BORAX: What it is, how to use it, and how to cook it down PROPERLY

Wait for it...wait for it...

I promise you there's a punchline coming in this one, but it takes about 4:50 of set-up to get to the punchline.

What Trent (as previously featured) shows us here is the results of heating borax as a flux in blacksmithing of steel. He shows, initially, what happens when you pour borax straight onto heated steel. The borax bubbles up and doesn't necessarily stick right when it's been poured. Because of that, according to Trent anyway, some blacksmiths will 'cook down' their borax resulting in glassy beads (as previously featured) that can then be pulverized and used as a non-bubbling flux.

I get the feeling that Trent might not exactly see this as a necessary step.

The science of what's happening seems to be pretty straight forward

First off, borax is actually sodium borate (or maybe sodium tetraborate) decahydrate (Na₂B₄O₇·10 H₂O) (source).

Initially, the puffing seems to be from a simple loss of the water of hydration (the ·10 H₂O part of the formula).

Then the borax seems to decompose into sodium metaborate and boric anhydride through this reaction (source & source).

Na₂B₄O₇ ⟶ 2 NaBO₂ + B₂O₃ 

This appears to be the same reaction that causes the creation of the borax glass beads.

When the beads pick up colors, one source says that the color comes from the formation of metallic borate compounds such as Co(BO₂)₂. That's not relevant to this video, though.

Here we just get to have fun seeing Trent rant again.