US3497401A

US3497401A - Patination of copper

Info

Publication number: US3497401A
Application number: US669287A
Authority: US
Inventors: Charles Keith Hanson; Charles I Whitman
Original assignee: INTERN COPPER RESEARCH ASS Inc
Current assignee: INTERN COPPER RESEARCH ASS Inc; INTERN COPPER RESEARCH ASSOCIATION Inc
Priority date: 1967-09-20
Filing date: 1967-09-20
Publication date: 1970-02-24
Anticipated expiration: 1987-02-24

Description

United States Patent U.S. Cl. 148-624 8 Claims ABSTRACT OF THE DISCLOSURE A method is described for producing a synthetic patina of brochantite and malachite on copper by immersion of the copper in an aqueous acidic solution at ordinary temperatures and pressures in which there are dissolved 2.5-10 g./l. of potassium chlorate and 0.520 g./l. of copper sulfate.

BACKGROUND OF THE INVENTION Field of the invention When copper and its alloys are exposed to the natural atmosphere, their surface colors will gradually change according to the copper composition and the atmospheric conditions. In the natural weathering of copper, the color changes gradually from the bright metallic pink of the new metal through red brown, dark brown, ebony, gray and gray-green to the ultimate blue-green patina. The color change of some copper base alloys follows an analogous pattern, though the rate of change and color produced may differ from those on unalloyed copper The gradual color change of the cupreous surface is due to the slow formation of a relatively thin surface layer of copper compounds produced by the chemical reaction of the cupreous surface with atmospheric constituents, such as, oxygen, carbon dioxide, hydrogen sulfide, sulfur dioxide, and sodium chloride in the presence of atmospheric humidity, dew and rain. Depending on the composition of the cupreous surface, the climate and certain atmospheric contaminants, this natural patina may appear on the surface only after exposure to the elements for decades or centuries.

In 1929, Vernon established patina to be a basic sulfate of the same composition as the mineral brochantite [CuSO -3Cu(OH) An exception to this is a product from a purely marine atmosphere, in which the basic chloride atacamite [CuCl -3Cu(OH) predominates. Where urban and marine conditions coincide, basic sulfate predominates over basic chloride. Basic carbonate, malachite [CuCO -Cu(OH) only appears to a minor extent.

The colors of the natural patina are very pleasing in appearance and are desirable in many architectural and decorative applications. The length of time required and the necessity for certain atmospheric and climatic conditions, however, render dependence on the natural process for commerical use impractical. Therefore, many artificial methods using chemicals or electrolytic processes were proposed to provide the cupreous surface with a pleasing and stable patina finish in a shorter length of time.

It has been proposed heretofore to develop a patina coloration on cupreous surface artificially by immersing such surface in various salt solutions. The chemicals, or mixtures of chemicals, using such solutions are intended to react relatively quickly with the cupreous surface to provide a surface layer of adhering, insoluble copper compounds. The color of this surface layer may vary considerably depending on the chemicals used and the "ice composition of the cupreous surface. Moreover, repeated applications are necessary with many of these prior processes, in order to obtain sufiicient depth of color.

Of the various methods developed, the electrolytic method of Vernon and the CABRA (The Copper & Brass Research Association in America) spray process have been considered to be the most promising methods. In the Vernon anodic method, an electrolyte consisting of magnesium sulfate, magnesium hydroxide, and potassium bromate in water is used for the patinating process. The deposit obtained, still requires aging to fully develop the patina. Because it is an electrolytic method, patinating treatment of large surface areas becomes difiicult if not impossible. The CABRA process requires a thoroughly clean and oxide-free surface before the application of an aqueous coloring solution consisting of ammonium sulfate, copper sulfate, and ammonia. The amount of ammonia used must be measured exactly since the ratio of ammonia to water is critical. The solution is spraycoated on the surface and allowed to dry before a second application can be made. The spraying and drying cycle must be repeated five or six times and often up to eight cycles are required. A properly sprayed finish appears glassy and requires weathering for the full patina to develop under suitable atmospheric conditions.

SUMMARY OF THE INVENTION It is the object of this invention to overcome the disadvantage of the prior methods by providing a method and a coating composition which have the advantage of greater simplicity and versatility and are capable of providing a synthetic patina of the basic sulfate and carboriate which is chemically and physically the same as the one produced by the natural aging process.

According to the present invention, no spray or electroplating equipment is required because the patina is applied by immersing the copper into the solution at ordinary temperature and pressure. Furthermore, only a single application is required and no fixing solution is necessar roadly stated the method of imparting a decorative patina finish to a cupreous surface in accordance with this invention comprises covering said surface at ordinary temperature and pressure with a layer of an aqueous acidic solution in which there are dissolved 2.5-10 g./l. of potassium chlorate, 0.5-20 g./l. of copper sulfate, up to 10 g./l. sodium sulfate and up to 2 g./l. of sodium bicarbonate. The solution is made acidic with sulfuric acid. The amount of each ingredient to be used in the solution can be varied within a wide range depending upon the type of finish desired.

We have found by using a solution of this invention a pleasing synthetic patina finish can be imparted to cupreous surfaces without the necessity of repeated applications required by prior methods. The process involves immersion of the copper into an aqueous acidic solution at ordinary temperature and pressure for a period of days.

X-ray diffraction analysis shows that a cupreous oxide (Cu O) layer is formed on the copper initially, generally within 12 hours. After a few days the cupreous oxide is gradually transformed into an outer layer of brochantite [CuSO -3Cu(OH) or malachite [CuCO -Cu(OH) depending on the particular solution used.

The solution is rendered acidic by the addition of an acid such as sulfuric acid until the proper pH is attained. There may be other substances present to obtain special advantage in rate and nature of crystal growth. Copper sulfate is essential to the second stage of the process where the cupreous oxide is slowly transformed to basic sulfate-brochantite. Sodium bicarbonate is essential to the second stage of the process where the cupreous oxide is slowly transformed to basic carbonate-malachite.

In order to further illustrate this invention, specific examples are described below.

EXAMPLE 1 A solution having the following composition was prepared.

G./l. Potassium chlorate, KClO 5 Copper sulfate, CuSO -5H 3.2 Sodium sulfate, Na SO Sulfuric acid to pH 3 The surface of the copper to be treated is cleaned with an abrasive pad to remove oxide, oil and dust. The cleaned Copper article then is immersed in the solution at ordinary temperature and pressure (by which is meant ambient room temperature and atmospheric pressure). Development of the synthetic patina begins immediately after immersing the copper into the aqueous acidic solution. After a few days the surface presents a vivid green appearance as the outer layer has been transformed into brochantite. The copper is removed on the twenty-first day and washed in a water bath.

EXAMPLE 2 A solution having the following composition was prepared.

G./l. Potassium chlorate, K010 l 5 Copper sulfate, CuSO -5H O Sulfuric acid, H 50 Treatment of a copper article in this solution is the same as in Example 1 but the exposure to the aqueous acidic solution is for nine (9) days at which time the outer layer of the copper has been transformed into brochantite.

EXAMPLE 3 A solution having the following composition was prepared.

G./l. Potassium chlorate, K010 5 Copper sulfate, CuSO -5H O 1.25 Sodium sulfate, Na SO 2 Sulfuric acid, H 50 Treatment of a copper article in this solution is the same as in Example 1 but the pH is adjusted to 3.5 and the exposure to the aqueous acidic solution is for twenty (20) days at which time the outer layer of the copper has been transformed into brochantite.

EXAMPLE 4 A solution having the following composition was prepared.

G./l. Potassium chlorate, KClO 5 Copper sulfate, CuSO -5H O 1.6 Sodium sulfate, Na SO 2.5

Sulfuric acid, H 80 Treatment of a copper article in this solution is the same as in Example 1 but the exposure to the aqueous acidic solution is for twenty (20) days at which time the outer layer of the copper has beeen transformd into brochantite.

In Examples 1 to 4 the composition may be varied in accordance with the table below:

G./l. Potassium chlorate, KClO 2.5-1O Copper sulfate, CuSO -5H O 0.5-20

Sodium sulfate, Na SO Up to 10 Sulfuric acid, H SO To adjust the solution to pH of 2.0-4.0 in which the cupreous surface is exposed for 9-21 days and is transformed into brochantite CuSO '3Cu (OH) 4 EXAMPLE 5 A solution having the following composition was prepared.

G./l. Potassium chlorate, KClO 5 Copper sulfate, CuSO -5H O 1 Sodium bicarbonate, NaHCO l Sulfuric acid, H

Treatment of a copper article in this solution is the same as Example 1 but the pH is adjusted to 6.1 and the exposure to the aqueous acidic solution is for eleven (11) days at which time the outer layer of the copper has been transformed into malachite.

In Example 5 the composition may be varied in accordance with the table below:

G./l. Potassium chlorate, KClO 2.5-l0 Copper sulfate, CuSO -5H O 0.5-2 Sodium bicarbonate, NaHCO 0.5-2

Sulfuric acid, H 80 ing the following composition:

G./l. Potassium chlorate, KClO 2.5-l0 Copper sulfate, CuSO -5H O 0.5-20 Sodium sulfate, Na SO Up to 10 Sulfuric acid, H 50 to adjust the solution to a pH of 2.0-4.0 whereby the cupreous surface is transformed into brochanite,

3. A method according to claim 1 in which the cupreous surface is exposed for 9-15 days in a solution having the following composition:

Sulfuric acid, H 80 to adjust the solution to a pH of 5.0-6.5 whereby the cupreous surface is transformed into malachite,

CuCO Cu (OH 2 4. A method according to claim 2 in which the solution contains about 5 g./l. of potassium chlorate, about 10 g./l. of copper sulfate and sulfuric acid to adjust the pH to about 3.

5. A method according to claim 2 in which the solution contains about 5 g./l. of potassium chlorate, about 3.2 g./l. of copper sulfate, about 5 g./l. of sodium sulfate and sulfuric acid to adjust the pH to about 3.

6. A method according to claim 2 in which the solution contains about 5 g./l. of potassium chlorate, about 1.25 g./l. of copper sulfate, about 2 g./l. of sodium sulfate and sulfuric acid to adjust the pH to about 3.5.

7. A method according to claim 2 in which the solu tion contains about 5 g./l. of potassium chlorate, about 1.6 g./l. of copper sulfate, about 2.5 g./l. of sodium sulfate and sulfuric acid to adjust the pH to about 3.

8. A method according to claim 3 in which the solu- OTHER REFERENCES tion contains about 5 g./l. of potassium chlorate, about Fishlock: Metal colouring, 1962 Robert Draper 1 g./l. of copper sulfate, about 1 g./1. of sodium bicar- 208 bonate and sulfuric acid to adjust the pH to about 6.1.

References Cited 5 RALPH S. KENDALL, Primary Examiner UNITED STATES PATENTS U.S. Cl. X.R.

3,284,249 11/1966 Osborn 1486.24