Resin is distilled by heating it in large iron retorts, when gases, water, acetic acid, resin spirit, resin oil, and resin pitch are left as a residue in the stills. The crude resin oil imported is too impure to be used except for the preparation of common lubricating greases. To purify the oil, it is first treated with sulphuric acid, washed with water, treated with soda, and again washed with water. It is then heated in a still and may be separated into portions of different gravity by collecting the portions distilling at different times in separate receivers. To do this, a hydrometer should be floated in the oil in the receiver, and the receiver changed as soon as the oil in it has risen to the gravity required. The next portion passing over will be collected separately and will have a higher gravity than the first one.