This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
Varnish is a solution of resin in oil, turpentine, or alcohol. The oil dries and the other 2 solvents evaporate, in either case leaving a solid transparent film of resin over the surface varnished. In estimating the quality of a varnish the following points must be considered: .
(1) Quickness in drying.
(2) hardness of film or coating.
(3) toughness of film.
(4) amount of gloss.
(5) permanence of gloss of film.
(6) durability on exposure to weather. The quality of a varnish depends almost entirely upon that of its ingredients; much skill is, however, required in mixing and boiling the ingredients together. Varnish is used to give brilliancy to painted surfaces, and to protect them from the action of the atmosphere, or from slight friction. It is often applied to plain unpainted wood surfaces in the roofs, joinery, and fittings of houses, and to intensify and brighten the ornamental appearance of the grain. Also to painted and to papered walls. In the former case, it is sometimes " flatted," so as to give a dead appearance, similar to that of a flatted coat of paint.
Gums are exudations from trees. At first they are generally mixed with some essential oil; they are then soft and viscous, and are known as balsams; the oil evaporates and leaves the resin, which is solid and brittle. Resins are often called " gums " in practice, but a gum, properly speaking, is soluble in water, and therefore unfit for varnishes, while resins dissolve only in spirits or oil. Gum-resins are natural mixtures of gum with resin, and sometimes with essential oil found in the milky juices of plants. When rubbed up with water, the gum is dissolved, and the oil and resin remain suspended.
The quality of the resin greatly influences that of the varnish. The softer varieties dissolve more readily than the others, but are not so hard, tough, or durable. Common rosin or colophony is either brown or white; the brown variety is obtained by distilling the turpentine of spruce fir in water; the white is distilled from Bordeaux turpentine. The principal resins used in good work are as follows : -
Amber, obtained chiefly from Prussia, is a light yellow transparent substance found between beds of wood coal, or, after storms, on the coasts of the Baltic; is the hardest and most durable of the gums, keeps its colour well, and is tough, but difficult to dissolve, costly, and slow in drying. Gum animi is imported from the East Indies; is nearly as insoluble, hard, and durable as amber, but not so tough; makes a varnish quick in drying, but apt to crack, and the colour deepens by exposure. Copal is imported from the East and West Indies and America, etc, in 3 qualities, according to colour, the palest being kept for the highest class of varnish; these become light by exposure. Mastic is a resinous gum from the Mediterranean; it is soft, and works easily. Gum dammar is extracted from the Kawrie pine of New Zealand, and comes also from India; makes a softer varnish than mastic, and the tint is nearly colourless. Gum elemi comes from the West Indies, and somewhat resembles copal. Lac is a resinous substance which exudes from several trees found in the East Indies; more soluble than the gums above mentioned; stick lac consists of the twigs covered with the gum; seed lac is the insoluble portion left after pounding and digesting stick lac; when seed lac is melted, strained, and compressed into sheets, it becomes shell lac; of these 3 varieties, shell lac is the softest, palest, and purest, and it is therefore used for making lacquers.
Sandarach is a substance said to exude from the juniper tree; resembles lac but is softer, less brilliant, and lighter in colour, and is used for pale varnish. Dragons' blood is a resinous substance imported from various places in dark brown-red lumps, in bright red powder, and in other forms; used chiefly for colouring varnishes and lacquers.
Solvents must be suited to the description of gum they are to dissolve. Boiling linseed-oil (and sometimes other oils, such as rosemary) is used to dissolve amber, gum animi, or copal. Turpentine for mastic, dammar, and common rosin. Methylated spirits of wine for lac and sandarach. Wood naphtha is frequently used for cheap varnishes; it dissolves the resins more readily than ordinary spirits of wine, but the varnish is less brilliant, and the smell of the naphtha is very offensive, therefore it is never employed for the best work.
Driers are generally added to varnish in the form of litharge, sugar of lead, or white copperas. Sugar of lead not only hardens but combines with the varnish. A large proportion of driers injures the durability of the varnish, though it causes it to dry more quickly.
Varnishes are classified as oil varnish, turpentine varnish, spirit varnish, or water varnish, according to the solvent used. They are generally called by the name of the gum dissolved in them.
Oil varnishes, made from the hardest gums (amber, gum animi, and copal) dissolved in oil, require some time to dry, but are the hardest and most durable of all varnishes; are specially adapted for work exposed to the weather, and for such as requires polishing or frequent cleaning; are used for coaches, japan work, for the best joinery and fittings of houses, and for all outside work. Turpentine varnishes are also made from soft gums (mastic, dammar, common rosin) dissolved in the best turpentine; are cheaper, more flexible, dry more quickly, and are lighter in colour than oil varnishes, but are not so tough or durable. Spirit varnishes or lacquers are made with softer gums (lac and sandarach) dissolved in spirits of wine or pyroligneous spirit; dry more quickly, and become harder and more brilliant than turpentine varnishes, but are apt to crack and scale off, and are used for cabinet and other work not exposed to the weather. Water varnishes consist of lac dissolved in hot water, mixed with just so much ammonia, borax, potash, or soda, as will dissolve the lac; the solution makes a varnish which will just bear washing; the alkalies darken the colour of the lac.
This requires great skill and care. Full details of the process are given in Spons' ' Encyclopaedia.' Here may just be mentioned one or two points useful in mixing varnishes on a small scale; but as a rule, it is better to buy varnish ready mixed when possible.