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Fine Powder PMC


#1

Hello Laura. Yes, there is a chemical process that can produce a
fine silver powder. The material looks like cement and is called
"cement silver". I don’t know what the particle size range of
cement silver is, but it probably varies widely. I doubt very much
it would be suitable for making PMC. James Binnion gave the answer
to the practicability of making your own PMC in his post of 8-14.

The process for cement silver is very old. It is still used by
assayers and others recovering values from scrap. The process is
hazardous for those not trained in proper chemical/laboratory
procedures. I will describe the process briefly, without the many
important details, to give those interested an idea of what is
involved in one of the several processes used for recovering silver
from sterling.

  1. The scrap sterling is cleaned to remove organic matter, foreign
    material, etc. It is then physically processed to produce pieces
    having as large a surface area as possible in a form that prevents
    the material from laying flat on the bottom of the reaction vessel.

  2. The prepared material is digested in 50 volume percent nitric
    acid. This must be done carefully to control the reaction rate. The
    reaction produces oxides of nitrogen, much of which is nitrogen
    dioxide-nitrogen tetraoxide (NO2-N2O4). Nitrogen dioxide-tetraoxide
    is a reddish-brown gas that is very toxic, suffocating and extremely
    corrosive. There are laws controlling the amount and procedures for
    the release of these gases. The presence of excess nitric acid is
    to be avoided. It is added a little at a time. When all of the
    scrap is dissolved, a piece of silver is added and then watched for
    any indication of further reaction. If reaction is observed it is
    allowed to proceed to completion.

  3. The nitric acid free reaction solution is diluted with three
    volumes of distilled water and filtered. At this point you have a
    blue-green solution that contains silver nitrate and cupric nitrate
    (copper(II)nitrate).

  4. The filtered solution is further diluted with tap water and a
    concentrated table salt solution is slowly added with stirring.
    From this point on it is desirable to to keep light as low as
    possible. The silver is precipitated as a white solid (silver
    chloride). The precipitate is allowed to settle and the supernatant
    liquid is tested to assure complete silver precipitation. The
    silver chloride is filtered off and then washed with hot water until
    the filtrate is colorless. A small pinch of the silver chloride is
    then usually removed and tested for the presence of lead or mercury.
    We would expect none using good clean sterling scrap.

  5. The washed silver chloride is kept wet/damp and is spread out in
    a glass dish and then covered with dilute sulfuric acid. The
    mixture is warmed slightly and zinc (preferably zinc powder) is
    added slowly with with vigorous stirring until the evolution of
    hydrogen gas (very flammable!) ceases. The slurry is filtered and
    washed on the filter with very dilute sulfuric acid (to remove any
    excess zinc) followed by water washes until the pH is about 7. The
    dried precipitate in the filter is good purity finely powdered
    silver that has a grey color. Ordinarily, the cement silver is
    mixed with flux and melted to cast an ingot.

Captain Blood
"Marlinespike Seamanship in Precious Metals"
@Alden_Glenda_Blood


#2

There are a few sites that sell fine silver casting grain or flakes.
Here are the links:

http://www.pmcconnection.com/index.htm and
http://artclayworld.com/cp/cgi-bin/cp-app.cgi

Hope this helps!
Sonje Mitchell
Moonlight Creations
Yarmouth Maine