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The E-Sylum:  Volume 9, Number 45, November 5, 2006, Article 23

ON THE USE OF LEAD IN ELECTORFORMING

Ed Krivoniak writes: "I have to take exception to Dick Johnson's
comment that lead does not conduct electricity. It DOES! If it did
not, your car would not have a lead acid storage battery in it whose
terminals are always lead. All metals conduct electricity! That is
the definition of a metal."

Dan Demeo writes: "Dick Johnson's comments regarding electrotypes
raised some hairs on the back of my neck.  As a retired chemist, I
recognize electrical conductivity (or low resistivity) as a necessary
characteristic of a metal.  Lead, as listed in my 25 year-old Handbook
of Chemistry and Physics, has a resistivity of 20.648 microhm-cm,
compared to copper, 1.6730 in the same units.  This makes lead some
12-13 times more resistive (less conductive) than copper, but
conductive nevertheless.

Non-metallic elements have resistivities several orders of magnitude
higher than this.  Some of the problems in using lead as a conductor
could relate to the oxides which form on its surface, making contact
difficult.

As a collector of both early U.S. and ancient coins, I have always
heard of lead in conjunction with electrotypes, but generally as the
meat in the sandwich, copper surfaces or shells for example, with a
lead core.

I have never heard whether the British Museum and others actually
used lead to join and fill their two thin copper or silver "faces",
but, given the relatively high melting point and low wettability by
lead, I would think they would have used lower melting alloys, perhaps
with lead as an ingredient, but also with tin, antimony, etc.
Differences in the "meat" layer might be useful in telling a BM
electrotype from others, for example.

I do know the British Museum and others developed excellent casting
and electrotyping techniques, but I have never seen this totally
explained.

I do know that many of the early museum and auction catalogs had
illustrations of casts of coins, rather than the coins themselves;
reflections and shadows are more easily controlled when photographing
an object with a matte surface, rather than a shiny metal object.
This must have taken a large amount of resources, especially for an
auction catalog."

Dick Johnson writes: "I received several comments to the item in
last week's E-Sylum on the fact electrotypes cannot be made of lead.
One of the best replies came from Daniel Demeo, a retired chemist.
Dan was correct in several of his statements including this one.
You can find lead INSIDE an electrotype, or as Dan said, the meat,
the internal composition between copper shells.

But collectors are incorrect when they often think the item was
cast or otherwise formed in lead first with the copper coated
afterwards. (Such a technique would be very indistinct and would
not have sharp detail of a struck piece or an electrotype.)  It
is just the opposite -- the copper shell electrotypes are made
first. Then the lead is filled in to make the item solid.

Lead is used for several reasons. It makes the item solid, or for
larger electrogalvanic casts, as galvano plaques, the lead is applied
to the back to add strength to the thin shells (that are often only
1/16th of an inch thick). The lead is always applied on galvanos to
the low points on the reverse because these would be the highpoints
on the obverse and most susceptible to damage (as a nose on a relief
portrait). Another reason is that lead is less costly than any metal
in which electroforms are made, copper, silver, gold.

To make a coin electrotype you must make two shells, one of each side.
The side with the greatest cavity is placed face down on a level surface
and molten lead is poured in minute amounts until it reaches the surface
of the rim of this shell. A tad bid more is added but not to run over.
It will "dome" up because of the meniscus characteristic of lead. The
other shell is "floated" on top of the lead. No air pockets must be
allowed between the lead and the shell. Placement of second side must
be in correct orientation to the other side or you will have a
"rotated reverse" mint error. Once the lead solidifies it becomes
a solid item.

When such items are cataloged in numismatics the correct term to use
is "lead fill-in." A diagnostic may (or may not) exist of a gray lead
color line around the center perimeter of the edge where the two shells
are joined. The edge is buffed and polished to eliminate the seam
(not always successful).

Interestingly, I have come across similar items made by embossing in
cheap imitation of electroforming. The two embossed shells were used
with an added "fill-in," not of lead, but of sand! How cheap can you
get?  I called this "ballast" in my catalog description."

  Wayne Homren, Editor

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