I just use silver flux, but from what I hear of brass, the glassy hard stuff is non-water-soluble. So just be sure to ask your supplier for the water soluble stuff, or buy the stuff sold by Henry James, or other bike suppliers since it will all be water soluble. The stuff that requires chipping off can lead to damaged tubes, and is a royal pain. Just in case you are using that stuff on something other than a bike, they make a useful air tool that fires out a hedgehog's worth of spikes for rattling off the flux.
If you have the right flux, then you just need enough water contact. Something like a wheelbarrow, or a tarp supported on a frame so you can get the whole frame wet long enough to fully loosen the flux. Take some of your glassy flux chips or a small part and immerse in warm water for an hour or so and see if the flux softens, if it doesn't you need to change your flux.