El albayalde es el carbonato de plomo básico 2PbCO 3 ·Pb(OH) 2 . [1] Es una sal compleja que contiene iones carbonato e hidróxido. La albayalde se presenta naturalmente como mineral, en cuyo contexto se la conoce como hidrocerusita , [1] un hidrato de cerusita . [2] Antiguamente se usaba como ingrediente para pintura con plomo y un cosmético llamado cerusa veneciana , debido a su opacidad y la mezcla suave y satinada que hacía con aceites secables. Sin embargo, tendía a provocar intoxicación por plomo y su uso ha sido prohibido en la mayoría de los países. [3]
White lead compounds known as lead soap were also used as additive for lubricants for bearings and in machine shops.[4] Lead soap was also used as an oil drying agent for paints made with drying oil or air drying paints made with alkyd resins. Lead is often used with cobalt driers. Lead free substitutes have been developed to replace this use of lead in paint.
What is commonly known today as the "Dutch method" for the preparation of white lead was described as early as Theophrastus of Eresos[5] (ca. 300 BC), in his brief work on rocks or minerals, On Stones or History of Stones. His directions for the process were repeated throughout history by many authors of chemical and alchemical literature. The uses of cerussa were described as an external medication and pigment.[6]
Clifford Dyer Holley quotes from Theophrastus' History of Stones[7] as follows, in his book The Lead and Zinc Pigments.
Lead is placed in earthen vessels over sharp vinegar, and after it has acquired some thickness of a sort of rust, which it commonly does in about ten days, they open the vessels and scrape it off, as it were, in a sort of foulness; they then place the lead over vinegar again, repeating over and over again the same method of scraping it till it has wholly dissolved. What has been scraped off they then beat to powder and boil for a long time, and what at last subsides to the bottom of the vessel is ceruse.[8]
Later descriptions of the Dutch process involved casting metallic lead as thin buckles and corroded with acetic acid in the presence of carbon dioxide. This was done by placing them over pots with a little vinegar (which contains acetic acid). These were stacked up and covered with a mixture of decaying dung and spent tanner's bark, which supplied the CO2, and left for six to fourteen weeks, by which time the blue-grey lead had corroded to white lead. The pots were then taken to a separating table where scraping and pounding removed the white lead from the buckles. The powder was then dried and packed for shipment or shipped as a paste.[9] One benefit of the process was that it was not necessary to dry the paste of white lead, removing its water. All that needed was to mill the paste with linseed oil, and the white lead would take up the oil and reject the residual water, to give white lead in oil.[citation needed]
La albayalde ha sido el principal pigmento blanco de la pintura al óleo clásica europea . Se ha afirmado que es en parte responsable del oscurecimiento de las pinturas antiguas con el tiempo, al reaccionar con trazas de sulfuro de hidrógeno en el aire para producir sulfuro de plomo negro . Otras autoridades lo cuestionan; La opinión más tradicional es que los pigmentos no permanentes y el barniz sucio (que a menudo se puede limpiar) son los más probables responsables del oscurecimiento. [ cita necesaria ]
La albayalde ha sido suplantada en su mayor parte en el uso artístico por el blanco de titanio , que tiene un poder colorante mucho mayor que el albayalde. [10] Los críticos argumentan que los sustitutos como el óxido de zinc y el dióxido de titanio son más reactivos, se vuelven quebradizos y pueden desprenderse. [11] [12] Los pintores de hoy en día utilizan menos la albayalde, no debido a su toxicidad directa; sino simplemente porque su toxicidad en otros contextos ha dado lugar a restricciones comerciales que dificultan que los artistas obtengan plomo blanco en cantidades suficientes. [13] A Winsor & Newton , la empresa de pinturas inglesa, se le restringió en 2014 la venta de su blanco en escamas en tubos y ahora debe venderlo exclusivamente en latas de 150 ml (5,3 imp fl oz; 5,1 US fl oz). [14]
En el siglo XVIII, las pinturas con plomo blanco se utilizaban habitualmente para repintar los cascos y suelos de los buques de la Royal Navy , para impermeabilizar las vigas y limitar la infestación por gusanos de barco . [15]
Entre los sinónimos de albayalde se encuentran el blanco Berlín, el blanco Cremnitz , el blanco holandés, el blanco escamas, el blanco flamenco, el blanco Krems, el blanco Londres, el blanco pigmento 1, el blanco romano, el blanco plateado, el blanco pizarra y el blanco Viena. [dieciséis]
Por motivos de toxicidad, estos colores Plomo Blanco sólo están disponibles en latas en la UE.
