I believe the purpose of coating the crucible with borax is to
separate the molten metal from the silica in the crucible. The borax
does not make the crucible last longer nor does it prevent cracks
from appearing, but it does flow into cracks filling, but not really
bonding them. What does prevent cracks is slow even heating and
cooling of the crucible. As you are using a kiln to melt the metal,
cracking should not be a problem unless you put cold metal into a hot
crucible, or place the crucible on a cold surface after pouring.
What you describe is consistent with using a kiln for melting metal.
The pool of borax at the bottom of your crucible is not a big problem
- the borax will melt and float on top of the molten metal and should
coat the crucible in the process. To coat the crucible more
completely using the kiln, the method you describe sounds like the