How You Can Find the Number of Karats in Gold
If you wish to find out if a piece of jewelry is made of gold,
touch it with a glass stopper which has been wetted with nitric
acid. If it is pure gold it will remain untouched and unchanged;
if not, the solution will become blue from the formation of
nitrate of copper. (False "gold" always contains copper.)
To find the number of karats in a gold object is not as easy as
testing for gold content. First, weigh the gold object. Then
break it up and mix it with seven times its weight in silver.
Beat this mixture into thin leaves and then add nitric acid. The
silver and copper will dissolve. Now fuse the remaining gold and
weigh it. Compare the first and last weights: the difference
will be equal to the number of karats of pure gold. Repeat
several times to check.