Like other highly nitrated organic compounds, picric acid is an explosive, hence its primary use. Nitric acid reacts with proteins to form yellow nitrated products. Heating the reaction mixture is sufficient to hydrolyze the amide back to the nitrated aniline. Nitrotoluene, methyl benzoate and benzoic acid are nitrated by prolonged exposure over a few days. Compounds related to soot, such as polyaromatic hydrocarbons (PAHs) and nitrated PAHs, are also reduced. Triethylene glycol dinitrate (TEGDN) is a nitrated alcohol ester of triethylene glycol. In pure form it is a white crystalline explosive, much like other fully nitrated polyols. Also, the more chlorinated or nitrated the compound, the more difficult it is to degrade. These color changes are caused by nitrated aromatic rings in the protein. Other possibilities include nitrated guar gum. Oxidation of fluorene gives fluorenone, which is nitrated to give commercially useful derivatives. Other sources report the contents as "a nitrated mixture of linseed oil and naphthalenes". Pyridine is nitrated to 3-nitropyridine and 4-nitropyridine. In September 1996 he and his group have successfully nitrated a sapphire. "Wrangel"s hull was made of nitrated steel.