[Jewelry making - Article 00377] - Re: [Orchid] Tumbago alloy

[Rick Martin]

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>     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


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