3722. Test Fluid for Silver-Plated Goods

3722.    Test Fluid for Silver-Plated Goods. For this purpose a testing fluid is prepared by adding pure nitric acid to powdered red chromate of potash, and mixing them in such a manner that a part of the latter remains in suspension, the whole being kept well stirred during the mixing. Equal parts by weight of each may be taken. The nitric must be quite free from hydrochloric acid, and have the proper degree of concentration, being neither too fuming nor too dilute; it should have a specific gravity between 1.20 and 1.25. When the mixture has been prepared for a few hours, and been stirred several times, the reddish - colored liquid is poured off from the residue and kept in a stoppered bottle.

3723. To Test Silver-Plated Goods

3723. To Test Silver-Plated Goods. The ordinary and very accurate method of testing of silver is founded upon the insolubility of chloride of silver in dilute acids and in water. This otherwise satisfactory test is, however, difficult to carry out when an article is very thinly plated. A drop of the test liquid (see last receipt) is then brought in contact with the metal to be tested, and immediately washed off again with water. If a visible blood-red spot remains, silver is present. This method requires only the following precautions: The metallic surface must have been quite cleansed from grease or varnish with spirits of wine - water must be poured over the treated surface before judging of the color, as that of the testing fluid is altered by the metal, and the red precipitate is not distinctly visible until the colored liquid has been washed off. The red spot can afterwards be very easily removed with the finger. By this method the slightest trace of silver in an alloy may be ascertained. When an article is suspected to be only thinly plated, a very minute drop of the testing fluid should be used. With no other metal or alloy does this red spot, so characteristic of silver, appear; in some cases the testing fluid only corrodes the surface of the metal, whilst in others colored precipitates are formed, which, however, cannot be confounded with those of silver. German silver brought into contact with the testing fluid affords no red spot after being washed. The spot will, however, have been strongly corroded. Britannia metal yields a black spot; zinc is strongly corroded; platinum is not attacked; lead gives a yellow precipitate; tin is strongly affected by the fluid; when the brownish-colored testing fluid is washed off, a yellow precipitate is perceived, which adheres tightly to the metal; copper is strongly attacked, a tarnished surface of this metal is brightened by the action of the acid.