If your crucible is nicely covered with borax, you'll not find any debris from earlier meltings.
The purpose of borax is to:
1.Prevend any loss of precious metals into the pores of the crucible
2.Collect the oxide of metals in the melting process.
3.Providing a smooth poor
Exception to this rule is when you melt filings. They like to form lots of little balls.
To my opinion, there is no need for having a wood stick "scraping" into that wanted layer of borax
The borax already does that job for you.
Liquid silver and oxygen like eachother very much and there is nothing wrong with that.
The problem is when the silver cools of liberating the obundancy of oxygen.
Copper like to adsorb this liberated oxygen producing firestain (cupricoxide) and exactly this is not what you like to have in silver. There is nothing you can to remove this firestain other then file/sand/polisch it down and turn the leftovers in for refinning.
Gold is much more forgiving then silver however, having firestain in gold is also possible.
Porosity is not only formed by tomuch heat or tomuch oxygen by the way.
Porosity can be caused by to small sprues and schrinking of the metal.
If you have fine castings using your method, by all means keep on doing it.
It is not up to me to change your way of working.
I only try to explain what the chemicals are doing for you, what the metals are doing when they are in a molten status and explain you briefly the process.