Walnut-oil or Nut-oil (p. 1413).—With regard to the drying powers of this oil, some authorities place it above linseed-oil, and others below it, whilst most of the older writers considered it as very little, if at all, inferior. We are inclined, from all accounts, to agree with the last-mentioned opinion, although almost all that comes into the Eoglish market is far inferior to linseed-oil. The cause may be that the palest oil has been chosen in the Continental market as the best as well as the dearest, but in ignorance of the fact that the best salad oils are generally the worst driers.
Ilerapseed-oil (p. 1391).—This is generally supposed to be about equal to linseed- and walnut oile in drying quality. It is said to make a very bad-smelling and deep-coloured boiled oil; but seems, frcm all accounts, to be much used in E. and N. Europe for paints and varnishes, though very little, if at all, in England.
Linseed-oil (p. 1393).—This oil, which from its high drying property, its cheapness, its steady supply, and its great constancy of quality (when free from any adulteration with non-drying oils), is universally employed in W. Europe for oil-varnishes, and boiled oil for paintiog, japanning, floor cloth, and all other purposes where a strongly-drying oil is required ; it is also the one prescribed in the oldest of all known recipes for varnish and for oil-painting.
All oil to be ueed in oil-varnishes or for boiled oil should be chosen as new, sweet, and free from rancidity as possible, and should be at once clarified and allowed to settle for a few months before being used, so as to deposit impurities, " mucosities," and the substances (if any) used in clarifyiug it, and, when clear, should be decanted from the dregs, and stored for use in slate or galvanized-iron tanks.
Many methods have been used for the clarification of oils (see p. 1459), to rid them of the gurnmy and mucous matters, water, saline substances, &c., derived from the seed during expression, and which, besides making the oil turbid, induce a kind of rancid fermentation, and much impair the keeping quality of the oil, and hinder its drying. The simplest of all these processes consists in heating the oil slowly up to 300° (572° F.) either alone or with the addition of 1-5 parts in 1000 of either caustic lime, carbonate of lime, calcined magoesia, or carbonate of magnesia, and keeping it at that temperature for one or two hours, and then allowing it to cool uncovered and undisturbed. The oil should then be transferred to a settling-taok to deposit and clarify.
When thus freed from the impurities arising from the seed, the oil will not eo easily get rancid, and will improve by keeping, becoming more pale, limpid, transparent, and drying, in proportion to its age. All oils prescribed in the recipes for oil-varnishes in the course of this article will be supposed to havo been clarified by this or some other process.
Resins.--In addition to what has been said under this head (pp. 1621-95), the following may be enumerated with especial reference to their use in varnishes.
Oil-varnish Resins.—(1) Amber (p. 1628) is the hardest and rnost difficult to fuse, but gives the most durable and resisting oil-varnish, of dark colour unfortunately. (2) The true copals (p. 1640) make the finest of all the oil-varnishcs, nearly as hard and durable as amber-varnish, and much paler in colour and more quick-drying. (3) The pseudo-copals,—kauri (p. 1666), and hard and soft Manilla (p. 1678), give varnish inferior to the true copale, but more easily made. (4) Sandarach (p. 1681), is almost unknown as an oil-varnish resin at the present day, its use beiog eotirely confined to spirit-varnishes; it is, however, equal to most of the true copals for varnish-making in point of hardness and general qualities, excepting colour. (5) The resins of the conifers, constituting the " rosin " of commerce (p. 1680), make poor weak varnishes, only fit for the comuaonest purposes.
All the above oil-varnish resins, with the exception of group 5, are quite insoluble in linseed oil, turps, and other essential oils, benzol and its homologues, petroleum, chloroform, &c., until they have uodergone destructive fusion at nearly red-heat, after which they dissolve freely in all.
Essence-varnish resins.—(1) All the copals, amher, sandarach, and the pseudo-copals, after fusion. (2) All the dammars (p. 1644), without previous fusion. (3) Mastic and other resins from species of Pistacia (pp. 1673, 1687). (4) Coniferous resins and turpentines. (5) The resins of copaiba and gurjun balsams (pp. 1639, 1651). These resins are soluble in essential oils and other hydrocarbons, and in ether, chloroform, &c., forming Classes A and B of Div. IV., called oleo-resinous or essential-oil varnishes, and ether varnishee.