2. Coal tar

The vegetable coal is placed in an airtight oven for almost 20 hours. The temperature at the heart of the coke thus prepared rises to 900 to 1,000°C, synthesizing countless chemical substances that are released in gaseous form.

After cooling, the volatile matter is separated into gaseous and liquid products, the latter in turn being separated into substances soluble in ammonia water and total tar.

Fractional distillation of total tar yields several cuts, from the lightest hydrocarbons (benzol, toluol...), through anthracene oils, to the bottom of the column, which may be tar or pitch, depending on its viscosity.