This is highly confusing to me. Firstly, what is "19 or 20k
White gold solders are notorious for being less resistant to the
action of polishing compounds than the white golds themselves, so
sizing seams tend to polish out, leaving a distinct indented line.
the 19 or 20K white weld solders are indeed higher karat, but
formulated so that although they melt just barely below the 14K or
18K white golds, they've got good color match, and are hard enough to
resist polishing on a par with the gold, so the seams then look a lot
better than with lower karat solders. They're called "weld" grades
because they melt just a little bit before the golds themselves, and
using them is almost like just fusing a joint without solder. Not so
useful for very delicate assemblies, unless you've got a great deal
Also, I thought that only lower karat golds could be used as
solders on a given object.
That is true IF the "solder" you are using is simply a standard
gold alloy, such that the only difference between the gold you're
assembling and the "solder" is that the solder has less gold. That
serves to lower the melting point, usually, so it's then usable as a
solder. But it's not a good one, many times, since normal gold
alloys aren't formulated for the same melting and flowing
characteristics as solders.
Solders work because either the proportions of the componants of the
alloys are different from the parent alloy being soldered, or the
solder has different or additional metals in the alloy, to lower the
melting point of the solder. Most gold solders today used fall into
two classes, the repair grade solders, and the 'plumb" solders.
Repair grade solders are indeed often (but not always) lower in gold
content than the alloy they're intended for. This sometimes allows
better color match, but usually it's done to lower the flow temps
even more, or otherwise make the solder easier to use. The "Plumb"
solders are just that. they contain the same amount of gold as the
alloys they're used with, and the lower melting point is achieved by
varying the composition of the other alloying metals without
changing the gold content. In some cases, Cadmium or zinc is added,
in others, well, other stuff is added, and usually the ratios of
silver to copper are played with too, to lower the melting point.
These sometimes, especially with easy grades, don't match the color
as well as one could wish, but they are full karat, making
hallmarking/stamping possible. For this reason, most new jewelery
is assembled with "plumb" solders. The "weld" high karat grades of
white gold solders are special cases, formulated not just for flow
temperature and color, but also for hardness, which is seldom a
probem with yellow gold solders.