Specific examples of (a) nitrites or nitrates of alkali metals or alkaline earth metals include alkali metal or alkaline earth metal salts of nitric acid, such as sodium nitrate, potassium nitrate, magnesium nitrate, strontium nitrate; and alkali metal or alkaline earth metal salts of nitrous acid, such as sodium nitrite, potassium nitrite, magnesium nitrite and strontium nitrite.