Ethan Johnson, age 13, of White Plains, N.Y., for his question:
HOW DO WE MAKE TAR?
Tar is a thick, black or dark brown oily material that is made by distilling coal, wood, bone, oils, fate or a number of other organic substances including various wastes.
Coal tar is a byproduct that comes from the manufacture of coke from bituminous coals. It is used to make solvents, chemicals for dyestuffs and a number of synthetic drugs.
Wood tar is a byproduct that results when charcoal is made. This is the tar that is used as one of the sources of acetic acid and acetone. It is also used in the manufacture of some medicines.
Tars donut dissolve in water and are therefore difficult to remove from clothing or your skin. Also, they have a burnt or smoky odor that many people consider to be unpleasant.
When you heat certain types of organic substances in an oven, vapors and gas are given off, and finally only black, solid char remains in the oven. This heating process is called "destructive distillation."
Vapors pass through water cooled tubes, which then condense them into liquid form. This liquid becomes thicker and more like tar as the oven temperature becomes hotter.
The tar part of the liquid separates from the part that is watery. The crude tar is usually heated and refined by distilling before use.
Large amounts of wood are destructively distilled to produce charcoal, chemicals and tars. Usually the wood used is not good for other purposes.
Tars have many uses. Pine tar comes from destructive distillation of dead trees and pins stumps and is added to natural rubber. It helps to make the rubber tough and durable.
Pins tar also is added to rope fiber to form oakum, a woolly material that is used to calk cracks in boats. Oakum is also used with lead by plumbers in making the joints in cast iron sewer pipes.
Such hardwoods as maple, birch and oak are destructively distilled in great amounts. The woods are forest waste, such as branches and crooked tress that are not suitable for lumber.
Charcoal is the main product from the destructive distillation of hardwoods, but hardwood tar also 1s as important byproduct. Often the crude tar is black, but it is often refined to an amber or white color. Important hardwood tar products include an oil used in separating metal ors from impurities and medicines used to make cough drops or veterinary remedies. In addition, a refined hardwood tar oil is used indirectly in making clear, glasslike plastic.
Coal tar obtained from the manufacture of coke is used on streets, in roofing materials and in preserving wood.
Blast furnace tar comes from the operations of pig iron blast furnaces when coal is used instead of coke to change the iron ors to iron metal.