Oil Varnishes

The preparation of fat or oil varnishes with the harder resins is generally attended with considerable difficulty; but there is as we have already mentioned, one way in which the difficulty may be lessened. By the aid of one of the powerful and very volatile solvents previously named, we prepare a spirit copal or amber varnish; we then add the required amount of 'manganese' oil and draw off the volatile solvent by distillation, thinning the resinous solution obtained with so much oil of turpentine as is necessary. If the copal or amber employed has been first roasted or fused, the varnish produced will be more or less dark in tint; it is on this account that the exposure of the powdered resin to the air in a flat porcelain dish for seventy-two hours, at a temperature (of 220º C.) which does not cause discoloration, is recommended. But if, on the other hand, the copal or amber be merely powdered, some part of it, and that a considerable part, will probably remain undissolved though swollen, and will therefore be wasted.

The following process, in the main identical with one recommended in the American edition of Mr. Erwin Andres' work on varnishes, yields a pale and durable varnish when Sierra Leone copal or other hard copal is employed, and is doubtless well adapted for the preparation of amber varnish also. As will be seen, it is based upon the preliminary partial solution of the hard resin in chloroform, or in light petroleum spirit of about the same boiling-point. It may be stated at once that the proportions of the five ingredients used are approximately 10 parts by weight of copal or other hard resin; 5 parts by weight of dried powdered glass or sand; enough chloroform to cover the above substances; 35 parts by weight of oil of turpentine, and 10 parts by weight of 'manganese' oil. The following is an outline only of the process. The copal, after having been powdered and heated to 220º C. for seventy-two hours, is mixed with the glass or sand, and introduced into a retort; chloroform in quantity sufficient to cover the mixture is added. After the lapse of twenty-four hours the dry oil of turpentine is poured in, and an upright condenser is attached to the retort. The retort is then heated to 50º or 60° C. for two hours, so that the chloroform continually returns to the mixture.

Then the contents of the retort are allowed to cool, and the condenser slanted downwards to allow of the chloroform being distilled over. This removal of the chloroform having been effected at a temperature so low that very little turpentine has come over, the remaining mixture in the retort is heated once more with the condenser in an upright position. The heat used must suffice to bring the oil of turpentine into vigorous ebullition - in an hour the whole of the copal should have dissolved. The 'manganese' oil wanted should now be heated to 100° C, and then the copal mixture, when it has cooled to 70º C., added little by little to it with constant stirring, the temperature of the oil being maintained at 90º to 100°. When the mixture is complete the source of heat is withdrawn, but the varnish is still stirred for twenty minutes. Then it is allowed to settle, until quite clear, in glass bottles, or, if an appropriate filtering apparatus is available, it is filtered. In the latter case a little hot oil of turpentine may be used to extract any copal solution which may remain with the powdered glass or the sand in the retort.

The older plan of preparing oil or fat varnishes with hard resins is still that usually adopted; but it yields products which are darker in colour than those obtained by the method just described, as the copal or amber used has been previously heated or even fused, whereby it has lost one quarter of its weight. One way of carrying out this plan consists in melting the copal in one vessel, and heating the oil until it commences to give off small bubbles in another; then half the oil is poured in a very thin stream into the melted resin, and incorporated therewith by constant stirring. Complete union having been effected between the two materials, the mixture is incorporated with the remainder of the hot linseed oil, any portions adhering to the vessel being afterwards dissolved by means of oil of turpentine; 30 parts of melted copal, 100 parts of linseed oil, and 70 parts of oil of turpentine, are proportions often employed in carrying out the process we are describing. This process may now be completed by adding to the solution of copal in linseed oil 1/4 of a part of manganese borate, stirring continually, and heating for two hours, or until the solution has acquired the character of a thick gold-coloured syrup which can be drawn out into threads.

This point having been reached, the heating is discontinued, and the contents of the boiler allowed to cool to 60° or 70º C, and then is added the warm oil of turpentine which has been used to dissolve out any of the copal solution clinging to the vessel in which that resin was melted. Finally, the remainder of the oil of turpentine is very gradually introduced with constant stirring. Copal varnish prepared in the above manner ought to dry in twelve hours or sooner. It is scarcely necessary to say that this method of preparing varnish with copal or other hard resin is one that no inexperienced person should attempt; not only is there some chance of partial or total failure, but there is serious risk of fire. An easier and less dangerous process requires a specially constructed heater, which is kept hot by a water-bath. Melted copal, copal or amber oil, 'manganese' oil, and oil of turpentine, are the materials used. They are all introduced together, and, as the temperature during the process of cohobation does not exceed 100°C, the time required is greater than in the previously described process.

A good copal or amber varnish ought to leave a film (on a sheet of glass) which combines the qualities of hardness and toughness. The toughness is given by the oil, the hardness by the resin. Such a film should not become fissured even when it has been exposed to sunshine during a year. Much of the copal varnish of commerce is not made from true copal or animé at all, kowdi or kauri resin (from Dammara australis), which is much easier to dissolve, being employed instead - the product, however, is decidedly inferior. Sometimes several resins are mixed together in the preparation of a so called copal varnish. A guarantee of genuineness, in which the name or names and proportions of the resin or resins employed is inserted, should always be demanded when buying copal varnish. This ought to be furnished by the varnish-maker himself, for artists' colourmen rarely prepare oil-varnishes themselves.