Testing Metals

Use extreme care in handling testing solutions as they are corrosive acids.  In case of skin contact, flush with large amounts of water. Then treat affected area with sodium bicarbonate (baking soda).  If swallowed, contact a physician or hospital at once.  In case of spills, treat with water and then sodium bicarbonate, (baking soda).

The page is printed with permission by Art Arbutine


Practice and more practice, along with a test kit and using common sense are the secrets to becoming an expert metals tester. 

Don't rely blindly on acid testing.   Use your eyes, hands, and head.  Observe the color of the gold.  Scrutinize the piece.    If it looks like junk jewelry and tests to be gold, something may be wrong.   Most karat stamps are correct, but don't rely completely on all of them.   Think!!  After repeated testing you should be able to develop an instinct for the different pieces you will encounter. 
Note: Non destructive tests mentioned here mean rubbing a streak  of  metal on  the  black  stone  which  will not  damage jewelry or other items.  Destructive tests mean taking a file or knife and filing or scraping through the surface layer of metal.

HOW TO USE THE TEST KIT.  If you don't have a test kit, visit our St. Petersburg store.

IMPORTANT!    Accurate readings will be difficult unless the test stone is thoroughly sanded with #240 to 400 sandpaper.  Remove accumulated metal streaks with the same sandpaper.  Breaking in a new stone:  After the stone has been very thoroughly sanded on both sides, take a squirt bottle of 14K acid and cover the entire surface one side at a time.  You will see numerous microscopic bubbles as the acid is absorbed into the stone.  After five minutes remove the acid and sand it again, and then coat it again with the 14K.  This is the only time you will have to do this.  You may hear many exotic methods of removing built up streaks on the stone, but simple sanding with 400 sandpaper is the best and actually will improve the testing surface for future metals testing.

TEST FOR 10K GOLD:  Rub item being tested on a well sanded smooth test stone, leaving a strong visible streak.  Apply 10K acid.  Observe the reaction.(1) If test mark fades completely, item has zero gold content.  If the test mark stays bright it is at least 10K gold.(2) If the test mark stays on the stone but fades to brown, the item is less than 10K but has some definite gold content.  To determine approximate gold content, proceed to the destructive test for 10K gold. Need 10k acid? Visit our St. Petersburg store.

DESTRUCTIVE TEST FOR 10K GOLD:  File into the metal.  Apply 14K (Not 10K) acid directly upon the cut.(1) Both white and yellow gold metal:  If the cut immediately and profusely bubbles very green, the item is gold filled or gold plated and the metal is copper, nickel or some brass alloy.(2) If the application of the 14K acid on to the cut or filed flat surface causes the metal to turn brown but after 10-15 seconds there are no green bubbles, then the item tested is true 10K gold. Observation of  the green bubbles is best examined with a ten power loupe.  This can also be confirmed by blotting the acid with a white Kleenex.  The green will be visible on the tissue.  If after 10-15 seconds there are only one or two green bubbles the item is 9K.  If there are numerous green bubbles after 10-15 seconds the item is about 8K.  If bubbles appear instantly the item is 7 karat or less.  If there are profuse bubbles the item is 6K or below.  Remember:  The  information in this paragraph is applicable only after the streak faded but not completely with the 10K acid.
Note:  True 10K white gold will very slightly bubble green when the 14K acid is applied.  This is normal due to the high nickel alloy.

TEST FOR 14K GOLD:  Rub item on test stone forming strong streak.  Apply 14K acid to the streak:

(1) If test mark stays on the stone with no fading after ten seconds the metal is at least 14K.  If it fades completely there is no gold content.

(2) If test streak stays on the stone but immediately fades to brown, the metal is considerably lower than 14K.  On most American jewelry it is very likely 10K.  Note: Some jewelry with a factory stamp may be actually slightly less than 14K.  Prior to about 1982 it was legal to stamp 13.5K gold as 14K. 13.5K will cause only slight change in the streak, causing it to lose some brightness and after a while turn a rusty yellow.  You will encounter some jewelry marked 14K P.  This means 14K “plumb” gold, exactly 14K gold. Need 14k Acid? Visit our St. Petersburg store.

Note:  14K “pink” or “rose” gold and 14K white gold usually tests a slight bit weaker than yellow gold, in most all tests.

TEST FOR 18K GOLD:  Rub item on a clean spot on a well sanded test stone.  Apply 18K acid.

(1) If test mark immediately goes “lights out” to brown the unknown is almost surely 14K (If it has passed the 14K tests.)  No further testing is necessary.

(2) If the test mark stays on the stone for 10 seconds or so and then slowly fades to brown is it likely just some under karated 18K.

(3) If the test mark is unaffected by the acid and remains bright on the stone, it is at least 18K.  If you are testing white metal and suspect the item is platinum, proceed to the test for platinum.  If an unmarked item has a more yellow color, and “heft” for size than is typical for 18K gold, refer to testing for 22-24 karat gold. Need 18k acid? Visit our St. Petersburg store.

TESTING FOR GOLD FILLED:   You may encounter items belonging to a customer that have no karat stamps and you suspect might be gold filled.  Ask the customer's permission to test it on the stone.  Rub a sharp corner or edge on the stone.  Then rub another strong streak, from the same edge or corner right next to the first streak.  Then do it a third time, all three next to each other.  Apply the same 14K acid to all the streaks.  If the second or third streak fades out or significantly changes color the piece is heavy gold filled.  On most gold filled one streak is all that is needed.  If it fades away completely there is no gold content.

TEST FOR 21 TO 24K GOLD:  Assaying 21-24K is the most difficult of all tests.  The best way is to rub a streak from a known 21 or 22K coin and a streak of the unknown, side by side.  Apply 22K or platinum acid.  The lower karat metal will fade first.  This takes some practice and educated guessing.  If the item tested is said to be 24K rub it on the stone.  If you get a nice smooth even streak it is not 24K. Pure gold will not “bite” into the stone. It will be impossible to get a nice even streak and will leave little flecks of gold on the stone. Need 22k acid? Visit our St. Petersburg store.

TEST FOR PLATINUM:  Rub item on stone.  Apply 22K or platinum acid.  Observe reaction. If item gradually fades to brown it is surely 18K, presuming is has already passed the 18K test.  If the white metal remains on the stone after a minute or so with no trace of fading, the metal is platinum.
Important notice!  This is where you should start using your head.  Genuine platinum should rub relatively soft on the stone and should have a much more than average heft or heavy feeling relative to its size.  If the item is so hard that it practically squeaks across the stone, it can't be platinum.  Iron and steel items will pass the stone test for platinum.  Use the magnet provided in the kit to eliminate these.  There are many other white non-magnetic metals that will give false platinum stone readings, such as metals used in stainless flatware, seladium class rings, chromium cobalt dental material, etc. etc . There are numerous thick heavy wedding bands out there that are factory stamped PLATINUM.  Unfortunately, for those who buy these, they are some sort of worthless white stainless metal.

PALLADIUM TESTING:  Rub item on stone and apply 18K acid.  If the metal has palladium content, the streak will very slowly turn a yellowish green color.  If streak fades completely it is not palladium.

TEST FOR SILVER:  Clean the surface of the test item.  The test will be better if the surface has been scraped with a knife or a file.  Apply red silver acid in the kit on the cleaned area; the brighter the red color, the higher the silver content.  Experimentation with the red acid will be helpful.  The shelf life of the red acid is sometimes a problem.  We actually prefer the following tests for silver. Rub unknown white metal on the stone.  A smoothly sanded stone is essential for this test.  Apply 18K acid to the streak, observe reaction.  If test mark turns to intense silvery blue color, there is strong silver content. If test mark turns to a blotchy uneven blue color it is silver plate.  If it dissolves completely it has no silver content.  Even silver coins that are just 10% pure will leave some faint blue streaks or faint blobs.   A false silver reading may be obtained  if the item being tested is very heavy silver plate or a Sheffield  (silver filled)  process.

DESTRUCTIVE SILVER TESTS:  Take a file or knife to a smooth flat area of item being tested.  Go as deep as possible depending upon the value of the item being tested and the circumstances.  Then drop 14K acid on the spot.  If the metal is 90% or higher the spot will turn a whitish milky gray color, it will likely be.925 sterling silver.  The whiter the color, the higher the silver content.  If the spot turns a noticeably darker gray color the silver is approximately 80%.  The darker the color the lower the silver content.  If the silver is below 60% green bubbles will form.  If the bubbles are immediate and very green and profuse the item is not solid silver.  Silver below 50% is seldom encountered in jewelry or silverware.

COMPARISON TESTS:  One of the best ways to determine karat values is to rub a standard item (test needles, or 14K band or 10K class ring, or 22K coin, etc.) on the stone and the unknown item next to it.  Drop the most applicable test acid on both streaks at the same time.  The weaker will fade first.  Some assayers keep a small 10K and 14K ring as standards.

TESTING FOR DENTAL MATERIALS:  Little testing is usually required for yellow dental gold as it is so obvious.  A large group of dental gold will average to about 16K gold, and most metals buyers accept dental material based on assumed 16K without any testing, but individual pieces vary.  It is usually useless to attempt to get an accurate karat number using the stone and acid as the alloy dental labs use  palladium, ruthenium, platinum,  and other exotic metals that make this material test higher than it  really  is.    Some white metal dental material contains fairly high gold content and some contains little to none.  Use the tests previously mentioned here, along with the heft test, to determine whether the white material contains gold. Palladium is often evident in dental materials.  Extremely hard, white metals used in more modern bridges are made of chromium cobalt and other very hard shiny, silvery metal with no scrap or other value.

BRITISH COMMONWEALTH .375 & .333 GOLD:

These are usually from Great Britain, Australia, New Zealand and S. Africa and are symbols for 9K and 8K gold.  We have tested some of these that were significantly under karated.  Use the destructive tests for 10K if in doubt here.

PLACER GOLD AND NATURAL NUGGETS:  These are very hard to figure as they contain mineral impurities which vary depending upon where they are from.  Most pacer flakes will assay at around 80% pure.  Most nuggets are higher karat but contain more impurities and assays will vary from 70-90%.  Many dealers buy both at 18K.  None will be 24K as is the common public belief.

CREMATORY DENTAL MATERIAL:  Dealers may encounter dental material recovered from crematoriums.  These are teeth and bridges that are burned dark and are sometimes partially or completely melted.  Use extreme caution here.  While most dental material is accepted at face, there are other prosthesis metals contained in bodies that are melted along with teeth but contain no precious metals.  Many of these look like melted teeth.  Some are copper nickel or brass and will bubble green when 14K acid is applied.  Each piece should be carefully tested. 

X-RAY RECOVERY SILVER:  Properly processed, dry x-ray recovery silver will assay to about 90-95%, but losses of 10% or more, before-after melt are common due to an amalgam that evaporates during melting.  The flaky, crusty, silverish material is always higher than the black dirt type that looks like potting soil.  Caution:  Buying the black dirt x-ray material is quite risky.  If it is improperly processed or if it is damp, the actual silver content per after-melt ounce can be quite low.

INDUSTRIAL CONTACT POINTS:  Most of these show good strength on all the silver tests, however most refineries will not accept them.  When they are melted  some toxic gas is released due to cadium and sometimes cyanide content.  It is best to pass on these unless you find some refinery or other dealer who will buy them from you.  Or  you can really buy them very dirt cheap.

MELTED DOWN LUMPS:  Take a triangular file and go deep.  Apply 14K acid to the cut.  If there are no green bubbles use the stone and acid to determine the karat.  The lump should be tested in more than one place as improper melting can cause different readings on different parts of the lump.  Large thick lumps are problems as the centers may contain non precious metals.

THAILAND BAHT CHAINS: These are identified by the higher karat yellow color, exceptional heavy heft, and the omega shaped clasp.  Some are marked 24K or .999, and others .965 near the clasp.  Some have no markings.  There are only two types.  One is what is called “Thai Gold”, which is .965 fine (23.16K).  The other type is pure gold (24K).  It is very easy to tell them apart.  The 23.16K will leave a nice continuous solid rub on the stone.  No further testing is usually necessary.  The 24K is so soft it will not bite into the stone, will not make a good streak and small flakes of gold might be evident on the stone.

JEWELRY MARKED 417:   Somewhere around 1980, jewelry marked 417 started to appear on the market.  Plumb 10K would be .416666 fine, so .417 should be an indication for plumb 10K, however this is not always true.  We have tested more than 20,000 pennyweights of jewelry marked 417 and it is our estimate that about 30% of it is actually 10K; 50% is 8-9K, and 20% is 7K or lower.  Use the 10K destructive test on all that is marked 417.

COMMONLY ENCOUNTERED BOGUS ITEMS:
HEAVY ITALIAN BRACELETS:  These are extremely well made, beautiful, usually multi colored pieces of jewelry.  They feel like gold due to their thickness and large width and have dual factory stamps, 750  18K.  The bogus ones are usually very lightly gold plated over base metal.  A simple 18K stone test is all that is needed.

THAILAND PRINCESS RINGS:  These are the dome shaped rings with multi layers of semiprecious stones.  The stones are almost always real and they are universally stamped 18K.  It is uncommon to find one of these that are actually 18K.  Most are 10K or much less.  A simple stone test is all that is needed.

HAND STAMPED GOLD JEWELRY:  Many rings, bracelets and other jewelry are non factory and are occasionally under karated by the maker.  A good example of this is the heavy Cuban link bracelets and necklaces coming out of Miami.  Many of these are stamped 18K and test stronger than 14K but much weaker than 18K.  The bad ones are typically about 16K.  Most factory stamps and hallmarked items are correct.  As you gain experience, you will get a feeling for the ones that require further testing, despite the factory stamps.

STAMPS AND HALLMARKS ON SILVERWARE:   Use extreme care in buying silverware items (bowls, trays, pitchers, etc.) stamped by hand.  They are almost sure to be silver plate and stamped to deceive.  Note: If a modern American silver maker places its name and trademark on a sterling silver item, it will be clearly marked STERLING.  Similar foreign items will be marked STERLING or 925.  English silver has the “passant lion” symbol for sterling somewhere included in the hallmark.  This is a little lion with one paw up.  Older European silver might be marked 835 or 800 (83.5% and 80%).  All of the silver just mentioned is rarely under karated.  If American or foreign silver items include the capital letters EP (Electro Plate), or EPC (Electro Plated Copper), or EPNS (Electro Plated Nickel Silver), or any combination of letters including EP, then the items are silver plated and not solid silver.  No testing is necessary.

UNSTAMPED SILVER JEWELRY:  American Indian jewelry is seldom stamped.  Most all Indians buy their stock from supply houses and most of it is good sterling silver.  Little testing is usually required if the piece looks right.  Mexican jewelry stamped 925-Sterling is usually OK but there are exceptions.  The fake stuff turns a silvery faint yellow color with age and is easy to spot.  A simple stone-18K test is all that is needed.  Well made properly stamped  American silver jewelry is seldom underkarated or bogus.  If it looks right, it probably is.  The magnet test is very important when buying a large lot of silver jewelry.  Use the magnet provided in the test kit.  Items that are attracted to it are not solid silver.  Again, with experience you will get a feeling about which items require actual testing.

POCKET WATCH CASES:  Many antique or vintage gold filled pocket watch cases have factory stamps or inscriptions that read for example, “14K Dueber Special”, which means the watches are filled with 14K gold.  Most but not all genuine solid gold watches will have a factory hallmark with the inscription, “Warranted 14K US Assay”.  Solid gold cases are relatively thin.  Gold filled watch cases are usually very thick and rigid.  Push on the back of the case with your thumbs.  If it “gives a little”, it has a very good chance of being solid gold.  If it is stiff as a board it is very likely gold filled.  The best gold filled cases are very heavy gold filled.  If testing on the stone use the multiple streak as outlined in the previous, Testing for 14K Gold Filled section.

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