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  Varnishes, their Nature and Manufacture - Manufacturer & Builder, 1869  

Under the general term varnishes are comprised those solutions of gums which, when allowed to dry in thin layers, leave the gum as an adherent covering, either in combination with the solvent or otherwise. They are employed in all instances where it is desired to give to solid surfaces a brilliancy which they would not otherwise possess.

When of good quality, varnish also preserves from time effects of air and moisture the articles to which it is applied. A good varnish should possess great hardness, combined with a certain degree of tenacity, so as to afford polish, and it should be unalterable by atmospheric influences. It may be colored or not.

The resins and gums usually employed are: mastic, amber, copal, shellac, sandarac, elemi, colophony, and asphalt. The menstrua are: ordinary and methylic alcohol, spirits of wood, spirits of turpentine, linseed and poppy oil.

 

 When alcohol-like solvents are employed, time products are termed “spirit varnishes;” and when oils are the vehicles used, “fat or oil varnishes” are produced.

Those for which alcohol, ether, or benzol have been used are least durable.

Those made by spirits are more durable, as the turpentine becomes partly oxidized and forms with the gum a tough and very durable compound of turpentine.

Such varnishes are seldom used alone, but generally employed in combination with oil varnishes. With respect to durability, especially in relation to heat and moisture, the latter are excelled by none.

The manufacture of spirit varnishes is exceedingly simple, it being only necessary to pulverize the gums finely and to heat them slightly with the solvent, while, in order to prevent balling together of the resins they are mixed with a uniformly coarse glass powder.

The solution itself is mostly effected in a distilling apparatus of copper, which is so arranged that the volatilized liquid may uninterruptedly flow back in a condensed state.

Great care ought to be paid to the assortment and selection of the resins. The lightest colored pieces should be reserved for fine and colorless varnishes, while the colored pieces should be put aside for the ordinary kinds.

Before using them, they should be washed in hot water to remove impurities, and then well dried. In order to obtain clear varnishes, it is also necessary to allow them to remain quiet for some time, after which they are filtered. If previously digested within freshly burned and cooled bone-black, they gain greatly in brilliancy.

The use of paper in filtering should be avoided, as the varnishes run too slowly through it, and moreover, they do not become perfectly clear. A far better plan consists in stopping the funnel within a bunch of cotton, and allowing the liquid to flow through the same.

In the preparation of fat or oil varnishes, for which copal and amber are most extensively used, great care ought to be paid to the selection of the oil, which should be cold-pressed, clear, and of considerable age. These three requisites being complied with, good varnishes may always be expected, provided proper care has in other respects been bestowed upon their manufacture. To cause the resins to be more easily taken up, they are to be previously melted.

The most convenient apparatus for this purpose consists of a copper cylinder which terminates at its lower end in a funnel, and may be well closed with a lid. When the cylinder has been filled with the gum, which is poured on a perforated plate resting in the funnel, it is set into a small furnace, in such a manner that the tube of the funnel may pass through the grates and through an aperture in the ash-box. Under the latter is placed a vessel containing the oil, which is heated by a charcoal fire. If the cylinder is now surrounded with glowing coals, the resin melts and drops into the oil, by which it is immediately dissolved.

The boiling of the oil as well as the solution of the resin is effected in copper pans, which should be more shallow than deep; they must be of a capacity sufficient for double the quantity of the charge of oil. The varnish should be made in dry weather; otherwise moisture is absorbed, and its transparency and drying quality impaired.

With respect to the mode of heating, a charcoal fire serves this purpose best, as it yields a uniform heat, and is not likely to overheat the sides of the pan. The pan is set into an iron ring, which should only be so wide as to leave the lower part of the kettle free. The oil is heated very gradually until it commences to simmer, when the scum is skimmed off. It was at this stage in the old method that the dryers (litharge, minium, oxide of zinc, etc.) were added. The heat is then gradually increased until the oil bubbles and emits a gaseous vapor, which inflames when a lighted taper is held near by.

During the boiling, it is a good plan to stir the oil constantly and to bring it as much as possible in contact with the air. This may be effected to a sufficient extent by ladling a part out and letting it again flow in a stream into the kettle. As to the quantity of copal which oil will bear, it is about one half its own weight - ten pounds of copal requiring at least twenty pounds of oil.

If the oil varnish is to be mixed with spirits of turpentine, the latter must be added during continual stirring. If the boiling-pan is not sufficiently spacious, the mixture must be effected in another vessel, and the spirits are to be added while the mixture of oil and resin is still hot.

The boiling-pan is finally well wiped out with some spirits of turpentine, and then scoured with a rag which has been dipped into powdered pumice-stone. In this manner all other utensils are cleansed, then rinsed with pure spirits of turpentine, and at last dried with a clean cloth.

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