The
Etruscans made granulated gold jewelry with granules, tiny gold
spheres as small as 1/300th of an inch, attached to a
gold base. It is hard to see how that could be done using solder
without flooding the join and converting a granule to a bump.
The technique they are believed to have used was described, not
for granulation but for delicate filigree, by Theophilus in the
11th century and Cellini in the 16th,
rediscovered and patented as “colloidal hard soldering” by
Littlechild in the 20th century. The idea is to glue
the granules down using a mix of a copper salt or oxide, a flux,
and, for Littlechild but not for Cellini, an organic glue. The
evidence that that is how it was done in classical antiquity is
that “chrysocolla,” the name of a semiprecious gemstone that is
a copper mineral, is Greek for “gold glue.” Pliny uses its name
Heat
in a reducing flame and the copper oxide reduces to a film of
metallic copper between granule and surface. At a temperature a
little below the melting point of the gold or silver the copper
soaks into the surface of the metal of the granule and the
sheet, turning it into a film of copper-gold or copper-silver
alloy, aka gold or silver solder, which fuses together. At a
slightly hotter temperature the whole piece melts, so you have
to remove the torch flame in time. Cellini says to watch for the
skin of the gold to move, which is what I try to do with my
silver.
You
need to use fine silver rather than sterling, or high carat
gold. For doing it with gold, which I have not tried, a modern
source[1]
recommends depletion gilding the surface to get it closer to
pure gold.
To
make the granules you cut your silver or gold in small pieces
and melt them with your torch on a charcoal block — they will
pull up into spheres. Herbert Maryon suggests doing it on
hemispherical dents in the charcoal so as to avoid slight flats
at the bottom of the granule.
Cellini’s
formula,
which I use, is six parts of verdigris (copper carbonate) to one
each of borax and salts of ammonia (sal ammoniac). I have only
experimented a little with adding glue. My modern source uses
hide glue diluted one to twelve in water to glue the granules
down.
The
most frustrating part of the process is when, in either pickling
or polishing the piece, some of the granules that you thought
were fused turn out to be only stuck down by the melted borax
and you have to redo them.
Good
luck.
11th century
granulation
Etruscan pin
and pin head, Metropolitan
Museum
Andalusian
Ring
One Silver
Granule and Video of attaching a granule.
A Granulated Cross: Original My
Recreation
