 530 °C) leads to boron trioxide:
change_case:capital_word_frequency[7]