Part 73

_Hist._ Bleaching is a very ancient art, as passages referring to it in the earlier sacred and profane writers fully testify. It had probably reached a high degree of excellence among the inhabitants of the first Assyrian empire, and was certainly practised in Egypt long before the commencement of written history. We may fairly assume that fine white linen formed part of the "raiment," which, together with "jewels of gold, and jewels of silver," and "precious things," Abraham sent as presents to the beautiful Rebekah and her family,[206] fully three centuries and a half before the Exodus. Subsequently, in Scripture, we have special mention of "fine linen, white and clean." Herodotus, the earliest Greek historian, tells us, that the Babylonians wore "white cloaks;"[207] and in Athenæus we read of "shining fine linen," as opposed to that which was "raw" or unbleached.[208] At this early period, and for many centuries afterwards, the operations of washing, fulling, and bleaching were not distinctly separated. The common system of washing followed by drying in the sun, adopted by the ancients, is a process which of itself, by frequent repetition, decolorises the raw materials of textile fabrics, and thus must inevitably have taught them the art of 'natural bleaching' of a character similar to that practised in Europe up to a comparatively very recent period. And this appears, according to the authority of ancient authors, to have been the case. Washing or steeping in alkaline and ammoniacal lyes, or in milk of lime, followed by exposure in the sun, formed the chief basis of their system; whilst woollens, then as now, were treated with soap and fuller's earth, or with potter's clay, marl, Cimolian earth, or other like mineral. Urine was highly esteemed among them; and we are told that in the time of the emperor Vespasian,[209] and undoubtedly long before it, cloths were sulphured. Indeed, according to Pliny, sulphuring was often had recourse to in ordinary washing, as well as in the bleaching process.[210]

[Footnote 206: Gen. xxiv, 53; B.C. 1857.]

[Footnote 207: Herod., i, 195.]

[Footnote 208: Athen., ix, 77.]

[Footnote 209: 'Hist. Nat.,' xxv, 57, &c.]

[Footnote 210: {Transcriber's note: Footnote omitted by publisher.}]

Bleaching continued to be practised with no essential change of its principles until the discovery of chlorine, to which we shall presently refer. In the last century Holland obtained the best name for bleaching. The process passed then to Ireland and Scotland, and thence into England. It was even customary to send goods from this country to be bleached in Holland. The first attempt to vie with Holland was made, in Scotland, in 1749.

The first steps towards the modern or chemical system of bleaching were the investigations of Berthollet on chlorine, in 1784, but which were not communicated to the French Academy until the year 1787. The knowledge of the use of chlorine as a bleacher was soon afterwards brought to this country by the Duke of Gordon, and by Prof. Copeland of Aberdeen, and through them was practically applied by Messrs Milnes of that place. About the same time James Watt, a correspondent of Berthollet, successfully introduced its use in the neighbourhood of Glasgow, and then generously laid a statement of the results before the Manchester manufacturers. In enforcing the importance of the new substance and process on these gentlemen, he was ably followed and seconded by Dr Henry. In 1798, Mr Charles Tennant, of Glasgow, obtained a patent for a new bleaching liquor prepared by saturating lime water with chlorine; and another, in 1799, for dry chloride of lime, a substance which is still preferred as a bleacher to all other preparations of chlorine. The new or continuous process of bleaching, as it is called, and that which is at present in general use in all the chief bleach-works of Lancashire, was introduced by Mr David Bentley, of Pendleton, and patented by him in 1828.

_Proc._ Bleaching is commonly said to be natural when exposure to light, air, and moisture forms the leading part of the process; and to be chemical when chlorine, chloride of lime, sulphurous acid, or other like substances are employed. In some cases, as with linen, the two processes are combined. The subject requires to be noticed under separate heads, depending on the material operated on:--

I. BLEACHING of =Cotton=:--Cotton is more easily bleached, and appears to suffer less from the process than most other textile substances. On the old plan it was first (1) thoroughly washed in warm water, to remove the weaver's paste or dressing; then (2) 'bucked' or 'bowked' (boiled) in a weak alkaline lye, or in milk of lime, to remove colouring, fatty, and resinous matters, insoluble in simple water; and after being (3) again well washed, was (4) spread out upon the grass, or bleaching ground, and freely exposed to the joint action of light, air, and moisture (technically called 'crofting'). The operation of 'bucking' in an alkaline lye, washing, and exposure was repeated as often as necessary, when the goods were (5) 'soured' or immersed in water acidulated with sulphuric acid, after which they (6) received a final thorough washing in clean water, and were (7) dried, finished, and folded for the market. From the length of the exposure upon the bleaching ground this method is apt to injure the texture of the cloth; and from the number of operations required is necessarily expensive and tedious. It is therefore now very generally superseded by the system of chemical bleaching briefly described below.

In the CHEMICAL SYSTEM of bleaching the goods are 'washed' and 'bucked' as on the old plan, then submitted to the action of a weak solution of chloride of lime, and afterwards passed through water soured with hydrochloric or sulphuric acid, when they have only to be thoroughly washed, and to be dried and finished, for the entire completion of the process.

The new or continuous process, before referred to,[211] is the method of chemical bleaching at present in the most general use; and, indeed, it has nearly superseded all other methods. In this system the pieces, previously tacked together endwise so as to form a chain, are drawn, by the motion of rollers, in any direction, and any number of times, through every solution to the action of which it is desired to expose them, and this entirely and completely under the control of the operator.

[Footnote 211: See 'Hist.' (_antè_).]

The following _Table_ exhibits an outline of the several operations in the improved form of the continuous process as practised by Messrs McNaughten, Barton, and Thom, at Chorley, and in most other large bleach-works:--

1. Preliminary operations:--_a_. The 'pieces'[212] are separately stamped with the printer's name, a solution of silver, or sometimes coal-tar, being employed for the purpose.

[Footnote 212: Usually about 30 yards each.]

_b._ They are tacked together endwise either by hand or a machine, so as to form one continuous piece of 300 to 350 yards in length, according to the weight of the cloth.

_c._ They are singed.[213]

[Footnote 213: Generally on one side only; but for goods to be subsequently finely printed, on both sides. For very fine printing the nap is sometimes removed by shearing instead of 'singeing,' an ingenious and effective machine being employed for the purpose.]

_d._ They are crushed into a rope-like form by drawing them through a smooth aperture,[214] the surface of which is generally of glass or porcelain--the rope-form being given them to enable the water and other liquids to penetrate the goods more easily, and to allow them to be laid in loose coils in the kiers.

[Footnote 214: Such apertures are also used instead of pulleys, in the transfer of the rope from place to place.]

2. The pieces are bucked or boiled in milk of lime[215] for 12 to 14 hours,[216] followed by rinsing or cleansing in the washing-machine.

[Footnote 215: The common proportions are about 1 lb. of quick-lime slaked and reduced to milk, with 2 galls. of water, for every 14 or 15 lbs. of cloth.]

[Footnote 216: This is done in a large iron boiler furnished with a perforated false bottom of wood, and technically termed a 'kier.']

3. They are soured in water acidulated with hydrochloric acid,[217] and again washed; similar machines being employed for each.

[Footnote 217: This dilute acid or acidulated water has usually the sp. gr. 1·010 or 2° Twaddle.]

4. They are bucked or boiled for 15 or 16 hours in a solution of resinate of soda,[218] and then washed as before.

[Footnote 218: Made with about 17 _lbs._ of soda-ash and 3 _lbs._ of resin, with water, 50 galls., to every 20 or 21 _lbs._ of cloth.]

5. They are chemicked by being laid in a wooden, stone, or slate cistern, when a solution of chloride of lime[219] is pumped over them, so as to run through the 'goods' into a vessel below, from which it is returned on them by continued pumping, so that the cloth lies in it for 1 or 2 hours; it is then washed.

[Footnote 219: Technically called 'chemick.' The strength of the solution is usually of the sp. gr. 1·0025, or 1/2° Twaddle.]

6. They are bucked or boiled, for 4 or 5 hours, in a solution of 1 _lb._ of crystallised carbonate of soda, dissolved in 5 galls. of water, to every 35 _lbs._ of cloth; and washed.

7. They are again 'chemicked,' as before; and washed.

8. They are soured in very dilute hydrochloric acid;[220] and then left on 'stillages'[221] for 5 or 6 hours.

[Footnote 220: Sp. gr. 1·0125, or 2-1/2° Twaddle.]

[Footnote 221: Low stools or props to keep them from the ground.]

9. They are, finally, thoroughly washed, well squeezed between rollers, dried over steam-heated tin-cylinders, starched or dressed, and finished.

This is the usual process for good calicos, Muslins, and other light goods, are handled rather more carefully; whilst for commoner ones the sixth and seventh operations are generally omitted. The whole usually occupies 5 days; but by using Mr Barlow's high-pressure steam kiers, it may be performed in two days. Yarns, &c., may be bleached in a similar manner by first looping the skeins together.

_Obs._ According to the most reliable authorities, the strength of cotton-fibre is not impaired by its being boiled for two hours in milk of lime, under ordinary pressure, out of contact with the air; nor, according to the bleachers, even by sixteen hours boiling at the strength of 40 _lbs._ per 100 galls. It is said that lime is less injurious than 'soda.'

Solution of caustic soda, sp. gr. 1·030, does not injure it, even by boiling under high pressure; but, in practice, soda-ash, or carbonate of soda, is used, and this only in the second bucking, and in the third, if there be one. The strength now never exceeds 25 _lbs._ of the crystals to the 100 _galls._, and is usually less.

Experiments have shown that immersion for 8 hours in a solution of chloride of lime containing 3 _lbs._ to the 100 galls., followed by souring in sulphuric acid of the sp. gr. 1·067, or for 18 hours in acid of 1·035, does not injure it.

By the improved method of previously treating the goods with lime or alkalies, little chloride of lime is required. Indeed, it is said that where 300 _lbs._ were formerly employed, 30 to 40 _lbs._ only are now used. At the same time it is right to mention, that though a solution at 1/2° Twaddle is usually regarded as the best and safest strength, yet in some bleach works, particularly for inferior and less tender goods, this is greatly increased, even up to 5°, the period of immersion being proportionately reduced, as it is not safe to expose the goods long to the action of such powerful solutions. With the higher strengths they are passed rapidly through the liquid with the calender, sufficient time only being allowed to soak them thoroughly; then immediately through the acid or souring, followed by washing as before.

In Scotland and Ireland the washing is generally performed by wash-stocks; whilst in Lancashire, dash-wheels, or washing machines with squeezers, are almost always used for the purpose.

Cotton loses about 1-20th of its weight by bleaching.

II. BLEACHING of =Linen=:--Linen may be bleached in a similar way to 'cotton,' but the process is much more troublesome and tedious, owing to its greater affinity for the colouring matter existing in it in the raw state. Under the old system, several alternate buckings with pearlash or potash and lengthened exposure on the field, with one or two sourings, and a final scrubbing with a strong lather of soft soap, constituted the chief details of the process. In this way a high degree of whiteness, though not an absolutely pure or snow white, was ultimately produced. Grass-bleaching or crofting is still extensively used for linen; but it is more generally employed only for a limited time, and in combination with a modification of the system at present almost universally adopted for cotton goods; whilst, in some cases, crofting is omitted altogether, and the bleaching conducted wholly by the latter process. The following _Tables_ exhibit the outlines of the new system as at present practised in Ireland and Scotland:--

_a._ For plain sheetings:--

1. They are bucked for 12 or 15 hours in a lye made with about 1 _lb._ of pearlash (or soda-ash) to every 56 _lbs._ of cloth, and washed.

2. Crofted for about 2 days.

3. Bucked in milk of lime.

4. Turned, and the bucking continued, some fresh lime and water being added; and washed.

5. Soured in dilute sulphuric acid at 2° Twaddle.

6. Bucked with soda-ash for about 10 hours, and washed.

7. Crofted, as before.

8. Bucked again with soda-ash, as before.

9. Crofted for about 3 days.

10. Examined, the white ones taken out, and the others again bucked and crofted.

11. Scalded or simmered in a lye of soda-ash of about only 2-3rds the former strength, and washed.

12. Chemicked, for 2 hours, at 1/2° Twaddle, washed, and scalded.

13. Again chemicked, as before.

14. Soured for 4 hours, as in No. 5; washed, and finished.

This occupies 13 to 15 days, according to the weather.

_b._ For shirtings, &c.:--As the preceding, but with somewhat weaker solutions.

_c._ For goods to be subsequently printed:--

1. Bucked in milk of lime for 10 or 12 hours.

2. Soured in dilute hydrochloric acid of 2° Tw., for 3 to 5 hours, and washed.

3. Bucked with resinate of soda for about 12 hours.

4. Goods turned, reboiled as before, and washed.

5. Chemicked at 1/2° Tw., for 4 hours.

6. Soured at 2° Tw., for 2 hours, and washed.

7. Bucked with soda-ash for about 10 hours, and washed.

8. Chemicked as in No. 5.

9. Soured, as at No. 6, for 3 hours; washed, and dried.[222]

[Footnote 222: The strengths of the solutions, when not otherwise stated, are about the same as those given under COTTON (_antè_).]

_Obs._ The chief difficulty in bleaching linen arises from the fact that its colouring matter is insoluble in acid or alkaline solutions until it has been long acted upon by light, air, and moisture, as in the common process of grass-bleaching. Chlorine hastens the operation; but, unfortunately, it can only be employed towards the end of the process; as when earlier used, the colour of the raw cloth becomes set, and irremovable. To obviate this difficulty Mr F. M. Jennings, of Cork, has lately[223] introduced the joint use of an alkali and an alkaline hypochlorite (chloride) in the place of the ordinary chloride of lime. He prepares a bath of solution of soda at 5° Twaddle, which he raises by the addition of chloride of soda (or of potash) to 6 or 7°, and in this he steeps the cloth (after the first bucking and souring) for some hours, heat, or constant squeezing between rollers, being had recourse to, to facilitate the action. Souring and washing follow, when the goods are again put into the alkaline and chloride bath, as before; after which they are soured, and bucked again with soda. These last three operations are repeated until the cloth is almost white, when crofting for one half to one fourth the time required by the usual method renders it fit for the final bucking, and finishing. Indeed, it is said that if the process be very carefully managed it renders crofting unnecessary.

[Footnote 223: Patent dated 1859.]

Raw linen loses about 1-3rd of its weight in bleaching.

III. =Silk=:--Silk is usually bleached by first steeping it, and then boiling it in solutions of white soap in water, after which it is subjected to repeated rinsings, a little indigo-blue, or archil, being added to the last water to give it a pearly appearance. When required to be very white (as for gloves, stockings, &c.), the goods are cautiously submitted, for 2 or 3 hours, to the action of the fumes of burning sulphur, and then finished by rinsing, as before.

_Obs._ Boiling or sulphuring is not required for the white silk of China. Raw silk loses from 4 to 5 _oz._ per _lb._ by bleaching.

IV. =Wool=:--In bleaching raw wool it is first deprived of the yolk or peculiar natural varnish with which it is covered. For this purpose it is steeped and stirred for about 20 minutes in rather warm water (135°--140° Fahr.), either with or without the addition of 1-4th part of stale urine; after which it is placed in baskets to drain, and soon afterwards thoroughly rinsed in a stream of water, when it is again allowed to drain, and it is hung up to dry. The further operations depend on circumstances, wool being sometimes whitened in the fleece, or in the yarn, but still more frequently and extensively not till woven. When it is intended to send it in the first two forms white to market, it is hung up or spread out, whilst still wet, and sulphured (see _below_); after which it is either at once rinsed for some time in cold water, or is previously treated with a very weak bath of soft soap.

In the case of woollen fabrics the operations of purifying or whitening the wool, beyond the removal of the yolk, are, for the most part, mixed up with the weaving and working of it. The pieces leave the hands of the weaver of a dingy grey colour, loaded with oil, dirt, and dressing. They then pass to the fulling-mill, where they are treated with fuller's earth and soap, often preceded with ammonia or stale urine, after each of which they are well washed out or scoured with cold water, and are then ready for the dyer. When it is intended to obtain them very white, or to dye them of a very delicate shade, they are commonly sulphured; after which they are washed or milled in cold water for some hours, a little finely ground indigo being added towards the end, to increase their whiteness; an addition also made when the cloth is sufficiently white without the sulphuring process.

The usual mode of SULPHURING woollen goods is to hang them up on pegs or rails, or, in the case of fleece-wool, to spread it about, at the upper part of a close, lofty room or chamber, called a sulphur-stove. In each corner of this room is set a cast-iron pot containing sulphur, which, after the introduction of the goods, is set on fire, when the door at the lower part of the chamber is shut tight and clayed. This is commonly done over-night; and by the morning, the bleaching being finished, the goods are removed, washed, and azured.

Sulphuring, unless very skilfully managed, imparts a harsh feel to woollen goods, which is best removed by a very weak bath of soap-and-water (lukewarm); but the action of soap in part reproduces the previous yellowish-white tinge. Milling with cold, or lukewarm water, tinged with indigo, is the best substitute.

_Obs._ Raw wool loses from 35 to 45% of its weight by scouring, and 1 to 2% more in the subsequent operations of the bleacher; the loss being in direct proportion to the fineness of the staple.

[asterism]The above are the four principal applications of the art of bleaching; but, in technical language, the words bleaching, bleacher, bleachery, bleach-works, &c., when employed alone, are understood to have reference only to cotton and linen. This has arisen from the enormous extent of these manufactures, and from the process of bleaching them forming a business entirely distinct from that of weaving, dyeing, or printing them. The following, with the exception of the first, are of comparatively minor importance and interest:--

V. =Materials for Paper=:--Old rags for the manufacture of paper, and paper-pulp, are now almost universally bleached with chlorine or chloride of lime; the former being generally used in France, and the latter in England. The process usually consists in (1) boiling in an alkaline lye to remove grease and dirt, (2) washing, (3) pressing, (4) deviling or tearing up the pressed cake into fine shreds or pulp, (5) chemicking, with agitation, for about an hour, in a clear solution of chloride of lime,[224] followed by (6) washing, (7) souring with dilute hydrochloric acid at 1 or 2° Tw., or treatment with a solution of some antichlor, or both, and (8) a final washing and pressing. For the common kinds of paper, the operations included in No. 7 are omitted; but unless the whole of the lime-salt be removed from the pulp, the paper made of it is liable to turn brown and become rotten by age. In some cases rags are bleached before being divided and pulped. Cotton-waste is bleached in a similar way to rags.

[Footnote 224: The 'strength' varies with the colour and quality of the rags. From 2 to 4 _lbs._ per _cwt._ of rags is a common proportion; but for dyed and printed rags as much as 7 or even 8 _lbs._ per _cwt._ are often employed. It is better, however, to prolong the process with a weaker solution, than to hasten it by using the chloride in excess. Large rectangular cisterns of wood, or of slate, are commonly employed as the bleach-vessels. Cisterns of wood, or brick-work lined with gutta percha or with asphalto-bitumen, are employed in some paper-mills, and answer admirably.]

In France, the chlorine, in a gaseous form, is passed from the generators into the bleach-cisterns containing the pulp, which in this case must be fitted with close covers.

=VI. Printed Paper=, as Books, Engravings, Maps, &c.--These when stained or discoloured may be whitened by (1) wetting them with pure clean water, (2) plunging them into a dilute solution of chloride of lime, (3) passing them through water soured with hydrochloric acid, and then (4) through pure water until every trace of acid be removed. This process may be further improved by further dipping them into a weak solution of some antichlor, and again washing them, before finally drying them. It is only rare and valuable original works or specimens of art that are worth this treatment, which, owing to the very nature of paper, requires considerable address to manage. In many cases a sufficient degree of renovation may be effected by simply exposing the articles, previously slightly moistened, to the fumes of burning sulphur, followed by passing them through a vessel of pure water.