The main cause of aging is exposure to high temperatures and contact with air that leads to oxidation, decomposition, polymerization and condensation of hydrocarbons.
Their accumulation leads to buildup of slurries, resinous and asphaltic compounds, coke, soot, various salts and acids in the oils.
They remove only mechanical impurities (metal particles, sand, dust, as well as tar, asphalt and coke-like substances, water).
In addition, clay treatment leads to higher resistance to oil oxidation at high temperatures and increased colour stability.
Chemical methods include acid and alkaline refining, drying with calcium sulphate or reduction with metal hydrides.