DOI Page / Citation link:
https://doi.org/10.11588/diglit.619#0081
12
NAUKKATIS.
of the scale is estimated at 6m2, the area is 1*3
square inches, the volume therefore 8m; .it -will
then weigh about 10 grains (being half red oxide
and half green carbonate), and of this 3 grains
will be gain by oxygen and carbonic acid; the
original weight therefore was 265*5 of metal. A
copper weight is now 139*9 grains; it has f of
its crust of green carbonate remaining, and f is
lost, and after cracking the remaining scale off, the
loss is 63*2 grains: therefore 42 grains have been
lost. Thus the gain is 28 grains of oxygen and
carbonic acid, and the loss 23 grains of copper.
But it also consists of red oxide to a depth of 85m
on an area of *45 square inch, or 35m volume
= 50 grains, and showing a gain, therefore, of
about 6 grains of oxygen. The whole gain then
is 34 grains, and loss 23, showing the original
weight was 129 grains. The allowance for crust
that has been lost requires particular care; in
many cases a thick coating may have been knocked
off (containing perhaps 10 grains of metal on a
weight of 130) without the appearance calling
attention to it; a careful scrutiny with a magnifier
is indispensable, and some familiarity with the
appearances of partly-cleaned weights, whose loss
may be still measured.
89. In the case of silver coins it is very desirable,
for all examples that are not worn, to ascertain the
original weight when buried. For this purpose
we need to free them from all matter which they
have accumulated, both in chemical combination
and in mechanical addition. The necessity of this
was strongly brought before me by the finds of
tetradrachms in good condition at Naukratis; and
the method devised for them was found to answer
admirably as a means of cleaning silver coins.
First, they are placed each in a separate cup of
weak hydrochloric acid, so long as bubbles of
carbonic acid are given off; this dissolves out
most, or all, of the carbonate of lime, which
usually is encrusted on coins. Secondly, they are
2 m is a convenient abbreviation for a thousandth of an
inch, lineal, square, or cubic.
placed in a strong solution of common salt, with a
piece of clean sheet zinc on either side; this gal-
vanically reduces all the chloride, sulphide, or
other compounds of silver (and any copper) in the
coins to the metallic state : the end of this stage
is shown by gelatinous oxide or carbonate of zinc
being formed in the liquid, and the coins must
now be tenderly handled, as the reduced silver is
often quite loose. Thirdly, they are washed as
soon as the zinc is removed, and placed in acetic
acid so long as any bubbles are formed; this dis-
solves any carbonate of lime fresh exposed, and
carbonate or oxide of zinc, and any scraps of zinc
that might become detached, without any risk
of attacking the finely-divided reduced silver.
Fourthly, they are washed; any silver which has
become detached is placed on the face of the coin
to which it belongs, and all the coins are baked
at well over boiling point. Fifthly, so soon as
they are cool they are weighed, thus ascertaining
the exact weight of the coin when buried. Sixthly,
the scale of reduced silver is then pushed off; very
generally it can be taken off with the finger-nail,
or, at the worst, it flakes off freely with an iron
point without touching the face of the coin. If
obstinate, a touch of strong nitric acid soaks the
porous silver, and loosens it without affecting the
denser silver of the coin below. In case any un-
dissolved carbonate of lime should be found below,
and the weight of that coin in particular is needed,
it should be weighed again; then dissolve the
lime with acetic acid, and weigh again, to find the
amount to be subtracted from the fifth stage
above. It is impracticable to try to collect the
scaled-off silver sufficiently carefully to weigh it;
much of it flies off in powder on brushing the coin.
This process will leave a far smoother face to silver
that is much corroded than the usual dissolving
of the chloride in ammonia, and it enables the
original weight of the coin to be ascertained.
90. The following arrangement of the weights,
attributing them to different standards, is almost
entirely based on their internal evidence, as there
NAUKKATIS.
of the scale is estimated at 6m2, the area is 1*3
square inches, the volume therefore 8m; .it -will
then weigh about 10 grains (being half red oxide
and half green carbonate), and of this 3 grains
will be gain by oxygen and carbonic acid; the
original weight therefore was 265*5 of metal. A
copper weight is now 139*9 grains; it has f of
its crust of green carbonate remaining, and f is
lost, and after cracking the remaining scale off, the
loss is 63*2 grains: therefore 42 grains have been
lost. Thus the gain is 28 grains of oxygen and
carbonic acid, and the loss 23 grains of copper.
But it also consists of red oxide to a depth of 85m
on an area of *45 square inch, or 35m volume
= 50 grains, and showing a gain, therefore, of
about 6 grains of oxygen. The whole gain then
is 34 grains, and loss 23, showing the original
weight was 129 grains. The allowance for crust
that has been lost requires particular care; in
many cases a thick coating may have been knocked
off (containing perhaps 10 grains of metal on a
weight of 130) without the appearance calling
attention to it; a careful scrutiny with a magnifier
is indispensable, and some familiarity with the
appearances of partly-cleaned weights, whose loss
may be still measured.
89. In the case of silver coins it is very desirable,
for all examples that are not worn, to ascertain the
original weight when buried. For this purpose
we need to free them from all matter which they
have accumulated, both in chemical combination
and in mechanical addition. The necessity of this
was strongly brought before me by the finds of
tetradrachms in good condition at Naukratis; and
the method devised for them was found to answer
admirably as a means of cleaning silver coins.
First, they are placed each in a separate cup of
weak hydrochloric acid, so long as bubbles of
carbonic acid are given off; this dissolves out
most, or all, of the carbonate of lime, which
usually is encrusted on coins. Secondly, they are
2 m is a convenient abbreviation for a thousandth of an
inch, lineal, square, or cubic.
placed in a strong solution of common salt, with a
piece of clean sheet zinc on either side; this gal-
vanically reduces all the chloride, sulphide, or
other compounds of silver (and any copper) in the
coins to the metallic state : the end of this stage
is shown by gelatinous oxide or carbonate of zinc
being formed in the liquid, and the coins must
now be tenderly handled, as the reduced silver is
often quite loose. Thirdly, they are washed as
soon as the zinc is removed, and placed in acetic
acid so long as any bubbles are formed; this dis-
solves any carbonate of lime fresh exposed, and
carbonate or oxide of zinc, and any scraps of zinc
that might become detached, without any risk
of attacking the finely-divided reduced silver.
Fourthly, they are washed; any silver which has
become detached is placed on the face of the coin
to which it belongs, and all the coins are baked
at well over boiling point. Fifthly, so soon as
they are cool they are weighed, thus ascertaining
the exact weight of the coin when buried. Sixthly,
the scale of reduced silver is then pushed off; very
generally it can be taken off with the finger-nail,
or, at the worst, it flakes off freely with an iron
point without touching the face of the coin. If
obstinate, a touch of strong nitric acid soaks the
porous silver, and loosens it without affecting the
denser silver of the coin below. In case any un-
dissolved carbonate of lime should be found below,
and the weight of that coin in particular is needed,
it should be weighed again; then dissolve the
lime with acetic acid, and weigh again, to find the
amount to be subtracted from the fifth stage
above. It is impracticable to try to collect the
scaled-off silver sufficiently carefully to weigh it;
much of it flies off in powder on brushing the coin.
This process will leave a far smoother face to silver
that is much corroded than the usual dissolving
of the chloride in ammonia, and it enables the
original weight of the coin to be ascertained.
90. The following arrangement of the weights,
attributing them to different standards, is almost
entirely based on their internal evidence, as there