Tannic Acid Coating for Rusted Iron Artifacts, formerly published under the title Tannic Acid Treatment – CCI Notes 9/5

CCI Note 9/5 is part of CCI Notes Series 9 (Metals)

Caution:
This Note discusses actions that will physically affect the object, and/or procedures that involve the use of chemicals. Exercise caution, and seek qualified assistance if in doubt.

Introduction

Iron objects are often covered with a layer of corrosion, which can vary from a light rust film to heavy, disfiguring scale. Corrosion is undesirable when it detracts from an object's appearance and usefulness for display. Also, because iron rarely corrodes in an even, regular manner, the corrosion layers are not as protective as those formed on other metals and they allow water vapour and oxygen to penetrate to the iron surface and react with the underlying metal (consult CCI Notes 9/1 Recognizing Active Corrosion).

Tannic acid is a complex organic acid found in plants. Different types of tannic acid are usually identified by the species of plant from which they come. When applied to iron, tannic acid reacts with the iron ions to form ferric tannate, a somewhat porous blue-black film whose degree of protection can be controlled to some extent by the method of application. It produces a uniform finish that enhances the appearance of an object (for more information on the effects of tannic treatment on iron, consult Selwyn 2004).

Tannic acid should not be used on objects originally intended to be brightly finished or painted. As tannic acid produces a blue-black surface, it is suitable for wrought and cast iron, on which such a finish is appropriate.

It is important to remember that the application of tannic acid will not eliminate the need for a museum environment, nor will it remove damaging soluble salts from actively corroding objects (consult CCI Notes 9/6 Care and Cleaning of Iron).

Preparing the Iron

Before applying tannic acid to rusty iron, the surfaces should be free of dirt, accretions, oils and grease (consult CCI Notes 9/6 Care and Cleaning of Iron). Composite objects, i.e. objects composed of different materials, should ideally be dismantled. If this cannot be done, ensure that the tannic acid does not touch any material other than the iron. Tannic acid can permanently stain materials such as wood, paper, wool, silk, leather, bone, horn and ivory.

It is not necessary or desirable to remove light rust films or heavy corrosion before applying tannic acid.

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Preparing the Acid Solution

Tannic acid is a light, fluffy powder. Its colour varies from brown to golden brown, depending on the grade and the manufacturer. Like many natural plant products, tannic acid is a mixture of polymers, which means that a uniform formula for tannic acid does not exist and that the composition can vary among available brands.

As with any concentrated chemical, tannic acid poses a potential health hazard. Be careful not to inhale the powder or bring the acid in contact with skin. A dust mask should be worn when weighing and mixing the dry powder. For convenience, it is recommended that a 10% (w/v) solution of tannic acid be prepared and portions used or diluted as needed. Keep the 10% (w/v) solution refrigerated to reduce the chance of it going mouldy.

Recipe for one litre of 10% solution of tannic acid:

  • 100 g tannic acid
  • 900 mL deionized or distilled water
  • 50 mL ethanol
  • approximately 2 mL dilute phosphoric acid (H3PO4)

Notes on the ingredients:

Tannic acid is very light, so a 100 g measure is bulky. A mixing vessel holding at least 1.5–2.0 L is therefore required.

Use deionized or distilled water; tap water may contain chloride ions and other dissolved salts that encourage iron corrosion.

Ethanol acts as a wetting agent that enhances the flow of the solution into porous corrosion layers and into all fissures on a corroded iron surface.

Phosphoric acid lowers the pH of the acid solution, and increases the amount of dissolved iron ions available for reaction with tannic acid. It is not necessary to be concerned about acid residues remaining on the iron, since phosphoric acid will react with iron ions to form ferric phosphate, which protects iron. Very little acid is introduced from this method, and there is no danger of excess amounts crystallizing on the surface.

Equipment:

  • a balance
  • a mixing container (glass), at least 1.5 L capacity, that can be heated
  • a scoop and stirring spoon or rod
  • an eye dropper
  • pH papers
  • a measuring device marked in millilitres (mL) (e.g. a graduated cylinder)
  • a stirrer/hot plate
  • personal protective equipment including gloves, a dust mask, eye protection and a lab coat or full apron

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Mixing the solution

Tannic acid stains, so protective clothing (gloves and a lab coat or full apron) are advised. Safety glasses or goggles must be worn. If eyeglasses make it difficult or impossible to wear normal safety glasses, a full face shield should be used. Tannic acid is a fine powder and, during preparation, a dust mask should be worn to avoid inhaling it.

Measure and mix 800 mL deionized or distilled water and 50 mL ethanol in a glass container. Plastic containers should not be used because they cannot be heated.

Weigh out the 100 g of tannic acid. If you do not have a balance, you may be able to borrow one from a school science lab.

Gradually add the tannic acid to the water/ethanol solution, stirring constantly. To accelerate the process, gently heat the solution on the hot plate. When the tannic acid has dissolved, add enough deionized or distilled water (approximately another 100 mL) to make a total volume of 1 L.

Test the pH of the solution using pH papers. If it is greater than 2.4, it will be necessary to lower the pH with dilute phosphoric acid.

Prepare a dilute phosphoric acid solution by pouring 9 mL distilled or deionized water into a glass container and adding 1 mL concentrated phosphoric acid, usually sold in an 85% (w/w) concentration. Always add acid to water, rather than vice versa, to avoid a violent reaction.

With the aid of a dropper, add a few drops of the dilute phosphoric acid to the tannic acid solution, stirring continuously. After adding about 10 drops, test the pH again. Continue the drop-wise addition of the acid until the pH decreases to between 2.2 and 2.4. This may require about 50 drops (2 mL) of the dilute phosphoric acid, depending on the initial pH of the tannic acid solution (consult Figure 1 for pH range). Unused dilute phosphoric acid can be washed down the drain with running water.

Schematic diagram showing pH ranging from pH 1, which is acidic, through pH 7, which is neutral, to pH 14, which is alkaline. Indicated on the diagram is the pH range for tannic acid solution, 2.2–2.4, which is acidic.
Figure 1. Schematic diagram showing pH 1 (acidic) through pH 7 (neutral) to pH 14 (alkaline). Indicated is the pH range for tannic acid solution, 2.2–2.4, which is acidic.

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Dilution of Tannic Acid Stock

To achieve an even coating of ferric tannate, it is best to apply several coats of a dilute solution of tannic acid rather than one concentrated coat. Ten percent tannic acid, as prepared above, is too concentrated to produce a good result. For most purposes, it should be diluted to between 2% (w/v) and 3% (w/v).

A simple method of dilution is to mix the 10% (w/v) solution 50/50 with deionized water, thereby yielding a 5% (w/v) solution. If desired, this 5% (w/v) solution can then be diluted 50/50 again with deionized water, yielding a 2.5% (w/v) solution.

Remember to test the pH of the diluted solution and, if necessary, add a few drops of dilute phosphoric acid until it is between 2.2 and 2.4.

Applying the Tannic Acid

Prepare a working space, preferably a table. Empty the table and cover it with polyethylene sheeting taped down at the edges. Two stiff bristle brushes, such as toothbrushes, are required: one to apply the acid and one to brush the object after the acid has dried. Because this procedure can be messy, ensure that the acid will not splatter surfaces that it could stain (e.g. wood, clothing). In good weather, the procedure can be performed outdoors. Protective clothing and safety glasses are advised.

Transfer a small amount of 2.5% (w/v) solution into a suitable container, such as a small Pyrex beaker. Heat the solution to about 50°C and brush it onto the object's surface. When this solution becomes black, discard it and start again with fresh solution.

Continue brushing as the solution dries in order to distribute the acid evenly and to introduce oxygen. This will produce an even colour.

Colour change will occur quickly, usually beginning in pitted areas. These areas resist further reaction as they become coated with an even, coherent film of ferric tannate. The tannic acid will then begin to react with less corroded areas. The presence of phosphoric acid helps establish an even reaction over the surface. Once the first coat has dried, the surface of the iron should be brushed with a stiff, dry brush to remove flaky areas of ferric tannate. Another coat of warm tannic acid should then be applied. Continue brushing as the acid dries.

Repeat these steps until a coating of the desired thickness and colour is achieved.

When dealing with case-hardened objects and steels, such as cutting tools, the tannic acid/iron reaction may be different around the working edges. Also, a different appearance in the ferric tannate should be expected at welds or joins. Heavy corrosion layers on objects will take longer to react and require more coats than will light rust or bare metal.

Some practice may be necessary to become familiar with the ferric tannate film and how to control its formation. By increasing the concentration of tannic acid from 2.5% (w/v) to 5% (w/v), a much darker, thicker film can be achieved. Excess ferric tannate will merely flake off the surface.

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Care of Objects Treated with Tannic Acid

Ferric tannate films are not vapour barriers. They will delay or retard corrosion by forming a film over iron, but this effect will not last indefinitely. The life of the film can be extended by:

  • storing the object in stable relative humidity (RH), preferably around 50% (consult CCI Notes 9/6 Care and Cleaning of Iron)
  • wearing gloves when handling the object
  • protecting the surface from dust and abrasion

Ferric tannate sometimes rubs off onto other materials. Surfaces treated with tannic acid should not be allowed to contact absorbent objects that stain easily, such as paper, textiles, leather, bone and wood.

If spots of rust begin to appear through the ferric tannate film, check the RH and carefully examine the object to ensure that the spots are not a result of chloride contamination (consult CCI Notes 9/6 Care and Cleaning of Iron). If recurrence of rust appears to be due to failure of the ferric tannate film, the object should be re-treated with tannic acid. It is not necessary to remove the original ferric tannate layer.

It is not necessary or advisable to put a protective finish, such as a wax, oil or lacquer, over a surface treated with tannic acid when the object is kept indoors under museum environmental conditions. Protective coatings like waxes and lacquers do not stop the transmission of water vapour and oxygen, only reduce them. When applied to porous corrosion layers, the coatings may be very difficult to remove if the object begins to actively corrode. A tannic acid coating will not protect against corrosion in an outdoor environment and will quickly degrade (Binnie et al. 1995).

For further information on storage of metal objects, consult CCI Notes 9/2 Storage of Metals.

Suppliers

Tannic acid:
chemical suppliers
Phosphoric acid:
drugstores, some school chemistry departments or chemical suppliers
Ethanol:
school chemistry departments or chemical suppliers
Brushes:
art supply or hardware stores
Distilled water:
drugstores or hardware stores
pH paper:
chemical suppliers

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Bibliography

Binnie, N.E., L. Selwyn, C. Schlichting and D.A. Rennie-Bisaillion. "Corrosion Protection of Outdoor Iron Artifacts Using Commercial Rust Converters." Journal of the International Institute for Conservation – Canadian Group 20 (1995), pp. 26-40.

Selwyn, L. Metals and Corrosion: A Handbook for the Conservation Professional. Ottawa, ON: Canadian Conservation Institute, 2004.


by Judy Logan
revised by Lyndsie Selwyn, Clifford Cook and Tara Grant

Originally published 1989
Revised 1997, 2007, 2013

Copies are also available in French.
Texte également publié en version française.

© Government of Canada, Canadian Conservation Institute, 2014
ISSN 0714-6221