Cleaning Metals

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The Cleaning and Protective Coating of Ferrous Metals Author(s): Geoffrey Michael Lemmer Source: Bulletin of the American Group. International Institute for Conservation of Historic and Artistic Works, Vol. 12, No. 2 (Apr., 1972), pp. 97-108 Published by: The American Institute for Conservation of Historic & Artistic Works Stable URL: Accessed: 11/05/2009 12:47
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Bulletin of the American

Group-IIC 12, No. 2, 1972






Geoffrey Michael Lemmer

of their corrosion products. The metals corrode easily, being attacked The corrosion products are ugly by oxygen in the presence of moisture. and deformation of an attacked object may be severe. The action of oxygen and moisture can be further compounded by the presence of salts which can cause local electrochemical activity, accelerating pitting, and mineralThe problem for the conservator is to remove and/or stabilize ization. the layer of corrosion products on the object, and to provide some method to prevent further corrosion. It is hoped that the following brief survey will assist to this end. I. Scope

able problems

Objects of iron and steel provide some of the mtost intractfor the conservator because of the variety and complexity

or coatings, but only the major ones. The chemistry will not be dealt with; this subject has been explored and their findings are readily available. presented of specific

Cleaning methods and protective coatings that have been described in the literature will be reviewed and subjected to comparative An attempt will not be made to review or test all known processes tests.

and causes of corrosion in great depth by others

made by the person doing the work and should be based on a precise knowlThe author has borrowed freely from the edge of all the factors involved. publications and work of others, and is much indebted to them. II. Cleaning

The results of the tests carried out by the author will be based on the author' s own observations together with comments The process examples. adapted for a specific object must be

Before cleaning can be settled upon, a thorough examination of the object must be made. This examination will be instrumental in Some objects that may selecting a cleaning method and a coating material. be completely mineralized, having no metallic core remaining, may have reached a stage of stability and only consolidation would be required. The surface is the first to be examined. Even though corroded iron and steel do not have the color range of corroded bronze objects, slight differences in surface texture and coloration can be noticed and may
Conservator of Glass, Ceramics, Winterthur Museum, Winterthur, and Metals, The Francis Delaware 19735. du Pont


on the surface either local indicate areas of active corrosion. Dampness in contact with as chlorides is a sure sign of active corrosion or overall, In the case of heavily iron give rise to hygroscopic corrosion products. corroded pieces, detailed examination with a magnet is recommended to ascertain the extent of the metallic core remaining. XOccasionally is a useful tool for assessing the internal condition of the radiography A needle or metal probe used with magnification can help reveal object.
hidden decoration or marks of identification which may be concealed by

the corrosion. is completed a course of action must After the examination be decided on. There are several basic methods of cleaning, with many will be broken down variations of each. For the sake of simplicity, cleaning of each. with a brief description into the five following classes The removal of the corrosion 1. Mechanical: product by use of abrading, wire brushing, shot blasting, chipping, picking, scraping, grinding, of these. or any combination In general, these methods are to be condemned for use on museum or between corrosion they do not differentiate Usually, objects. while corrosion The high areas of a piece can be damaged, metal. there are some and pores. still remains in the crevices However, if used in moderation; that are acceptable mechanical processes of treatment, picking away of gentle brushing during other classes and the use of a sharp needle to speed up a process, deep pits with This last mentioned is amplified in the an Airbrasive system. specific test section. 2. This class of cleaning can cover a wide range of different Chemical: There are the commercial solutions. paste and liquid rust solvents as well as other acids and complexing acid based on phosphoric that are neutral in pH, probably are solvents, agents; proprietary Several chemical agents are treated or chelating agents. sequestering under the specific tests. is to bury the object in The procedure Reduction: Electrochemical the metal the object is made of; zinc metal baser than a sacrificial This is done in an inert container such as enamel, is usually chosen. is then covered iron. Everything ceramic, glass and sometimes ten to twenty percent caustic soda soluwith an electrolyte, usually This procedure tion, and heated to promote the chemical activity. which can then be brushed away. will soften the corrosion layer, The solution is spent rather quickly, usually one-half to one hour, is continued until all The process and must be replaced with fresh. The amount of heat applied is dependent is removed. of the corrosion A heavy adz head with a continuous on the strength of the object. could be brought to a boil without fear of core remaining metallic belt buckle may require damage, whilst a flimsy or discontinuous to say, caution is needed with Needless the use of a steam oven. is hazardous, as are the fumes. this method as the caustic electrolyte




Reduction: This class of cleaning is similar in operaElectrolytic tion to electrochemical reduction except that the EMF (Electromotive rather than being generated during Force) is supplied externally the process. reduction is probably the most widely Electrolytic one of which is the probability used, but it has some drawbacks, of an object with a discontinuous core being mechanically weakened. variations of this procedure have been developed: different Many materials for the anode plates, and different electrolytes both caustic and acid. Other: This class involves any other method not already outlined. Some of the procedures, of course, overlap. Here expansion and contraction a. Heating and Cooling: will cause some of the corrosion to flake off. Not to be recommended, but is sometimes useful in separating of objects corroded large masses together. b. Ultrasonic Baths and Probes: The ultrasonic such as probes, the Cavitron unit, are classified with the mechanical processes. The unit has been found to be of very limited use in our work. tank cleaning, Only limited tests have been made with ultrasonic and more work needs to be done. Preparation



Each sample used for the specific cleaning tests was one-half of a lock plate with an even layer of corrosion on the surface. Each sample was prepared for testing by grinding away the corrosion from one layer corner so as to have a bare metal surface. An identification letter was the appearance of this area at the constamped into this area. By observing clusion of the tests any damage to the metal could be observed. Specific SAMPLE A: Class of Cleaning: Type: Procedure: Results: Class of Cleaning: Type: Procedure: Cleaning Tests

None None None Control





Chemical Chelating Agent, sodium gluconate Soaked in a solution of 700 ml. water, 70 gm. sodium gluconate, 70 gm. sodium hydroxide. heated Alternately to 190?F and cooled for a period of 24 days, scrubbed with a soft brush several times during treatment. The bulk of the corrosion was removed with no indication of attack to the metal.




Class of Cleaning: Type:

Procedure: Results:

Chemical Brookstone Rust Remover No. 1595 Co., Dept. C, 9 (Brookstone Brookstone Peterborough, Bldg., N.H. 03458), pH 6.75, probably a complexing agent. on the container. Followed instructions had been removed, Most of the corrosion However, except some thick deposits. the metal has also been attacked to number the extent that the identification was almost obliterated. Chemical Rust-Removo (Ace Chemical Co., 4401 Ill. 60640) North Ravenswood, Chicago, acid type. on container for a Followed directions period of twenty days. on the area treated All of the corrosion but there is evidence was removed, of attack to the metal. Electrochemical at elevated Caustic soda-zinc process temperature. Standard had been removed All of the corrosion with no indication of attack to the metal. Electrolytic Standard Standard All of the corrosion with no indication metal.



Class of Cleaning: Type: Procedure: Results:



Class of Cleaning: Type: Procedure: Results:



Class of Cleaning: Type: Procedure: Results:

had been removed of attack to the



Class of Cleaning: Type: Procedure: Results:

Chemical Naval Jelly (Woodhill Chemical Sales Ohio 44128, Stock Cleveland, Corp., No. NJ-l), acid type. on container for a Followed directions period of twelve days. was removed from All of the corrosion but there was evithe area treated, dence of attack to the metal. Chemical acid Thioglycolic As outlined in the article by T. Stambolov and B. Van Rheeden in Studies in ConVol. 13, No. 3. servation,



Class of Cleaning: Type: Procedure:



All of the corrosion was removed, but some fine pitting was produced in the metal. Chemical acid Phosphoric immersion for 16 hours 10% All of the corrosion was removed, some fine pitting to the metal. Chemical Rust Remover Jelly de-Rust, (Flexabar New Products, Inc., Northvale, Jersey; available from Edmund The pH is less than one. Scientific). The label states "Contains Acid". The sample was immersed for 24 hours rather than coated. All of the corrosion was removed with the exception of some deep pockets. Very fine pitting to the metal. Other Ultrasonic tank Immersed in a solution of Edmund Scientific S. N. Cleaner, Phosphoric 71200, P-401, approx. 5%, 5 minutes at 120?F. This cycle (20 kilohertz). was repeated six times over a period of several days, the object remaining in the solution between cycles. Most of the corrosion has been removed, but the metal itself is heavily pitted. NOTE: The pitting of the metal, I was caused by the action of believe, the acid cleaner and not the effect of ultrasonic radiation. I feel that a great deal more work is required in the area of ultrasonic cleaning and that the above test is not indicative of the potential of ultrasonic cleaning. Abrasive Industrial Unit, Model K Airbrasive, S. White Industrial (Penwalt--S. available from the Precision Products; Sales Company, 104 West Joppa Road, Baltimore, Maryland 21204).



Class of Cleaning: Type: Procedure: Results: Class of Cleaning: Type:



Procedure: Results:



Class of Cleaning: Type: Procedure:




Class Type:

of Cleaning:


Procedure: Results:

As per instructions using 27 micron aluminum oxide. All of the corrosion was removed in The metal was about 20 minutes. surface. left with a micro-pitted of the surNOTE: The micro-pitting face could be reduced even further by such as ten using a finer abrasive, micron aluminum oxide. of Cleaning


be tolerated; the use would be applicable.

If the loss of any metal from an object cannot and electrolytic of electrochemical reduction


If very small and delicate objects made up of several different is the Airbrasive besides metal are involved, materials process for the equipment is conapplicable, although the expense involved siderable. If large and bulky objects are to be treated with cost as a factor some of the acid treatments and if some loss of metal is acceptable, soluthe phosphoric-acid-based would be preferred, particularly tions.


of cleaning methods can be used to Many other combinations It is hoped on ferrous metal objects. solve a variety of corrosion problems as a guide to formulate that the reader will use the information presented methods for treating specific objects based on additional testing. III. Protective Coatings

of cleaning and proper washing of an iron Upon completion or steel object, a protective coating needs to be applied in order to halt A number of different coating materials the outbreak of further cor.rosion.
have been tested at Winterthur and the results are included in this report.

of coatings No special criteria were used in the selection were made. points were kept in mind when the selections 1.

to test, but several They are:

or nearly so. This rules out the The coating must be transparent, of pigmented coatings that are so widely used in selection large industry. From a museum point of view reversibility removal is of prime importance, although, was tested. epoxy formulation of the coating for future silicone one crosslinking



are also to be considered. and application Ease of availability and small museums do not have the resources Most private collectors available. of a chemical laboratory


The tests described in this survey were performed on polished ferrous metal samples of two different types and shapes, and in two different types of atmospheres under accelerated aging conditions. accelerated of how a coating However, aging tests may not be indicative is going to perform under normal conditions. The results must be considered with this in mind. Procedure Test chamber a. specifications: loose enough to allow for Testing Coatings

An aquarium with a fitted aluminum hood, air circulation in and out of the chamber

b. A 250-watt light source mounted inside the top of the hood. This provides light for photography and the heat source for cycling the environmental conditions c. d. A suitable liquid sired atmosphere in the bottom of the chamber to provide the de-

Controls on the heat source to be cycled from ambient 106?F and 35% R. H. (These placed hydro-thermograph weeks prior to the start of

to enable the atmosphere condition of 73?F and 88% R. H. to extremes of were obtained with a recording figures in the chamber, and operated for two actual testing. )

Test A: 1. 2. 3. 4. 5. soft iron cylindrical sections cut from the barrel of Sample material: a model 1816 common musket, filed down to bare metal, and polished. Prior to coating the samples were cleaned with petroleum benzine. hung

Each of the samples were coated with the material to be tested, from a glass rod, and placed into the test chamber. Ordinary tap water hood put on. was placed in the bottom of the chamber

and the

The chamber was cycled twice per week day for a period of twenty-nine At the end of five days acetic acid was added to the water to days. make a 1% solution. B: material: mild steel strips, wire brushed, and polished. and coated

Test 1. 2.


The samples were on the identification

with 1, 1, 1-trichloroethane degreased end with epoxy (Epo-Tek 301).



acid was placed in of 2000 ml. water and 10 ml. acetic A solution and the chamber once per weekthe bottom of the chamber cycled days. day for thirty-three C: that plain water was used for twentywas cycled


as Test B, except and procedures Same material The chamber acid solution. instead of the acetic three days.

TABLE I List and Sources COATING NO. 1 2 3 4 5 6 7 8 9 11 12 13 14 15 16 17 18 19 20 NAME Sperm Whale Oil 3 in 1 Oil Vaseline Petroleum Jelly Jelly 50% Picreator Renaissance Products, Picreator Ltd. Enterprises, 44 Park View Gardens, London N. W. 4 available at most Sodium Benzoate, Chemical Supply Houses Standard Oil of New Jersey The Butcher Polish Company, Boston, Mass.; Available at most Hardware Stores Borden, Inc., Dept. CP, New York City; Available at most Hardware Stores Rohm-Haas, Independence Square, Phila., Pa. Union Carbide (any local distributor) Frank A. Hoppe Division, Penquin Pa.; also Industries, Inc., Parkesburg, most Gun Shops W. Canning & Company, Ltd., Birmingham 18, England E. I. du Pont de Nemours & Co., Inc., Wilm., Del.; also most Auto Supply Stores Amway Corp., Ada, Mich.; Local Distributor (See 16) Midland Div., 9001 Kinsman Dexter Corp., Ohio (Code-292-7374) Road, Cleveland, Div. of C. J. Webb, Inc., CRC Chemicals, Pa.; Also many Auto Supply Dresher, Stores Div. of Economics Magnus Chemical, Inc., Garwood New Jersey Laboratory, of Materials Used in Coating & SOURCE Tests


3 in 1 Oil - 50% and Vaseline Petroleum Micro-crystalline (Renaissance) Wax

Dan & Wes Kindig, Lodi, Ohio or Dixie Gun Works, Union City, Tenn. Inc., New York City, Boyle-Midway, Available at most Hardware Stores J. Strikland & Co., Memphis, Tenn., Available at most Drug Stores (See 2 and 3)






Wax plus Micro-crystalline 5% Sodium Benzoate Fanox Butcher' s Hard Paste Spray Wax


Ea Ea

Krylon Acrylic Acryloid B-72



PVA-AYAC Hoppes Gun Grease Frigilene (Cellulose Acetate)




Du Pont Rally Car Wax Amway Shoe Spray Amway Silicone Polish




Silicone Crosslinking (Dow XR-6-2121) CRC 3-36 Magnus FF-111





TABLE Results Method of Coating No. Application* 1 a 2 a 3 a 4 a 5 c 6 c 7 a 8 c
9 11 c

II on Coatings B Points 5 5 3 Test C Coupon No. Points 2 1 3 4 4 3 5 3 6 5 7 5 8 3 3 9 10 3 4 12 6 13 14 1 15 5 Total Points
12 :

of Tests

Test A Coupon No. Points


Test CouponNo. -

2 3
4 5


8 9
10 13 14

6 6 6 6 6 6 6 6 2

3 2 4


8 9

3 6 6 6
5 3 5

12 13 14 15 16 17 18 19 20

b b

c c

15 16
17 18 19


b Application
a. b. Protective Protective

20 21 22
coating coating

6 6 6 6 6 6 6
5 5

13 14 15 16 17 18 19


5 6 5 6 5 6

17 18


20 21


3 2 5 1 4

15 12 12 17 17 15 14 8 13 18 12 17 12 15 13 17


wiped on with a soft diluted to the proper lint-free cloth and the excess and applied with consistency wiped off. a brush.

c. d.

Protective printed on the container. coating applied according to the directions The sample heated up and dipped into the protective coating, which is heated to a liquid state, and the excess material wiped off.

The coating has run due to the elevated tected in some areas. "' Material formulated as a temporary The amount
is indicated

temperature, coating.


the coupon less


protective of corrosion

Test Rating


on a coated
of points

coupon after the

of the tests

by the number

5 4 3 2 1

6 points
points points points points point

total or near total corrosion
75% - 100% corrosion 50% - 75% corrosion 25% - 50% corrosion

less than 25% corrosion no corrosion or very little

The total number of points for each coating were added together. Presumably the coating with the lowest total should be the best, but this is not always the case because some coatings performed erratically.


Test 1.


of Protective

Coatings deal, the even though

If the object to be coated is going to be handled a great grease and oil types of coatings would not be desirable, in general they tested quite well.


The coatings seem to perform much better in a non-polluted control can improve therefore, proper climatic atmosphere; resistance of almost any coating. corrosion



in proprietary The ingredients coatings are not always listed on the of the coating an unknown factor. the permanence container, making the ingredients are subject to change without notice. Moreover, the argument for periodic testing of proThis further strengthens tective coatings. Bibliography


D. M. D., "Some Methods for Proand Thacker, L., Cripps, E.S., Museums Journal, Vol. 54, No. 2 Cleaned Iron Objects", tecting (May, 1954) pp. 32-36. Vera and Hodges, Henry, "A Metallurgical of Two Iron Examination Swords From Luristan", Studies in Conservation, Vol. 13, No. 4 (November, 1968) pp. 215-223. R. M. and Bradley, York: Reinhold, 1962. W. W., Protective Coatings for Metals, New



of Corrosion to Ancient Metals "Principals Cushing, Daniel, Applicable and Methods of Identifying Corrosion of Science Products", Application in Examination of Works of Art, Boston: Museum of Fine Arts, 1967,

pp. 53-65.

T. W., Biek, L., and Wormwell, I. F., "The Role of Tannates Farrier, and Phosphates in the Preservation of Ancient Buried Iron Objects", Journal of Applied Chemistry, Vol. 53 (February, 1953) pp. 80-84. Rutherford J., "Mineral Alteration Products Gettens, Recent Advances in Conservation, edited jects", 1963, pp. 89-92. Butterworths, Gibson, Bethune M., "The Use of the Airbrasive Studies in Conservation, logical Materials", 1969) pp. 155-164. "Restoration of Modern Henson, William, 49, No. 10 (June, 1971) pp. 13-17. on Ancient Metal ObLondon: by G. Thomson,

Process for Cleaning EthnoVol. 14, No. 4 (November, Museum News, Vol.



E. and White, T., "The Protection of Metals Knowles, Journal of the Oil & Colour Chemists Association, 1958) pp. 10-23.

with Tannins", Vol. 41 (January,

and the Study of Art Objects", Norton, John T., "Metallography Application in Examination of Science of Works of Art, Boston: Museum of Fine Arts, 1967, pp. 13-19. Organ, R. M., Design for Scientific D. C.: Smithsonian Institution, Conservation 1968. of Antiquities, Washington,

Museums Journal, Vol. Organ, R. M., "The Washing of Treated Bronzes", No. 5 (August, 1955) pp. 112-119. (This entry is included because 55, outlined in it are in general applicable the washing and testing procedures to all metals. ) in the Conservation J. B., "The Use of Polyphosphate Pelikan, Complexes Studies in Conservation, Vol. 9, No. 2 of Iron and Steel Objects", (May, 1964) pp. 59-66. of Iron with Tannin", J. B., "Conservation Pekikan, Vol. 11, No. 3 (August, 1966) pp. 109-116. Mendel, History Peterson, 19 65. Institution, Plenderleith, London: Under the Sea, Studies D.C.: in Conservation, Smithsonian


of Antiquities H. J., The Conservation 1956. Oxford University Press,

and Works of Art, of Metals in the Paris: UNESCO,

H. J. and Toracca, Plenderleith, The Conservation Tropics", 1968, pp. 237-249.

G., "The Conservation of Cultural Property,

and of Metallic Antiquities and Conservation Stambolov, T., The Corrosion Central Research - A Preliminary Amsterdam: Works of Art Survey, N. D. Laboratory for Objects of Art and Science, T. and van Rheeden, B., "Notes on the Removal of Rust from Old Stambolov, Vol. 13, No. 3 Acid", Studies in Conservation, Iron with Thioglycolic 142-144. (August, 1968) pp.

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