Connect a bubbler so that air is bubbled continuously through the mixture. It should be removed as completely as possible because excess amounts of manganese in the final varnish can cause defects in the dried film. The thick polymerized turpentine must now be made into a resin suitable for making varnish. The thick turpentine is a mixture of many complex substances derived from the terpenes. It is only necessary to evaporate the volatile substances in a cooking process to free the desirable terpene resin.
Extreme care is required, however, since the mixture contains some hydroperoxides.
These substances are very unstable and when heated change to acids with the liberation of considerable heat. The reaction is very exothermic and the fumes are heavy. The reaction begins at about 125 degrees Celsius and ceases when all of the hydroperoxides are expelled. Connect an electric hot plate in an open outside area. Place the pan on the hot plate and gently heat observing the temperature increase on a thermometer.