Pagina-afbeeldingen
PDF
ePub

linseed oil forms the peculiar artists' medium known as megilp.

Manila Copal.-This resin is the product of various species of trees growing in the Philipine Islands. It is gathered by the inhabitants and exported, chiefly from Manila, the principal town of the islands; hence it is called Manila copal. The use of the term copal is misleading, as it does not resemble the copals in its properties. The only similarity is that, like these, it is of vegetable origin.

Manila copal occurs in small pebble-like pieces of a pale brownish-yellow colour, varying slightly in different samples. Its specific gravity is about 1062. It differs very much from the true copals in all its properties; it is much softer; melts at from 230° to 250° F.; and at a temperature a little above this begins to distil over, the distillate consisting of both acid water and an oil having a very empyreumatic odour.

Manila copal swells when placed in petroleum spirit, and a little dissolves; it is soluble in ether, and partially soluble in benzene, chloroform, and turpentine; in alcohol it dissolves to a turbid solution. Manila copal which has been melted is much more soluble than fresh copal, the fused gum being freely soluble in all solvents except alcohol, which does not dissolve it any more freely than the fresh resin. It is used in making spirit varnishes, more for the purpose of giving elasticity to other resins than for any brilliancy and hardness it possesses of itself. It leaves a fairly durable coat behind it, and is much used in the preparation of enamel paints.

SPIRIT-VARNISH RESINS.-This group of resins comprises those which are soluble in alcohol or methylated spirit and they form an important and valuable group. The spirit-varnish resins include:

[blocks in formation]

Some of these have already been described, leaving but a few that require to be dealt with.

Lac is a most important product from many points of view. In its origin it differs from the resins previously described. Lac is a resinous incrustation found on the twigs of many species of Indian trees, the number and variety of which has not as yet been properly ascertained. The following is a list of the most important lac trees :-Palas or dhak tree, Butea frondosa; the peepul tree, Ficus religiosa; the koosum tree, Schleichera trijuga;

Acacia arabica, Acacia catechu, various species of Croton (lacciferum, draco, sanguiferum), Butea superba, various species of Ficus (elastica, cordifolia, venosa, villosa, indica, glomerata, &c.), Mimosa cinerea, &c. All the lac trees have a very gummy or resinous sap. The lac is not a direct product of the tree, but is formed from the sap by the female of the lac insect, Coccus lacca. The insect punctures the bark of the tree and commences to secrete the lac, forming it into cells in which it lays its eggs; the insect becomes in time completely imbedded in the lac, breathing by means of fine filaments which it sends to the surface for that purpose; when it has laid its eggs it dies; after the young insects are hatched they puncture the lac and swarm over the twig or tree branch; the males impregnate the females, which latter then proceed to secrete lac as their ancestors did before them; the insects do not move from the portion of the tree on which they first swarmed. The tree supplies nourishment to large quantities of lac insects, but at the expense of its own vitality; for, after a time, it begins to decay and then it ceases to be able to support any more insects. The distribution of the insects from place to place is probably, in the absence of any effort by the insects themselves, effected by the agency of other insects and of birds who carry the young from tree to tree; of late years artificial propagation has come into existence.

The principal portion of the lac of commerce is grown in India, but it is also obtained from other Asiatic countries. The lac of Siam has a great reputation for quality. It is also found in Ceylon, Burmah, China, Malay archipelago, and other localities. In India lac grows principally in the province of Bengal, the capital of which is a great emporium for lac in all its states; in this province the jungle districts of Chota Nagpore, Orissa, and Beerbhoom, are the chief localities. It is collected twice a year, from about the middle of October to January, and from the middle of May to the middle of July. In the Scinde State it is found abundantly in the forests surrounding Hyderabhad, where it chiefly grows on the babool tree, Acacia Arabica; the tree, however, is not attacked by the insects while it is in the full vigour of its growth, but as soon as it begins to wither the insects attack it and thrive well upon it, eventually killing it. The period of gathering the lac extends from October to the following April. It is mostly consumed locally, being used to produce the famous lacquered ware for which Hyderabhad is noted. Assam is a great lac country, and lac is collected in large quantities; much is used locally, but still a large quantity finds its way to Calcutta for export. In the Central Provinces large quantities of lac are pro

duced, in the east districts especially. Jubbulpore is a great centre for lac; the district itself supplies a great deal, and much comes into the city from Rajpore, Bilaspore, Mundla, and Sangor; most of it is used locally in the manufacture of ornaments of various kinds, and the rest is sent to Bombay for export. Sumbulpore and Mirzapore are also places of note in the lac trade. Sobhapore, in the Hoshungabad district of the Central Provinces, is a large centre of the lac trade, the lac coming into it from Futtehpore, Bankheri, the Nerbuddha Hills, Nursinghpore, and other places. The towns of Hoshungabad and Babai are also places of trade in lac. From Oudh comes supplies of lac which is found principally in the forests of the south-eastern districts, where it grows chiefly on the Ficus religiosa. In the Punjab lac is found in large quantities, although of inferior quality; it is sent to Calcutta for export. There are many other localities where lac is obtained in large or small quantities; some of it is used locally for a great variety of purposes, and the rest is sent to one of the principal seaports for export to Europe.

Lac comes into commerce as

[blocks in formation]

Another lac product is lac dye, which at one time was largely imported into Europe for dyeing purposes, but since the introduction of the coal-tar colours it has completely fallen into disuse.

1. Stick lac is the crude product just as it is taken from the trees; it rarely comes into English commerce, as it does not pay to export useless twigs of trees. Stick lac is in the form of short pieces (about 2 to 3 inches long) of the twigs with the lac incrusting them; it is cut into these lengths for convenience of carrying, &c. Stick lac is sent from the forests and jungles, where it is collected, to the different towns for manufacture into lac products. The process of manufacture is very simple, and is carried out in a very crude manner, although of recent years improvements have been made with the view of increasing both the quantity and quality of the products.

The process of lac manufacture consists in first separating the lac from the woody portion of the stick lac; this is done by placing the stick lac on a table and passing a roller over it; the lac being brittle is broken and separated from the wood almost entirely; the little that is left adhering to the wood is picked off by hand. The wood is thrown away, the lac is taken, broken into small pieces about inch in size, placed in large tubs with

warm water, and worked by the workmen getting into the tubs and treading; this process extracts the colouring matter from the lac, and is continued until the wash waters remain clear. The lac left behind is known as "seed lac." The liquor is boiled down dry, and the resulting solid mass is formed into cakes and sold as "lac dye."

2. Seed lac. This is the partly manufactured product obtained, as described above, by treating stick lac with water; this is sometimes sold for various purposes, but it is mostly manufactured into the other lac products.

3. Shellac.-This is the principal lac product and the variety which is mostly used in this country for varnish making. Shellac is made from seed lac as follows:-The seed lac is dried and placed in large bags made of cotton cloth of a medium texture. Two men take hold of the bag of seed lac, one at each end, and hold it in front of a charcoal fire; the heat of the fire soon melts the lac, which flows out of the bag, the men assisting the flow by twisting the bag from each end in opposite directions, so as to squeeze the liquid lac out of the bag; the molten lac drops into a trough placed in front of the fire. A cylinder (made of different materials at different localities) of wood, with the upper half covered with brass, or it may be made of porcelain or of the finely polished stem of the plantain. This cylinder is set in a somewhat inclined position, and the operator, taking up a ladleful of the molten shellac from the trough, pours it on the upper surface of the cylinder, while an assistant, by means of a leaf of plantain, spreads the melted material over the surface of the cylinder. It soon sets, when, by means of a knife, it is stripped from the cylinder, and is then ready for sale as "shellac."

The best quality of shellac has a pale and bright orange colour, and is hence known as 66 orange shellac;" but in the market many qualities are recognised distinguishable from one another by their colour, by their freedom from dirt and grit, and by their transparency.

4. Button lac is only different from shellac in form. Instead of being made into thin sheets the melted shellac is poured on to plates in such a manner that it sets in the form of large round flat pieces, which, owing to their thickness, appear of a dark brown colour; but are of a dark ruby colour on being looked through.

5. Garnet lac is very similar to button lac, but is made into thick flat pieces, which, in colour, resemble those of button lac. Usually the quality of lac made into button or garnet lac is not so good as that from which orange shellac is made.

For making varnishes either shell, button, or garnet lac may be used, but the latter two are only used for making the commoner

kinds of shellac varnish, where colour is not so much an object; for the best qualities of spirit varnishes only orange shellac is used.

Of late years some improvements have been adopted in one or two large lac factories in India. The lac is better washed, whereby more colouring matter is extracted from the crude lac; then, the appliances for melting and straining and converting the lac into its shell form are considerable improvements on the primitive methods described above.

Lac comes into commerce in three forms-shellac, in thin flakes of an orange colour, varying a little in shade and transparency; button lac, in large round flat masses of a dark colour; and as garnet lac, in irregularly shaped flat pieces of a dark ruby colour. It is rather brittle and easily broken up into small pieces. Occasionally it is artificially coloured with orpiment or mixed with rosin, but such adulterations are rare.

Lac is incompletely or only partially soluble in alcohol or methylated spirit, forming a turbid brownish-orange solution, which is largely sold as French polish and varnish for cabinet and other work; it is soluble in amyl alcohol.

Lac is only partially soluble in ether, chloroform, and turpentine, while it is insoluble in petroleum spirit. It is soluble in solutions of caustic potash, and of caustic soda to dark red solutions. In borax solution and in weak ammonia it is also soluble, and such solutions are sometimes used as water varnishes. One point of interest in the solubility of shellac in such alkaline liquors is that the colouring matter is first dissolved away from the resin proper, leaving the latter of a pale colour; this property is taken advantage of in preparing white shellac. Chlorine passed through alkaline solutions of lac throws down the resin free from colour. Lac has a specific gravity of 1·113 to 1·214, the darker varieties being the heavier.

Crude stick lac freed from woody matter contains 66.67 per cent. of resin, 6 per cent. of wax, 6 per cent. of gluten, and 10.8 per cent. of colouring matter. In shellac five distinct resins have

been separated (1) resin soluble in alcohol and ether; (2) resin soluble in alcohol, but insoluble in ether; (3) resin slightly soluble in alcohol; (4) a crystallisable resin; and (5) an uncrystallisable resin. These constitute about 90 per cent. of the shellac. There are in addition (6) fatty matter; (7) wax; (8) gum; and (9) colouring matter.

Bleached or White Shellac.-Shellac may be bleached in two or three ways. One method is to boil ordinary shellac in a weak solution of carbonate of potash, and when dissolved passing a current of chlorine through it; the lac precipitated is collected, melted under water, and then, while soft, pulled so as to give it

« VorigeDoorgaan »