Only the naphtha and a sufficient quantity of the heavier oils are run off to bring the residue to the required density, which may be 29° to 32° B. for oil for large journals or those revolving at moderate speeds, or 26° to 29° B. for small, high-speed journals, or for the interior of steam-engine cylinders.
Anything approaching destructive distillation or "cracking," is carefully avoided, as it would result in the reduction of the viscosity of the product.  Only the naphtha and a sufficient quantity of the heavier oils are run off to bring the residue to the required density, which may be 29° to 32° B. for oil for large journals or those revolving at moderate speeds, or 26° to 29° B. for small, high-speed journals, or for the interior of steam-engine cylinders.  Professor Peckham states that this process is used for a large propor tion of the lighter-grade oils of Western Virginia and Ohio, and for the whole product of the Smith's Ferry district. "