Borax is a natural mineral mined from the earth made of boron, sodium, oxygen and water.
When you add water to elmer's glue the PVA, being unstable, starts to dissolve in the water.
When you add the wet borax, it is slightly acidic, and it reacts with the PVA to crosslink.
This crosslinking causes the guk to undergo an irreversible gelation reaction much like when an egg boils to become a hard boiled egg. It is safe to handle, but hey! let's not eat it...OK?
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