GB2124367A

GB2124367A - Precious metal testing

Info

Publication number: GB2124367A
Application number: GB08316286A
Authority: GB
Inventors: Edward Ernest Davis
Original assignee: FINE METALS CORP Pty Ltd
Current assignee: FINE METALS CORP Pty Ltd
Priority date: 1982-06-18
Filing date: 1983-06-15
Publication date: 1984-02-15
Also published as: JPS5957162A; JPH0217075B2; GB8316286D0

Abstract

A method of testing the purity of precious metals when alloyed with base metals requires stroking the alloy with a mildly abrasive dark-coloured acid resistant stone of MOH hardness from 7 to 8, in order to transfer a sample of the alloy to the stone, then either addition of nitric acid of a concentration appropriate to dissolve the expected base metal content of a 9 carat sample or addition of a mixture of hydrochloric and nitric acid for samples greater than 9 carat and equating persistence of the sample colour with purity as warranted or disappearance of the sample colour with the need to test further. A test kit is disclosed containing a sample taking stone and a series of acid reagents.

Description

SPECIFICATION Precious metal testing This invention concerns a method of testing the purity of precious metals, especially gold and silver, as used in jewellery and a convenient test kit for carrying out the method.

Both silver and gold are hall-marked for the purpose of expressing the content of the gold or silver in a particular item of jewellery. Counterfeiting of hallmarks is not unknown and in some countries the designation of the carat rating of gold and the percentage purity of silver in general is arbitrary or simply lax.

If a dealer wishes to check for himself the real content this tends to involve a subjective test in which nitric acid is applied to the surface of, say, a gold article. The discolouration of the acidified area which follows is greater in dependence upon the quantity of silver or base metal with which the gold is alloyed, commonly copper, zinc and nickel. The test result is comparative and its accuracy therefore depends upon the experience and judgement of the tester. After the test the discolouration must be removed by polishing.

The disadvantages of this established method are obvious.

One aspect of this invention provides a method of testing the purity of precious metals when alloyed with base metal comprising: (a) taking a sample of the alloy metal by stroking the alloy metal with a mildly abrasive, relatively dark-coloured, acid resistant stone having a MOH hardness from 7 to 8; (b) applying to the sample a test liquid which for samples of the order of 9 carats contains nitric acid of a concentration appropriate to dissolve the expected base metal content of the sample, or for samples of an order greater than 9 carats, contains a mixture of nitric acid and hydrochloric acid of a concentration appropriate to dissolve the expected base metal content of the sample together with a predetermined part of the gold content;; (c) equating persistence of the sample colour with purity as warranted or conversely equating partial or total disappearance of the sample colour with a need to test further.

Another aspect of this invention provides a test kit for testing the purity of precious metals when alloyed with base metal comprising: (a) a sample taking, mildly abrasive, relatively dark coloured, acid resistant stone with a MOH hardness from 7 to 8; (b) a container containing, for samples of 9 carats, nitric acid of a concentration capable of dissolving the expected base metal content or for samples of greater than 9 carats, a mixture of nitric acid and hydrochloric acid of concentrations capable of dissolving the base metal content of the sample together with a predetermined part of the expected gold content.

The choice of stone is critical, particularly as regards hardness. The darker the stone the more visible is the sample rubbed from the article. The mineral acid mixture for gold contains hydrochloric acid and nitric acid. Preferably the proportions of the acid mixture and the overall normality of the mixture are adjusted for each of the commonly used purity levels of the precious metal concerned, for example in gold jewellery, 9 carat and 18 carat.

A preferred embodiment of the invention is now described with reference to the accompanying drawings in which: FIG. 1 is a plan of the quartz sampler and a side view of the dropper bottle; FIG. 2 is a plan of the quartz sampler bearing an alloy sample rubbed from a piece of jewellery; FIG. 3 is the same as Fig. 2 but showing the addition of a drop of test liquid just after addition; FIG. 4 is the same as Fig. 3 but showing the result after 50 seconds on a sample whose purity is less than that warranted.

Referring now to the drawings, a tablet 2 of black onyx 10 mm x 10 mm x 4 mm has a matt surface which is rubbed against a 9 carat gold bracelet. A series of visible yellow scratches is left on the tablet (best seen in Fig. 2). Only a small quantity of the bracelet need be rubbed away to give the sample. A dropper bottle 6 with a dropper 8 contains 6.6 Normal nitric acid which is prepared from concentrated nitric acid supplied to laboratories at 63%.

A single drop of nitric acid is placed on the sample as shown in Fig. 3. After a few seconds the wetted part of the sample may grow paler. After 60 seconds or so the reaction has stopped and the sample remains.

Three other bottles are supplied for 14, 18 and 22 carat alloys these being the values commonly used by manufacturing jewellers. The mixtures of nitric acid and hydrochloric acid are shown below.

These are prepared from concentrated nitric acid at 63% and hydrochloric acid at 36% as supplied to laboratories.

14 carat test mixture 1.35 ml of 36% hydrochloric acid is made up to 1000 ml with 8.35 N nitric acid made by diluting 63% nitric acid.

18 carat mixture 11.0 ml of 36% hydrochloric acid is made up to 1000 ml with 8.3 N nitric acid made by diluting 63% nitric acid.

24 carat mixture 149 ml of 36% hydrochloric acid is made up to 1000 ml with 9 N nitric acid made by diluting 63% nitric acid.

If the bracelet is warranted 9 carat it contains a large percentage of base metal. It is the base metal which produces most of the yellow colour in the sample mark on the tablet. The nitric acid only dissolves the base metal and leaves the gold intact. If it is not even 9 carat the sample will dissolve because more base metal is present than should be the case for a 9 carat alloy and the gold component becomes harder to see.

If the sample remains undissolved it is 9 carat or better. As a check, a drop of acid from the 14 carat mixture is applied to the same mark. If it dissolves, the bracelet is 9 carat or better and probabiy falls between 9 and 1 3.

If a sample is warranted 1 8 carat a drop of 18 carat mixture is applied to the mark. If it remains it is 1 8 carat or better and testing with the 22 carat solution follows as with 9 and 14 described above.

The concentration of the acid mixture is calculated to be chemically equivalent to alloys of one carat less than each of the 9, 14, 18 and 22 carat alloys which are used in the jewellery trade. The 22 carat mixture is capable of dissolving all of a 22 carat sample in time but not in the 2 minutes allowed for the test. During the test period only the base metal and part of the gold are dissolved.

The ratios of the acid mixes as percentages are as shown below: Nitric acid Hydrochloric acid Carat % 9 100 0 14 99.865 0.135 18 98.9 1.1 22 85.1 14.9 Percentage variations of about 5% is acceptable.

Any deposits which remain can be rubbed off easily with the help of a mild abrasive cleaner.

Kits for platinum, rhodium and silver are prepared likewise.

We consider the advantages to be: 1. the article itself is not discoloured or etched by the test; 2. the test is more specific than the prior art method described above; 3. the results are easily judged by testers with no previous experience; 4. for gold the test can discriminate a two-carat difference.

Claims

1. A method of testing the purity of precious metals when alloyed with base metal comprising: (a) taking a sample of the alloy metal by stroking the alloy metal with a mildly abrasive, relatively dark-coloured, acid resistant stone having a MOH hardness from 7 to 8; (b) applying to the sample a test liquid which for samples of the order of 9 carats contains nitric acid of a concentration appropriate to dissolve the expected base metal content of the sample, or for samples of an order greater than 9 carats, contains a mixture of a nitric acid and hydrochloric acid of a concentration appropriate to dissolve the expected base metal content of the sample together with a predetermined part of the gold content; (c) equating persistence of the sample colour with purity as warranted or conversely equating partial or total disappearance of the sample colour with a need to test further.

2. A test kit for testing the purity of precious metals when alloyed with base metal comprising: (a) a sample taking, mildly abrasive, relatively dark coloured, acid resistant stone with a MOH hardness from 7 to 8; (b) a container containing, for samples of 9 carats, nitric acid of a concentration capable of discolouring the expected base metal content or for samples of greater than 9 carats, a mixture of nitric acid and hydrochloric acid of concentrations capable of dissolving the base metal content of the sample together with a predetermined part of the expected gold content.

3. A test kit as claimed in claim 2 wherein the stone is synthetic quartz.

4. A test kit constructed and adapted for use substantially as herein described with reference to and as illustrated in the accompanying drawings.

5. A method of testing the purity of precious metals when alloyed with base metal substantially as herein described with reference to the accompanying drawings.