Fireplace crystals change the color of a flame because they contain chemicals that produce vivid colors when burned. For example, copper chloride produces a bright blue flame when it burns, while potassium chloride burns with a purple light and strontium chloride turns a flame bright red.
While there are a number of commercial preparations that change the color of a fire, some colors can be achieved easily with household products. Borax powder burns with a yellow-green hue, while the magnesium sulfate in Epsom salts turn flames a bright white.
Crystals and other substances may be poured carefully into open flames, or they may be layered prior to ignition to produce a patterned light show. Throwing these substances into a fire can produce a vivid light show, but care should be taken with flammable powders to avoid the possibility of flash-over and fire. In addition, most compounds should only be used in a wood fire, as a gas-fed fire may cause them to react differently.
Fire colorants should only be used in a fireplace or other decorative fire. They should not be used in a cooking fire, because incomplete combustion of the colorant materials may produce toxic smoke that could contaminate food cooked over the flames.