As pigment, lead(II) chromate (PbCrO4, "chrome yellow"), Lead(II,IV) oxide, (Pb3O4, "red lead"), and lead(II) carbonate (PbCO3, "white lead") are the most common forms. Lead is added to paint to speed up drying, increase durability, maintain a fresh appearance, and resist moisture that causes corrosion. It is one of the main health and environmental hazards associated with paint. In some countries, lead continues to be added to paint intended for domestic use, whereas countries such as the U.S. and the U.K. have regulations prohibiting this, although lead paint may still be found in older properties painted prior to the introduction of such regulations. Although lead has been banned from household paints in the United States since 1978, paint used in road markings may still contain it. Alternatives such as water-based, lead-free traffic paint are readily available, and many states and federal agencies have changed their purchasing contracts to buy these instead.
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History
Lead white was being produced during the 4th century BC; the process is described by Pliny the Elder, Vitruvius and the ancient Greek author Theophrastus.
The traditional method making the pigment was called the stack process. Hundreds or thousands of earthenware pots containing vinegar and lead were embedded in a layer of either tan bark or cow feces. The pots were designed so that the vinegar and lead were in separate compartments, but the lead was in contact with the vapor of the vinegar. The lead was usually coiled into a spiral, and placed on a ledge inside the pot. The pot was loosely covered with a grid of lead, which allowed the carbon dioxide formed by the fermentation of the tan bark or the dung to circulate in the pot. Each layer of pots was covered by a new layer of tan, then another layer of pots. the heat created by the fermentation, acetic acid vapor and carbon dioxide within the stack did their work, and within a month the lead coils were covered with a crust of white lead. This crust was separated from the lead, washed and ground for pigment. This was an extremely dangerous process for the workmen. Medieval texts warned of the danger of "apoplexy, epilepsy, and paralysis" from working with lead white.
Despite the risks, the pigment was very popular with artists because of its density and opacity; a small amount could cover a large surface. It was widely used by artists until the 19th century, when it was replaced by zinc white and titanium white.
The dangers of lead paint were considered well-established by the beginning of the 20th century. In the July 1904 edition of its monthly publication, Sherwin-Williams reported the dangers of paint containing lead, noting that a French expert had deemed lead paint "poisonous in a large degree, both for the workmen and for the inhabitants of a house painted with lead colors." As early as 1886, German health laws prohibited women and children from working in factories processing lead paint and lead sugar.
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Toxicity
Lead paint is hazardous. It can cause nervous system damage, stunted growth, kidney damage, and delayed development. It is dangerous to children because it tastes sweet, therefore encouraging children to put lead chips and toys with lead dust in their mouths. Lead paint is dangerous to adults and can cause reproductive problems in men or women. Decreases in sperm production in men have been noted. Lead is considered a possible and likely carcinogen. High levels may result in death.
Regulation
The European Union has passed a directive controlling lead paint use.
In Canada, regulations were first enacted under the Hazardous Products Act in 1976 that limited lead content of paints and other liquid coatings on furniture, household products, children's products, and exterior and interior surfaces of any building frequented by children to 0.5% by weight. New regulations on surface coating materials, which came into force in 2005, further limit lead to its background level for both interior and exterior paints sold to consumers. Canadian paint manufacturers have been conforming to this background level in their interior and exterior consumer paints since 1991. Nevertheless, a Canadian company, Dominion Colour Corporation, is "the largest manufacturer of lead-based paint pigments in the world" and has faced public criticism for continuing to export paints to countries where its uses are not tightly regulated.
The United States' Consumer Product Safety Commission (CPSC) banned lead paint in 1977 in residential properties and public buildings(16 Code of Federal Regulations 1303), along with toys and furniture containing lead paint. The cited reason was "to reduce the risk of lead poisoning in children who may ingest paint chips or peelings." For manufacturers, the CPSC instituted the Consumer Product Safety Improvement Act of 2008, which changed the cap on lead content in paint from 0.06% to 0.009% starting August 14, 2009.
In April 2010 the U.S. Environmental Protection Agency required that all renovators working in homes built before 1978 and disturbing more than six square feet of lead paint inside the home or 20 square feet outside the home be certified. EPA's Lead Renovation, Repair and Painting Rule (RRP Rule) lowers the risk of lead contamination from home renovation activities. It requires that firms performing renovation, repair, and painting projects that disturb lead-based paint in homes, child care facilities and pre-schools (any child occupied facility) built before 1978 be certified by EPA and use certified renovators who are trained by EPA-approved training providers to follow lead-safe work practices.
Lead paint is not prohibited in India. A 2015 study found that over 31% of household paints in India had lead concentration above 10,000 parts per million (ppm), which far exceeds the BIS standard of 90 ppm for lead in paint.
Lead paint in art
Oil paints
In art, white lead paint is known as "flake white" or "Cremnitz white". It is valued for the ease of handling and resilience the lead confers to oil paints. Lead white paint dries relatively quickly to form a strong, flexible paint film. Lead-based white is one of the oldest manufactured pigments. It was the only white pigment available to artists in appreciable quantities until the twentieth century, when zinc-white and titanium-white became available. Industrially produced lead white, the typical pigment from the 19th century until its ban, was thought to be inferior to traditionally fabricated forms, which had larger "flake" particles that conferred ease of handling.
Titanium and zinc whites are far less toxic than lead white and have largely supplanted it in most fine arts applications. Safety regulations have also made lead white more expensive and difficult to obtain in some regions, such as the EU. However lead white oil paints are still produced and in use by artists who prefer their unique handling, mixing, and structural qualities.
Flake white has various drawbacks, including a tendency to become transparent over time. It also blackens in the presence of certain atmospheric pollutants, although this can be reversed.
Water-based paints
Lead is not a traditional pigment in water mediums, as zinc is superior for works on paper, as is calcium hydroxide (slaked lime) for frescos. Lead-based paints, when used on paper, often cause the work to become discolored after long periods of time; the paint's lead carbonate reacts with hydrogen sulfide in the air and with acids, which often come from fingerprints.
Substitutes
Titanium
Paint manufacturers have replaced white lead with a less toxic substitute, titanium white (based on the pigment titanium dioxide), which was first used in paints in the 19th century. Titanium dioxide is considered safe enough to use as a food coloring and in toothpaste. It is also a common ingredient in sunscreen. When used in paints today, it is often coated with silicon or aluminum oxides for durability. Titanium white has far greater opacity and tinting strength than lead white, and it can easily overpower most other pigments if not mixed carefully. Titanium white has been criticized for leading to "chalkiness" in mixtures, and for allegedly decreasing the permanence of organic pigments mixed with it due to its high refractive index.
Zinc
Zinc white is less opaque and weaker in tinting strength than either titanium white or lead white. It is commonly used to lighten mixtures subtly while maintaining transparency. Although zinc white is the standard white in watercolors, its structural soundness in oils has been debated. Zinc white dries slowly and creates a relatively inflexible paint film. Critics of the pigment argue that its use leads to excessive cracking and delamination, even when used sparingly.
Source of the article : Wikipedia
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