Hi phung chu,
I changed the name of the topic slightly, but this may answer your
question:
The quickest way is to ask an experienced jeweller to sort them out.
He or she will be able to tell just by looking at the solders and by
the way they melt. They will also tell you if it is worth the trouble
in the first place. “If in doubt, throw it out” applies specially to
solders.
If you have the time and/or there is a lot of solder, then try this
approach:
-
Sort out the silver, yellow gold, and white gold into different
groups. 18ct Yelllow gold is very yellow, 9ct is pale yellow or
pinkish. (I don’t know about 14ct, it is an odd alloy in Australia
and I use 18ct solder on the 14ct and 15ct alloys). If you are not
sure which is silver or white gold, then test for silver using a drop
of nitric acid. Silver will react, white gold will not. -
Do a melt test using a small snippet from each sample in the
group. Arrange the snippets on a small sheet of metal so that you
know which piece the snippet came from. Apply flux, make sure that
all the snippets are in contact with the sheet of metal, then heat
the sheet of metal evenly all over and see which snippet melts
first, second, and so on. If using a flame to heat the metal then the
accuracy of the test depends on heating the whole sheet equally, and
is best done by heating from under the sheet. -
Easy solder will melt first and spread around the most. Hard
solder will melt last and only spread a little at a much higher
temperature.
If you have known samples of solder, then do the above test to
compare the unknown snippets to the known snippets. The snippets that
melt in the same way as the known samples are the same as the known
samples.
Cheers, Alastair