Borax on strong heating loses its water of crystallization and then shrinks forming a

transparent glassy bead of sodium metaborate $\left(\mathrm{N} \mathrm{aB} \mathrm{O}_{2}\right)$ and boric anhydride $\left(\mathrm{B}_{2} \mathrm{O}_{3}\right)$ as shown

in reaction given below:

$\mathrm{Na}_{2} \mathrm{B}_{4} \mathrm{O}_{7} \cdot 10 \mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{Na}_{2} \mathrm{B}_{4} \mathrm{O}_{7}+10 \mathrm{H}_{2} \mathrm{O}$

$\mathrm{Na}_{2} \mathrm{B}_{4} \mathrm{O}_{7} \rightarrow 2 \mathrm{NaBO}_{2}+\mathrm{B}_{2} \mathrm{O}_{3}$

Boric anhydride, being non-volatile displaces more volatile acidic oxides and combine with basic oxides present to form metaborates which are identified through their characteristic colours.