On Sat, 08 Nov 2008 16:21:49 -0500, Ed Huntress wrote:

"LLBrown" wrote in message
...
Here is what goes on.... I bend a thin wall brass tube to shape then I
have to melt the bending medium out of the pipe with a torch. This
causes a black scaling to show up on the outside of the pipe. To
remove this I dip the pipe in a 50/50 mix of muratic acid and water.
This in turn plates the pipe with very thin copper that I have to buff
off.

I know the acid is leaching copper and plating later pipes but what is
the science behind this? Why does it plate?

LLB in Laredo

It isn't plating. It's de-zincifying the brass. Muriatic eats zinc fast
and leaves a porous copper mess behind, which is very weak. When you
buffed it, you polished right through the porous copper again and got
down to the parent metal.

Apparently much of the "Aztec Gold" that the Spanish went chasing after
was naturally occurring copper-silver-gold alloy that they made their
objects out of, then soaked in a suitable solvent (one such "suitable
solvent" being, IIRC, **** that had been left in the bowl until it was
smelly -- I don't know if I'm _that_ dedicated of a metalworker).

The solvent would dissolve the copper and silver, leaving porous gold
behind. Then the craftsman would beat on the thing to crush the gold
into a nice solid-looking layer.

This is all recollection of an article read a looooong time ago, so if
someone has a more accurate reference share it, by all means.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html