Here's the second and final part of Dick Johnson's Encyclopedia of Coin and Medal Terminology entry on Finish and Finishing.
Oxidation (actually a sulphatization). The wire basket containing the abrasive-blasted medals are then immersed in a bucket containing the darkening chemical (as ammonium sulfide) for the next step of antiquing. The wire basket is rotated by hand holding the handle, swishing it around submerged in the liquid so it comes in contact with as much of every medal's surface as possible.
Time is quite critical. The chemical acts quickly. The medals turn black almost immediately. The sulfur from the ammonium sulfide combines with the surface molecules of the bronze or silver to form copper sulfide or silver sulfide. The metal reacts instantly with the sulfide, the process of sulphatization (unfortunately it is called oxidation, which implies oxygen, but the reactive ingredient is the sulphur, no oxygen is required). This chemical action occurs in from five to ten seconds.
The medals are again changed in color (as well as their surface changed chemically). From gray abrasive blasted, they are now black. The operator can control the blackening somewhat. In the first few seconds bronze turns dark brown, then black (silver turns black immediately). The longer the medals are left in the solution, the darker (blacker) they become, reaching a maximum blackness in the ten seconds.
The wire basket containing the medals is removed from the bucket containing the darkener and immediately immersed in water. This stops the chemical action. Then fresh water is run over the medals to wash away any remaining darkening chemical, and any residual abrasive. The wet medals are removed from the wire basket, placed in a tote box and moved to the relieving station.
Relieving and highlighting. Dry ground pumice in very fine grain size is mixed with water until it has the consistency of thick soup. The operator douses a slurry of this
mixture on the surface of a darkened medal while he holds it in his hand. It is then buffed by hand on a motorized muslin buffing wheel by lifting the dripping medal upwards under the rotating wheel. (The wheel is rotating counterclockwise, otherwise the operator would get a face full of slurry. A splash guard behind the buffing wheel collects the spun off slurry instead.) It is certainly understandable why this equipment is called wet wheel and the process wet-coloring.
The operator rotates the medal by hand then makes a couple of passes under the buffing wheel. The buffing action heats up the medal. By adding more pumice slurry he lowers the temperature so his fingers can control the medal. He repeats the slurry and buffing process as often as necessary for each side of the medal. Then he will buff the edge, rotating it while the revolving buffing wheel is in motion. Skill comes in handling the slippery moist medal, and not dropping it, while making an even two-tone color on both sides, with an edge to match.
The buffing wheel removes the darkened surface in areas where it can reach – flat surfaces and high areas of the detail. Since it does not touch the surface in the crevices and the corner areas it does not remove the darkened surface there. Thus the completely relieved medal would be light (the color of the original bronze or silver) on the flat and high surfaces, and the contrasting dark color in the crevices and corners.
This total process creates the light-and-dark, two-toned effect to give it its highlighted appearance. The optical effect is to make the object's relief in greatest contrast for the human eye to perceive. Highlighting accomplishes this.
Drying and lacquering. After their treatment under the wet wheel, the medals are
washed under running water of all the pumice slurry. They are then laid on a flat rack accommodating a dozen or so medals to be dried. The rack is then placed under ultraviolet lights (or in an oven) to completely remove all moisture.
Moisture is the enemy of the next step – lacquering. A wet spot, the tiniest
bit of water, or a person sneezing on the medal's dry surface, will, in time, cause dark spots to form on the medal's surface under the lacquer (it is even called water spot). Every effort is made to dry the medals thoroughly, both sides, then get them in the lacquer spray booth at once.
The rack is laid on a turntable in the spray booth. A spray gun – the best method
For applying lacquer – is passed over the medals once back and forth. The rack is rotated one quarter turn. The medals are sprayed again, once. Thus the medals have received the minimum amount of lacquer but in two directions, once horizontal, once vertically.
This thin coating of lacquer is perfect. It is so thin (less than a few thousandths of an inch) it cannot be seen with the naked eye. With any additional spraying the lacquer would build up in the corners and be unsightly. Dipping causes this more so, and brushing (the only other ways of applying lacquer) leaves brush marks. Spraying is ideal.
Applying the light lacquer coating while the medals are still warm from their heat lamp drying treatment will give a matte appearance to their surface. If a glossy finish is desired additional passes of the spray gun with a heavier coating will increase the reflectiveness to give the surface a shinny appearance.
The lacquer is usually clear. However, the lacquer can be colored, by adding metal powders (bronze powders), dyes or tints. Thus the spraying can keep the color of previous metal coloring (with transparent lacquer), or change it drastically, or only slightly.
With one side of the medals sprayed both directions, an empty rack is placed over the medals, and by holding the two racks tightly together, the operator can flip this over, to lay the second rack on the turntable and spray the second side of the medals. The lacquer drys almost instantly, the finishing of the medals are now complete and need only
to be carried to the next workstation for any mounting, fabricating, edge lettering or numbering, or for inspection and packaging.
Imitating gold and silver. Not surprising, a lot of the activity in the finishing department is to make metal objects appear more attractive or desirable. Base metal items are often desired to be colored gold or silver. Undoubtedly this is a carry over from the jewelry field where so many of these processes were developed. Objects are struck or cast in base metals for the lower metal cost and given a surface, or coating, or finish, to appear as the more expensive noble metals.
While some metal compositions are chosen to imitate gold – brass, goldene,
oroide – several methods imitate gold with layering (as rolled gold), clad, or electroplating with the outer layer actually gold. nickel silver is a composition in imitation of fine silver. But silver can even be imitated with silver wash, silvered, silverclad, and silverplate. The finishing department must be able to work with all of these, but most often utilizes gold and silver plating.
Electroplating. A well equipped finishing department for medal manufacturing will require both gold and silver electroplating tanks. These are used for finishing medallic items with the required surface color by plating. Usually, because the finishing foreman has the knowledge of electrolysis, he is also in charge of making galvanos by electrolysis even though this is a production of the item, not just the finishing (unless the galvano is required in a further production step, as a dieshell, in which case it is not required to be finished).
Medallic items to be plated are placed on racks which are immersed in the electrolyte solution. Anodes of the pure medal – gold or silver – are also in the solution. The items to be plated are the cathode. Electric current is turned on and, by the process of electrolysis, the metal is leached from the anodes and deposits on the cathode (the medallic items). A further description of this process is given under the entry on electroplating.
Patina finishes. While oxidizing (sulphatization) with a darkening chemical and highlighting (by relieving) is but one method of finishing a medal, a medal can also be finished with a patina. Patina finishes offer a range of surface protection – in a spectrum of possible colors! With a quantity of browns and greens for bronze items and grays and blacks for silver. The choice of a patina is an artistic one, it should be made to harmonize with the design or amplify the theme. It is often made in consultation between the sculptor or creator of the design and the finishing department foreman.
Each patina finish has its own methodology, and these techniques of application may range across a broad spectrum of activities. The variables concern: (1) the temperature of the medal, (2) the temperature of the patina solution, (3) how a solution is applied, (4) how long a solution is applied, (5) the type of applicator (if any), (6) any wet wheel application, (7) how a medal is supported while work is performed on it, (8) how the solution is allowed to set or dry, (9) pH of an acid solution, others.
There are, perhaps, forty or more commonly used patina formulas for medals. This despite the 1,126 different patinas listed in Hughes and Rowe, the standard work on patinas for the field of metal coloring. The authors recommended only 344 of these formulas and techniques.
The Society of Medalists issues, created by Medallic Art Company, all have different patina finishes up to about issue #96. Although some are slight modifications the attempt of the Society was to issue a different patina for each issue. This medal series exhibits, therefore, examples of what can be done in a modern finishing department. An even more dramatic series was the Religions of the World, issued by Presidential Art Medals in which 15 different medals, each a different religion, each with a different patina.
The entry under patina lists the colors, chemicals, and names of the more popular patinas. The reader is directed there for these specific patinas.
Enameling adds color. The finishing department must also be able to do enameling as well as other finishing processes. Enamel adds color by applying tiny colored glass beads in an enclosed area on the surface of a medallic item. The cloisonné process requires fences or sides of relief to contain these loose beads, but an astute designer can work these raised surfaces into the overall design. Separate color beads are added for each different color.
With the medal laying flat and all areas to be colored with enamel filled with loose glass beads the piece is then fired. It is placed in an oven which melts the beads which become hardened and fixed in position. Enamel is essentially colored glass applied to a metal object. It is the most ideal way to add different colors to a medallic item, the colors are permanent and solid. However, since the enamel is glass it is susceptible to breaking, particularly near the edges.
Fabricating and mounting. To complete a medallic item the finishing department is required to do any final steps in its manufacture by fabricating – bringing together any additional assembly, like placing a ribbon drape on a medal – or mounting the item, as on some other object. Further details on these can be found in the entries on fabricated and mounting.
Thus we have observed that in contrast to coins, which require no finish (that's why it's called coin finish), medals require and can be adorned with a great variety of finishes. It is the finishing department that accomplishes all these functions.
To read the complete entry on the Newman Numismatic Portal, see:
Finish and Finishing
To read the earlier E-Sylum article, see:
VOCABULARY TERM: FINISH AND FINISHING, PART 1
Wayne Homren, Editor
The Numismatic Bibliomania Society is a non-profit organization
promoting numismatic literature. See our web site at coinbooks.org.
To submit items for publication in The E-Sylum, write to the Editor
at this address: email@example.com
To subscribe go to: https://my.binhost.com/lists/listinfo/esylum
Copyright © 1998 - 2021 The Numismatic Bibliomania Society (NBS)
All Rights Reserved.
NBS Home Page
Contact the NBS webmaster