Heat it until it reaches the eutectic melting point. Sorry, I must have missed the point of the question, why would melting zinc & copper be any different from melting lead ?
With the melting point of copper being 1083 and zinc having a boiling point of 907, I wanted to know the best way to melt the 2 together to form an alloy.
Make sure you understand zinc's volatility and take proper precautions. Zinc generates fumes well below its boiling point. Adding zinc means you will be generating hazardous fumes in molten metal. It is nothing that can’t be managed but best if it doesn’t catch you by surprise.
If you put the two metals together in the crucible and heat it, you will find liquid metal forming below the melting point. This will be the eutectic composition.
There is no reason to fully melt the Cu first. You would be better off if you can melt a small amount of brass to start with. You will keep the melt temp lower and loose less metal.
Don't underestimate the Zn fumes. You don't want to be breathing this.
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Rust never sleeps
Neither should your protection
Best to use brass scrap or master alloys (if available).
Starting materials should be small pieces & mixed to enable rapid alloying to lower the zinc activity & vaporization.
Preheat the melting pot or crucible (a gas flame works), load the charge, and cover with an inch or so of borax. Acts as a flux, holds the zinc vapor in, and is inexpensive.
w/o some premixing of the solid pieces and the use of flux, zinc losses are high:
"The appropriate amount of suitable copper alloy scrap is weighed and transferred into an electric furnace where it is melted at about 1,920°F (1,050°C). After adjusting for the amount of zinc in the scrap alloy, an appropriate amount of zinc is added after the copper melts. A small amount of additional zinc, about 50% of the total zinc* required, may be added to compensate for any zinc that vaporizes during the melting operation..."
My be a stupid question but I'll ask anyway, am I wasting my time by trying to melt the copper with a blow torch?
I have a small amount of copper scrap (very small pieces) and I want to melt the copper down but cannot afford a kiln/furnace and only have a propane blow torch and I have heated the copper in a metal dish for a good few minutes and it goes orange but no further.
Orange color is maybe 750 oC whilst copper melts at 1084.6 oC. You will need welder's goggles to observe. For Cu-25%Zn, about 100 oC lower.
What metal do you have that may be inert toward copper? Graphite or the cheaper clay-graphite may be better. Use a crucible with a lid to hold in heat & zinc vapor (along with the flux).
swall's answer is most practical.
It takes some investment, and more heat than from a handheld propane burner, to melt copper alloys. The least expensive set-up:
And, what is the point of simply melting the alloy? To cast white hot liquid metal w/o proper equipment & experience may lead to the emergency room of the local hospital.
Definitely not something to screw with unless you intend to put the effort into doing it safely. Mind you, people have been doing this for thousands of years- that's what got me interested in it (hence my moniker on this forum)
The picture looks remarkably like my own set-up, except I have a 12x12 hard firebrick tile as a hearth/base, my burner is driven by a small blower, and the insulating firebrick box is wrapped in a batt of mineral fibre insulation.
The trick is to have a proper crucible and a safe means to lift it out of the box- don't bother even trying to do this in a "metal dish".
The borax flux is essential not only to keep the zinc losses down but also to flux (dissolve) the oxides from the scrap you're melting. It's a bit tricky to skim the borax off the melt prior to pouring- still figuring that one out.
I've got a Graphite crucible and the tongs to remove and pour, I am getting some firebricks tomorrow to build the kiln and next month I am going to buy a proper kiln.
My boss has given me a plaster paris (gypsum) mould and said the brass (around 850oc) would be able to cast in that but I am a bit skeptical.
You need to remove moisture from the mold before using. Slowly heat & bake the plaster-of-Paris mold at 200 oC. Plaster-of-Paris as cast is CaSO4.2H2O. Drying below 160 oC removes 3/4 of the water, then final drying at 160-200 oC removes the remaining water.
Foundries have special plaster mixes that are more permeable than plaster-of-Paris; this lets air escape from the mold better during filling.
Melting point of brass depending on chemistry is 900-940C. you need to superheat this by 50C atleast. You will then be in the temperature range of 1000-1020C. So please check if you are adequately equipped to handle molten metal at this temperature.
Any presence of moisture in the mold can be disastrous.
" All that is necessary for triumph of evil is that good men do nothing".
Edmund Burke
Heed kenvlach and arunmrao's advice: a wet mould will very dangerous.
I do all my castings in sand- roughly 20:3 mix of silica sand and bentonine (or ground cat litter), moistened gently to the point where a clump of sand squeezed in the hand can be broken cleanly in half without crumbling.
