1) Any ideas how I messed up?
You likely didn’t. Prips should be made up so it’s fully in
solution. All you need do is add a bit more water.
From your (mis)spelling of Prips flux, with 3 "p"s, I’m going to
guess you found the formula to mix it, in the orchid archives,
perhaps from an older posting I might have put in. I say that
because I recall making that typo once or twice, and it seems to have
become somewhat entrenched. (John (“Jack”) Prip spelled his name with
only two Ps. one at the front, and one at the end. ) Anyway, in some
of those first messages where I posted a formula for mixing Prips
flux, I specified an amount of water that was a bit short, with the
idea that one would need to add just a little more to keep it fully
dissolved, resulting in saturated solution. Apparently I didn’t make
that clear, or got those numbers wrong, since the formula I recall
posting (not sure where I got it, but no matter) does indeed not
have enough water, so when the mix cools, a bunch of it drops back
out of solution and crystalizes.
The fix is simply to add more water. Heat it up again til it all
goes back into solution, and if it still crystalizes out on cooling,
well, add more still. The exact concentration is not at all important
beyond changing the amount of total liquid you have to spray to get
good flux coverage. The only important thing is the ratio of the
three componants to each other. 3 parts boric acid to 2 parts each of
borax and TSP. Some school programs routinely mix it up at a quarter
the concentration (a gallon of water instead of a quart) simply
because students tend to use too much, so making it dilute reduces
the waste. Personally, I like it as concentrated as possible to
reduce extra effort in spraying.
Racking very old brain cells, I know that adding a dry crystal
into a supersaturated solution can do this, which I don't think I
had as I put the lid on the pot to wash down the sides while
boiling. Where I do think I may have screwed up is having recently
washed the jar and there still being a drop or two of water in the
bottom when I poured in the cooled solution. Also, while very
unlikely, it's not 100% impossible I miscounted the 6T Boric acid.
You’re overthinking it. The formula you followed likely didn’t
specify quite enough water, so on cooling, some of the chemical
simply crystalized back out. Nothing special about that. I seem to
recall those old posts said a mix of 120 grams boric acid, and 80
grams each borax and TSP, in a quart of water. Easy to remember
numbers, but not enough water. For a better and still easy to
remember formula Instead, try 90 grams boric acid and 60 grams of
each of the others, in a LITER of water (that’s more than a quart).
That formula will fully dissolve first time out, and is reasonably
close to saturated. At least it works for me. If for some reason you
still get crystals remaining, well, guess what… Add more water.
If by chance the formula was not from one of my posts, well, then
two comments. First is that Jack Prip spelled his name without two
p’s at the end. Just one. The second is that the fix is the same. Add
more water.
2) Best ways to remedy?
add more water. 
(broken record, huh?)
Oh, and don’t worry too much about it, OK? So long as all the
chemical is dissolved, it’s pretty much impossible to ruin this.
Making Jello for desert is more complex (there, too much water really
will mess it up.)
Jar is sturdy but questionably tempered--kitchen storage not lab
gear--and the old-fashioned style with wire binding on the lip.
Have a double boiler, and assorted sizes of steamer insert pots but
am trying to keep kitchen and chem gear separate so hopefully I can
manage this with a smaller pot like the one I used for boiling the
flux.
I keep the mixed stuff in a liter sized plastic bottle,. it looks
much like one of those nalgene labware bottles, and might well be,
but the printing left over on the bottle betrays the fact that when I
first bought that bottle, it held shampoo from the dollar store… I
use this bottle because I had it, and it’s cheap, and if you drop it,
unlike your glass jar, it won’t break. And the lid seals well. And
did I mention, it was cheap?
For actually applying/spraying the flux, I use a smaller plastic
bottle, nicely hand sized, who’s cap has a hole to fit the feed tube
of the mouth atomizer type sprayer I use. Again, it’s some plastic
product bottle I salvaged from somewhere and repurposed to the
task…
As to boiling the water to dissolve the chemicals, don’t freak out
too much about chem safety and all. The borax is laundry detergent,
the TSP is the major componant in dishwasher powder detergent, and
boric acid, while not a detergent, is in the same class. You don’t
want to drink it, but paranoia isn’t required here. just wash and
rinse the saucepan you use when you’re done. This is not like the
sorts of caution you might need with, say, electroplating chemicals
or even sparex pickle, which is quite caustic. These things simply
aren’t. Well, the TSP if mixed strongly is a strong alkali, but
again, this isn’t highly toxic stuff. Just clean up after yourself.
Do note that the above somewhat cavalier sounding advice is limited
to mixing Prips flux. It is not general advice for any other
chemicals one might encounter. It’s just that these three don’t pose
a threat so long as you are able to do the dishes when done to the
same standard you’d use after cooking dinner. Two of the three
chemicals might even be in your dishwasher detergent anyway…
(though the borax is more common in laundry detergent…)
Don’t worry too much about exact procedure. This isn’t rocket
science. All you need to do is dissolve those chemicals in a
sufficient amount of water so they stay in solution when it cools.
Even the heating bit is just to speed it up. With a bit of time, the
correct amounts of chemical would dissolve more slowly if just added
to cold water and allowed to sit. There are no actual chemical
reactions or the like going on. It’s just like stirring some sugar
into your tea. Nothing more complicated than that. (except the flux
doesn’t taste so good, and you probably shouldn’t drink it.)
Hope that helps.
Peter Rowe