patina
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Patina ( ital. Patina "thin layer", " varnish ") is a surface (structure and color) created by natural or artificial aging.
Basics
The patina is a layer that arises from the weathering products of the surface, sedimentation of airborne particles in the air ( dust , aerosols ) and the chemical reaction products of these substances:
- Oil paintings patinate because the varnish, which is now color-neutral and transparent as far as possible, yellows due to oxidation (the Italian name is derived from this). This is annoying for the color effect of the picture.
- A sculpture that is exposed to the effects of the weather is given a unique, often very attractive surface due to chemical or corrosive influences. Here the patina is desired as long as it is not unsightly.
- Wall paintings are exposed to heavy patination. A particular problem is that of the fresco , because its manufacturing principle means that the patina also sinters in over time after the layer of paint . It may therefore no longer be possible to remove it without destroying the fresco (or its protection).
The patina is considered evidence of the age of an object. This has two effects:
- in restoration
- Patination as artificial aging.
The patina in the restoration
In modern restoration , the patina is counted as part of the original substance, as it documents the object's history like few other aspects of the state of preservation. The preservation of monuments has distanced itself from a concept of “shining in new splendor”, but cleaning an object as part of the restoration is inevitable. The extent to which the patina should be preserved has to be considered for each object.
Patinate
Because the patina so emphasizes the age of a surface, it can be used on the one hand as an artistic means of expression to take the “hard” new thing away from an object, or in the counterfeit to cover up the new. When restoring, newly added parts are often patinated in order to minimize the contrast between old and new and to create a cohesive overall impression. In contrast to the counterfeit, the difference should remain recognizable on closer inspection. In contemporary art , patina is often used to underline the artistic effect of a sculpture. As a result, a sculpture gains plasticity , the surface appears more lively.
To patinate, the surface is often coated with a layer of paint or glaze and then processed using various techniques (scratching, brushing, polishing, etc.). Lows are usually emphasized with a darker color, while the highs are emphasized with a light color. The following materials in particular are used to produce the artificial patina: oil glaze , acrylic , wax , stain , spirit glaze as well as dyes and pigments .
Patination as "artificial aging" is also common in stage construction , costume design and model making .
Patina on metals
gold and silver
The patina of gloss gilding and - silver-plating is different from other metal Patinae: The natural patina, in particular gold, can be artificially produced little, but neither gold nor silver can be dissolved. Therefore, metal coverings can only be patinated using an oily technique. This is used in restoration when repairing gilding, where the fresh surface would be overly intrusive.
Copper and copper alloys
Copper patina, colloquially also known as verdigris , are copper ( carbonate - sulfate - chloride ) - hydroxide mixtures, urates or salts of other organic acids (basic copper compounds) that are formed by corrosion on components made of copper that are exposed to the weather, or by pickling copper and copper alloys such as bronze . The green color of copper components is solely due to the water-insoluble oxidation products of copper, which form a weather-resistant, firmly adhering, non-toxic, stable and self-healing protective layer. So it is not actually "verdigris", a trivial name for copper acetate , which because of its solubility in water has no protective properties.
Natural patina consists mainly of basic salts crystallized in layer grids (brucite type) . These include the basic copper sulfate , but less carbonate and chlorides ( one - and two-valued ). The typical color of the copper salts of the matte green patina includes color -Nuancen the turquoise copper chloridhydroxid over the green-blue to deep blue hydroxide (Bremer Green / Blue) accordingly - malachite and azurite in carbonates - and the high blue sulfate (bluestone) to black Copper sulfide and oxide .
Weather-related emergence
Natural copper patina gradually develops in the atmosphere under the influence of CO 2 , SO 2 (near the sea also chlorides ) and other substances that are contained in the air in addition to water vapor .
The color of the patina depends not only on environmental influences but also on the composition of the metal, which can lead to undesirable effects when different alloys are used on an object: the equestrian statue, reconstructed from 1989 on the Deutsches Eck in Koblenz , still showed a uniform, artificial appearance when it was erected generated patina; However, due to the different composition of the bronze used for the individual cast parts, it changed over time so that the individual parts are now differently colored and clearly distinguishable. Another example of weather-related copper patination is the Statue of Liberty , and many copper-roofed church towers are green thanks to their patina.
Artificial copper patina
There are different processes for patinating copper. You can do the copper
- Place together with a bowl with hydrochloric acid and lime (e.g. pieces of limestone or marble ) under a bell jar for a long time, during which carbon dioxide is formed,
- alternately immerse in ten percent ammonium sulfate solution for twelve hours and allow to air dry,
- Brush with a solution of 13 grams of ammonia and 6 grams of clover salt on one liter of water,
- Brush with a solution of sulfur liver (potassium polysulphide) in water. Black copper (II) sulfide is formed .
There are numerous historical recipes for the patination of roofing, which contain, among other things, horse manure , which leads to urate formation .
Brass
Fire patination of brass , see also: Ikora .
iron
Iron alloys can be burnished . A dark layer is applied to protect the surface or to give it an antique look.
Wrought iron
Wrought iron that is not exposed to the elements can be given an even surface and protected from rust by applying oil or wax.
If beeswax or another wax is rubbed or brushed onto the workpiece, which is still hot or reheated, it spreads into all pores of the material. Vegetable or mineral oils distribute themselves well even when applied cold. However, applying it to hot steel can create a darker surface. It should be noted that oils, fats and waxes can ignite during application, depending on their flash point .
Rough surfaces can also be rubbed with a cotton cloth when they are hot in order to obtain a sooty black surface, which can then be fixed by applying a drying oil.
aluminum
Aluminum can be given a brownish patina by brushing it with oil, for example linseed oil or olive oil, and then heating it to up to 400 ° C.
Tea patina
When brewing tea in a teapot, a brown coating, also known as a patina, is often formed on the inner walls. In professional preparation as well as in tea ceremonies , the teapot is only washed out with water without detergent, so that the pot does not absorb any foreign flavors. Often that tea patina is left at that.
See also
literature
- Georg Buchner: Metal coloring. Handbook for chemical, electrochemical and mechanical metal coloring together with a description of the historical development of the same. 6th increased and improved edition. M. Krayn, Berlin 1922.
- Georg Buchner: The metal coloring and its execution. With special consideration of chemical metal coloring. 2nd improved and increased edition. M. Krayn, Berlin 1901 ( online ).
- Oskar P. Krämer: Recipes for metal coloring and metal coatings without a power source. Including preparatory work: grinding, polishing, scratching, pickling, burning, degreasing. With a list of chemicals for metal coloring with colored sample boards in the original colors of the colored metals according to the recipes described. 4th supplemented edition. Leuze, Saulgau 1964 (5th edition, unchanged reprint of the 4th edition with an additional and detailed reference to footnotes. Ibid 1977).
- Thomas Brachert: Patina. The benefits and disadvantages of the restoration. Callwey, Munich 1985, ISBN 3-7667-0778-7 .
- German Copper Institute (Ed.): Chemical coloring of copper and copper alloys. 5th edition. Emphasis. Deutsches Kupferinstitut, Düsseldorf 2010 ( online (PDF; 3.51 MB) ).
- Alessandro Pergoli Campanelli: The value of patina on the antiques market - Affinities and relationships between conservation theories and buyers' taste : NEWS IN CONSERVATION, (31), 2012.
Web links
- deutschlandfunk.de , essay and discourse , June 5, 2017, Thomas Palzer : Patina - the future of finitude
Individual evidence
- ↑ Kurt Wehlte : Materials and Techniques of Painting. 4th edition. Urania, Stuttgart 1990, p. 775 ff.
- ↑ https://www.kupferinstitut.de/de/werkstoffe/anendung/bauwesen/dach-und-wand/allgemeines.html
- ↑ Little tea customer