[ramchip]

I would guess there's more than borax in it:

> Traditionally borax was used as a flux for brazing, but there are now many different fluxes available, often using active chemicals such as fluorides[9] as well as wetting agents. Many of these chemicals are toxic and due care should be taken during their use.

https://en.wikipedia.org/wiki/Flux_(metallurgy)#Brazing_and_...