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In the sixteenth century it became fashionable to add a layer of silver onto brass clock dials using the process of displacement plating that is described in the procedure below.

Materials | Procedure | Examining the Reactions

Brass is an alloy, which means it is composed of two or more metals. When composed of 85 percent copper and 15 percent zinc, it resembles gold in color. When silvered, its surface color changes to white. Silvering was not only undertaken for decorative purposes, but also to ensure that the black numbers of the dial could be easily read against its white background. Examples of silvered clock dials can often be found in museums. Many have a dark tarnish, which is the result of the silver oxidizing in reaction with sulfur-containing molecules that are always present in low concentrations in the air. Today, silvered clock dials are often protected with a clear coating to prevent oxidation.

How did people obtain the chemicals needed for displacement plating a few hundred years ago? They relied on nature. Cream of tartar is a byproduct of wine fermentation. Sodium chloride can be made by evaporating ocean water. Silver chloride is created by heating a mixture of silver nuggets (mined silver ore), salt (sodium chloride), and water; or it can be mined as the mineral cerargyrite, or horn silver. (Early names for silver chloride were Lac argenti and Luna cornea.) Hematite or agate, which are naturally occurring stones, were used as burnishing tools, and saliva was applied to the brass to assist with burnishing.

Brass clock dial with silvered surface. Detail from the Tall Case Clock 1930-124-1 illustrated in the displacement plating section.

Materials

  • Gloves and safety glasses
  • Silvering powder (300 mg portions in Eppendorf tubes)*
  • Small, plastic weigh boat
  • Knit cotton cloth
  • Deionized water in a wash bottle
  • Brass piece to be silvered (any copper-based metal may be used: brass, bronze, or copper)
  • Waste container
  • Liquid soap (or saliva may be substituted)
  • Burnishing stone (hematite or agate)
  • 1 part silver chloride, AgCl
  • 2 parts cream of tartar, potassium bitartrate: KO2CCH(OH)CH(OH)CO2H
  • 3 parts sodium chloride, NaCl (table salt)
Additional materials needed to prepare the silvering powder.
  • 3 spatulas or scoopulas
  • Balance
  • Mortar and pestle
  • Brown or amber jar for storage of the silvering powder (AgCl is light sensitive)
  • Eppendorf tubes for dividing the powder into 300 mg portions

* To prepare the silvering powder, combine the following chemicals (salts). See the supplementary information section in the appendix for further instruction.

Note: the waste from this experiment may be washed down the sink.

Procedure

Please read through the entire procedure before you begin.
  • Put on gloves and safety glasses.
  • Using your thumb, carefully open the Eppendorf tube containing the silvering powder. Pour the powder into the weigh boat.
  • Wrap your finger with cotton cloth and dampen the cloth with water from the wash bottle.
  • Dip the dampened cloth into the weigh boat and lightly tap your finger on the powder until it forms into a paste.
  • Wait for 30 seconds.
  • Pat small amounts of the silver powder onto the brass. Use the cloth to apply the powder evenly over the entire surface and gently rub the surface in an overlapping circular motion. Do not press down on the surface. (At this point, the yellow colored brass surface should start to appear grayish white—the silver is appearing! Try to spend the same amount of time rubbing each area: start at one corner and work your way across the entire surface.)
  • After the entire surface appears to be grayish white, stop rubbing. More rubbing will not add more silver; it will make the silver coating uneven.
  • Wash the brass sheet using water from the wash bottle, collecting the water in the waste container.
  • Dry the brass with a dry, clean part of the cloth.
  • To assist with burnishing, place liquid soap or saliva on the brass. Then burnish the silver coating by rubbing a small piece of highly polished, tumbled hematite or agate over the surface until the silver is shiny. Do not press down on the stone—doing so will only scratch the surface and remove silver instead of making the silver reflective.
  • Wipe off any materials left on the brass with a dampened clean cloth.

Examining the Reactions

The text in the Chemical Finishing Techniques section explains the basic process of displacement plating, but does not address the role of the reactants. For example, what roles do sodium chloride and cream of tartar have in the reaction? Below are some detailed explanations.

A slow start
Why did we wait for thirty seconds? We waited to allow the three salts that make up the silvering powder—silver chloride, potassium bitartrate (cream of tartar), and sodium chloride—to dissolve (the three reactions are listed below). However, because silver chloride is not very soluble in water it never completely dissolves; you probably observed the presence of white crystals throughout the experiment.

  1. Dissolution of silver chloride:
    AgCl → Ag+ + Cl-

  2. Dissolution of cream of tartar:
    K●C4H5O6→ C4H5O6- + K+

  3. Dissolution of sodium chloride:
    NaCl → Na+ + Cl-

    It is important that the salts dissolve because they facilitate the displacement reaction by increasing the ionic strength of the solution. Some of the ions from these dissolved salts also directly participate in the displacement reaction.

    The displacement reaction
    Recall that the brass alloy used here is 85 percent copper and 15 percent zinc. We’ll ignore the zinc part of the brass alloy since it reacts in a similar way to the copper in this displacement reaction. From the Metal Activity Series, we know that copper will donate electrons to silver because it is more easily oxidized than silver. Written below is the overall displacement reaction between silver ions and copper metal that produces silver metal and copper ions:

  4. Silver displaces copper:
    Ag+ + Cu0 → Ag0 + Cu+

    The role of chloride ions (the importance of a fresh [unoxidized] brass surface)
    For silver to deposit on a copper surface, we need a fresh metallic copper surface, Cu0. In air, the surface of copper is always oxidized, mostly to cuprous oxide, Cu2O, with a few layers of cupric oxide, CuO. Note that copper is in the plus-one, Cu+, oxidation state in cuprous oxide and in the plus-two, Cu2+, oxidation state in cupric oxide. The chloride ions (from sodium chloride and silver chloride) react with these copper oxides to dissolve and remove them from the surface of the copper. On the surface, Cu+ (in cuprous oxide) reacts with chloride ions to form cuprous chloride salts (5). Some of the cuprous chloride salts are insoluble in water, but they undergo a disproportionation reaction into cupric chloride, which is water soluble, and copper (6). The cupric chloride salts dissolve (7), and when they do the underlying copper surface, Cu0, is exposed.

  5. Formation of cuprous chloride salts:
    Cu2O + 2 Cl- + H3O+ → 2 CuCl + OH- + H2O

  6. Cuprous chloride disproportionates, forming cupric chloride and copper:
    2 CuCl → CuCl2 + Cu0

  7. Dissolution of cupric chloride salts:
    CuCl2 → Cu2+ + 2 Cl-

    Recall that the overall displacement reaction is: Cu0 + Ag+ → Ag0 + Cu+

    So we need to have Cu0, not Cu+, that is to say we need copper metal, not oxidized copper, for the reaction to proceed from left to right. The chloride ions drive the displacement reaction in two ways: 1. by directly forming Cu0 as a product of the disproportionation reaction (6) and 2. by exposing underlying Cu0 when cupric chloride dissolves (7).

    The role of tartrate ions, which drive the displacement reaction forward
    Tartrate is a complexing agent, meaning that it can bind to metal ions, like copper ions from the dissolution of cupic chloride.

  8. Cu2+ + n C4H4O6→ [Cu●(C4H4O6)n]2-n
    (where n≥3)

    By complexing (or binding) the copper ions produced in reaction (7), reaction (8) is driven to the right according to Le Châtelier’s Principle. Therefore, the complexing agent ensures that when copper ions are produced they are bound by tartrate. When bound, they diffuse to expose a fresh copper surface that is activated for the displacement reaction with silver ions.

To summarize the purpose of the reactants: the dissolved salts assist the electrochemical displacement reaction by increasing the solution’s ionic strength, the chloride ions dissolve oxidized copper, while the cream of tartar complexes the dissolved copper ions. Together, these reactants drive the overall displacement reaction.

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