Posts Tagged ‘cathode’

So Important for Medal Making!

“TANKS for the memory!” Sorry to start with a pun, but your plating tanks are very important for modern medal manufacturing. With apologies to Bob Hope’s theme song, your plating tanks are helping to create medals which provide very fond memories for a lot of medal recipients.

Plating – the process of electrochemical deposition of one metal on another –provides a versatility that is so necessary for the full spectrum of metal finishes required for modern medals. It has been used for 160 years in America and has replaced an even earlier technology of firegilding that was practiced by the Renaissance craftsmen in Europe and Japanese swordmakers in the Orient.

Metals of an inexpensive alloy or composition, such as copper or bronze, are plated with a more expensive metal, like gold or silver. Thus the plated objects are manufactured of a low-cost alloy but given the finish, color, and texture of the more expensive metal.

Medals can even be struck in silver and then goldplated. This is called vermeil (pronounced ver-MAY, the word and custom is French, of course). The end result is the color of gold with the sheen of silver. It also has the weight of silver which is more than that of any base metal, but less than that of solid gold.

Plating uses the same technology as electrolysis, which I wrote about in MONDAY REPORT #14 – August 2, 2010. Tanks similar to those used for casting electroforms, die shells, and galvanos, are also used for electroplating. Both technologies pay homage to Moritz Herman Jacobi (1801-1874) the German engineer who developed the process of electrolysis, and to Luigi Galvani (1745-1798) the Italian scientist who first observed electric current.

Both processes use direct current electricity at very low-voltage. A circuit is created from a rectifier which converts commercial alternating current electricity to that low-voltage direct current. (Why not use alternating current from a normal electrical outlet? – Well, it would deposit the metal in one instant and remove it the next as the electric current alternates back and forth!)

From that rectifier the direct current passes to a bus bar placed over the tanks. It is wired to a bar from which positively charged anodes are suspended. The current passes through a liquid solution (containing a salt of the same metal as the anode, the metal being plated). The current continues through the negatively charged cathode – which receives the metal being deposited – and back to the rectifier, completing the electrical circuit. When the electricity is flowing, metal is forced from the anodes into the solution and plated out of solution onto whatever is acting as the cathode.

The metal base must be thoroughly cleaned and free of corrosion, dirt, and grease in order for electroplating to create a uniform new surface over it. The new metal must bond with the base metal. Metals that alloy well together, such as copper and gold, are easily bonded by plating; those that do not – for example gold does not bond to iron or aluminum – must first be electroplated with a metal that does. Thus iron is first electroplated with copper or nickel before it can be goldplated.

Metals intended to be plated are chemically cleaned or degreased to remove any corrosion or contamination. At this stage the metal is said to have an activated surface, and it is highly receptive to tarnish. The medals should be placed in the plating tanks as soon as possible, before any tarnish starts. (Or they can be placed in a cream of tarter bath as a holding solution until they are ready to be plated.)

The size of the tank is determined by the size and number of the objects to be plated at one time. Formerly, at Medallic Art in New York and Danbury, the largest tanks were three feet deep by six feet long. These were used for electroforming and could make a copper tablet up to that size maximum.

For plating medals the tanks could, of course, be smaller. Tanks for silverplating were of a size to hold a rack of medals, six to ten medals at a time, so the silver tank was deep enough to accommodate that rack. I don’t remember its dimensions, but it could be less than two feet square. The gold tank was the smallest of all, as gold medals were plated one at a time.

The tableware industry was the first to embrace plating. This occurred in the England where two cousins, George and Henry Elkington, obtained Jacobi’s process and, developing it further, patented silverplating in 1840. The technology passed to America, first at Scovill Manufacturing in Waterbury, then in 1847 to Rogers Brothers for use in silverplating tableware.

But it was the jewelry industry in America that developed electroplating to a fine art, even using other metals to enhance the aesthetic beauty of their products. Rhodium, iridium, palladium, and nickel have all been electroplated on jewelry items.

Silverplating any object gives it all the surface characteristics of silver. It does not have the weight or fineness of solid silver, of course, but resembles silver in all other aspects. Silverplating of bronze medals is widely done in medal manufacturing and continues to be important at the present time.

As the cost of silver rises, clients may come to request their solid silver medals be replaced by silverplated base metals to reduce costs. In the past, however, when silver was less than $2 an ounce, it was less costly to strike medals initially in solid silver that were smaller than silver dollar size (1½-inch). It was less costly than adding the separate silverplating step. Today as silver is more costly, even those 1½-inch medals could be more cost effective to strike in bronze and silverplate.

Silverplated medals do not need to be edge-lettered to identify their base metal. All precious metal objects, in contrast, need to be hallmarked – to identify the fineness by inspection alone. This came about since silverplated items look so similar to solid silver. Hallmarking was established to indicate precious metal content. A law passed in 1906 requires American made products to be so marked.

Solid silver medals can be identified in words or numbers: STER, STERLING, 925 or .925 all mean sterling silver, which is 92.5% silver. Solid silver is marked FINE, 999 or .999, since it is at least 99.9% silver.

Goldplating is most often accomplished on bronze or copper alloys (which are, in fact, called “gilding metal”). While most metals can be gilded, some will not bond with gold – iron and aluminum are examples. Even so, these can be given in intermediate plating with a metal, like copper or nickel, and then goldplated.

Brass is also a popular composition for goldplating. Both metals have a similar yellow hue, and this is a reason for the choice of brass. Even if the goldplate wears away or a small portion is removed to reveal the underlying brass, it may not be that noticeable because of the similar color.

Yellow gold is free of impurities, but tiny amounts of other elements in natural gold will color (change the hue) slightly. White gold has silver or platinum impurities, pink or red gold has copper impurities, green gold has iron impurities, and the very rare black gold has bismuth impurities.

Each of these, except black gold, can be obtained by goldplating, either in the tank at the time of goldplating, or afterwards. Yellow gold is obtained by “pure” goldplating without any other metal ions in the plating solution. While white gold could be plated with silver or platinum, the same results can be obtained with tin (as sodium stannate) or nickel in the plating solution. Copper is introduced into the solution for a pink or red gold. For a green gold effect, the object is plated with silver (potassium silver cyanide) and lead acetate.

All of these techniques have been used in the jewelry industry, and there is no reason similar techniques cannot be applied to medals.

Other concepts within the subject of electroplating need to be mentioned. Listed below are those terms with a brief definition:

Anodized Aluminum. A coating, actually a plating on aluminum which, unlike other plating, can be done in a variety of colors.

Bleed, Bleeding. Exposure of the base metal of a plated piece, from wear or from too thin a plating (occurring from flash plating).

Bright Plating. An enhanced coating by addition of more than one type of metal anode in the electroplating process.

Electroless Gold. Coating by immersion in liquid gold baths.

Electrolysis. The physical and chemical process of passing a low voltage direct current through a liquid electrolyte containing a solution that carries ions of anodic metal to deposit on the cathodic work.

Electroplating. A very thin coating of metal applied by electrolysis on a coin or medal to improve its surface metal, color or finish.

Firegilding, Firegilt. An early form of goldplating where a mixture of gold and mercury is applied to the surface to be plated, then the object is heated to drive off the mercury.

Flash Plate. Very thin plating; minimum required to cover areas desired.

Gilding, Gilt. Covered with gold; the application of gold to an object by any process.

Goldwash. A light goldplating after a piece has been bright dipped or polished.

Hard Goldplate. An enhanced goldplating by the addition of a small amount of other metal in the process of goldplating.

Heavy Goldplate. A special goldplating in which a tiny amount of silver – as one part per 50 of gold – is introduced into the electrolysis to effect a harder gold surface; also called bright plating or hard goldplate.

Immersion Gold. A coating of gold by melting the precious metal and inserting a medallic item into the dip solution.

Mordant Gilding. A process of depositing gold on a metal surface, much like firegilding.

Mosaic Gold. A very cheap gold coloring process using stannic sulphide.

Parcel-gilt. Partly gilt; goldplated only on a part of a medallic item for the effect of a contrast of metals or their color.

Reverse Plating. A striping away of an unwanted layer of surface metal; the removal of the outermost layer from a plated item, usually silver or gold in unsightly condition; electrostripping.

Sheffield Plate. An early form of silver bonded to copper by fusion, then rolled to desired thickness.

Silverclad. A base metal bonded or clad with silver, of which the silver may be pure, sterling or other alloy.

Silvered. A metal finish, actually a coating, of gray color with a metallic luster.

Silverplate, Silverplating. Depositing a layer of silver on a piece of base metal by the process of electrolysis.

Silver Strike. A term in electroplating meaning the flash plate of silver prior to other metals being electroplated on top of this.

Test Cut. A purposely made cut into the edge of a numismatic item by unthinking people to ascertain the item’s base composition, usually of a plated piece.

Vermeil. Goldplated silver or silver gilt.

X Gold. Extra heavy goldplate.


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