Basic care – Recognizing metals and their corrosion products

by Bart Ankersmit, Martina Griesser-Stermscheg, Lyndsie Selwyn and Susanne Sutherland

Introduction and authors

Introduction

The PDF format of this text is also available for purchase as a spiral-bound colour-print booklet from Lana Chan, Parks Canada.

Objectives

  • Help staff responsible for historic houses, museums, churches
  • Act as a visual guide to the problems and damage occasionally observed in metal collections
  • Offer tips for the proper care of metal objects
  • Provide reference material for further reading

Authors

Bart Ankersmit , Conservation Scientist
Cultural Heritage Agency of the Netherlands
PO Box 1600
3800 BP Amersfoort
Telephone: +31 033 – 421 71 89
E-mail: b.ankersmit@cultureelerfgoed.nl
Martina Griesser-Stermscheg , Conservator
University of Applied Arts Vienna, Conservation Department (in German only)
Salzgries 14/ 3-5
A-1010 Vienna
Austria
Telephone: 0043-1-71133-4815
E-mail: martina.griesser@uni-ak.ac.at

Lyndsie Selwyn , Senior Conservation Scientist
Canadian Conservation Institute
1030 Innes Road
Ottawa ON K1B 4S7
Canada
Telephone: 613-998-3721 or 1-866-998-3721
Fax: 613-998-4721
E-mail: lyndsie.selwyn@canada.ca

Susanne Sutherland , Conservator, Inorganics and Composites
Retired from Parks Canada

Recognizing metals and their corrosion products

Recognizing copper

Copper and Copper Alloys (Brass and Bronze)

  • colour of bare metal: various shades of yellow (pure copper is reddish)
  • not magnetic (but brass-plated iron will be magnetic)
  • brass is an alloy of copper plus zinc
  • bronze is an alloy of copper plus tin

Copper objects

Copper vase.
Freshly polished copper objects have a reddish-orange colour. Examples of objects made from copper include coins, roofing, cookware and pipes. Shown here is a copper vase. Photograph courtesy of the Canadian Conservation Institute.

Brass objects

Partly patinated brass.
Freshly polished brass is usually yellow, although sometimes it may be red. Shown here is brass that has been partly patinated. Other examples of brass objects are candlesticks, military accessories, and musical and scientific instruments. Photograph of partly patinated brass is courtesy of Martina Griesser-Stermscheg.

Bronze objects

Cast bronze figure with a natural tarnish layer.
Although polished bronze is normally yellow, objects made with bronze have often another colour because they have tarnished or been intentionally patinated. Examples of bronze objects are statues, bells and Chinese mirrors. This photograph shows a cast bronze figure with a natural tarnish layer. Photograph courtesy of Martina Griesser-Stermscheg and the University of Applied Arts Vienna.

Copper corrosion

Copper and Copper Alloys (Brass and Bronze)

  • tarnish (or patina) is black
  • more serious corrosion is green or blue-green (may be red underneath)
  • fingerprints easily stain polished metal
  • organic acids react with copper to form green corrosion products

Tarnished copper

Copper pot with the left side tarnished and the right side just polished.
Polished copper gradually tarnishes. This darkening is caused by sulfur-containing gases (e.g. from pollution, rubber, wool) in the air. This photograph shows a copper pot with the left side tarnished and the right side just polished. Photograph courtesy of the Canadian Conservation Institute.

Tarnished brass

Polished brass candlestick on the left and a tarnished brass candlestick on the right.
Polished brass can also tarnish. This photograph shows a polished brass candlestick on the left and a tarnished brass candlestick on the right. Photograph courtesy of the Canadian Conservation Institute.

Fingerprints on copper

Fingerprints on a copper surface.
Fingerprints on a polished surface are caused by touching it with bare hands. This photograph shows fingerprints on a copper surface. Photograph courtesy of Martina Griesser-Stermscheg.

Polish residue on copper alloys

Polish on the underside of a brass object.
Residue of liquid polishes can leak through holes and be left beneath an object. This photograph shows polish on the underside of a brass object. Photograph courtesy of Bart Ankersmit, Netherlands Institute for Cultural Heritage and the Royal Tropical Institute, Tropenmuseum, Amsterdam.

Residue in crevices of copper alloys

A close up of green copper corrosion, on a copper pot, located in the join where the brass handle joins the pot.
Residue of abrasive polishes can remain in crevices or engraved areas because of inadequate rinsing. The green colour is copper corrosion staining the white abrasive. This close-up photograph shows green copper corrosion on a copper pot, located in the join where the brass handle joins the pot. Photograph courtesy of the Canadian Conservation Institute.

Local spots of green corrosion on copper alloys

Waxy green corrosion that has formed on a brass surface adjacent to leather.
Local green spots on copper alloy objects may be caused by exposure to organic acids from coatings on leather. This photograph shows waxy green corrosion that has formed on a brass surface adjacent to leather. Photograph courtesy of Bart Ankersmit and the Army Museum, Delft

Green corrosion on outdoor copper alloys

Green patina on an outdoor bronze sculpture.
Green copper corrosion products form on copper alloys when they are exposed outdoors. This photograph shows this green patina on an outdoor bronze sculpture. Photograph courtesy of Martina Griesser-Stermscheg and the University of Applied Arts Vienna.

Bronze disease on archaeological copper alloys

Bronze disease on an archaeological copper alloy blade.
Bronze disease appears as light-green spots of corrosion and is caused by the accumulation of salt (sodium chloride) during burial. This photograph shows an example of bronze disease on an archaeological copper alloy blade. Photograph courtesy of the Canadian Conservation Institute.

Recognizing iron

Iron and Iron Alloys (Wrought and Cast)

  • colour of bare metal: silvery-gray
  • almost always magnetic (certain stainless steels are not magnetic)
  • historic wrought iron is almost pure iron (<0.1% carbon) with glass inclusions
  • steel is iron plus 0.2 to 2% carbon
  • cast iron is iron plus 2 to 4% carbon
  • stainless steel is iron plus chromium and nickel

Wrought iron objects

A corroded wrought iron chain.
Corroded wrought iron objects have a wood-like structure. This photograph shows a corroded wrought iron chain. Other examples of wrought iron objects are nails, rails and anchors. Photograph courtesy of Henry Unglick, Parks Canada, Ottawa.

Steel objects

A putty knife that has a steel blade.
Uncorroded steel objects have a silvery-gray colour. Many tools and machinery (lathes, milling machines) are made from steel. This photograph shows a putty knife that has a steel blade. Photograph courtesy of the Canadian Conservation Institute.

Cast iron objects

A cast iron frying pan.
A cast iron hand grenade.
Uncorroded cast iron objects have a silvery-gray colour. Examples of cast iron objects include pots, pans, fire grates, stoves, cannons and cannon balls. The photograph on the left is a cast iron frying pan, and the photograph on the right is a cast iron hand grenade. Photographs courtesy of the Canadian Conservation Institute.

Stainless steel objects

Stainless steel cutlery.
Stainless steel objects have a silvery-gray colour and do not easily rust. Stainless steel is used to make cooking utensils, cutlery, decorative architectural hardware, and for many other industrial purposes. Stainless steel cutlery is shown in the photograph. Photograph courtesy of the Canadian Conservation Institute.

Iron corrosion

Iron and Iron Alloys (Wrought and Cast)

  • corrosion is rust-coloured (red, yellow, red-brown)
  • rapid rusting (flash rusting) caused by sudden increase in relative humidity
  • drops of liquid on iron or dry, hollow shells are evidence of contamination by salt (chlorides)

Stable corrosion (stable rust)

A historic vehicle with many of its iron components covered with rust.
Stable rust is red-brown in colour, usually well adhered, and is caused by exposure to moisture sometime during an object's lifetime. Shown in this photograph is a historic vehicle with many of its iron components covered with rust. This rust will remain unchanged as long as the relative humidity remains low. Photograph courtesy of the Canadian Conservation Institute.

Damage by ongoing rusting

Rust pushing off a dark coating.
Ongoing rusting is caused by high humidity. The continued formation of rust causes flaking (or flaking of paint if the surface was painted), surface deformations and spalling. This photograph shows rust pushing off a dark coating. Photograph courtesy of Martina Griesser-Stermscheg.

Local rusting

Local rusting on an iron blade.
Local rusting appears as small areas of rust on bare (uncorroded) iron. It is caused by water remaining in one spot. It might also be caused by the local accumulation of dust or dirt. This photograph shows local rusting on an iron blade. Photograph courtesy of John Watt, Parks Canada, Winnipeg.

Fingerprints on bare iron

Touching unrusted iron can result in fingerprints or, as shown in this photograph, handprints.
Touching unrusted iron can result in fingerprints or, as shown in this photograph, handprints. Photograph courtesy of Joosje van Bennekom, Stedelijk Museum, Amsterdam.

Flash rusting on iron

Rapid rusting on a steel putty knife. The rust that forms is bright orange.
Flash rusting is rapid rusting on iron just after exposure to high humidity (e.g. a flood). The rust that forms is bright orange. This photograph shows rapid rusting on a steel putty knife. Photograph courtesy of the Canadian Conservation Institute.

Delamination of iron

As shown in this photograph of an archaeological spade from the 1st century.
Prolonged corrosion of iron, especially if contaminated with salts, can result in delamination, as shown in this photograph of an archaeological spade from the 1st century. Photograph courtesy of Ronny Meijers, Museum Valkhof, Nijmegen, Netherlands.

Weeping iron (droplets of water)

Weeping iron. This is the formation of small, round droplets of water at high relative humidity.
Weeping iron is the formation of small, round droplets of water at high relative humidity. These droplets become covered with a thin film of rust. It is caused by salt contamination in objects (these are usually archaeological). Photograph courtesy of Nestor Gonzalez, H.L. Hunley Project, Warren Lasch Conservation Center, South Carolina.

Weeping iron (dry, hollow shells)

Hollow shells.
After weeping has occurred and the relative humidity has dropped, an iron object is often covered with hollow shells of rust. This photograph shows an example of hollow shells. Photograph courtesy of the Canadian Conservation Institute.

Recognizing silver

Silver Plate and Silver Alloys (Sterling Silver)

  • colour of bare metal: silvery-gray (highly reflective when polished)
  • not magnetic
  • possibly stamped to indicate sterling silver or silver plate
  • sterling silver is an alloy of 92.5% silver and 7.5% copper
  • silver plate stamped E.P.B.M. is silver plated onto Britannia metal (an alloy of mainly tin with some antimony and copper)
  • silver plate stamped E.P.N.S. is silver plated onto nickel silver (an alloy of mainly copper with some nickel and zinc)

Sterling silver objects

Two pieces of sterling silver flatware.
Sterling silver is used to make dishes, jewellery and liturgical objects. This photograph shows an example of two pieces of sterling silver flatware. Photograph courtesy of the Canadian Conservation Institute.

Silver-plated objects

The back of a silver-plated fork stamped with the letters E.P.N.S.
Objects plated with silver are less expensive than those made with sterling silver. Examples of silver-plated objects include flatware, serving dishes, jewellery, liturgical objects, candlesticks and trophies. This photograph shows the back of a silver-plated fork stamped with the letters E.P.N.S. (indicating "electroplated nickel silver"), which means that the silver has been electroplated onto a nickel-silver alloy (a copper-rich alloy containing nickel and zinc, but no silver). Photograph courtesy of the Canadian Conservation Institute.

Silver corrosion

Silver Plate and Silver Alloys (Sterling Silver)

  • tarnish is black if thick
  • tarnish can be the colours of the rainbow if thin
  • fingerprints easily stain polished silver

Tarnish on silver alloys

The spoon on the left has been cleaned (no tarnish) and the spoon on the right has not been cleaned and is covered with tarnish.
Tarnish on silver is the gradual discolouration of silver. A thin layer of tarnish looks yellow. Tarnish is caused by sulfur-containing gases (e.g. from pollution, rubber, wool). The spoon on the left has been cleaned (no tarnish), and the spoon on the right has not been cleaned and is covered with tarnish. Photograph courtesy of Bart Ankersmit and the Groninger Museum, Netherlands.

Interference colours on silver

Red and blue interference colours on the outer edges of a silver object.
Thin tarnish layers on silver can vary from yellow to red to blue before the layer becomes black, its final colour. This photograph shows red and blue interference colours on the outer edges of a silver object. Photograph courtesy of Margo Brunn, Provincial Museum of Alberta.

Fingerprints

Tarnished silver with fingerprints. The inset shows a close-up of a fingerprint.
Fingerprints are caused by touching silver with bare hands. This photograph shows tarnished silver with fingerprints. The inset shows a close-up of a fingerprint. Photograph courtesy of Sandra Santesso, Parks Canada, Winnipeg.

Loss of plating

Detail of the loss of the silver layer from a silver-plated copper tray caused by repeated cleaning with an abrasive polish.
Damage to a plating layer or to surface detail can be caused by cleaning with an abrasive polish or by excessive handling. This photograph shows detail of the loss of the silver layer from a silver-plated copper tray caused by repeated cleaning with an abrasive polish. Photograph courtesy of the Canadian Conservation Institute.

Residue in crevices after polishing silver

The white areas show polish residue.
Polish residue on a silver object. Polish residue can be left on silver objects after abrasive polishing if the object was not rinsed well enough. The residual material is often trapped in crevices and can be white or stained green if copper is present under the plating or in the silver alloy. The white areas in this photograph show polish residue. Photograph courtesy of Martina Griesser-Stermscheg.

Recognizing tin

Tin Plate and Tin Alloys (Pewter)

  • colour of bare metal: silvery-gray
  • not magnetic (but tin-plated iron will be magnetic)
  • modern pewter is mainly tin plus some antimony and copper
  • old pewter may also contain lead

Tin-plated objects

A funnel made with tin-plated iron.
Tin-plated iron is used to make tin cans, kitchen utensils and dishes. Shown in this photograph is a funnel made with tin-plated iron. Photograph courtesy of the Canadian Conservation Institute.

Modern pewter

A dish made from modern pewter.
Modern pewter is an alloy of mainly tin, with some copper and antimony, but no lead. Pewter is easily cast and is used to make jewellery, dishes, vases, candlesticks and trophies. This photograph shows a dish made from modern pewter. Photograph courtesy of the Canadian Conservation Institute.

Tin corrosion

Tin Plate and Tin Alloys (Pewter)

  • tarnish (patina) is a darkening of bare metal
  • tarnish on leaded pewter is dark gray
  • tin corrosion products are either white or black
  • rust on tin plate is corrosion of the underlying iron

Rusting of tin plate

Red iron rust showing through the shiny layer of tin plating.
Tin-plated iron often suffers from the rusting of iron. This photograph shows red iron rust showing through the shiny layer of tin-plating. Photograph courtesy of Susanne Sutherland.

White corrosion on tin

Archaeological tin on which white corrosion is causing the dark paint to delaminate.
Tin corrodes when exposed to moisture. This photograph shows archaeological tin on which white corrosion is causing the dark paint to delaminate. Photograph courtesy of Bart Ankersmit and the National Museum of Antiquities, Leiden, Netherlands.

Pewter objects

A pewter tankard with its dark gray patina.
Old pewter usually contains lead and gradually darkens with time to form a dark gray patina. This photograph shows an example of a pewter tankard with a dark gray patina. Photograph courtesy of the Canadian Conservation Institute.

Recognizing lead

Lead and Lead Alloys (Solder)

  • colour of bare metal: dull silvery-gray (cannot be polished)
  • not magnetic
  • objects are relatively heavy
  • soft solder (mixtures of lead and tin)
An ingot of pure lead.
Lead has a low melting point and is easily cast. It has been used to make weights, communion tokens, inexpensive jewellery, components in ship models, type metal for printing, solder, ammunition and toys. Shown in this photograph is an ingot of pure lead. Photograph courtesy of the Canadian Conservation Institute.

Lead corrosion

Lead and Lead Alloys (Solder)

  • tarnish (or patina) is dark gray
  • corrosion products are usually white
  • lead is susceptible to corrosion by acetic acid (often from wood products)

White corrosion versus stable patina on lead

Lead sheet at the bottom of model on the right and left hand side.
Lead gradually darkens with time to form a stable dark gray patina (see the lead sheet at the bottom of model on the right-hand side). Lead suffering from active corrosion turns white (see the lead sheet at the bottom of model on the left-hand side). Photograph courtesy of Bart Ankersmit and the Maritime Museum, Amsterdam.

White corrosion on lead

A stained glass angel with lead cames.
A close-up showing white corrosion on the lead, except on a solder joint.
White corrosion on lead is usually caused by exposure to volatile organic acids such as acetic acid (vinegar). The photograph on the left shows a stained glass angel with lead cames. The photograph on the right is a close-up showing white corrosion on the lead, except on a solder joint. Photograph courtesy of the Canadian Conservation Institute.
Lead communion tokens after storage in wood drawers.
This photograph shows lead communion tokens after storage in wood drawers. The lead tokens covered with white corrosion and surrounded by white particles are suffering from active corrosion; the other tokens are not. Photograph courtesy of the Canadian Conservation Institute.

Gray crystals on lead alloys

Gray crystals of lead formate on a wheel on a toy after nine years in a large sealed display case.
Corrosion on lead can be caused by formic acid. This photograph shows gray crystals of lead formate on the wheel of a toy after nine years in a large sealed display case. Photograph courtesy of the Glenbow Museum.

Problems and solutions

When in doubt, consult a conservator.

Table 1. Problems and Solutions
Problems Solutions
Damage caused by handling Minimize handling, wear cotton gloves
Damage caused by abrasive cleaning Clean only when necessary; use softest abrasive for the job (e.g. paste of precipitated calcium carbonate and water)
New corrosion of metal under plating Isolate object in a dry environment
Loss of plating layer Leave alone to save information about history of use
Condensation inside sealed plastic bag
  1. Break seal by cutting small holes in bag to promote ventilation; or
  2. add a drying agent (e.g. dry silica gel) to the bag and then reseal
Blue-green corrosion on copper alloys inside wood case Add ventilation to case; consider painting case
Tarnish on silver Store inside a sealed plastic bag; better yet seal inside a plastic bag containing dry silica gel and activated charcoal
White or gray corrosion on lead Provide better ventilation to dilute source of problem (volatile organic acids) and consult a conservator about safe corrosion removal and disposal
Difficulty finding conservation resources See reference section below. See Canadian Conservation Institute (CCI) Notes (9 series) available on the CCI website.
Difficulty finding conservation advice When in doubt, consult a conservator. For more information, see the Canadian Association of Professional Conservators (CAPC) website.

Appendix: Factors of deterioration

The following factors can affect the deterioration of objects in collections. They are often used as a basis for assessing the risks to a collection:

Contaminants and pollutants

  • accelerate metal corrosion
  • contaminants may be from:
    • sulfur-containing gases (e.g. food, polluted air)
    • cleaning chemicals (especially aerosols)
    • soot, dust and dirt
    • degrading plastics

Fire

  • loss of collection

Handling (and other physical forces)

  • corrodes metals (from salts and acids on bare hands)
  • damages objects (e.g. scratching, dents, breakage)

Incorrect relative humidity

  • corrodes metals above about 65% relative humidity

Incorrect temperature

  • damages sensitive material (e.g. wood) associated with metals

Light

  • fades light-sensitive material (e.g. ribbons) associated with metals

Pests

  • attack organic material associated with metals

Security

  • loss of portable, valuable or rare objects

Water

  • corrodes metals
  • water may come from:
    • burst pipes
    • melting ice
    • leaks because of heavy rain and wind
    • floods
    • condensation

References and acknowledgements

References

  • Canadian Conservation Institute. Recognizing Active Corrosion . CCI Notes 9/1. Ottawa: Canadian Conservation Institute, .
  • Canadian Conservation Institute. Storage of Metals . CCI Notes 9/2. Ottawa: Canadian Conservation Institute, .
  • Canadian Conservation Institute. The Cleaning, Polishing and Protective Waxing of Brass and Copper . CCI Notes 9/3. Ottawa: Canadian Conservation Institute, .
  • Canadian Conservation Institute. Basic Care of Coins, Medals and Medallic Art . CCI Notes 9/4. Ottawa: Canadian Conservation Institute, .
  • Canadian Conservation Institute. Care and Cleaning of Iron . CCI Notes 9/6. Ottawa: Canadian Conservation Institute, .
  • Canadian Conservation Institute. Silver – Care and Tarnish Removal . CCI Notes 9/7. Ottawa: Canadian Conservation Institute, .
  • Canadian Conservation Institute. Mechanical Removal of Rust from Machined Ferrous Surfaces . CCI Notes 9/8. Ottawa: Canadian Conservation Institute, .
  • Drayman-Weisser, T. "Metal Objects." pp. 108-121 in Caring for Your Collections (editor H. Whelchel). New York: Harry N. Abrams, .
  • Selwyn, L. Metals and Corrosion: A Handbook for the Conservation Professional . Ottawa: Canadian Conservation Institute, .
  • Tétreault, J. "Display Materials: The Good, The Bad and The Ugly." pp. 79-87 in Exhibitions and Conservation . Edinburgh: Scottish Society for Conservation and Restoration, .

Acknowledgements

The authors thank Lana Chan and Liz Croome from the Western and Northern Service Centre of Parks Canada in Winnipeg for their ideas, comments and help.

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