Metals

Spot-tests

Spot-tests are used to distinguish different metals which make up an alloy. Simple instructions and a list of the tests are provided, to help you identify metal objects in your collections. It is important to note that these tests are only qualitative in nature—they will not tell you the relative amounts of the different metals in an alloy.

General instructions

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Do the tests in unobtrusive spots, because marks may be left.

Remove protective coatings such as lacquers and waxes, otherwise there will be no reaction and no identification can be made.

The test papers to be used with the electrolysis are best cut into triangles. They are easier to handle this way and, if cut reasonably small, will leave less of a mark on the metal object being tested.

When wet—but not soaking—with distilled water or salt solution, the papers should be shiny.

The alligator clip needs to be attached firmly to an area of reasonably solid metal.

The test paper in the tweezers should be put on the surface of the object, about 1cm away from the clip.

Don't let the metal tweezers contact the surface of the metal. The electrical contact must be made through the wet test paper.

Rinse wet spots of electrolyte, for example, NaCl—sodium chloride—or other reagents with distilled water, and dry them.

The battery should be stored disconnected from the tweezers and clip—to avoid accidental discharging if the two should touch.

Tests for identifying metals in an alloy

Antimony—Sb

Procedure: dip a small piece of antimony test paper in diluted HCl—hydrochloric acid—and apply it to the object.

Results: the presence of Sb is indicated by an orange colour. The reaction is complete in five seconds on pure Sb using 2% HCl. Surfaces containing trace Sb will be much slower. Gold—Au—and silver—Ag—surfaces remain unaffected by the test. Lead—Pb—is slightly darkened, copper—Cu—and iron—Fe—corrosion products change colour slightly.

Copper—Cu

Procedure: wet a small piece of commercial test paper—Cuprotesmo—with distilled water and place it on the surface of the metal.

Results: the copper metal or Cu⁺and Cu²⁺ions cause the pale yellow paper to turn pink/purple. This test works particularly well on corroded or patinated areas, and leaves no trace of the test. On highly polished or new surfaces, the reaction is much slower.

Gold—Au

Procedure: dip a small triangle of plain, filter paper in a saturated solution of sodium chloride in water. Electrolyse for less than 15 seconds. Some darkening will probably be evident if copper is present. Leave the paper on the spot until it is slightly dried, to ensure that gold is on the surface of the paper. Then dip it into a mixture of 20% tin chloride—SnCl₂—in 15%HCl.

Results: the paper turns black if gold is present.

Iron—Fe

Procedure: for corroded objects, dip a small square of dipyridyl test paper in distilled water and place it on the surface of the object. This leaves no visible effect on the object. For uncorroded objects, dip a long piece of dipyridyl test paper in NaCl saturated solution so that the paper is wet but not saturated—electrolyse. The paper should be long, to prevent confusion with any colour reaction which occurs with the steel of the tweezers.

Results: if iron is present, the white test paper turns red after several seconds—for corroded and uncorroded objects.

Nickel—Ni

Procedure: dip a small piece of nickel test paper into a saturated solution of NaCl. Electrolyse for about five seconds.

Results: on drying, the following colours may be observed: pink-red for nickel, brown for iron, green for copper and yellow for gold.

Silver—Ag

Procedure: wet a filter paper with 10% potassium chromate K₂CrO₄. Electrolyse for one second or less.

Results: if silver is present, a red silver chromate Ag₄CrO₄forms in the spot of the metal. This mark, if small, can be polished off very easily.

Tin—Sn

Procedure: dip a small piece of filter paper in a saturated cacotheline solution—0.6% aqueous. When the cacotheline dries slightly, dip the filter paper in NaCl saturated solution. Electrolyse.

Results: if tin is present, the paper turns purple. Shiny surfaces become matte and dark after two second of electrolysis.

Zinc—Zn

Procedure: dip a small piece of filter paper in sodium hydoxide NaOH—5–10% solution—and apply the paper to the surface of the object for five to ten seconds. Electrolysis is recommended. When the sample has been absorbed in the filter paper, place this paper in the centre of a larger filter paper, making a wet spot. Wash this spot with successive drops of dithizone/carbon tetrachloride, CCl₄.

Results: if zinc is present, the edges around the sport turn pink—not orange. Wipe off immediately any NaOH remaining on the metal of the object. This can be done with the same filter paper being used for the dithizone reaction. Shiny zinc surfaces may be slightly darkened or dulled after electrolysis. There is a small effect on some copper corrosion products, and the NaOH droplet causes a shiny spot to develop on lead.

If you have a problem relating to the care of metals, contact a conservator. Conservators can offer advice and practical solutions.