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Lost lead casting


#1

I have been studying about early Medieval metalworking and jewelry,
particularly Insular jewelry. One thing that keeps popping up is the
speculation that lead models may have been used like wax models to
make castings. The idea being that lead would carve more sharply than
the types of wax available at the time. I believe this is very
plausible if the models are pressed into a clay matrix and then
physically removed, but the idea of burning out lead just doesn’t seem
like a very good idea. Forget the obvious health risk, these same
metalsmiths routinely used Mercury amalgam for gilding. A clean
melt-out/burn-out just does not seem very likely. Whenever I have
melted lead there is always a nasty bit of slag floating on the top. I
imagine this residue remaining in the mold and contaminating the
cast. If lead could be used this way it would seem like it would work
better on a larger scale.

I am involved in a project that logically suggests that I test the
theory by trying it. I would much rather discover that someone else
has already proved it works or failed to make it work. If anyone has
seen anything about someone actually trying this, or better yet has
done it themselves, please let me know.

Stephen Walker


#2

A couple things that would probably counter indicate the use of lead
for lost pattern mold making:

Lead has a very high thermal expansion which would probably crack
molds during melt out. Wax can and does crack improperly designed
and dewaxed modern shell molds.

Lead easily forms a lot of oxide dross on heating that can
contaminate the mold surface and cause detail loss and pitting.

Incomplete dewaxing of modern molds have surface degradation
problems from carbon residue.

Burn outs of natural objects leaving carbon and ash residues also
cause surface degradation.

Lead is the basis for fire assaying in which gold and silver in ores
are dissolved in metalic lead reduced from lead oxide by heating. The
lead is then boiled off by continued heating in air. It looks to me
like pouring gold or silver into a dross contaminated mold would
possibly result in a reverse reaction contaminating the object
surface metal. The surface would probably then degrade overtime and
be fairly obvious.

Then there is Occam’s razor: The simplest way is probably the way it
was done.

Bees wax and rosin are ubiquitous on most of the inhabited earth
surface. They can be compounded into modeling -carving waxes that
produce very fine work today. The Japanese Mitsuro wax modeling is an
example of work with these totally natural substances from some time
in the past to the present.

It appears that lost wax -clay mold casting developed independently
at many locations in the world and it worked any place there was wax,
clay and the beginnings of metallurgical knowledge.

Investment casting as an industrial process developed during and
after WW2. By 1950 The low melting CERRO bismuth based low melting
alloys and various solders had been investigated for lost pattern
molding and abandoned. At the same time ( late 1940’s) lost frozen
mercury was used for a while as a real process when the current
investment casting methods were being developed. It was a patented
process in actual use in the early 50’s at least. I don’t know when
it was discontinued. I remember concern over mercury vapor toxicity
as an industrial hygiene issue in 1962 and not before then. But?

Are you familiar with “Pirotechnia” by Biringuccio, Vannoccio.
(1540) ? This is one of the earliest books printed. The is first
technical book and is a good compilation of the metal arts until
that time-- and not a bad reference even today. Cellini was
rediscovering lost wax casting at the same time.

DE RE METALLICA. Agricola, Georgius. the second metals book was
printed in 1565 and has a little different coverage

I think these two cover the state of the art then pretty well and
lead as a modeling media isn’t mentioned. Cellini was rediscovering
lost wax casting at the same time. It now seems that the Dark ages
were not as dark as we were taught.

While these books in print are not old as knowledge may go. They
probably are a fair practical representation of realistic, successful
metallurgical practices until then. There may have been something
different in China . But??

People seem to have gotten around a lot more than we were taught and
there was knowledge transfer.

I believe any unmolded lead patterns would have not survived ambient
exposures. There is no way to tell if a process was tried and
abandoned or just lost. To test a premise you don’t have to make
special models just start with some commercial wax patterns.

Today non destructive SEM based surface analysis techniques can
analyze for this type contamination even below the surface.

In spite of the ease of doing analysis with the only practical
constraint being cost, I doubt if there would be any one willing to
expose their objects to analysis

If you have the credentials --Maybe.

jesse


#3
 I have been studying about early Medieval metalworking and
jewelry, particularly Insular jewelry. One thing that keeps popping
up is the speculation that lead models may have been used like wax
models to make castings. The idea being that lead would carve more
sharply than the types of wax available at the time. I believe this
is very plausible if the models are pressed into a clay matrix and
then physically removed, but the idea of burning out lead just
doesn't seem like a very good idea. Forget the obvious health risk,
these same metalsmiths routinely used Mercury amalgam for gilding.
A clean melt-out/burn-out just does not seem very likely. Whenever
I have melted lead there is always a nasty bit of slag floating on
the top. I imagine this residue remaining in the mold and
contaminating the cast. If lead could be used this way it would
seem like it would work better on a larger scale. 

I’ve studied a bit on Medieval metalwork myself. I agree that most
likely lead alloy models were used to as masters for a clay or sand
mold. Many of the surviving metal work from recent digs like the
London and York digs (which has more examples of middle and lower
class objects) reveal examples of pieces (especially brooches and
buckles made off of a common form with minor modifications done in
the casting process).

Ron Charlotte – Gainesville, FL
@Ron_Charlotte1 OR afn03234@afn.org


#4

I went ahead and tried “lost lead” I had a burnout going overnight
so I made up a little test flask with a piece of lead 15 x 30 mm x
2mm, with a little engraving on it in Satincast 2000. It worked, but
not very well. It did not contaminate my other flasks, which I had
some anxiety about. Figuring that the oxides would cause problems, I
scraped the surface, but not the edges. The edges of the casting
show a crust of crud, but the surface at the top of the piece,
farthest from the sprue, show good detail and clean metal. The
bottom shows obvious contamination. If you wanted to, I think you
could work out a way to make this useful. Given the really good
carving wax we have today I don’t see this as being in anyway useful
for us, but if you wanted to carve sharp detail a thousand years ago
lead models would be a lot better than beeswax. So, if you can base
any kind of conclusion on one test like this I would have to say
that the use of lost lead casting in early medieval times is a
viable theory.

Stephen Walker