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Coloring Finishing And Painting Wood
The mineral thinners which are now almost exclusively employed for thinning varnishes are often called mineral spirits, tho special terms such as varnolene or solvent naphtha are sometimes used. The thinner chosen enters into the problem of spreading varnish, but it evaporates quickly and leaves little residue: consequently, most varnishes contain only cheap mineral spirits, either entirely or in large part.
335. Thinning Synthetic Varnishes.—Persons who have felt the easy flow of varnishes of the past are tempted to thin any synthetic varnish that they have in order to make it spread easier—more like the varnishes of just a few years ago. Thinners should not be added to quick-drying varnishes because there is danger of precipitation of the gums and serious varnish trouble. Manufacturers of present-day varnishes prepare them for use just as they are found in the cans. No addition of thinners of any kind is allowable without danger, except when mixing and dipping varnishes are being used. Cold varnishes do not spread easily. The remedy is heat. Place the varnish pot on a hot brick or in a container of hot water some minutes before use and the flow under the brush will be greatly improved.
336. Varnish Cooking.—The methods of cooking fossilresin and synthetic-gum varnishes vary to some extent, but heat treatments are necessary in making all of the drying-oil varnishes. Fossil resins in general must be heated to a very high heat and held at some great temperature until about 20% of the molten liquid has been driven off by evaporation. Temperatures of 600° F., or above, are commonly used. This heat treatment or melting changes the fossil resins and causes them to be soluble in drying-oils—usually linseedand China-wood oil—which are poured into the hot resin mixture away from the fire, the temperature of the oil varying with different types of varnish. The batch of resin is then recooked in order to cause the resin mixture and drying-oils to combine thoroly.
Ester-gum varnishes do not require any prolonged cooking or high heat. China-wood oil must be cooked at a temperature between 525° F. to 560° F. preferably with part of the ester-gum. Later on, the remainder of the estergum is put into the kettle and the batch is properly reeooked, tho at a lower temperature. When partly cooled, the varnish is thinned with mineral spirits or some similar cheap, volatile liquid.
Synthetic resins are usually cooked at the top heat with suitable drying-oils, especially China-wood oil with a small portion of linseed-oil; but a considerably lower cooking heat is employed for them than for fossil-resin or ester-gum varnishes.
The actual or real properties of varnishes of all types' are determined to a considerable degree by the method of cooking used. The same component ingredients can be so modified by different cooking operations that the final varnishes produced may be quite different in their characteristics.
