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Resins
Accroides, or Black Boy Gum.-A resin derived from several of the Xanthorrhea species of trees; it comes to us mainly from Australia.It is obtained both from exudations and by extraction from crushed bark and wood.
Red and yellow varieties come on the market.
Both are transparent and dissolve in alcohol, giving strong yellowish solutions, mainly used for colouring spirit varnishes.
Amber .-A fossil resin found in the green sand beds stretching from Norfolk, along the Dutch coast, the shores of the Baltic, across Siberia, and also found in America.
Somewhat kindred fossil resins are found over limited areas in Asia. Practically all the amber found in commerce comes from Holland and the Baltic.
It has a glossy appearance, and may be clear or cloudy, ranging in colour from pale straw to bright yellow, light brown to orange or greenish brown, and in some cases is almost black and opaque.
In the trade besides the terms" transparent" and" cloudy" we find these resins classed as " shining" (pale yellow or greenish), " bone colour" (dull white), and " bastard" (dark and opaque).
It is insoluble in water, ammonia, petroleum spirit, benzol, acetic acid, and carbon bisulphide, but partly soluble in alcohol, turpentine, ether, and certain essential oils. Amber is, however, seldom used for varnishes to-day, on account of its high price and scarcity, and so-called" Amber" varnishes seldom contain amber, the term being mainly taken to denote the colour.
Animi.-This resin, which comes from Zanzibar, and is sometimes known as "Zanzibar Copal," is (with the exception of amber) the hardest of the fossil resins.
It is pale yellow in colour and nearly transparent. It is very expensive and, in consequence, used only for very highgrade varnishes.
Benzoin or Benjamin Gum.-A balsamic resin derived from Styrax benzoin, this comes from countries in the Indian Ocean.
It is derived by tapping the trees, and the exudation for the first three years forms first-class resin, yellowish white, soft and fragrant.
After that, the quality falls off and the resin is darker in colour and hard. Siam benzoin is sold in flattish opaque tears," and in large amber masses enclosing white patches.
It is brittle and melts at 1000 C. Sumatra benzoin is marketed in large dark blocks enclosing white masses, and is hard.
These resins are used in varnishes.
Copal.- This name is given to several kinds of semi-fossil resin, chiefly derived from Trachylobium verrucosa trees, and found in the ground near their roots.
These trees grow mostly in West and Central Africa.
It is insoluble in water and alcohol, ether and chloroform.
When ground fine and digested in ether it swells, and on being melted gives off an oily liquid.
The residue sets into a brittle mass which is soluble in ether, turpentine, chloroform, etc.
Copal is useful for varnishes required to dry with a hard lustrous surface.
Dammar (or Damar).-A resin derived from Dammara orientalis, and imported from Singapore in nodules of 1 to 2 in.
It is clear white to straw colour, but is sometimes covered with a powdery crust, is soluble in turpentine, ether, petrol spirit, and oil, while both alcohol and amyl alcohol convert it into a white jelly.
Penang Dammar is similar to the above, and both are valued for dissolving in oils to produce coach- and cabinet-makers' varnishes; for pictures and paper it is dissolved in benzol, which gives a clear, hard, but rather friable varnish.
Dragon's Blood.-A resin excreted by the Calamus draco tree, growing in Eastern Asia.
It is imported in cylindrical sticks, 12 to 14 in. long, and! to I in. thick, but is usually marketed in a ground form.
It is of a blackish-brown colour, of opaque appearance in the mass, but crimson when sliced thin.
It is soluble in alcohol, benzol, petroleum spirit, shale spirit, etc. Besides the resins coming from the East, dragon's blood is also exported from Socotra, Zanzibar, Teneriffe, and Mexico.
Elemi.-A resin which is known under two names:
(I) Manila gum elemi is derived from Canarium commune, growing in the Philippine Islands.
The best quality is white, the second grey.
It is naturally soft and granular, becomes plastic at between 75° to 80° C., and melts at 120° C. It is partially soluble in alcohol and more thoroughly in ether.
(2) Mexican elemi is derived from Amyris elimifera, is darker and heavier than Manila elemi, and comes from Mexico, Brazil, and Mauritius.
It is particularly useful for mixing with shellac or sandarac when it is desired that a varnish should retain elasticity.
Gamboge.-A resin yielded by the Garcinia morella tree, this has a bright-yellow colour.
It is soluble in alkaline solutions.
Colouring matter is extracted by maceration in ether, which, on evaporation, leaves a transparent, vitreous, and brittle mass, reddish orange in appearance, but giving a deep yellow after being powdered.
Formerly used for colouring varnishes, it is not permanent to light and has mainly been superseded by artificial dyes for this purpose.
Kauri, or Cowree.-A fossil resin found in New Zealand in lumps sometimes attaining over 100 lb. in weight.
Alcohol, turpentine, and benzol convert it into a gelatinous mass; ether dissolves it completely.
Kauri is used in the preparation of high-grade varnishes and is expensive.
Lac (Shellac).-This resin is not a natural exudation from a tree but is prepared from the secretion of insects which feed on the sap of a variety of trees which grow almost exclusively in India.
It varies in colour from yellow to orange.
The deposits of lac are formed mainly on twigs and small branches, and in this condition are known as "stick-lac."
After being separated from the wood, the product is refined by native labour, purified, graded according to colour, melted by heat, and, after it has set In sheet form, is sold as "shellac."
The best quality is orange in colour and is known as " orange shellac."
Other forms in which it is marketed are" button lac," which is the same as shellac but made in thick, round lumps, and" garnet lac," which is similar to button lac but is in thick, flattish pieces.
By treating shellac with hot alkaline solutions, the colouring matter can be removed and it is then sold as " bleached" or" white" shellac.
Shellac is soluble in alcohol and is the chief resin used in the preparation of spirit varnishes and French polishes.
It is insoluble in turpentine or oil, and for this reason is widely employed for" knotting," used by the painter for preventing resinous exudations from knots, or colours subject to bleeding, from affecting paint or varnish coatings applied on top of them.
Mastic.-A resin obtained from several lentisc trees, growing in Northern Africa.
It is soluble in turpentine, alcohol) amyl alcohol, and acetone, but not soluble in ether.
It is a compound in fine varnishes for pictures, etc., and when mixed with turpentine and linseed oil forms the megilp used as a medium for water-colours.
The cake variety, in large pieces, is the best.
Small tear mastic is also used for varnishes and is known as gum juniper.
Rosin.-A brittle resin obtained as a residue after distillation of turpentine.
It is transparent to translucent, pale yellow to brown.
The best quality is known as " window glass," the second as "amber."
"Virgin rosin" is the crude resin before distillation, known also as gum thus, which is used in the preparation of some of the coarser varnishes.
Rosin softens at 75° C. and melts- at 100° C.
It is soluble in amyl alcohol, turpentine, ether, acetone, benzol, petroleum spirit, coal-tar naphtha, but not in water. When distilled it yields rosin spirit and rosin oil.
It is used for cheap varieties of varnishes and varnish-paints, to which it imparts initially a high gloss, but it forms films which are not durable and which soon crack and disintegrate.
Sandarac.-A North African resin, yielded by Callitris quadrivalvis trees; it comes on to the market in cylindrical pieces; its melting-point is 150° C. Sandarac is soluble in alcohol, methylated spirit, amyl alcohol; only slightly so in benzol and petroleum spirit.
It is employed in the preparation of spirit and oil varnish, giving a hard bright surface.