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Displacement Plating Silver Onto Brass

Preparing the Silvering Powder
The ratios of the reagents (chemicals) are not critical in the reaction since they are all present in excess. The following variations also produce excellent results:
  • 1 part silver nitrate (instead of silver chloride)
  • 1–2 parts potassium bitartrate (amount can vary) or potassium sodium tartrate (instead of potassium bitartrate)
  • 2–3 parts sodium chloride (amount can vary)
Begin by weighing the chemicals listed above into the ratios needed. Then the mixture should be ground to a fine powder using a mortar and pestle. To provide each student with the correct amount, divide the powder into 300 mg quantities and place into Eppendorf tubes. Approximately 300 mg of silvering powder will easily cover an area of approximately 5 cm2 (using the reagent ratios and method described in the experiment).

Alternate Displacement Plating Activity
The concept of displacement plating can also be demonstrated by simply immersing an iron screw into a solution of copper sulfate (0.5 Molar). Copper will spontaneously plate onto the iron after a few seconds of immersion. Several different metal/plating solution combinations may be tried; students may create a matrix using their results to deduce the order in the Metal Activity Series.

Creating a Design in Silver
Demonstration of displacement plating on a piece of stenciled brass.
A variation of the experiment is to create a design when silvering the brass. The students will need stencils, which may be made with craft foam sheets or purchased, and clear spray paint. The stencil is placed on top of the metal and the entire piece is then spray painted. Spray painting should be conducted outdoors or in a fume hood. Allow the spray paint to dry for several hours before silvering. When the silvering powder is applied, the areas of the brass covered with spray paint will not silver. However, if the silvering powder is rubbed with too much force, the paint may delaminate from the brass and the design will be lost. Therefore students should be cautioned to rub the powder gently onto the brass.

 

Hands-Free Silver “Polishing”

Creating Tarnish
If you don’t have a piece of well-tarnished silver, the amount of tarnish can be increased using eggs or mayonnaise. In a sealed container or bag, place the silver piece(s) together with hard-boiled eggs (peeled and sliced) or real mayonnaise for at least twenty-four hours. The sulfur released from the eggs or mayonnaise will tarnish the silver. Silver pieces for this experiment may be purchased from a thrift store; electroplated silverware is usually readily available.

Determination of Gold Karat

Preparing the Aqua Regia
Personnel experienced in handling acids should prepare the aqua regia. In a fume hood, start with one part deionized water (about 10 mL), add one part (10 mL) concentrated nitric acid, and then add three parts (30 mL) hydrochloric acid. Always add acid to water. Aqua regia will evolve chlorine gas (Cl2) during storage so it is highly recommended that any unused portion of the mixture be neutralized and discarded after completing the experiment. If stored, it should be for no longer than a few days and the mixture must be in ambient conditions in a glass container with a glass stopper (NOT in a sealed container).

Disposing of the Aqua Regia
To properly dispose of unused aqua regia it must first be neutralized in a fume hood with a basic solution. One such procedure is to add sodium bicarbonate a drop at a time to the aqua regia until it is completely neutral. Once neutralized, it can be rinsed down the sink.

Recommended Demonstration: Electroplating

A demonstration of electroplating contrasts with the displacement plating activities in that one requires electricity and one does not. Students should be reminded of the interplay between scientific innovation and art: the invention of electrical apparatus facilitated advances in the finishing of metallic art.

A simple copper plating solution can be made using 150–250 g/L copper sulfate and 40–120 g/L of concentrated sulfuric acid in deionized water. A thick copper wire or plate connected to the (+) terminal serves as the anode, while a metal object, connected to the (-) terminal of the power supply, such as a silver plated spoon, serves as the cathode. Students enjoy observing the color change on the spoon as copper is plated onto it. The copper plate will appear dull after plating because electroplated copper is microscopically rough. Students may burnish the spoon by gently rubbing it with hematite to make it smooth and reflective. It is also an interesting demonstration to reverse the electrodes (connect the spoon to the [+] terminal and the copper wire to the [-] terminal) to strip off the copper plate. After removing the copper, the silver spoon appears dull and may be shined by burnishing with hematite.

Note: The waste from this experiment must be neutralized before washing down the sink.

Hunting for False Damascene

This activity presents an inquiry-based method for students to learn about a finishing technique, which differs from a traditional “show and tell” method. Rather than leading students to a particular object in the gallery and then explaining the finishing technique, students are presented first with the finishing technique and then are asked to find an object in the gallery that accurately represents what was described.

Practice Using Scientific Language

Students may increase their understanding of scientific language by performing exercises such as the one provided here, which requires an exact description of a metal work of art. One of the categories asks the student to describe the color of the piece. Students commonly identify the color of metal as “gold” or “silver.” We have provided the visible color spectrum (red, orange, yellow, green, blue, indigo, violet) and a gray scale to encourage students to be more exact. For example, specifying “yellow” for gold or brass objects and “grayish-white” for objects of silver or other white metals. The grayish-white pieces can then be compared to the gray scale to estimate the value of gray. A metric ruler is included so that students can experiment with the metric system when approximating the dimensions of their chosen work of art.
 

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