Posted by Allan Lang on October 17, 1998 at 05:26:15:
In Reply to: Re: Copper behaviour posted by Jeremy Kriewaldt on October 14, 1998 at 17:10:41:
Up to now, I had never considered why the smelting failed.
But after digging out my 30 year old TextBook of Mettallurgy (A R Bailey MacMillan 2ed 1964) I think that because of the furnace design and Chemistry of the process the copper would not have oxidised.
I agree that the ore was in the form of pyrites
a) I'm sure Dick would recognise metallic copper
b) Capt Flint identified it as pyrites in Chap 33.
The most likely form to be mistaken for gold is Chalcopyrite, a combination Copper/Iron pyrite (CuFeS2).
In refining of pyrites it is first heated until it melts at about 1000 Celsius to form a molton Iron/Copper/Sulphur "matte".
This is probably the stage Dick reached with the blowpipe, breaking down the brassy-yellow crystalline form to amorphous black sulphides. (Point 1)
Because of the presence Iron and Sulphur the copper itself won't oxidise.
To produce copper, air actually has to be blown into the matte in a furnace.
This first oxidizes the Iron, which goes into the slag, leaving a copper/sulphur mix.
Then the sulphur is oxidized and the copper collects at the bottom of the furnace.
The problem with the S.A.D Mining Co furnace would therefore be too little oxygen, it being consumed by the charcoal before reaching the crucible.
The molten matte in the crucible would remain unchanged no matter how long it was heated.
What they thought was a "few lumps of slag" was probably the remains of the matte. (Point 3)
I think Dick was wrong about uneven heating.
Completely surrounded by hot charcoal, the crucible would be more evenly heated tahn in normal use over a burner.
Despite his denial, John probably broke it when he loaded charcoal into the furnace.
Actually, I think the crucible was broken by authorial decision.
If it hadn't broken, the large lump of solidified matte would have definitely established that it wasn't gold, and ruined the subsequent story plot.