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Asunto:RE: [gr_mulhacen] Sobre el Bicarbonato
Fecha:Martes, 4 de Febrero, 2003  08:39:21 (+0100)
Autor:"Gutiérrez Nuez, Francisco" <francisco.gutierrez>

RE: [gr_mulhacen] Sobre el Bicarbonato

Ante la pregunta de Ronaldo sobre limpieza de cobres adjunto estos dos archivos desgraciadamente en inglés . Si fuera de mucho interés general me ofrezco a traducirlo con tiempo.

Cleaning Copper

Acid usually etches and damages copper, sometimes turning it to a rosy red. Ammonia can result in the eventual disintegration of copper. Rough specimens of copper can be soaked in a weak solution of acetic acid.

Keep in mind the Triple-A Rule when working with any acid any where: Always Add Acid. This means you should always add the acid to whatever you are using to dilute the acid. Never ever add the other material to the acid.

Mix one part of glacial acetic acid, which is almost pure acetic acid, with ten parts of water. Be careful not to breathe the fumes. Prepare the mixture in a well-ventilated area. Soak the specimen in this solution, rinse thoroughly in clear water, and use a stiff bristle brush to remove any loose particles.

A more elaborate method of cleaning native copper, used by the British Museum, is recommended if the specimen is nicely crystallized. Mix one part by weight of stick-form sodium hydroxide (also known as caustic soda or soda lye) with three parts by weight of crystallized Rochelle salt (sodium potassium tartrate). Be careful: this mixture is strongly caustic and will burn moist skin if touched. How do you know if your skin was moist? By the burning sensation. Use appropriate gloves.

Dissolve the two chemicals in a glass container, using 20 parts of water, preferably distilled. When not in use, keep the solution in a closed jar to prevent evaporation. Tie a bare copper wire around the specimen to be treated, and suspend the specimen in the solution. Tie the other end of the wire to a stick, which can be place across the top of the vessel so that the specimen is left immersed. As the copper compounds dissolve, the solution will turn bluish. Raise and lower the specimen every half hour to stir the solution. When it is thoroughly clean, rinse it thoroughly in running water to remove all alkali. Soak the specimen for an additional half hour in a pail of water. If it is not nicely crystallized, the specimen may be scrubbed with scouring powder to bring out the metallic luster. Rinse in running water and allow it to dry in air.

Many specimens of native copper are obscured by black or green surface coatings which hide the beauty of the mineral. Black coatings on copper are cupric oxide -- tenorite to you mineralogists. Green coatings are generally combinations of copper sulfate (brochantite) and copper chloride (atacamite). On statues and pre-Columbian art, the green coating is termed patina and is generally regarded as a very desirable indicator of authenticity. On mineral specimens, the coating is generally regarded as offensive and most collectors want it off.

A number of techniques have been developed for cleaning copper. Some work better than others. Most involve the use of caustic solutions; so be careful. Work in a well-ventilated area, preferably outdoors.

At any rate, here is a short compendium of copper cleaning techniques ...

Make a paste using flour, salt, and vinegar. Brush it on, let it sit a while, then rinse it off. The acetic acid in the vinegar causes the tarnish to slowly dissolve. Stubborn coatings may require more than one application.

Another method of cleaning copper has been sucessfully used in at least one major museum. In a sealable glass container, mix one part caustic soda (sodium hydroxide) with three parts rochelle salt (sodium potassium tartrate). To this add 20 parts of distilled water and carefully stir until the chemicals are dissolved. Suspend the copper specimen in the solution with copper wire.

Some folk remedies for cleaning copper include scrubbing with buttermilk, using an ammonia/soapsud mixture, or (believe it or not) Toni permanent-wave solution without the neutralizer. Other folk remedies use catsup, olive oil, or baking soda with ammonia.

Some have used more drastic measures to clean copper. Most of these involve the use of very strong acids or potassium cyanide; however, these methods have been known to cause undesirable color changes in the copper (or you if you're not careful) and won't be detailed here.

Whatever method you use, be sure to throroughly rinse the specimen after cleaning and remember to dispose of all solutions safely. And once cleaned, consider protecting the freshly cleaned copper surface with lacquer to prevent further tarnishing.

Salu2 , Paco

-----Mensaje original-----
De: Rolando Gioia []
Enviado el: sábado 1 de febrero de 2003 0:59
Asunto: Re: [gr_mulhacen] Sobre el Bicarbonato

Ola Jose ,
Quero saber se alguem pode me dar uma
informação , como se limpa cobre nativo ,
pois os meus já estão ficando preto ,
se algum dos amigos tiver a resposta por favor ,
deixe - me saber.
Um abraço,

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