On Tue, 05 Jun 2007 09:10:53 -0700, Doc wrote:

When soldering copper pipe, what does the flux do to cause the solder
to get drawn into the joint? Something to do with capillary action?

What he said. It keeps the copper from oxidizing, or 'rust'ing. And wen
you add the heat it would oxidize faster. Solder does not take to copper
oxide as I understand it. so flux helps to keep the weld area clean.