After warming orthoboric corrosive at a temperature of $370 \mathrm{~K}$ or above, it is changed over into metaboric corrosive and, upon additional warming, yields boric oxide $\left(\mathrm{B}_{2} \mathrm{O}_{3}\right)$.

$\mathrm{H}_{3} \mathrm{BO}_{3} \rightarrow \mathrm{HBO}_{2} \rightarrow \mathrm{B}_{2} \mathrm{O}_{3}$