Hi David and David-
I have been in learning mode for a long time, and generally just lurking about, but since getting into castings, I thought it best to get out into the world and compare notes with others making their own engine castings.
Brass seems like it has to be watched to prevent overheating.
My first brass pour was a bit of a surprise when the fumes began burning when I opened the furnace and the fresh air met with the fumes.
I had been monitoring the temperature with a pyrometer, but was not sure exactly at what point to pour.
There seems to be a fine line between getting the melt hot enough to pour, but not so hot as to start fume combustion, and not so hot as to start boiling off all the zinc.
The fumes are pretty bad, watch out for those.
I held the temperature slightly lower on the next brass melt, and did not have much trouble with burning fumes on that pour.
Edit:
I was confused about brass and bronze for a long time.
Brass is generally an alloy of copper and zinc, and bronze is generally an alloy of copper and tin.
There are various grades and mixtures for brasses and bronzes, and some have a mixture of copper and both tin and zinc, but generally either the larger percentage of either zinc or tin will determine the characteristics of the metal.
Brass is more tricky to melt and pour due to the fact that the zinc can boil off first if the mix is overheated.
Bronze is more forgiving since the tin is not prone to boil off like zinc, but there are different grades of bronze, and one of the more popular grades of bronze for casting sculptures has a very low machinability.
For model work, expecially engines, I look at the machinability rating first and foremost, and try to keep it in the 80-100% range for any metal I use.
Cast iron is very machinable, and my favorite as far as machinability and use for engines.
Bronze, especially bronzes with a small amount of lead make good bearing material, but be careful to avoid the fumes from leaded metals, and don't handle the chips from leaded metal, or breath dust from grinding/sanding leaded metals.
Brass does not hold up well for bearings in my opinion, it is too soft.
Brass does give that bright yellow look, and can be polished to a miirror finish.
I don't like to machine brass since it is terrible about grabbing the tool bit, usually right near the end of the machining work, and throwing the piece across the room.
Bronze has more of a red tint.
Cast iron can also be polished to a mirror finish very easily, and generally machines to an excelent finish with a sharp tool bit.