[Jewelry making - Article 00249] - Re: [Orchid] The Beilby Layer

[leonid Surpin]

========[ Invite a Friend - http://www.ganoksin.com/invite.htm  ]========

    I can give it a try as it relates to jewellery. 

    In order to make material ( gold, silver, gems ) to reflect light,
    the size of the irregularities on the surface must be smaller that
    the 1/2 of the wavelength of light used to observe the surface.
    Visible spectrum is between 3500 and 6500 angstrom. This means that
    if we want to achieve reflective surface using abrasion, the
    particle size of our medium must be less than 1750 angstrom. Diameter
    of the atom approximately 10^-11, 1 angstrom is 10^-10, so we talking
    about a particle made of less than 17500 atoms. Very tall order
    indeed. 

    I do not know if it is possible to manufacture such medium with the
    present technology, but it was definitely out of reach in the
    beginning of 20th century and before that; but craftsmen were able
    to produce reflective surfaces nevertheless. 

    G. T. Beilby was first to propose theory to explain how reflective
    surface can be achieved without use of super-fine abrasives. Beilby
    was trying to explain mechanism used in polishing crystalline
    material. At first his theory was rejected simply because it cannot
    be proven, but later 2 scientists Bowden and Hughes accepted his
    theory and it was published as Beilby-Bowden theory of polishing.
    That theory was challenged again and finally rejected after Crowcroft
    published Demise of Beilby-Bowden Theory of Polishing. However,
    Beilby theory still holds in polishing of metals. Beilby correctly
    observed that polishing generate heat. Heat causes high points of the
    surface to melt and flow into lower points thereby creating smooth
    surface capable of reflecting light. As it relates to gem polishing
    the theory was rejected because in gem polishing water is used which
    is very efficient heat conductor and hypothesized melting would
    never take place. Present theory is based on the idea that in the
    presence of water the hydroxyl formation take place, microscopic
    fractures develop in hydroxyl layer, that causes flaking of the
    hydroxyl layer and the surface is reduced to reflective state. But in
    metal polishing no water is used. The heat generated does cause the
    changes proposed by Beilby and resulting layer is called as Beilby
    layer. 

    Practical importance: 

    If we finish surface using abrasive medium, under magnification the
    surface would look like a surface of the moon. When such surface is
    heated, the oxide formation takes place; the oxides are dissolved by
    the flux and the result is deepening of the surface irregularities.
    After pickling the surface becomes matte no matter how well it was
    polished prior. 

    If Beilby layer was achieved on the surface, as long as re-melting
    and re-crystallization would be prevented, the surface will remain
    in the original state after soldering. 

    Problem is that achieving Beilby layer uniformly requires long time
    and the process does not fit within cost structure of most
    businesses and therefore it is not taught in modern schools. This 
also
    explains why it is so difficult to polish platinum, since the melting
    point is much higher then gold. 

Leonid Surpin.
____________________________________________________________________
T h e   O r c h i d   L i s t
Open Electronic Forum for Jewelry Manufacturing Methods and Procedures
____________________________________________________________________
Orchid FAQ:
~ http://www.ganoksin.com/orchid/faq.htm
Orchid Archives:
~ http://www.ganoksin.com/orchid/archive
Orchid Galleries:
~ http://www.ganoksin.com/orchid/gallery.htm
Invite a Friend:
~ http://www.ganoksin.com/invite.htm
____________________________________________________________________
Tips From The Jeweler's Bench - Article Archive
~ http://www.ganoksin.com/borisat/tip_sear.htm
The Jeweler's Selected Bibliography List
~ http://www.ganoksin.com/jewelry-books
Buy Orchid Jewelry:
~ http://www.ganoksin.com/shop
____________________________________________________________________
-Unsubscribe:
-Email: orchid-request AT ganoksin.com Body=unsubscribe subject=blank
____________________________________________________________________