Wool Dyeing. Lecture II. (Continued)
Indigo Vats.
Woad Vat.
Extracts of Indigo (Indigo Carmine).

Practical Magazine 23, 1876

Pidempi artikkeli jaettu erillisiin osiin.

(Chemistry applied to the Arts, Manufactures, &c.)

A course of Lectures by George Jarmain.


The importance of this colouring agent in the woollen trade cannot be over-estimated. Our finest and most expensive cloths are dyed with it, often with it alone, and frequently it forms the base upon which other colours are dyed.

Indigo has been known and used as a dye from the earliest times of cloth manufacture. It was employed by the Egyptians at a very early period, for it is found on mummy cloths; and the ancient Hindoos, Chinese, Greeks, and Romans were known to use it.

Indigo is a product of the vegetable kingdom, and is some times found in small quantities in animal secretions. Many attempts have been made to produce it artificially for industrial purposes, but as yet without result. The success which has attended the preparation of artificial alizarine, however, encourages the hope that at no distant day the production of artificial indigo for the use of the dyer will be added to the triumphs achieved by chemists in the production of tinctorial substances.

The plants from which indigo is derived belong chiefly to the family Leguminosae, and genus Indigofera. The species most cultivated and esteemed are — I. tinctoria, I. disperma, I. anil, I. argențea. The production of indigo is not confined to the Indigofera, for it is furnished also by the woad plant, Isatis tinctoria, belonging to the Compositæ family, and by other plants.

Indigo does not appear to exist in the plant in either of its well-known conditions, but is a product of a species of fermentation, to which the leaves are subjected during its preparation, the details of which I will not enter upon.

Asiatic indigoes are produced in Bengal, Oude, Coromandel, Manilla, Madras, and Java.

African, in Egypt, Mauritius, and Senegal.

American, in Guatemala, Caraccas, Mexico, Brazil, and Carolina.

The Bengal, Java, and Guatemala varieties are most esteemed. The Java is preferred for the manufacture of indigo carmine on account of its purity of colour. Its percentage of colouring matter is not very high, and the woollen dyer consequently prefers varieties richer in colour.

The best varieties of Bengal indigo have a deep blue violet colour. They are fine grained, and adhere to the tongue. They are easily pulverized, and have a coppery hue when rubbed with the nail. Sp. gr. an average of '769. The colouring principle upon which the value of indigoes mainly depends ranges from 12 to 72 per cent.

The examination of indigo, so far as it concerns the dyer, includes: —

  1. The determination of the percentage of mineral matter.
  2. Its tinctorial power, compared with a standard sample of good indigo, or with indigotin, the pure colouring matter of indigo.
  3. The amount of indigotin it contains as compared with a standard sample.
  4. Its physical characters, such as its specific gravity, colour, ease with which it powders, appearance when rubbed with the nail, action on the tongue, fracture.

1. The mineral matter is determined by burning in a platinum capsule over a spirit lamp or Bunsen flame a known weight, say 20 grains, of the powdered indigo, until all carbonaceous matter disappears, and weighing the ash. If 20 grains have been taken five times the weight of the ash will give the percentage of mineral matter. The combustion will be complete in about an hour, but will be much facilitated by breaking the coke and stirring it up at intervals with a platinum wire. Good indigo does not leave more than 10 per cent. of ash, and the best 13engal seldom more than 7 per cent.

2. Its tinctorial power is found, as compared with the standard sample, by placing five grains of each in a 2-ounce porcelain dish, pouring on each five cubic centimetres of strong sulphuric acid, or, better still, of fuming sulphuric acid, and digesting them for an hour or two at a gentle heat. Water is then added, and they are run into the 100 cubic centimetre measure, and then emptied into a beaker or flask to mix the solution. Five cubic centimetres of the sample under examination are then taken up with a pipette and run into 100 cubic centimetres of water placed in a glass cylinder, or tall white glass bottle, and the solution of the standard sample is run into another 100 cubic centimetres of water from a graduated pipette or from the burette, until a colour is obtained equal to that of the sample under examination. The relative amounts of solution required to produce the same depth of colour give the relative tinctorial values of the indigoes. Fifty cubic centimetres of the remainder of each solution are placed in a small pan, dish, or beaker, along with 50 cubic centimetres of water, and swatches of woollen cloth of equal weight are dyed in them, the heat being raised nearly to the boil. The depth of colour obtained in each case will give the relative tinctorial values of the indigoes.

3. To obtain the amount of indigotin, as compared with a standard sample, pour 10 cubic centimetres of the solution (as obtained in 2) into a pint of water contained in a flask or beaker, and run in from the burette a solution of permanganate of potash, until the blue colour disappears and the solution be comes golden yellow. Treat 10 cubic centimetres of the standard solution in the same way. The number of cubic centimetres of permanganate required in each case gives the relative amount of pure indigotin in each sample. A convenient strength of solution of permanganate is made by dissolving five grains of the crystals in one litre of water.

Solutions of bichromate of potash or of bleaching powder may also be used for destroying the colour of the indigo solutions, and the percentage of indigotin calculated from the amount of solution required in each case.

4. The examination of the physical characters of the indigo needs no comment.

Indigo, as met with in commerce, is insoluble in water, alkali, and dilute acids, and its treatment for bringing it into solution, in order that it may be used as a tinctorial agent, depends upon the fact that it is rendered soluble by the action of nascent hydrogen, whereby it is converted into white, or so-called reduced indigo, which is then soluble in alkaline fluids, such as lime, soda, or potash. This is effected by various means, according to the class of goods which are required to be dyed with it. The vessels in which the goods are to be dyed are called vats, and the substances used to reduce or hydrogenize the indigo give the name to the vat. These vats may conveniently be divided into — 1, Cold vats; 2, warm vats.

The cold vats are used for dyeing cotton or other vegetable substances; warm vats are invariably used for wool and woollen goods.

The following equation will explain the conversion of insoluble blue indigo into soluble white or reduced indigo: —
2C4H5NO (Indigo blue.) + H2 = C16H12N2O3 (Indigo white.)

Two molecules of the former being converted into one of the latter by the nascent hydrogen.

The following table exhibits the various materials and means employed to bring about the conversion, and the name of the vat: —
Indigo Vats.
Copperas vatColdCopperas, lime, and soda or potash.Cotton and vegetable substances.
Woad vatWarmWoad, madder, bran and limeBy fermentation 140°Fah.Wool and woollen goods.
Pastel vatDittoPastel, madder, bran, limeBy fermentation, not beyond 158° Fah.Ditto
Pastel potash vatDittoPastel, madder, bran, lime, potash; and, for some classes of goods, molassesBy fermentation 120°Fah.Ditto
Urine vatDittoUrine, salt, madderUseful for small dyers.
The German soda vatDittoBran, carbonate of soda, lime, molasses.By fermentation 120°Fah. Wool and woollen fabrics.
Schutzenberger and Lalande's vat.Warm or cold.Zinc dust, bisulphite of soda, lime.By simple admixtureWool and woollen in warm vat; cotton in cold.

In the copperas vat, in which a mixture of copperas and lime is employed, the nascent hydrogen is produced as follows: — The lime precipitates ferrous hydrate (hydrate protoxide of iron) from the copperas thus: —
FeSO4 (Copperas) + CaH2O2 (Slaked lime.) = CaSO4 (Sulphate of lime.) + FeH2O2 (Ferrous hydrate.)

The ferrous hydrate then takes up oxygen from water:
2Fe2H2O2 (Ferrous hydrate.) + 2OH2 (Water.) - Fe2H6O6 (Ferric hydrate.) + H2 (Nascent hydrogen.)

Soda and potash, which are sometimes used in place of lime, act in the same manner.

In the fermentation processes, the woad, pastel, bran, and madder act as the ferments, by virtue of the nitrogenous matter which they contain; the sugar of the madder and the farina of the bran pass successively into grape sugar, lactic acid, and finally into butyric acid, the production of the latter being accompanied by the evolution of hydrogen and carbonic acid, the following equation representing the change:
2HC3H5O2 (Lactic acid) = HC4H7O2 (Butyric.) + 2CO2 (Carbonic acid gas.) + 2H2 (Hydrogen.)

The nascent hydrogen thus produced is the reducing or hydrogenizing agent.

In Schutzenberger and Lalande's vat, the hydrosulphite, formed by the zinc acting on bisulphite of soda, decomposes water, taking up oxygen from it and liberating the hydrogen, itself passing into a higher state of oxidation. These changes are at present not very well understood.

The only vats employed in England for the dyeing of wool, so far as I can learn, are the woad vat, and that of Schutzenberger and Lalande. There seems to be a strong objection amongst English dyers to the employment of soda or potash, in consequence of vats being made caustic by the use of these alkaline carbonates along with lime. Caustic potash or soda is exceedingly prejudicial to the soundness and handle of the wool, espe cially when it is heated along with it; and Schutzenberger and Lalande's vat was not at first well received, because soda was used as the solvent of the reduced indigo, and many complaints were made that it rendered the pieces rotten. This is, however, now avoided by the use of lime only as the solvent, too great an excess of lime being carefully avoided.


The indigo mill is a very important apparatus. Unless the indigo is thoroughly ground great loss of that material will take place. Indigo can only be reduced when brought into the condition of an impalpable powder. The mill is either constructed of stones, somewhat like those for grinding corn, and worked by a perpendicular shaft, or it consists of four iron rollers, which are rolled backwards and forwards in a trough by a cradle motion. The indigo is ground with water to a pulp, the operation being continued for several days. The pulp must feel perfectly smooth, and contain no gritty particles.

The vat is a cylindrical iron vessel, having a diameter and depth of 6 ft. 6 in. by 6 ft. 6 in. to 7 ft. 6 in. by 7 ft. 6 in. It is encased by an iron jacket or by brickwork, and heated by steam, the steam space in the jacket being about 2 in. wide, and extending from the bottom to within 4 ft. of the top. Sometimes the vats are heated by a coil of copper pipe placed inside the vat, which is an economical arrangement. Direct fire heat is now but seldom employed.

Rakes are used to stir up the contents of the vats. The rake head is about 15 in. long by 5 in. wide, and the handle is 10 or 12 feet long. The rake is lowered down to the bottom of the vat, and a portion of the sediment is drawn up with a jerking motion when the head approaches the surface. Three or four men work up the vat in this manner vigorously for about ten minutes.

When not in use, the vats are covered by a cover, in three pieces, made of battened three-quarter-inch boards. This keeps in the heat, and prevents oxidation of the vat liquor to a great extent.

A net of half-inch meshes is lowered down into the vat when wool has to be dyed. The wool is gently moved about in the net by means of a "stang," care being taken that it is not lifted out of the fluid.

When the wool has been in the vat a sufficient length of time, it is lifted out of the net and placed in a bag made of strong net. The wool is then wrung in the bag, and the liquid returned to the vat.

Pieces, when placed in the vat, are moved about by means of an instrument called a "hawk," one of which is held in each hand, and the piece is pulled towards the operator. It must not be raised above the liquor. In order to prevent the piece from sinking down into the sediment at the bottom of the vat, an iron hoop of the same diameter as the vat, covered with a net having 4 or 5-in. meshes, is lowered down about a yard below the surface of the liquid, and suspended there.

A movable squeezing machine, consisting of two iron rollers 6 in. in diameter, fixed in a framework, serves the purpose of removing pieces from the vat. One or both rollers may be covered with vulcanized india-rubber.

Materials used in the Woad Vat - Woad, Madder, Bran, Lime Indigo.

Woad consists of the fermented leaves of the woad plant, Isatis tinctoria. This article is grown and manufactured largely in Lincolnshire. The seeds are sown in early spring, and when the plants are large enough they are transplanted in rows. When the plants are from 3 to 6 inches high, the outer leaves are twisted off by children, and then placed under a shed. Usually three crops are thus gathered during the season. The manufacture of the woad is carried on in the winter. The leaves are mixed with lime and urine, and thrown in a heap to ferment. The heap is repeatedly turned over with shovels to prevent excessive fermentation; this turning of the heap is continued until fermentation ceases. The woad is then packed in barrels and sent to the market. The present price of woad is from A. 26 to £30 per ton. (Sample exhibited.)

Madder contains sugar and nitrogenous matter in addition to its colouring matters. The sugar passes into the butyric fermentation, and furnishes part of the nascent hydrogen; the nitrogenous matter acts as a ferment. The ordinary crop madder is employed. The use of madder in the fermentation vats is not essential, and in many establishments it is not employed. The colouring matter of the madder gives a special shade to the indigo which some manufacturers like.

The bran used is the ordinary bran obtained in grinding wheat. It contains nitrogenous matters and starch; the former assist as a ferment, and the latter passes during the fermentation through the various stages of glucose, lactic acid, and butyric acid, nascent hydrogen being produced at the latter stage.

Newly burnt lime slaked and riddled is thrown into the vat in the powdering condition. Its employment serves the purpose of neutralizing the butyric acid as fast as it is formed. When the addition of lime is neglected too long, the butyric acid and other matters pass into the putrid fermentation, which acts de structively upon the indigo.

The indigoes found to be the most economical and effective for woollen dyeing are the medium qualities of Bengal, ranging, at the present time, from 4s. to 5s. per lb. The violet copper coloured varieties, having a low specific gravity, are prepared. (Samples exhibited.)

Preliminary Operations.

When the indigo-dyed goods are required to have a special "bloom," the wool or woollen fabric has a "bottom "put on it. Camwood, barwood, Saunders'-wood, cudbear, archil, or archil paste, are the colouring matters which are employed for this purpose. Camwood is the best for wool. The woods give a much more permanent bloom than the weed colours, cudbear and archil, which are somewhat fugitive.

The goods are boiled with the colour for an hour or an hour and a-half, no mordant being used. The quantity of ware is varied according to the depth of shade of bottom required. 16 lbs. of camwood or 12 lbs. of cudbear for 100 lbs. of wool are good proportions for a full bottom.

Of course the blooming is not the only object which the dyer has in view; economy of indigo is an important matter to him.

It is important that the goods to be dyed should be thoroughly cleansed from all dirt, grease, and soap, or otherwise the dye will be taken up unequally.

Setting the Woad Vat.

The following is the method adopted in the woollen districts of the West Riding:
Dimensions of Vat, 7 ft. diameter by 7 ft. deep. – Materials.
5 cwt. Lincolnshire madder, at 26s ... 6£ 10s. 0d.
3 pails of bran, weight 18 lbs. ... 0£ 1s. 1d.
22 lbs. slaked lime in dry powder ... 0£ 0s. 2½d.
2½lbs. madder ... 0£ 0s. 10½d.
24 lbs. of indigo, at 5s ... 6£ 0s. 0d.
[TOTAL] ... 12£ 12s. 2d.

The woad is chopped and pounded with a spade until the pieces are no larger than a hen's egg. It is thrown into the vat, which is then nearly filled with water. The contents are now heated by turning on the steam, until a temperature of 140° to 150° Fah. is obtained. The vat is stirred up three or four times at intervals of fifteen minutes, and then left overnight. The woad will now be soft and pulpy. Next morning put in the bran, madder, indigo reduced to a smooth paste by grinding, and half the lime; rake up, cover over, and leave till next morning. If the fermentation has commenced, the vat will present the following appearances –A slight froth will rise to the surface when the bottom is slightly stirred with one rake. The liquid, when stirred, will appear green yellow with blue veins or streaks, the green colour predominating. A coppery blue scum called "flurry" will appear on the surface of the vat. A portion of the bottom drawn up with a rake will show signs of fermentation, and will smell slightly sour. A piece of wool put into the liquid for a short time will be dyed. If these appearances present themselves the vat is progressing favourably, and about a quart more lime may be added, and the whole well raked up. If no signs of fermentation appear, more time must be given before adding the lime, which in this case would only have the effect of retarding the fermentation. If the vat matures satisfactorily, a quart more lime may be added every two or three hours, stirring up well after each addition. The coppery blue flurry will increase in quantity, and the liquid, when stirred, will be green yellow with blue streaks. Then heat up to 140°, cover, and leave overnight. Next morning (the third morning), if the fermentation has proceeded satisfactorily, the vat may be used for dyeing. It is very important that great caution should be exercised in the addition of lime, for if the vat be overdosed fermentation will be arrested, and there will be considerable difficulty in re-establishing it. The vat will then have to be left probably for several days before the fermentation reappears. On the other hand, if the addition of lime be neglected too long, and the fermentation passes to the putrid stage unchecked, the indigo will be lost, or, as the dyers say, it will "run away." The liquid exhales an agreeable odour when the progress of the setting is proceeding satisfactorily, but the odour changes when too much or too little lime is present. Much depends on the care and skill of the dyer, for no description of the process can accurately convey to the mind of the uninitiated the mode to be adopted in the varying conditions to which the fermenting indigo vat is subject. An experienced indigo dyer can exercise perfect control over the condition of his vat.

The following account of the setting of two indigo vats last week, of which I took notes, may be useful:
The vats were intended for the dyeing of broad cloths and wool of a full blue.

Dimensions of vats, 6ft. 6in. by 6 ft 6in. On Wednesday, at noon, the vats were filled with water, and 4½ cwt. of crushed woad were put into each vat; the liquid was heated to 140°Fah., and stirred up once. On Thursday morning they were heated up again to 140°, and kept at that temperature all day. During the day they were raked up vigorously by four men four times. At five o'clock, p.m., 25 lbs. of indigo, well ground to a smooth paste, 14 lbs. madder, 5 quarts of lime, and 4 pails of bran (2 gallons each), were added, and the whole well stirred up and left over night. On Friday morning at nine o'clock a light froth rose on the surface when the vat was gently stirred up with one rake. The bottom was in a slight state of ferment and smelt rather sour. The liquid appeared green yellow on the surface with blue streaks when it was stirred by the hand. (Sample of wool dyed in it exhibited.) One quart of lime was added to each vat, and the contents well stirred for ten minutes as before. It was then covered up. At 11 a.m. and at 1 p.m. another quart of lime was given, and the contents raked up as before. A considerable quantity of coppery blue flurry had now accumulated on the surface, and the liquid appeared of a dark greenish yellow with blue streaks when disturbed by the hand. The odour was favourable and the fermentation steady, being kept under by the lime. At 3 p.m. the vats were warmed to 135° and another quart of lime added and again well stirred up, appearance and odour favourable as before. At 5.30 the vats were in a condition fit to use for dyeing. (Sample dyed exhibited.) Three pints of lime were given to each vat and 5 lbs. more indigo; they were well stirred up. Temperature 130°. They were covered up and left over night. At 9 a.m. on Saturday two sixty-yard pieces of narrow doeskins were put into each vat, and after they had been worked for 1¼ hours I took off a sample and left them. (Sample exhibited.)

The Dyeing Offerations.

Pieces to be dyed a full blue are hawked in a strong vat from twenty minutes to two hours, according to the class of goods — heavy and closely woven fabrics requiring more time for the colour to penetrate into the centre than lighter and more open goods. The material is then drawn out of the vat, and after exposure to the air for some time, in order that the reduced indigo may become oxidized and pass into the blue and insoluble condition, the goods are passed into a weaker bath to dye up to the shade required. For light-blue colours, such as are given for woading for blacks, a single working through a moderately weak bath for about twenty minutes will be sufficient.

For dyeing wool, a vat which shows the fermentation to be in a moderately active state is found to be the best, but pieces are dyed more perfectly in a vat in which the fermentation is kept under.

When the goods are withdrawn from the vat, they are of a fine green colour, which rapidly passes to blue. The green colour is due to the instant oxidation of a portion of the yellow coloured reduced indigo, which thus produces a mixture of blue and yellow.

The formation of indigo blue from the reduced indigo is explained by the following equation:
C16H12N2O2 (Indigo white.) + O (Oxygen from the air.) = 2C8H5NO (Indigo blue.) + OH2 (Water.)

For keeping up the strength of the vat, about 15 lbs. of indigo are added every other day, more lime and bran being added every day. After finishing the day's working of the vats the necessary addition of bran, lime, and indigo is made, and the vats well raked up and heated up to 140°Fah.

The following table gives the quantities and cost of materials used in a large dye-house, for the woad vat for dyeing dark indigo blues on cloth. (Sample exhibited):
6,533 lbs. indigo, at 5s. 6d. per lb ... £1,796 11s. 6d.
68 cwts. lime, at 1s. per cwt. ... £3 8s. 0d.
19¾ packs bran, at 14.s. 6d. ... £14 6s. 4d.
142½ lbs. madder, at 4¼d. ... £2 10s. 6d.
56½ cwts, woad, at 30s. ... £84 15s. 0d.
12 cwts. fuller's earth. ... £1 16s. 0d.
[TOTAL] £1,903 7s. 4 d.

Percentages of total cost:
Indigo ... 94'37
Fuller's earth ... 10
Woad ... 4'45
Madder ... '13
Bran ... '76
Lime ... '18
... 99.99

Cloth dyed with the above materials, 159,180 lbs. Cost per pound, 367 pence.

It is noteworthy that the materials required to reduce the indigo only cost 5,52 per cent. of the whole.

Schutzenberger and Lalande Vat.

This vat stands in marked contrast to the woad vat already described, as regards time and the number of vats required. The reduction of the indigo is almost instantaneous; in fact, the reduced indigo may be prepared beforehand, and then it is only necessary to add it to the vat, and proceed with the dyeing in the manner already described. The reduction is performed in the following manner: —

Materials required. — Bisulphite of soda, at 45° Tw., costing 9s, per cwt. Zinc dust, at 23s. per cwt. Slaked lime in powder, sifted, or in the form of cream of lime. Indigo, ground in water, in the form of pulpy paste, without any gritty particles.

Utensils for the preparation of the reduced Indigo. — A small tub, or pail, in which to prepare the hydrosulphite. A cask in which the indigo is reduced. Barrels in which to store the reduced indigo. The same vats and utensils which are employed with the woad vat.

To prepare the reduced indigo proceed as follows: —
Pour bisulphite of soda solution into a pail or small tub, and add zinc dust until the smell of sulphurous acid disappears, stir ring up well until this takes place. The zinc dust required will be from one-fifteenth to one-tenth of the weight of the bisulphite of soda. The solution so produced is called hydrosulphite.

Place your indigo paste in a cask or tub; add a little cream of lime, and heat to 120° Fah. Run in sufficient hydrosulphite to reduce the indigo, which may be known when the surface of the mixture assumes a dark green appearance; then add more cream of lime to dissolve the reduced indigo, when the mixture then becomes yellow. It may be known when sufficient lime has been added by taking out the stirring rod, which will quickly assume a bronzed, coppery appearance. If too much lime has been added, the indigo becomes brown and is partly destroyed, a smell of ammonia being given off.

The creamy fluid is then run into barrels for use.

The principle on which the indigo is reduced has already been given. (A small sample of indigo was reduced.)

The Dyeing Offerations.

The vat is filled up with water, which is heated up from 130° to 140°Fah., and the necessary quantity of reduced indigo run in from the store casks. The dyeing may be carried out exactly on the same plan as with the woad vat; the vat may, however, be used continuously, as it may be fortified at any moment by the addition of reduced indigo. If the indigo in the vat should become oxidized by exposure to the air, and by the air which the goods carry in with them mechanically, it may be reduced by adding hydrosulphite, and also lime, if necessary, to the vat. As there is not such an accumulation of solid matter at the bottom of the vat as there is in the woad vat, vessels having a much less depth may be employed.

The dyer very quickly learns how to manage this vat. The operations are simple and perfectly under control, and they do not, therefore, require an apprenticeship to learn how to manage them, as is the case with the fermentation process.

Opinions of Dyers on the Hydrosulphite Vat.

Since the year 1873, when it began to be used in Yorkshire, the experiences of woollen dyers with this vat have been very varied. It has been abandoned by some and continued by others.

The advantages claimed for it are — that it is easy to manage; that it dyes worsted goods clean; that it can be used either for light or dark blues; that the services of an experienced blue dyer at high wages can be dispensed with; that it is rapid.

The disadvantages attributed to it by those who have aban doned it are — that it is much more expensive than the woad vat on account of loss of indigo in washing; that the goods handle thinner when of the same weight and texture, although the strength of the goods is not impaired, as tested by Hebden's machine; that the colour is not quite so good, being somewhat green; that it requires one-third more red bottom than the woad vat; that it spoils the face of goods.

The heaviest charge against it, however, seems to be the cost, but this may be very greatly lessened by recovering the indigo which washes off, by passing the goods through milk of lime before the washing. The lime dissolves the reduced indigo which has not become fixed to the fibre. The green colour can be removed by passing the goods through very dilute sulphuric acid in the washing machine.

The following table gives the cost of dyeing dark blues by the hydrosulphite vat by a dyer who wished to use it, but was deterred by the cost. It bears unfavourable comparison with the cost of materials by the woad vat. The cost of labour in working the two vats may be assumed to be about the same:
240 lbs. Indigo at 5s. 6d. ... £66 0s. 0d.
1,978 lbs. Bisulphite of soda at 1d. ... £8 4s. 10d.
195 lbs. Zinc dust at 23s. per cwt. ... £2 0s. 1d.
30 lbs. Fuller's earth ... £0 1s. 0d.
Lime ... not given
[TOTAL] £76 5s. 11d.

Percentages of fofal cost:
Indigo ... 86'84
Bisulphite soda ... 10'62
Zinc dust ... 2'63
Fuller's earth ... nil.
Lime ... not given
... 100'09

Cloth dyed with above material, 3,420 lbs. Cost per lb., 5¾ d.

The materials required to reduce the indigo cost 13,25 per cent. of the whole.

(Wool and swatches of cloth were then dyed with the hydrosulphite vat.)

Extracts of Indigo (Indigo Carmine).

These are sulphuric acid derivatives of indigo obtained by dissolving indigo in strong, or even fuming, sulphuric acid. The extract having a purple colour, contains chiefly sulpho purpuric acid, C10H10N2O2,SO3, the blue extract contains sulph indigotic acid, C8H5NO,SO3. The production of these two varieties depends upon the strength of the acid used, and the temperature and time of contact.

The acid, or "sour extract," may be made by gradually adding 1 lb. of well-powdered indigo to 12 lbs. of the strongest commercial sulphuric acid, stirring well during the addition. The mixture is then kept for two days at a temperature of about 140°Fah. A little of the mixture dropped into water should completely dissolve. The mixture is poured into water, and is then ready for use. The best qualities of indigo and the refined article should be used in these preparations.

The sweet extract is made by neutralizing an acid extract with carbonate of soda and chalk, adding also salt. The sulphopurpuric and sulphindigotic acids form soda salts, which are insoluble in solutions of alkaline salts; the colouring matter can therefore be precipitated, filtered off, and washed. The following process gives a good sweet extract: — Add gradually 9 lbs. of the strongest sulphuric acid to 1 lb. of the finest quality of indigo finely ground and sifted, stirring well all the time; set aside for a week at from 60° to 70° Fah.; then add a solution of 10 lbs. of common salt and 15 lbs. of soda crystals; then ½ lb. of powdered chalk may be added, and the whole well stirred up and thrown upon a filter, and washed with a solution of salt till the washings run through colourless.

The purple extract is made by using one-third as much acid and leaving them in contact only a short time.

These dyes require no mordant for wool; they dye best in an acid bath, and the addition of sulphate of soda is found to be beneficial in clearing the liquid and producing evenness of shade.

They are not used as self colours on woollen goods, but enter largely into the composite colours of goods which do not require to be scoured afterwards.

They are not, therefore, fast colours, and cannot be compared with indigo colours dyed by vat processes, which possess a degree of permanence which has long given them a deserved popularity, as they remain unaffected alike by heat and cold, sunshine and rain, acid and alkali.

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