The resins used in making oleo-resinous varnish are of vegetable origin, and some are said to be collected from the trunks of living trees; but by far the most of them are found in the earth, the trees which produce them having died, fallen, and decayed, and the lumps of resin having become gradually buried in the soil; where they have remained for hundreds or, perhaps, thousands of years. In some cases, these lumps are found to contain insects, which were buried in them when they had the consistency of a soft balsam; and their antiquity is shown not only by hardening of the resin, but also by the fact that the insects, of which hundreds of sorts have been found, belong to species now extinct. These resins, which are found in tropical and sub-tropical regions, are collected by the natives and sold to the traders, who sort and clean them before shipment. The price of standard kinds have doubled in the last fifteen years, the supply failing to keep pace with the increasing demand.

The apparatus of the varnish-maker is simple. It consists of a cylindrical, flat-bottomed copper kettle, 3' in diameter and 3' deep, loosely mounted on a low, four-wheeled truck which may be drawn about by the workman. The kettle is provided with a loose cover in which holes are made to receive the stirring-rod and to facilitate the escape of gases. The stirring-rod is a slender steel rod with a wooden handle, about 5' long. The thermometers used in regulating the temperature of the kettle's contents are about 3' long, protected by well-made brass cases.

In the kettle the operator first puts the resin, 100 lb. being the standard charge, though 125 lb. is more frequently used. The kettle is then wheeled over a hot coke fire which is built in a fire-pit, under, or adjacent to, a chimney with a flue of from 10 to 25 sq. ft. cross section. This chimney creates a draught of great volume, necessary not so much to carry off the products of combustion, for very little fuel is used, as to remove the vapors generated in melting the resin, which are very acid and irritating, as well as highly inflammable. The kettle is kept over the fire about half an hour, or until the resin is all melted, which the varnish maker knows by feeling the liquid to be smooth as he stirs it, and by looking at drops of it which he withdraws on the stirring-rod. The temperature rises to about 600° or 700° F.

When all is melted, the kettle is drawn from the fire, the cover lifted off, and a quantity of hot linseed oil is gradually added. If it is desired to make a hard and brilliant varnish, the amount of oil may not be more than 8 gal. or 10 gal. to the 100 lb. of resin weighed before melting; but if it must have more elasticity, more oil is used, and for a varnish to be exposed to the weather, as on the outside of a carriage or a yacht, as much as 25 or 30 gallons is employed.

When the oil is all in, the kettle is put back on the fire and the mixture is cooked until its components have thoroughly united. If this operation is not sufficiently carried out, the resulting varnish will lack some of the durability that it should normally possess; if carried too far, the product will be too thick, and will require thinning with spirits of turpentine to an excessive degree, causing too thin a final film.

When properly cooked, the kettle is removed from the fire, and when it has cooled down to about 300° F. enough spirit of turpentine is added to make the varnish, when cold, of the proper consistency to flow out under the brush. The amount of cooking, the range of temperatures, and the quantity of turpentine needed are all empirically determined for each kind of varnish. It is then kept in tanks for several months, when it is found to be clear and ready for use.