India or Chinese ink is always used in making the tracing of a mechanical drawing, because of its permanence, its distinct blackness, and because it is waterproof. Moreover, India ink, because of its heavy composition, is less liable to "spread" and cause a blot. It is a mechanical mixture of pure, dense lampblack and a solution of gelatin, gum, or agar-agar. (Agar-agar is a gelatinous substance obtained from seaweed.) This mixture forms a black paste which is dried and pressed into cakes. It was formerly the custom to use it in this form, but at the present time it is easily obtainable in the liquid form ready for use.
Should the draftsman, however, buy India ink in the cake form, he can easily prepare it for use by shaving off a portion of the cake into water and stirring the mixture thoroughly.