Pigments, lead

Red lead pigments are one of the oldest and most popular types of anticorrosive pigments, mainly used for primers for metals. They are indirect inhibitors and require reaction with a selected resin system. When used in linseed oil or other oleoresinous binders, they react with acidic groups in the resin and form lead soap, which has an inhibitory effect. They are no longer used in the coating industry due to the toxicity associated with lead. 

Basic lead silicochromate is composed of basic lead chromate and basic lead silicate firmly bound to a silica core. It was used in coatings for metal protection in structural steel, and finer grades were used in electrocoat paints. Because these pigments contain both lead and hexavalent chromium, due to toxicity and human health reasons, they have lost their commercial importance. 

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Lead(II) sulphide is oxidised to lead(II) sulphate this reaction has been used in the restoration of old pictures where the white lead pigment has become blackened by conversion to lead sulphide due to hydrogen sulphide in urban air ... 

Lead sulfide is used in photoconductive cells, infrared detectors, transistors, humidity sensors in rockets, catalysts for removing mercaptans from petroleum distillates, mirror coatings to limit reflectivity, high temperature solid-film lubricants, and in blue lead pigments (82). 

The pigment is the principal agent in the electrochemical control of corrosion by primers (see Section 14.3). Probably the best known anticorrosive pigment is red lead. When used in conjunction with linseed oil as the binder it gives very good primers which will perform well over relatively poorly prepared (manually abraded) steel surfaces. Present-day use of red lead (and lead pigments, generally) in paints has been drastically curtailed as a result of understandable pressure from the environmentalists. 

Zinc chromate and zinc tetroxychromate have also been used successfully in anticorrosive paints. Both pigments function by releasing chromate ions which passivate the steel surface. In common with lead pigments, those... 

Thenard A process for making white lead pigment (basic lead carbonate) by boiling litharge (lead monoxide) with lead acetate solution and passing carbon dioxide gas into the suspension. 

Eor our purposes, the relevant point to emphasize is that the different lead pigments yield different reductive responses [126], Detailed electrochemical analysis indicated that, for a given electrolyte and material, the peak potential and the morphology of the cathodic signals depend slightly on the shape and size distribution of solid particles [73-78]. 

Table 3.1 Diagnostic criteria for characterizing lead pigments via voltammetry of microparticles using deposits of the pristine pigments on parafiSn-impregnated graphite electrodes. Data from square-wave voltammograms at a potential step increment of 4 mV, square-wave amplitude of 25 mV, and frequency of 15 Hz. All potentials refer to AgQ (3M NaCl)/Ag. Electrolyte, 0.50 M acetate buffer, pH 4.85...

Fig. 3.5 Representation of (E3/4 — E1/4) vs. — Eonset) for lead pigments, model paint spec-...

Fig. 3.7 Plot of (Eonset — Ep) vs. W] 2 for the peak at —0.45 V for different lead pigment plus binder specimens lead white plus proteinaceous binders squares), minium plus proteinaceous binders (solid squares), lead white plus oils (triangles), and Naples yellow plus oils (solid triangles). From square-wave voltammetric data using the conditions described in the caption of Fig. 3.3...

Fig. 3.8 Plot of the potential separation between the peaks at —0.45 and —0.65 V, AEp vs. Wj/2 for peak at —0.45 V for different lead pigment plus binder specimens solid squares = casein or bovine gelatin plus lead white or minium = squares lead white and Naples yellow plus poppy oil triangles = sunflower plus lead white and Naples yellow solid rhombs = egg plus lead white and minium. From square-wave voltammetiic data using the conditions from Fig. 3.3...

Domenech A, Domenech-Carb6 MT, Mas X (2007) Identification of lead pigments in nanosamples from ancient paintings and polychromed sculptures using voltammetry of nanoparticles/atomic force microscopy. Talanta 71 1569-1579. 

Domenech A, Domenech-Carbo MT, Mas X, Ciarrocci J (2007) Simultaneous identification of lead pigments and binding media in paint samples using voltammetry of microparticles. 

According to EEC Council Directive for the labelling of preparations [3.161] in conjunction with the 21st adaption to EEC Council Directive 67/548, Nota 1, [3.160] such materials containing more than 0.5 % lead, are labelled in the same way as the pure lead pigment, with a skull and crossbone (T) and the corresponding R and S phrases. 

Another issue affecting the architectural paint industry is the remediation of homes, buildings, and structures that contain lead-based paint. Lead poisoning in children has been linked to ingestion of paint dust or paint chips that contain lead pigments and this has resulted in U.S. government regulations to reduce the lead content in paint to no more than 0.06%. 

Because of the long-term and widespread use of lead, it is one of the most ubiquitous of the toxic metals. Exposure may be through air, water, or food sources. In the United States the major industrial uses, such as in fuel additives and lead pigments in paints, have been phased out, but other uses, such as in batteries, have not been reduced. Other sources of lead include lead from pipes and glazed ceramic food containers. 

Carbon black has uses similar to some inorganic compounds. For example, it is the most widely used black pigment. It is also an important reinforcing agent for various elastomers. It is used in tires and other elastomers and is a leading pigment in inks and paints. 

Page 5 Old paintings contain toxic lead pigments and should be kept in sealed vaults say environmental activists. 

The formation of the white spots was not linked to the use of lead white but to other lead pigments, namely red lead and lead-tin yellow which are thought to be much more chemically reactive. So does all this mean that eventually all the oils of oil paintings will slowly decompose The answer is no and the experts now believe that while some white spotting is inevitable it is a self-limiting process which will eventually cease before any real visual changes take place. 

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