Hardwood Finishing

The use of varnish is indicated by old ac counts dating back to the thirteenth century; but it was probably in those days employed chiefly for the protection of pictures, or for use on musical instruments, or for special purposes, and appears not to have been used for finishing ordinary woodwork of buildings, since there is no vestige of it on any of the old roof trusses or other timbers of the cathedrals or other examples of Gothic architecture that have come down to us. These seem either to have been finished with oil or to have been left unfinished and to have gradually darkened by time and exposure. Wax was also used for finishing some of the medieval woodwork, and its use has continued down to the present time, no method of finishing equaling it for softness and delicacy.

Kinds of Varnishes. Varnishes are of two classes—oil varnishes and spirit varnishes. The first are made from certain resins known in the trade as "varnish gums" (although, strictly speaking, they are not gums) and linseed oil, and thinned with spirits of turpentine. Cheaper grades of varnish are made from ordinary rosin (the resin of the long-leafed pine), and are sometimes thinned with benzine. Within recent years considerable tung or China wood oil has been used in the manufacture of certain grades of varnish. Oil varnishes dry or harden by the chemical change of the linseed oil contained in them to form linoxyn by combining with the oxygen of the air.

Spirit varnishes are made by dissolving the resin or other substance (resins are chiefly used) in a volatile liquid, such as alcohol or spirits of turpentine. When a varnish of this kind is spread over any surface it dries or hardens by the evaporation of the volatile liquid, and the resin is then left spread over the surface in a thin film, the liquid simply having served as a mechanical means for spreading.

The character and quality of an oil varnish depend largely on the kind of gum or resin used in manufacturing it. Most of the varnish gums are fossil resins, being dug up from the earth, and are sometimes found in localities where the trees producing them have long since passed away. Still some inferior grades of varnish are made from the softer gums taken from the living trees. Ordinary rosin is much used in the manufacture of cheap varnish, especially in connection with some of the harder gums. As a general thing, the harder the resin used for making the varnish, the greater will be its dura bility, especially under severe conditions of exposure. Varnishes for exterior use should always be made of specially selected hard gums, and great care must be used in proportioning and other details of their manufacture. Every operation, in fact, in the work of varnish mak ing requires the unceasing attention of a skilled workman, who must depend largely upon ex perience to guide him.

In making varnish the selected and cleaned resin is put into a large copper kettle mounted on wheels and fitted with a long iron handle by which it can be readily moved. This kettle is then rolled over a grate fire in a large fireplace built under a peculiar-shaped square stack, tapering upward, to carry off the fumes. When

the gum has been thoroughly melted, the var nish kettle is rolled off the fire, and the requi site quantity of linseed oil is slowly added and stirred in, the kettle being then placed over a slower fire for several hours. The material is continually stirred with rods, which are intro duced through apertures left in the cover of the kettle for the purpose. The workman judges from the appearance of the mass adher ing to the rod as he draws it from the kettle now and then to examine it, placing a drop or two on a sheet of glass, whether the varnish has been cooked long enough or not.

In order to make the varnish dry rapidly enough to satisfy practical requirements, a cer tain percentage of driers must be added either to the oil before it is placed in the kettle or during the after process of cooking the varnish. The driers most commonly employed are com pounds of lead or manganese salts with common rosin. In general it may be said that the quicker a varnish dries, the shorter will be its life; and that if a varnish is desired which shall have the maximum of durability under severe exposure, it must necessarily be slow-drying. As it is essential that a varnish should set within a reasonable time in order to avoid, as much as possible, injury from dust, it is necessary to make a compromise with durability, and for this reason a varnish containing no driers would be of little practical value.

After the varnish has cooked for the requi site length of time, it is taken off the fire and the kettle is run into the thinning room, where the varnish is allowed to cool down a little, but not enough to chill it; and then the volatile thinners, consisting of turpentine, benzol, ben zine, solvent naphtha, or whatever else may be used, are added, pouring them in slowly and constantly stirring so as to prevent undue loss by evaporation. When the proper degree of fluidity is obtained, the varnish is pumped off into overhead tanks, and is then run into filter ing presses, where any impurities that may have been mechanically mixed with the resin are removed. It is then pumped into storage tanks in warm rooms, where it is allowed to remain in order to ripen, settle, and properly age. The longer the varnish remains in these tanks, the better it will be and the greater will be its durability. Many varnish manufacturers keep the varnish in storage for from six months to a year before putting it upon the market. This, of course, means the locking up of considerable capital, and it involves a large tankage capacity. Hence a well-made and well-aged varnish cannot be bought at a low price; the price must necessarily be considerably in excess of the cost Gums Used in the Manufacture of Varnish. The resins used in the manufacture of oil var nishes are known to the trade as copals or gum copal, and are distinguished by the locality from which they are obtained.