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Tumbago alloy


#1
    I have a very interesting book entitled "Sweat of the Sun and
Tears of the Moon" that covers Gold and Silver in Pre-Columbian Art

Great. What does it say about tumbago? The copper/gold alloy similar
to shakudo. I understand they did a lot of work using this then
depletion gilded it to look like pure gold.

Brian

B r i a n A d a m
e y e g l a s s e s j e w e l l e r y
Auckland NEW ZEALAND
www.adam.co.nz


#2

In his book Emmerich has a long list of bibliographies used in his
research, to which he frequently refers.

Under the section covering Casting, alloying and gilding, one of the
first mentions of using copper when smetlting gold is on page163
when he mentions notes by Sir Walter Raleigh when he described
Indian metalworking in Columbia in 1596.

On page 164 a direct quote.

  "Sir Walter Raleigh's observation conerning the "part of
  copper" added to the gold, without which the Indins could not
  work it, has been thought by some to account for the great
  importance in pre-Columbian metalllurgy of the gold-copper
  allooy called tumbaga by the Spaniards. In a well-reasoned
  article Root (listed in the biography section) has concluded
  that tumbaga was in fact used so widely because it gave the
  Indians a way to use the copper they had in the manufacture of
  metal objects which would still be gold in color". 

Phyllis Richardson


#3

I have found two other sources on this maybe in NZ libraries too!!.

They are not academic monographs but in a popularized form published
by Time -Life “The Metalsmiths” (1974) part of the emergence of man
series which has the best coverage.

“Noble Metals” (1984) part of the planet earth series. Time -
Life did an exceptionally well researched and written job with their
many special book series. Mesoamericans apparently alloyed up to
about 50% copper with gold. Native ore would also contain silver
and they apparently depletion guilded out not only the copper but
also the silver. Most of the art was melted into ingot and hauled
to Spain or lost at sea.

Libraries may still have them but the may have been removed because
of low circulation. A major problem in US libraries today.

jesse


#4

Tumbaga was used in pre-Columbian times (roughly from 600 A.D. on)
from Central America to Peru and Chile as a generic term for any
combination of gold and copper. It could range from 95% copper to
95% gold, although tumbaga or guanin gold was usually made by adding
10 to 30% copper to gold. Tumbaga usually contains 5 to 10% silver as
well, which occurred naturally in the gold and wasn’t intentionally
added. That�s the definition given in “Sweat of the Sun, Tears of
the Moon” and another source I have.

There were several reasons tumbaga was popular. A primary one is
that 70% gold/30%copper will melt at around 800 C., much lower than
gold or copper separately. That’s important because melts were done
in large clay pots using a team of men huffing on blowpipes.
(There�s an amazing Moche clay urn I’ve seen that shows this
procedure in detail). Molten metal then flowed from a hole in the
bottom of the vessel into open molds made from stone or clay. These
molds have been found archaeologically from Mexico to Chile.

The lost wax casting techniques of these peoples were very
sophisticated as well. They routinely cast hollow objects and bimetal
part silver-part gold objects using complex one-time molds made of
ground charcoal, sand and clay. Depletion gilding was routinely used
to decorate the surfaces of objects made from low-gold alloys. The
amount of gold used in tumbaga depended on the metal�s availability.
Objects from gold-rich areas like Calima and Tolima in Colombia, for
instance, were large and contained purer gold while most pieces from
the Muisca and Tairona regions were smaller, less pure and depended
on gilding for appearance.

Tumbaga had another interesting use as well. Some cultures like the
Moche placed small tumbaga ingots in the mouths of their high-ranking
dead prior to burial. I�ve encountered this same ritual using
various metals while studying the burial practices of several other
ancient cultures around the world but am not clear on its
significance.

In the bibliography of “Sweat of the Sun, Tears of the Moon,” the
following sources are listed:

Coggins, Clemency Chase. “Artifacts from the Cenote of Sacrifice
Chichen Itza, Yucatan.” Ed. Clemency Chase Coggins Vol. 10/No. 3
Cambridge, The President and Fellows at Harvard College, 1992.

Jones, Julie. “The Art of Precolombian Gold: The Jan Mitchell
Collection.” Ed. Julie Jones. Boston Little, Brown and Company,
1985.

Nottebohm, Karl-Heinz. “A Second Tlaloc Gold Plaque from Guatemala.
Notes on Middle American Archaeology and Ethnography.” Vol. 2
Numbers 31=60. New York AMS Press, 1969.

Weaver, Muriel Porter. “The Aztecs, Maya, and their Predecessors;
Archaeology of Mesoamerica.” Third Ed. San Diego, Academic Press,
1993.

I have a wonderfully detailed article, “Gold of El Dorado:
Technology of Ancient Colombian Gold” by Clemencia Plazas and Ana
Maria Falchettie de Saenz that appeared in �Natural History�
(November 1979, Vol. 88, No. 9, pp. 36-46).

If it�s still in print, “Royal Tombs of Sipan” by Walter Alva and
Christopher B. Donnan is a beautifully produced and illustrated
showcase of Moche gold and silver work. It was published in
conjunction with a traveling exhibition from UCLA�s Fowler Museum.
The ISBN of my softbound (expensive enough!) is 0-930741-30-7.

Rick Martin
MARTIN DESIGNS


#5

Rick, thank you so much for your on the tumbaga alloys.
It has piqued my interest!

I never paid much attention to pre-Columbian gold before (I know,
shame on me), but after reading your post about the casting method,
I just have to get some of the books you listed! (I love ancient
metalworking techniques, and this will dove-tail nicely with the
European methods I am more familiar with.)

–Terri


#6
    Tumbaga was used in pre-Columbian times (roughly from 600 A.D.
on) from Central America to Peru and Chile as a generic term for
any combination of gold and copper. 

Thank you for that detail, Rick.
Do you know what their source of the acid was for depletion gilding?
Rhubarb leaves perchance?

Cheers
Brian

e y e g l a s s e s j e w e l l e r y
Auckland NEW ZEALAND
www.adam.co.nz


#7
Do you know what their source of the acid was for depletion
gilding? Rhubarb leaves perchance? 

I’m no expert on gilding – I’ve never done it – but here’s what
the sources in my files say.

“Sweat of the Sun, Tears of the Moon” says: “The acid solution used
to eat away the copper was a yellow earth, thought to be a highly
corrosive hydrated ferric sulphate mixed with salt. However, there
are similar accounts of the same process in South America using an
acid mixture drived from plants. Depletion gilding was also used on
sheet metal in a selective manner to produce bi-colored designs.”

In “Gold of El Dorado: Technology of Ancient Colombian Gold,” the
authors go into more detail.

They speak of the “mise en coleur” technique explained this way:
“When heated, a gold-copper alloy oxidizes to form a layer of copper
oxide which can be removed by an acid solution…in Ecuador and
Colombia, plants of the Oxalidaceae family were used to make the acid
solution.”

The “Archive General de Indias” in Colombia describes working a
tumbaga piece “until it was finished…and then the herb they brought
to give it colour was crushed on a stone…and placed in a small pot
which they brought in and added water and ground white salt and
stirred all together (then they polished, heated and quenched it in
the solution several times…and in this way it attained the colour
and finish it should have.”

That method was said to work well for pieces containing at least 30
per cent gold. If the tumbaga had less than 30% gold they used
another method called “superficial parting.” The technique was
essentially the same except they “used a corrosive agent of mineral
origin, such as iron sulfide, instead of the organic acid made from
oxalis plants.”

These cultures didn’t value gold for gold’s sake in the same way as
the Europeans. Sometimes gilded pieces were painted as the finishing
touch! While not directly to the point of gilding, I think the
following comment is interesting:

“The Indians took special care in the finishing of pieces that were
used for personal adornment. Gold was treasured by the most eminent
individuals of a tribal society, and there were clear social
distinctions concerning the quantity and quality of gold ornaments
various members of a group were permitted to own. It would be a
mistake to think that the ‘material value’ of gold was introduced
entirely by the Europeans. But to this must be added a religious
value. Gold was given to the gods in the form of votive offerings;
it was buried with the dead; and many gold objects served as special
symbols in the various rites and ceremonies of the Indian peoples.”

If viewed that way, the practice of placing tumbaga ingots in the
mouths of the dead (mentioned in my previous post) makes more sense
to me.

Rick Martin
MARTIN DESIGNS