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Tibetan casting methods


#1

hi everyone,

I have just returned from five wonderful weeks inside China and
Tibet. Whilst in a small town in Tibet I watched a jeweller cast
an item. She heated up a flask about three inches high, got the
gold rolling around on to top of the plaster, and then when she
was happy with the swirl of the gold, she quickly upended a can
containing a stiff mixture over it, held it in place for about
ten seconds, and then proceeded normally. The end result was a
beautiful fine pair of earrings. The client had evidently
watched the whole process. Unfortunately I came in at the
heating point! Amongst her tools was a piece of honeycomb
soldering pad! I had a look at the can mixture. It looked like a
brown rubbery clay mixed with some sort of fibre. Unfortunately
we could not communicate because of the language difference!!

I have three questions, does anyone know what the canned mixture
could be? How would she have melted the gold in the first place
on top of the plaster investment? Granules would drop down,
unless it was a larger nugget. Is this a form of steam casting?

Felicity in West oz.


#2

Tibetan casting…Chinese casting.

It is interesting to learn about “primitive” techniques that
work as well or better than the hi tech ones we are more
familiar with.

When teaching in Taiwan a few years ago, I sought out jewelry
supply stores. One had a catalog with pages of rings shown.
These corresponded to shelves of tiny flasks (about 1" diameter
and 1.5" tall). These were already invested and the wax was
already evacuated. Each flask was labled with the style number
and the amount of gold required. The flasks had a conical
opening, like a sprue base, leading to a very little hole,
leading to the cavity. Jewelers choose the pattern they want
from the catalog then purchase a mini flask, place their gold in
the conical opening, heat it and because the opening is very
small, it does not drip down into the cavity. When just the
right temperature in reached and the metal is liquid, they place
a water absorbant cap on it. This expands into steam and
instantly forces the gold into the cavity.

Alan Revere
Revere Academy of Jewelry Arts
San Francisco


#3
   Amongst her tools was a piece of honeycomb soldering pad! 

This may well have been salvaged from an automotive catalytic
converter, which is what those honeycomb ceramic things were
actually originally developed for…

   I had a look at the can mixture. It looked like a brown
rubbery clay mixed with some sort of fibre. Unfortunately we
could not communicate because of the language difference!! 

Probably just some bark or perhaps wadded up cloth, or
something. Doesn’t much matter, the key is that it’s wet. You
can use a wad of wet newspaper just as well.

   I have three questions, does anyone know what the canned
mixture could be? 

Who knows. It’s not critical to the process, only whatever was
convenient.

   How would she have melted the gold in the first place on top
of the plaster investment? Granules would drop down, unless it
was a larger  nugget. Is this a form of steam casting? 

They key to this, which IS indeed steam casting, is that the
item to be cast is sprued with small diameter sprues, often
several of them. For us, the normally recommended way to do it
is to take your wax model, add whatever normal types of sprues
you add, but keep them short, like about 1/4 inch. top that
with a ball of wax, somewhat larger than the sprue diameter. To
the ball, now add a number of 14 guage wax wires, spaced at least
their own diameter apart, and sufficient number of them that
their total cross sectional area is at least that of your main
sprue into th object. These are then affixed to your sprue
base. After investing, use a spoon to reshape the button from
it’s conical shape to a round bottom hemishpere. The spoon will
be cutting through the 14 guage wax wires too. Don’t carve
deeper than those wires go. What you’ll end up with is a round
bottom crucible shape in the investment, with a “drain” in the
bottom formed of those multiple small holes, rather than a single
large sprue hole. The key to steam casting is that the molten
metal has sufficient surface tension that it will not drop down
such a small hole without a push. You add your solid metal in
large enough chunks so tiny bits don’t drop down the holes.
When it melts, the molten metal stays in the crucible, instead of
plugging the holes. Only when you seal the top of the crucible by
bringing down that moist can, and generating steam pressure, does
the metal then get forced down past those fine holes, into the
ball reservoir, and from there, to the sprue and model cavity.

The amazing thing about steam casting is how well it can work.
We’re so used, as western jewelers to using our high tech methods
and machines, that we are surprised at how easy casting can be
with litterally no equipment at all. My first castings, after
college, and before I could afford professional equipment, for
example, were burned out on my kitchen stove. Was a gas burner.
A clay flowerpot, lined with several layers of aluminum foil,
was placed over the flask directly on the burner, and turned on
high. several hours later, the flask was nicely burned out and
at casting temp…

Another low tech casting technology that would amaze you with
it’s simplicity and the sophistication of it’s results is the
methods used in West Africa for casting gold, bronze, etc. The
models are often just beeswax, collected directly from the bees.
Rolled into very thin wax wires, and tiny balls, these things are
then coiled into the most delicate and beautiful forms. After
the models are made, and sprued, the get “invested” in a mix of
plain old clay, and organic matter like dried grass, etc. (the
initial coat over the model is fine clay, no straw). the is
formed, with successive layers, to form a mold, and the end of
the sprue is built up into a crucible shape as well. After the
clay has thoroughly dried, this thing is placed in a fire
(charcoal) and the wax burned out. It’s cooled slowly, and the
casting metal is placed in that open crucible shape, and more
clay is then built up over it to close the cavity. You end up
with a dumbell shape, one end of which is a hollow cavity with
the casting metal enclosed, and the other end is the model
cavity, the two connected by a sprue. Sounds complated, but do
keep in mind that so far, all the casting materials are dug
either from a beehive, or from the ground, except perhaps the
metal, and that too, I guess… Anyway, now this thing, after
again drying the clay, is placed again in the fire, this time
with the metal containing end down. The straw/organic content of
the clay makes the mold porous enough to allow fumes to exit, as
well as providing a nice reducing atmosphere for the melting
metal. When the metal is melted, judged by the color of fumes and
flames surrounding the mold, the whole dumbell is simply
inverted. The molten metal runs down into the mold area by
gravity alone. A look through any of the texts showing the
historical Ashante cast gold and bronze work will demonstrate
just how well this technology works. The reducing nature of the
mold keeps metal cleaner than our normal investing procedures,
and the high metal and mold temps allow a complete fill even with
fine sprues and very delicate filigree models. Some of these
things would be a real challenge to cast consistently using our
normal methods…

Hope this helps.

Peter Rowe

Peter Rowe


#4

I find all this just a little bit more than fascinating. I am a
gross novice at all jewelry techniques, but the origin of my
interest is my love for history and the middle ages… Yep, i am
in the SCA.

So my question is, can any of these low tech casting methods be
documented to a time past 1600? Can anybody help me with
bibliographic sources, or even better, locations/pictures of
items created in these manners?

Dominus Vobiscum,

Greg Vernon

aka, Father Augustine de Peregrine, Incipiant Canton of Houndsford, Barony
of Forgotten Sea, Proud Kingdom of Calontir, and a would be 10th century
jeweler.


#5
can any of these low tech casting methods be documented
to a time past 1600?...	 

Assumng that “past 1600” means “before 1600,” I can offer the
following references (heavy on the Ancient Near East, my field of
study, and it also has most of the earliest evidence for casting,
both artifacts and texts).

(1) I assume you are already familiar with the Dover paperback,
On Divers Arts – Theophilus, translated by John G. Hawthorne
and Cyril Stanley Smith (1979). It has a section on the Art of
the Metalworker, which includes some on medieval
casting.

(2) The work by Cyril Stanley Smith, A Search for Structure,
(MIT Press; 1981) has a chapter entitled, “The Early History of
Casting, Molds, and the Science of Solidification.”

(3) Most recently, the book, Ancient Mesopotamian Materials
and Industries
, by P. R. S. Moorey (Oxford; 1994 – to be
republished by Eisenbrauns this summer), has a chapter on
metalworking (pp.216-301) which includes comments on
Mesopotamian casting techniques from the fourth to the first
millennia B.C. Happy reading! By the way, what does SCA stand
for?

Judy Bjorkman
@JLBjorkman


#6
   So my question is, can any of these low tech casting
methods be documented to a time past 1600?  Can anybody help me
with bibliographic sources, or even better,  locations/pictures
of items created in these manners? 

The ancient greeks were happily using lost wax casting. A lot
more sophistated than just jewelry scale, too. Try larger than
life hollow shell bronze statues of zeus…

Lost wax casting is a common technique known since antiquity,
and never lost to western jewelers after the fall of the roman
empire. While at times, much jewelry was fabricated (made from
hammered sheets and wire, it’s lighter and conserves precious
metal, after all) you can find cast items representing most
historical periods, and in fact, most cultures. Precolumbian
cultures in central america, for example, were using lost wax
casting techniques. And the Ashante casting techniques I
mentioned in my last post are also known to go back hundreds of
years to around the middle ages or so too. Some of the most
ancient castings may not have been lost wax, but just poured into
open molds, stone molds, or sand molds, techniques which also
have continued to be used, even to this day, in one form or
another. But lost wax techniques were an early discovery…

Peter Rowe


#7

Hello Gregory,

In a word YUP! Steam casting is easily documented way way back.
For great info on medieval and renaissance jewelry techniques
see if your local library (or check Amazon.com) has a copy of
Benvenuto Cellini’s “Treatise on Goldsmithing and Sculpture” it
has been reprinted by Dover. Also helpful is "On Divers Arts"
by Theophilus. Both of these books were written by renaissance
and medieval masters. For pictures of period pieces I suggest you
start with Dame Joan Evan’s “A History of Jewelry 1100-1800)
also published by Dover. If you can find it, Hugh Taite’s
’Jewelry 7000 Years” is also excellent. Another excellent
source of ideas is to pay close attention paintings/statuary done
in the time period of your interest. Keep in mind though that
sometimes paintings are allegorical and can depict stuff that
would never be used in real life. If you have a large art
museum close by they sometimes have a few pieces of period stuff.

I hope this helps you get a start, it is always nice to see
someone else interested in historical stuff.

Nikki (Currently slaving over a wax based on a signet ring found
on Bosworth field an believed to be the signet of Richard III)


#8

If I remember correctly, National Geographic ran a rather
lengthy article on the discovery of a burial mound/area
somewhere in Eastern Europe near the Republic of Georgia in
Russia. These folks were a ‘horse culture’ from 2500 - 3000
years ago. The personal adornments and the adornments for their
horses absolutely blew me away. The gold castings were
magnificent and highly detailed. They said that the castings
appeared to have a Grecian influence and talked of a known
Grecian enclave on the North East coast of the Black Sea dating
from about 2000 B.C.!

Skip

Skip Meister
@Skip_Meister
Orchid Jewelry Listserve Member
N.R.A. Endowment &
Certified Instructor
in all disciplines
Certified Illinois D.N.R.
Hunter Ed, Instructor
ICQ 37319071