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Denatured alcohol or methylated spirits is ethanol that has additives to make it poisonous, extremely bad tasting, foul smelling or nauseating, to discourage recreational consumption. In some cases it is also dyed.
Denatured alcohol is used as a solvent and as fuel for spirit burners and camping stoves. Because of the diversity of industrial uses for denatured alcohol, hundreds of additives and denaturing methods have been used. The main additive has traditionally been 10% methanol, giving rise to the term "methylated spirits". Other typical additives include isopropyl alcohol, acetone, methyl ethyl ketone, methyl isobutyl ketone, and denatonium.
Different additives are used to make it difficult to use distillation or other simple processes to reverse the denaturation. Methanol is commonly used both because its boiling point is close to that of ethanol and because it is toxic. In many countries, it is also required that denatured alcohol be dyed blue or purple with an aniline dye.
In the United States, the Alcohol and Tobacco Tax and Trade Bureau (TTB), formerly known as the Bureau of Alcohol, Tobacco and Firearms (ATF), regulates sales of alcohol, more specifically ethanol. Spirits (liquor, wine, beer, etc.) are heavily taxed. In order to not pay spirits taxes on alcohol that is not meant to be used as a spirit, the alcohol must be denatured. Specially denatured alcohol is denatured alcohol that follows specific formulations given by the TTB. The reason for adding the denaturant is to prevent the alcohol from being consumed as a spirit. There are approximately 50 formulations provided by the TTB, most of which are indeed toxic to be ingested by the human body, but not all.
Denatured alcohol is not, in itself, a preferred product — that is, it is not something that would be normally demanded if given the alternative of normal ethanol. Denatured alcohol and its manufacture are a public policy compromise. The supply and demand for denatured alcohol arises from the fact that normal alcohol (which in everyday language refers specifically to ethanol, suitable for human ingestion as a recreational drink or extractive medium for medicinal tinctures) is usually very expensive in comparison with similar chemicals, being highly taxed for revenue and public health policy purposes (see sin tax). If pure ethanol were made cheaply available for fuel, solvents, or medicinal purposes, some people might drink it.
Denatured alcohol provides a solution to permit legitimate use and manufacture of ethanol, whereby cheap ethanol can be made available for non-consumption use without the risk of its being converted for consumption. The process creates an ethanol-containing solution that is not suitable for drinking, but is otherwise similar to ethanol for most purposes. As a result, there is no duty on denatured alcohol in most countries, making it considerably cheaper than pure ethanol. As a consequence, its composition is tightly defined by government regulations that vary between countries.
There are several grades of denatured alcohol, but in general the denaturants used are similar. As an example, the formulation for completely denatured alcohol, according to 2005 British regulations was as follows: 
Completely denatured alcohol must be made in accordance with the following formulation: with every 90 parts by volume of alcohol mix 9.5 parts by volume of wood naphtha or a substitute and 0.5 parts by volume of crude pyridine, and to the resulting mixture add mineral naphtha (petroleum oil) in the proportion of 3.75 litres to every 1000 litres of the mixture and synthetic organic dyestuff (methyl violet) in the proportion of 1.5 grams to every 1000 litres of the mixture.
Denatured alcohol has a variety of common uses:
In the United States, small amounts of denatured alcohol are used in many consumer products such as toothpaste, where they are labeled as "SD alcohol XX", where SD stands for "specially denatured" and XX is the formula used in the denaturing process that specifies the denaturants. These formulas for denatured alcohol are found in 27 CFR part 21 of the Code of Federal Regulations. Some of these formulas, such as SD alcohol 38-B, are designed to be unpalatable but otherwise non-poisonous; they are used in applications like mouthwashes where some amount of incidental ingestion is expected. (The specific denaturants in formulas 37 and 38-B closely resemble the active ingredients in alcohol-based mouthwashes like Listerine.)
Despite its poisonous content, denatured alcohol is sometimes consumed as a surrogate alcohol, which can result in blindness or death if (as was typical) it contains methanol. For instance, during American prohibition, Federal law required methanol in domestically manufactured industrial alcohols, and on Christmas and the two following days, roughly at the midpoint of that "Great Experiment", 31 people in New York City died of methanol poisoning. To help prevent this, denatonium is often added to give the substance an extremely bitter flavor. Substances such as pyridine help to give the mixture an unpleasant odor, and emetic (vomiting) agents such as syrup of ipecac may also be included.