This section is from the book "The Tinman's Manual And Builder's And Mechanic's Handbook", by Isaac Ridler Butt. Also available from Amazon: The Tinman's Manual And Builder's And Mechanic's Handbook.
The following are the chief Resins employed in the manufacture of Varnishes.
This resin is most distinguished for durability. It is usually of some shade of yellow, transparent, hard, and moderately tough. Heated in air, it fuses at about 549°; it burns with a clear flame, emitting a pleasant odor.
This is imported from the East Indies. The large, transparent, pale-yellow pieces, with vitreous fracture, arc best suited for varnish. Inferior qualities are employed for manufacturing gold-size or japan-black. Although superior to amber in its capacity for drying, and equal in hardness, varnish made from anime deepens in color on exposure to air, and is very liable to crack. It is, however, much used for mixing with copal varnish.
This is a gum-resin but little used in varnishes, on account of its costliness.
This resin is synonymous with arcanson and rosin. When the resinous juice of Pinus sylvestris and other varieties is distilled, colophony remains in the retort. Its dark color is due to the action of the fire. Dissolved in linseed oil, or in turpentine by the aid of heat, colophony forms a brilliant, hard, but brittle varnish.
This is a gum-resin of immense importance to the varnish-maker.
It consists of several minor resins of different degrees of solubility.
In durability, it is only second to amber. When made into varnish, the better sorts become lighter in color by exposure to air.
Copal is generally imported in large lumps about the size of potatoes. The clearest and palest are selected for what is called body-gum; the second best forms carriage-gum; whilst the residue, freed from the many impurities with which it is associated, constitutes worst quality, fitted only for japan-black or gold-size.
In alcohol, copal is but little soluble; but it is said to become more so by reducing it to a fine powder, and exposing it to atmospheric influences for twelve months. Boiling alcohol or spirit of turpentine, when poured upon fused copal, accomplishes its complete solution, provided the solvent be not added in too large proportions at a time. The addition of camphor also promotes the solubility of copal; so likewise does oil of rosemary.
This is a tasteless, inodorous, whitish resin, easily soluble in oils. It is not so hard as mastic, with which it forms a good admixture.
This is a resin of a yellow color, semi-transparent, and of faint fragrance. Of the two resins which it contains, one is crystallizable and soluble in cold alcohol.
This constitutes the basis of spirit-varnish. The resin is soluble in strong alcohol aided by heat. Its solution in ammonia may be used as a varnish, when the articles coated with it are not exposed more than an hour or two at a time to water.
This is a soft resin of considerable lustre. The two sorts in commerce are, in tears and the common mastic; the former is the purer of the two. It consists of two resins, one of which is soluble in dilute alcohol. With oil of turpentine, it forms a very pale varnish, of great lustre, which flows readily, and works easily. Moreover, it can be readily removed by friction with the hand; hence its use for delicate work of every description.
This is a pale, odorous resin, less hard than lac, with which it is often associated as a spirit-varnish. It consists of three resins differing as to solubility in alcohol, ether, and turpentine. It forms a good pale varnish for light-colored woods; when required to be polished, Venice turpentine is added to give it body.
Of the solvents of these various resins, little need be said. In the manufacture of varnishes, great care, as well as cleanliness, are required. The resins should be washed in hot water, to free them from particles of dust and dirt; they should be dried and assorted according to their color, reserving the lightest shades for the best kinds of varnish.
The linseed-oil should be as pale colored, and as well clarified as possible. New oil always contains mucilage, and more or less of foreign matters; as these prevent the regular absorption of oxygen, the oil requires preliminary treatment. The common plan is to boil it with litharge; but such oil varnish is inferior to that prepared with sulphate of lead.
The best method is to rub up linseed-oil with dry sulphate of lead, in sufficient quantity to form a milky mixture. After a week's exposure to the light, and frequent shaking, the mucus deposits with the sulphate of lead, and leaves the oil perfectly clear. The precipitated slime forms a compact membrane over the lead, hardening to such an extent that the clarified oil may be readily poured off.
This is of very extensive use. The older it is, the more ozonized, the better it is. Turpentine varnishes dry much more readily than oil varnishes, are of a lighter color, more flexible and cheap. They are, however, neither so tough nor so durable.
This is employed as the solvent of sandarach and of lac. The stronger, coeteris paribus, the better.
These are used for the cheaper varnishes. Their smell is disagreeable. The former is, however, a better solvent of resins than alcohol.