How to Make and Use a Precipitated Calcium Carbonate Silver Polish – Canadian Conservation Institute (CCI) Notes 9/11

A reminder before polishing

Check for marks that may help to identify a silver alloy or silver plating, and look for worn areas that may help to identify the base metal under the plating. Consult curators, other experts and available documentation to determine if there is niello, fire scale or an intentional patina. The decisions to remove tarnish from silver, on how much tarnish to remove and on the final appearance should be made jointly by a conservator and a curator. Polish only when necessary because too much polishing can remove the silver from a plated object or details from a silver alloy. Avoid polishing silver with an intentional patina, and minimize polishing on areas containing fire scale or niello.

Procedure to use the polish on tarnished silver

1. Prepare a suitable workspace, including a work surface padded with soft cotton. For further details, see Long (1999).
2. Put on a pair of disposable nitrile gloves before handling silver or chemicals.
3. Remove dust with a soft natural fibre brush. Ensure the brush is clean, as dust particles can scratch the silver. Use a new brush or one that is reserved for cleaning silver. Wash and dry the brush before it is used the first time. If the brush has a metal ferrule, cover the ferrule with tape so the metal cannot scratch the silver. Brush lightly. For further advice on cleaning, see Long (1999).
4. Use plastic wrap to cover any materials that should not be exposed to water or solvents (bone, wood or ivory, for example). In some silver trays, the rims are strengthened with iron, which must not be allowed to get wet. (A magnet can be used to check for iron at the rim of the tray.) Do not allow water to get into hollow handles or other hollow parts of an object.
5. Clean the silver with a few drops of mild detergent and water to remove any dirt and grease. Dry well.
6. Optional: If necessary, use ethanol, acetone or mineral spirits to remove adherent grease, candle wax or other organic residues. An organic solvent may also remove some of the tarnish, if the tarnish contains a significant amount of organic material (Hallett et al. 2003).
7. Prepare a calcium carbonate paste, and transfer some of it to a small part of a cotton pad (about 5 cm by 5 cm) as was done in the test on acrylic.
8. Polish the silver surface with the cotton pad by gently rubbing the surface. Use a circular motion where possible.
9. Replace the cotton pad when it is soiled. Always use a clean pad to wipe through the carbonate paste.
10. Stop often to wipe off, rinse and inspect the area being polished. Do not assume that the object needs more polishing just because a clean pad turns dark with polishing. Even after all the tarnish has been removed, the pad will still turn dark as the abrasive removes silver from the surface.
11. Do not spend too much time polishing any one area. It is better to go over the entire object several times rather than polish one area too much at once. If one area is polished too much, the whole object may have to be polished to the same level.
12. Once polishing is done, rinse the surface to remove fine particles of precipitated calcium carbonate. If possible, rinse under running water; otherwise use wet cotton flannel or cotton swabs. (Take care not to scratch the silver with cotton swabs made with wooden applicator sticks as wood may scratch silver.)
13. Dry the object well with soft absorbent cloths such as cotton flannel. Make sure there is no water left in crevices. Pat dry rather than wipe.
14. Examine the object under magnification and remove remaining traces of calcium carbonate with a soft brush or a cotton Selvyt polishing cloth. Use a small soft natural bristle brush to clean residual powder from grooves.

Some steps of the polishing procedure are demonstrated on a tarnished silver-plated tray in Figures 1 to 3. Figure 1 shows a strip of unpolished silver between two areas that have already been polished, rinsed and dried. In Figure 2, the cotton pad is rubbed on the tarnished strip. The resulting polished surface is shown in Figure 3. Some tarnish has been left in the engraved design to bring out the details.

Additional information

Removing material from recessed areas

It can be difficult to remove tarnish from recessed areas and grooves such as an engraved design. Using cotton swabs or other small soft tools can help to polish challenging areas. However, it may be beneficial to leave some tarnish (even if it is not original) in the recessed areas as it provides definition to a design and keeps a historic appearance rather than making the object look "new." The decisions to remove tarnish, on how much to remove and on the final appearance should be made jointly by a conservator and a curator.

As much residual polish as possible should be removed from the recessed areas because it disfigures the object. Figure 4 shows an example of white residue from an unknown polish that has built up after repeated polishing.

Intentional patination and other aspects of an object's history

Before polishing an object, determine whether the silver object might have been intentionally patinated. For example, between 1840 and 1950 a surface finish that was popular in Europe and America was called "oxidized silver" (Rudoe 1993). An object was immersed in a sulfur-containing solution to give a richer and older appearance. After this intentional tarnishing, the object was often lacquered to prevent further unintentional tarnishing. Another method used to intentionally enhance engraved designs was to fill them with niello, a lustrous dark material made from one or more metal sulfides (Selwyn 2004). Even in modern pieces, patination may be used to highlight marks on an object. Unfortunately, many such items have lost their original surface colour through polishing. Consult with a curator or other museum staff to identify oxidized silver, niello or intentional patination and to decide whether the surface should be polished.

Another aspect of the object's fabrication to consider is fire scale, also known as fire stain. It is a copper oxide layer (or scale) formed on sterling silver (a silver alloy containing 7.5% copper) when an object is being made (Mass and Matsen 2012). Fire scale develops when the sterling silver is annealed by heating. The heating step is followed by pickling in dilute sulfuric acid, which removes the fire scale on the surface but may leave some fire scale buried underneath a surface layer of nearly pure silver. This buried layer can be uncovered later by repeated polishing. Then it appears as an irregular dull grey area, best seen if the object is viewed against a white surface. Once fire scale has been uncovered, further polishing may expose an even larger area of fire scale, rather than removing the small area already exposed.

Before polishing, also consider whether the scratches on the object are an important part of the object's history. The scratch pattern indicates both use and repeated polishing and gives an impression of the age of the object. If the historical scratch pattern is removed through excessive polishing, this information will be lost. Excessive polishing could produce a mirror finish, which might be inappropriate on a historical object.

Distinguishing silver plate from sterling silver using hallmarks and other marks

Many different techniques and materials are used in making silver objects, but overall the objects can be separated into two categories: silver plate and silver alloys. Silver plate is a term used in two ways, to refer either to an entire object (usually tableware), or to a thin layer of silver deposited on a base metal. In this Note, the term is used to refer to the silver layer. The most familiar silver alloy is sterling silver, sometimes called 925, which contains 92.5% silver and 7.5% copper by weight. (Pure silver, also called fine silver, is seldom used to make objects because it is too soft.)

Before an object is polished, it is important to try to distinguish between a silver alloy and silver plate. For silver-plated objects, it is useful to identify the underlying base metal. (For example, the base metal could be lead and, therefore, hazardous.) With plated objects where the plating has already been damaged, it might be better not to polish, to avoid the risk of removing more of the plating.

Silver objects are often stamped with marks to indicate the silver composition or to distinguish between a silver alloy and silver plate. Sometimes these marks are required by law. In many countries sterling silver and other silver alloys must be identified in an inconspicuous area with one or more small stamps called "hallmarks" (Selwyn 2004). In Canada, hallmarks are not legally required, but any marks put on a silver object must conform to the Precious Metals Marking Act. (See also Guide to the Precious Metals Marking Act and Regulations.) Figure 5 illustrates the hallmarks on the back of a sterling silver spoon.

Different techniques have been used to apply a thin layer of pure silver to a base metal (La Niece 1993) to produce silver plate. To identify silver plate, first look for marks. Plated objects may be stamped with words such as "silver plate," or "silverplated" or with letters S.P. or E.P. The letters E.P. indicate that the object has been electroplated, a technique (first patented in 1840) where a thin even layer of metal (silver in this case) is deposited on any metal by an electrochemical process (Raub 1993). The letters E.P. may be followed by a word or by other letters to indicate the base metal: E.P. lead, E.P. copper, E.P. brass, E.P.B.M. for electroplated Britannia metal (an alloy of mainly tin with copper and antimony) or E.P.N.S. for electroplated nickel silver (an alloy of mainly copper with nickel and zinc but no silver). Figure 6 shows the mark on the back of an E.P.N.S. fork. Other marks that can identify a silver-plated object are those indicating a company that produced only silver plate. Detailed lists of marks used on silver alloys and silver plate are available; see for example the Online Encyclopedia of Silver Marks, Hallmarks & Makers' Marks, Silver Hallmarks and Marks and Silverplate Marks. Some advice on identifying plating done by techniques other than electroplating is given by Beentjes (2007).

Recognizing the base metal in a silver-plated object

Another way to identify silver plate is to look for worn or over-polished areas where the base metal is showing. Base metal is often seen at edges and corners or at the highest parts of an object where metal has been removed by touching and polishing. Figures 7 to 10 show some severe examples.

Figure 7 shows where the reddish colour of the underlying copper is visible on a silver-plated copper tray. The detail in Figure 8 reveals the golden yellow colour typical of brass in a goblet that was silver-plated. This golden colour can easily be mistaken for a thin layer of tarnish.

Figure 9 shows the detail of a silver-plated lead candlestick. The dull grey areas are lead. Lead is hazardous, and special precautions should be taken. Wear gloves, label the object as containing lead and treat any cloths used for polishing as hazardous waste. For further guidelines on dealing with lead, see CCI Note 1/8 Lead in Museum Collections and Heritage Buildings. (Lead is also present in some silver objects in decorations; for example, in trays similar to the one in Figure 7, the decoration along the edge could be made of a lead alloy.)

Figure 10 shows an example of a fork where some of the silver plating has been removed, revealing the base metal, which is nickel silver. Although duller than silver, nickel silver is similar in colour, making it useful as a base metal, since the loss of the plating metal is less noticeable than on a base metal with a contrasting colour, such as copper (Figure 7).

Identifying the presence of a coating

Silver objects may be coated with a lacquer or wax to form a barrier that helps to slow down tarnishing (Selwyn 1990). A relatively new coating on silver will be transparent and may be difficult to see. As lacquers age, they may yellow, shrink and crack. For further information on lacquers on silver, see Pouliot et al. (2015) and references therein.

On a lacquered silver object, tarnish will develop at defects in the lacquer, such as at cracks or scratches, or in areas that were not lacquered properly. Figure 11 shows an example of a lacquered silver-plated tray with dark tarnished areas and scratches.

Coatings can sometimes be recognized by comparing the reflectance of the object to an uncoated object or by looking for damage in the coating, such as tarnished scratches. One test for a coating is to measure the electrical resistance of the surface using a multimeter with an ohms function. Start with a piece of uncoated metal, and lightly touch the probes from the meter to the metal surface. Apply enough pressure until the resistance drops to a low value, indicating that the surface is conducting. Then touch the probes to the coated object, using about the same pressure. A coated surface will show a high resistance, probably off scale on the meter. Do not press too hard or move the probes on the surface; otherwise the probes could scratch through the coating to the metal underneath. If a coating is present, it must be removed before the silver is polished and a new coating applied after.

Precipitated calcium carbonate (precipitated chalk)

Precipitated calcium carbonate is available from some conservation suppliers such as Talas (calcium carbonate USP ultrafine, particle size 1.6 μm) and Conservation Support Systems (particle size 1.2 μm).

Selvyt polishing cloth

Cotton Selvyt polishing cloths are available from some jewellery suppliers and conservation suppliers. One example is Lacy & Co.

Lint-free cotton pads

Webril Cotton Handi-Pads, made by Fiberweb, are available from conservation supply companies, such as Talasor Carr McLean. Lint-free cotton pads are also available from print suppliers, such as Mark Andy Print Products (A.B. Dick brand), or laboratory suppliers, such as VWR International.

• Fiberweb
• Talas
• Carr McLean
• Mark Andy Print Products
• VWR International

Aluminum oxide powder

Aluminum oxide powder, Micropolish II (1.0 μm or 0.3 μm particle size) or MicroPolish (0.05 μm particle size) is available from Buehler.