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Student's Dictionary
A mix of topics, references and commentary by Iris.
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G
Gold
    Palladium (Pd) Alloyed with Gold
    Adding palladium to gold has been around since the turn of the 20th century, but is becoming more popular. The reasons for palladium's popularity are the white color, its less expensive than platinum, and it is very malleable. Like nickel, palladium serves as the bleaching agent to achieve a white gold color.

    Adding palladium to gold has many benefits but there are challenges. Casting palladium white gold is very challenging because it has extremely high flask and melt temperatures. When casting palladium white gold we recommend using platinum investment rather than the normal gold investments.

    When fabricating with palladium white gold sheet or wire, again the challenge is to use a higher temperature. Because of this challenge we recommend using low temperature platinum solder instead of karat white gold solders when soldering.

    From David H. Fell & Company, Inc.
    Winter 2006 Volume 16 Number 1

    Silver (Ag) Alloyed with Gold
    When silver is added to gold in sufficient quantities the result is a greenish gold. Like the other metals, the higher the silver content, the greener the gold is.

    Compared to red and white karat gold alloys, green gold alloys have many casting and fabrication benefits. Because of the silver content, green gold has a higher flow rate, which means a redution in the flask and casting temperatures. Another benefit of green gold, when cast, is it can be cooled either by quenching from red heat, naturally air colling, or quenching from black heat.

    Fabricating in green gold is superb because the silver content makes sheet and wire extremely malleable. With the silver content, green gold is highly suitable for reticulation.

    From David H. Fell & Company, Inc.
    Winter 2006 Volume 16 Number 1

    Zinc (Zn) Alloyed with Gold
    Zinc isn't often used as the primary constituent in an alloy. It is used as an additive to couteract some of the negative attributes of other metals. When zinc is combined along with silver, copper, nickel or palladium it lowers the melting temperatures of the alloys. Other quanlities of zinc are its ability to enhance the color of low karat gold, increase the whiteness of white gold, and act as a deoxidizer.

    Typically, many karat gold alloys will have some percentage of zinc in it. If there is zinc in the alloy, reticulation will not be possible. And if there is more than 2% in an alloy then enameling will not be possible.

    From David H. Fell & Company, Inc.
    Winter 2006 Volume 16 Number 1
Q
Quenching
    Quick Quench
    When using traditional lost-wax casting processes, conventional wisdom suggests that it's best to wait about 15 to 20 minutes (or at least until the red color is gone from the button) before quenching a cast flask. However, when casting palladium, this standard practice could negatively impact your results, says Teague. "It is best to quench a palladium flask immediately after casting 0 and I do mean literally taking the flask out of the casting machine once it has stopped spinning and putting it directly into the quench tank with practically no time lapse," he says. With nearly every other precious metal alloy, this would cause cracking. With palladium, the opposite happens: It prevents grain growth by forcing the grain to form quickly, and it prevents the alloying components from migrating to the grain boundaries. It also reduces the amount of oxygen that migrates into the metal through the button when it is removed from the casting machine. By decreasing the amount of oxygen absorbed through the button, you reduce the overall oxidation of the palladium. In the case of some alloys that contain ruthenium, it helps reduce the common cracking problem caused by the ruthenim oxidation.

    MJSA/Journal
    August 2008 (pg 46)

    Air-Cool Before Quenching...
    When soldering jewelry with large flat surfaces, always allow the piece to air cool before quenching. Tossing a hot piece directly into the cold water may crack or distort the flat surface, thus ruining it.

    From Lapidary Journal's Jewelry Journal Question, Answers, & Tips by Tom & Kay Benham, Contributing Editors November 2006 (pg 8)
S
Silver
    Hill Tribes Silver
    Hill Tribes Silver, comes from the northern hills of Thailand and the jewelry and components are "completely handmade of silver." They add that "these assay to about 96-98% purity." And the items are "made by indigenous tribe, thus preesrving their ancient heritage and traditions." Disclosure is one of the characteristics of the Fire Mountain Gem's company that I value, i.e. they inform you of the quality of their goods. Note that 96% to 98% translates into .960-.980 on a 1,000 point scale of silver purity.

    Fire Mountain Gems
    April-June 2006 catalogue (pgs 4 & 5)

    Silver Soldered (or Fused) to Gold. Why You Shouldn't....
    Gold on Silver (110): an embedded-atom-method study.

    Abstract: Calculations for trends in the energy of various configurations of small quantities of gold deposited on a silver (110) surface have been carried out using a simple nearest-neighbor embedded-atom-method model. The primary result is that it is energetically preferable for the outermost plane of atoms to be silver rather than gold. Isolated gold atoms on top of a (110) surface can readily exchange with silver atoms, but pairs of gold atoms are tightly bound and impede interchange. Gold atoms have little interaction with ledges parallel to tight-packed rows on the (110) surface but bind strongly to ledges parallel to (001) and to kinks. Kinetic effects should play a major role in determining the actual configurations found in experiments, with little initial barrier to the interchange of single adatoms with surface atoms on the (110) surface, but with limited interchange with increasing coverage. Clustering of gold on this surface occurs for the same reason that gold (110) surface reconstructs: the small ratio of gold (111) to - (110) surface energies. The primary unique property of gold that lends to all of these effects is the large positive Cauchy pressure, i.e., the large ration of the bulk modulus to the average shear modulus. This leads to a large curvature in the embedding function of gold so that the energy penalty is greater in gold than in silver to be given fraction away from its equilibrium electron density. It is also found that, depending on the range of local volume dependence as given by the range of the electron density function used in the model, there may be a significant repulsion between vacancies and free surfaces one or two atomic layers below the surface.

    From the IOP Electronic Journals
    Print publication: Issue 5 (September 1994)
    R A Johnson 1994 Modelling Simul. Mater. Sci. Eng. 2 985-994
    Dept. of Mater Sci. & Eng. , Virginia Univ., Charlottesville, VA, USA
    To read the entire article, go to
    http://www.iop.org/EJ/abstract/0965-0393/2/5/004 for PDF.




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