Thermal measuring and testing – Thermal testing of a nonthermal quantity
Reexamination Certificate
1998-03-09
2002-08-13
Gutierrez, Diego (Department: 2859)
Thermal measuring and testing
Thermal testing of a nonthermal quantity
C374S005000
Reexamination Certificate
active
06431748
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the nondestructive determination of the composition of a material when comparing the thermal properties of a sample of the material with the thermal properties of a standard of a similar material, said standard or substitute having a desired composition.
The invention herein described has use for nondestructive qualitative determination of composition of a variety of materials and its use is discussed mostly with reference to precious metals, such as gold and silver.
BACKGROUND OF THE INVENTION AND PRIOR ART
Background
The rise in trading in recent years of precious metals, such as gold and silver, as commodities, and the rise in their unit prices has increased the need for an economical, fail-safe mechanism for determining nondestructively, the purity of such materials. Since gold, like silver, is often transferred or sold by persons not particularly knowledgeable about such precious metals to one of greater knowledge, it is important that some way be found to detect forgeries and ascertain the purity of such precious metals that avoids the costly and time consuming methods, several of which are outlined below, a way that is nondestructive, fast and accurate.
By way of background, that which follows was paraphrased from
The collector 's Dictionary of the Silver and Gold of Great Britain and North America
, Michael Clayton, World Publishing Company, 1971.
Pure gold is extremely heavy in proportion to its volume and also very soft (malleable), and if pure, is referred to as ‘24 carat’. Silver is not so malleable and only approximately half the weight of a piece of gold of similar volume. Both are too soft to use in their pure form and must be hardened by the admixture of base metals, usually copper, though silver may be used with gold. If both silver and copper are added to gold it becomes pale and green in color. The fact that adding a 50 percent alloy of copper to silver retains a silvery appearance can easily lead to fraud without protection. In general, the best proportions of gold and alloy are 22 parts pure gold to 2 parts alloy, but this can be varied so that 18, 15, 12, or 9 carats (or parts) are balanced by an alloy making up 24 parts. These balanced fractions of pure gold and alloys are legally used and obviously the less pure gold parts, the cheaper the finished product. With silver, only two standards are permitted in Great Britain, 925 parts out of 1000 as normal Sterling standard and 958.3 parts as the higher Britannia standard. By the same measurement, 18 carat gold is the equivalent of 750 parts fine gold to 1000 (24 carats). For most of the English gold coinage and all since 1672, the fineness has never been below 22 carats (916.66 parts gold to the 1000). Since the nineteenth century in the United States, the coin standard is 900 parts pure silver to 100 parts alloy.
In order to protect the buyer of gold and silver, a system of testing, or assaying, and checking the quality and standards of an object is necessary. This can be done by comparison (touch), weight, or chemical means. The first demands considerable visual skill as the object to be tested and a piece of known quality are both stroked across a piece of basanite, a hard flint-like slate, and the resulting streaks compared. In the second test, weight, small portions of the object to be assayed are scraped from each piece, wrapped in lead (lead and silver are also used to wrap gold) and heated in a bone-ash crucible. As the heat is applied lead and other base metals oxidize and are absorbed by the crucible, known as a ‘cupel’; the balance is then weighed and compared with the weight of the original scrapings.
In the case of gold, which is also wrapped in silver, a further process is required whereby the silver is finally removed by placing it in hot nitric acid. This method was first recorded in 1495. If on completion of these tests, the gold or the silver are found to be below the lowest permitted standards, the marks which would guarantee their quality, ‘hall-marks’, as they are known, are withheld and the objects under examination are crushed and returned to the maker. The third is a simple method and applicable only to silver, but requires some reasonable idea of the quality of the metal being tested. This involves the dissolving of the weighed scrapings, also known as ‘diet’, in nitric acid and the addition of a standard solution of sodium chloride (common salt); at a certain point the cloudy liquid clears and silver chloride is precipitated. A comparison of the original weight of the silver sample and the quantity of saline solution required to do this enables the fineness of the metal to be assessed.
Historically, as indicated above there are a number of methods used to determine the composition of metallic materials that can be classified as comparative as well as destructive. A comparative method is one, as the name implies, that requires a comparison to a known reference material. A destructive test is as the name implies and needs no explanation. The descriptions of pertinent testing methods that follows are all comparative tests and are categorized as ‘destructive’ or ‘nondestructive’. The following paragraphs, under the heading “Prior Art”, discuss appropriate examples of these.
Prior Art
Destructive Tests
Some of the more modem methods, than those described above, that have been developed and in use today to determine alloy content of metallic materials are: optical emission spectrography, spectrometry, x-ray fluorescence spectrometry, atomic absorption spectrometry, plasma emission spectrometry and combustometric analysis to determine particular elements. Such methods are not only costly, but usually require a sample from the test piece, and thus are somewhat destructive.
A primary example of a destructive test is the standard prescribed by the American Society for Testing and Materials, (ASTM) Test Method B 562-95, “Standard Specification for Refined Gold”. This test method examines samples taken from the melt before pouring the casting of gold. The standard utilizes, for 99.5 percent purity, a test method for chemical analysis by cupellation fire assay. If there is a disagreement between the manufacturer and the purchaser the specified test will then be in accordance with ASTM Test Method E 1446, “Test method for Chemical Analysis of Refined Gold by Direct Current Plasma Emission Spectroscopy”.
The standard for testing silver, which is also destructive, is that given by the ASTM Test Method B 413-89, “Standard Specification for Refined Silver”. This method requires that the samples be taken from bars by drilling six holes and the chemical composition is determined in accordance with ASTM Test Method E 378 “Test Method for Spectrographic Analysis of Silver by the Powder Technique”.
Portable electronic gold testers that measure the carat value of gold are also available, such as those described in U.S. Pat. Nos. 4,799,999 and 5,218,303, authored by Medvinsky and Radomyshelsky. These patents describe a method for determining the assay of gold alloy, utilizing an electrochemical process. The specimen gold is wetted by an electrolyte, and a small current anodizes the surface of the specimen for a metered period of time. A potential sensing device is then applied to the charged surface, and a potential decay is observed. The potential decay information is compared with empirical data and by interpolating the potential with the empirical data a determination of the carat quality of the gold alloy may be determined. This same method may be used for other precious metals, employing different electrolytes, empirical standards, and potentiometers.
There are two additional patents, U.S. Pat. Nos. 5,128,016 and 5,080,766, authored by Moment and Nelson, that essentially utilize the same technique with some variation as those indicated above.
Criticisms of these gold testing devices are that they are slightly destructive, are surface sensitive only, will not detect plating or gold overlay, and will leave a mark on items that are of 14 cara
Gutierrez Diego
Pruchnic Stanley J.
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