Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/840

822 1em

A solution of peroxide of hydrogen (ILO.,) has been used with great success in the restoration of valuable prints, as well as for cleaning and reviving oil painting darkened by the action of sulphurous vapours.

The bleaching of wool and animal fibres generally is a much simpler and less important operation than is the whitening of vegetable fibres. Wool is covered with a peculiar varnish or greasy matter which impairs its qualities, and which it is the object of the bleacher to remove. To this varnish the name of &quot; yolk &quot; or &quot; suiut &quot; is given. It is a fatty unctuous matter, chiefly derived from the cutaneous perspiration, but, no doubt, also secreted by the pores of the wool itself ; and it imparts that peculiar heavy odour to sheep with which all must be familiar. From the researches of Vauquelin it would appear that this unctuous varnish consists chiefly of a kind of soap, together with a small quantity of waxy matter, a peculiar odorous animal substance, a mixture of potash salts, and a little lime. This varnish, in consequence of its soapy nature, is soluble in water, so that washing in pure water would remove the greater portion of it; but it is found more advantageous to remove it by the process termed &quot;scouring.&quot; Scouring is performed by means of an ammoniacal lye, prepared of river or other soft water mixed with stale purified urine, which is found to contain a large quantity of ammonia, upon which its action probably depends. The mixture is heated by steam to a temperature at which the hand of the workman can be easily held in it for a considerable time. In this bath the wool is left for from half an hour to two hours, according to the quantity of greasy matter it contains. It is then to be taken out and drained into a basket, so that the drainings may drop into the vessel in which it was steeped, that nothing may be lost. It must now be completely rinsed by exposing it in baskets to a continuous stream of clear water, while a workman is perpetually employed in stirring it with a pole, till the water passes off perfectly clear. As a substitute for urine pig s dung is sometimes used, and various other substances have been proposed and introduced, such as ammoniacal salts, soda-ash, phosphate of soda, and soluble glass. Recently a machine, Petrie s wool-washer, has been introduced for scouring wools. It consists of a range of three or four long tanks, clean water entering at one end of the series and flowing through the whole. The wool is introduced at the end of the range where the water escapes, and where it is consequently most highly charged with the impurities of the washing process, and it is carried forward from one tank to another till it is lifted out at the point where the pure water enters. It is known that the wool is properly scoured by its filaments being smooth, long, slender, white, and perfectly free from foreign substances, and not having lost their natural tenacity. If this scouring be properly done there is no need of further washings in soaps, or otherwise, till the wool is subjected to the process called &quot; sulphuring ;&quot; and in pc hit of fact it is very rarely passed through any other pro cess. Some, however, recommend for the finer wools, where a very delicate white is wished, that they should be passed through one, two, or more baths of soft soap. No caustic alkaline lyes can be employed, as they destroy the wool altogether, dissolving it, and forming with it a kind of soap. The process of sulphuring is applied to yarns and woven goods only when they are intended to be finished white or light bright colours. Formerly, the method of sulphuring woollen goods was to expose them in a close apartment to the vapour of burning sulphur. The goods were hung on poles, and when the chamber was filled, a quantity of sulphur placed in very flat and broad dishes was allowed to burn away gradually in the chamber, while every aperture by which the vapour could escape was carefully closed. After exposure to the sulphurous acid vapours from six to twenty-four hours the bleaching process was complete, and the goods withdrawn from the chamber. The same process is now much more expeditiously performed by Thorn s sulphuring process. The goods- are passed on a long chain up and down over a series of rollers in a small chamber filled with sulphurous acid vapours, and a few minutes suffice for the operation. Sulphite of soda acidified with hydrochloric acid is also used in France for the bleaching of woollen fabrics. Cloth which is to be finished white after the sulphuring process is run through a bath containing some indigo carmine, which increases the brilliancy of the white. When it is to be dyed it is treated with dilute sulphuric acid, thoroughly washed, and dried.

Raw silk is covered with a kind of varnish, the nature of which was first thoroughly investigated by M. Roard. He showed that this varnish, instead of being a gum, as was usually believed, resembled a mixture of bees wax and oil, with a resinous colouring matter, and in raw silk constituted 23 or 24 per cent, of the weight. The varnish is soluble in water, and affords a solution which forms a lather like soap. The yellow varnish is of a resinous nature, and is insoluble in water, but is soluble in alcohol. The waxy substance exists in all silks, but the whiter the silk the less wax does it contain. The comparative composition of yellow and white raw silk is shown by M. Mulder s analysis:—

Yellcrw. White. Fibroine 53 37 54 04 Gelatine 20 66 19 08 Albumen 24 43 25 47 Wax 1-39 Ml Colouring matter 05 Fatty and resinous matter 010 - 30 This varnish, or &quot; gum,&quot; as it is technically called, gives the silk a stiffness and elasticity which, for many of the purposes to which silk is applied, it is desirable to remove. This is called &quot; ungumming &quot; by the bleachers of silk. Though many different processes have been suggested for this purpose, none seems to answer so well as the old pro cess of scouring in a weak solution of soap. If, however, the silk be kept in the soap too long after the varnish is removed, it begins to lose bcdy, and has its qualities impaired, becoming dull, stiff, and discoloured, in conse quence of being partly dissolved. White or yellow silks may be completely scoured in one hour in the soap bath, using about 15 lb of water for each pound of silk, and a suitable quantity of the finest soap. The soap and silk should be put into the water half an hour before it is brought to the boiling point, and then be boiled one hour. They are then removed, wrung out, washed in pure water, and eith .r exposed to the vapour of sulphur or passed through a solution of sulphurous acid gas in water. The following is the process usually followed by the scourer of silks. A quantity of water is put into a boiler over a fire, and for every 100 Eb of silk to be scoured, 30 K&amp;gt;