I'm just not sure what kind of luck you're going to have melting metal inside a metal "crucible". Having done probably hundreds of melts and pours, castings, ingots, etc. in precious metals, gold& silver(don't know if I've ever poured brass or copper...don't think so.), I think you'd do better to have a proper melting dish, something made of ceramic with a pouring handle. Like this:
https://www.amazon.com/Ceramic-Cruci...e+melting+dish
You'd coat the inside with flux, usually borax which puts a glass-like coating on it. Heat it red hot with a torch, move the torch, sprinkle borax, put the torch back to melt the borax. Do that in differing spots until the whole dish is coated.
Then as suggested, you flux the metal as you melt as well. Start heating the metal with a torch, when it starts to melt, move torch away, sprinkle on a pinch of flux, then direct flame from torch back on the metal. When it is molten you pour it into your.... uhh, you didn't mention what you were using to pour into but typically you'd use an ingot mold. Preheat your mold first before pouring the molten metal in. If the metal forms a vapor lock against a cold mold it can explode and sling molten metal flying in every direction! Don't ask me how I know that. (Exciting day that was!)
Here are a couple of ingot molds:
https://www.amazon.com/Reversible-Ad...rds=ingot+mold
https://www.amazon.com/Ingot-Mold-31...rds=ingot+mold
Heat the mold with a torch until it's hot to the touch, too hot to hold in your hand but not blistering hot. All of this is assuming you have a torch of some type. Oxy-Acetelyne will work as will a large propane tip like some use for forge burners (not the little pipe soldering things with the little bottle) You'll also need some glasses or goggles used for brazing.
https://www.amazon.com/Hobart-770129...razing+glasses
After all that you get to hammer and file for a while to get it useable for bolster material.
Now having said all that, You know, brass sheet for bolsters, or copper for that matter, really isn't that expensive. Expecially when you consider your time, equipment costs, and breathing those fumes.
http://www.knifemaking.com/product-p/cp541.htm
http://www.knifemaking.com/product-p/cpc41.htm