The Dyeing of Woollen Fabrics Part 26

[Ill.u.s.tration: Fig. 31.--Yarn-drying Machine.]

After being wrung, squeezed or hydro-extracted the goods are ready to be dried. In the case of yarns this may be done in rooms heated by steam pipes placed on the floor, the hanks being hung on rods suspended from racks arranged for the purpose.

[Ill.u.s.tration: Fig. 32.--Cloth-drying Machine.]

Where large quant.i.ties of yarn have to be dried it is most economical to employ a yarn or drying machine, and one form of such is shown in figure 31. The appearance of the machine is that of one long room from the outside, internally it is divided into compartments, each of which is heated up by suitably arranged steam pipes, but the degree of heating in each compartment varies, at the entrance end it is (p. 209) high, at the exit end low. The yarn is fed in at one end, being hung on rods, and by suitable gearing it is carried directly through the various chambers or sections, and in its pa.s.sage the heat to which it is subject drives off the water it contains. The yarn requires no attention from the time it pa.s.ses in wet at the one end of the (p. 210) machine and comes out dry at the other end. The amount of labour required is slight, only that represented by filling the sticks with wet yarn and emptying the dried yarn. The machine works regularly and well.

The drying is accomplished by circulating heated air through the yarns, this heating being effected by steam coils; fresh air continually enters the chambers while water-saturated air is as continually being taken out at the top of the chamber. One of the great secrets in all drying operations is to have a constant current of fresh hot air playing on the goods to be dried, this absorbs the moisture they contain, and the water-charged air thus produced must be taken away as quickly as possible.

#Piece Goods.#--The most convenient manner of drying piece goods is to employ the steam cylinder drying machine such as is shown in figure 32. This consists of a number of hollow tin or copper cylinders which can be heated by steam pa.s.sing in through the axles of the cylinders, which are made hollow on purpose. The cloth to be dried pa.s.ses round these cylinders, which revolve while the cloth pa.s.ses. They work very effectually.

CHAPTER VIII. (p. 211)

EXPERIMENTAL DYEING AND COMPARATIVE DYE TESTING.

Every dyer ought to be able to make experiments in the mordanting and dyeing of textile fibres for the purpose of ascertaining the best methods of applying mordants or dye-stuffs, the best methods of obtaining any desired shade, and for the purpose of making comparative tests of dyes or mordanting materials with the object of determining their strength and value. This is not by any means difficult, nor does it involve the use of any expensive apparatus, so that a dyer need not hesitate to set up a small dyeing laboratory for fear of the expense which it might entail.

In order to carry out the work indicated above there will be required several pieces of apparatus. First a small chemical balance; one that will carry 50 grammes in each pan is quite large enough, and such a one, quite accurate enough for this work, can be bought for 25s. to 30s., while if the dyer be too poor even for this a cheap pair of apothecaries' scales might be used. It is advisable to procure a set of gramme weights and to get accustomed to them, which is not by any means difficult.

In using the balance always put the substance to be weighed on the left-hand pan and the weights on the right-hand pan. Never put chemicals of any kind direct on the pan, but weigh them in a watch-gla.s.s, small porcelain basin, or gla.s.s beaker (which has first been weighed), according to the nature of the material which is being weighed. The sets of weights are always fitted into a block or (p. 212) box, and every time they are used they should be put back into their proper place.

The experimenter will find it convenient to provide himself with a few small porcelain basins, gla.s.s beakers, cubic centimetre measures, two or three 200 c.c. flasks with a mark on the neck, a few pipettes of various sizes, 10 c.c., 20 c.c., 25 c.c.

The most important feature is the dyeing apparatus. Where only a single dye test is to be made a small copper or enamelled iron saucepan, such as can be bought at any ironmongers may be used; this may conveniently be heated by a gas-boiling burner, such as can also be bought at an ironmongers or plumbers for 2s.

[Ill.u.s.tration: Fig. 33.--Experimental Dye-bath.]

It is, however, advisable to have means whereby several dyeing experiments can be made at one time and under precisely the same conditions, and this cannot be done by using the simple means noted above.

To be able to make perfectly comparative dyeing experiments it is best to use porcelain dye-pots (these may be bought from most dealers in chemical apparatus), and to heat these in a water-bath arrangement.

The simplest arrangement is sketched in figure 33; it consists of a copper bath measuring 15 inches long by 10-1/2 inches broad and (p. 213) 6-1/2 inches deep; this is covered by a lid on which are six apertures to take the porcelain dye-baths. The bath is heated by two round gas-boiling burners of the type already referred to.

The copper bath is filled with water which, on being heated to the boil by the gas burners, heat up the dye-liquors in the dye-pots. The temperature in the dye-pots under such conditions can never reach the boiling point; where it is desirable, as in some cases of wool mordanting and dyeing that it should be so high, then there should be added to the water in the copper bath a quant.i.ty of calcium chloride, which forms a solution that has a much higher boiling point than that of water, and so the dye-liquors in the dye-pots may be heated up to the boil.

An objection might be raised that with such an apparatus the temperature in every part of the bath may not be uniform, and so the temperature of the dye-liquors in the pots might vary also, and differences of temperature often have a considerable influence on the shade of the colour which is being dyed. This is a minor objection, which is more academic in its origin than of practical importance. To obviate it Mr. William Marshall, of the Rochdale Technical School, has devised a circular form of dye-bath, in which the temperature in every part can be kept quite uniform.

The dyeing laboratories of Technical Schools and Colleges are generally provided with a more elaborate set of dyeing appliances.

These in the latest constructed consist of a copper bath supported on a hollow pair of trunnions, so that it can be turned over if needed.

Into the bath are firmly fixed three earthenware or porcelain dye-pots; steam for heating can be sent through the trunnions. After the dyeing tests have been made the apparatus can be turned over and the contents of the dye-pots emptied into a sink which is provided for the purpose.

Many other pieces of apparatus have been devised and made for the (p. 214) purpose of carrying on dyeing experiments on the small scale, but it will not be needful to describe these in detail. After all no more efficient apparatus can be desired than that described above.

Dyeing experiments can be made with either yarns or pieces of cloth, swatches as they are commonly called; a very convenient size is a small skein of yarn or a piece of cloth weighing 5 grammes. These test skeins or pieces ought to be well washed in hot water before use, so that they are clean and free from any size or grease. A little soda or soap will facilitate the cleansing process.

In carrying out a dyeing test the dye-pot should be filled with the water required, using as little as is consistent with the dye-swatch being handled comfortably therein, then there is added the required mordants, chemicals, dyes, etc., according to the character of the work which is being done.

Of such chemicals as soda, caustic soda, sodium sulphate (Glauber's salt), tartar, bichromate of potash, it will be found convenient to prepare stock solutions of known strength, say 50 grammes per litre, and then by means of a pipette any required quant.i.ty can be conveniently added. The same might be followed in the case of dyes which are constantly in use, in this case 5 grammes per litre will be found strong enough.

Supposing it is desired to make a test of a sample of Acid Red, using the following proportions, 2 per cent. dye-stuff, 3 per cent.

sulphuric acid and 15 per cent. Glauber's salt, and the weight of the swatch which is being used is 5 grammes, the following calculations are to be made to give the quant.i.ties of the ingredients required:--

For the dye-stuff, 5 (weight of swatch) multiplied by 2 (per cent. of dye) and divided by 100 equals (5 x 2) / 100 = 01 gramme of dye.

For the acid we have similarly (5 x 3) / 100 = 015 gramme of (p. 215) acid.

For the Glauber's salt (5 x 15) / 100 = 075 gramme of Glauber's salt.

These quant.i.ties may be weighed out and added to the dye-bath, or if solutions are kept a calculation can be made as to the number of cubic centimetres which contain the above quant.i.ties, and these measured out and added to the dye-bath.

When all is ready the bath is heated up, the swatch put in and the work of the test entered upon.

Students are recommended to make experiments on such points as:--

The shades obtained by using various proportions of dye-stuffs.

The influence of various a.s.sistants: common salt, soda, Glauber's salt, borax, phosphate of soda in the bath.

The influence of varying proportions of mordants on the shade of dyeing.

The value of various a.s.sistants, tartar, oxalic acid, lactic acid, sulphuric acid, on the fixation of mordants.

The relative value of tannin matters, etc.

Each dyer should make himself a pattern book into which he should enter his tests, with full particulars as to how they have been produced at the side.

It is important that a dyer should be able to make comparative dye-tests to ascertain the relative strength of any two or more samples of dyes which may be sent to him.

This is not difficult but requires considerable care in carrying out the various operations involved.

05 gramme of each of the samples of dyes should be weighed out and dissolved in 100 c.c. of water, care being taken that every (p. 216) portion of the dye is dissolved before any of the solution is used in making up the dye-vats. Care should be taken that the skeins of yarn or swatches of cloth are exactly equal in weight, that the same volume of water is placed in each of the dye-pots, that the same amounts of sulphate of soda or other dye a.s.sistants are added, that the quant.i.ties of dye-stuffs and solutions used are equal, in fact that in all respects the conditions of dyeing are exactly the same, such in fact being the vital conditions in making comparative dye-tests of the actual dyeing strength of several samples of dyes.

After the swatches have been dyed they are rinsed and then dried, when the depths of shade dyed on them may be compared one with another. To prevent any mistakes it is well to mark the swatches with one, two, three or more cuts as may be required.

It is easier to ascertain if two dyes are different in strength of colour than to ascertain the relative difference between them. There are two plans available for this purpose; one is a dyeing test, the other is a colorimetric test made with the solutions of the dyes.

#Dyeing Test.#--This method of ascertaining the relative value of two dyes as regards strength of colour is carried out as follows. A preliminary test will show which sample is stronger than the other; then there is prepared a series of dye-vats, one contains a swatch with the deepest of the two dyes, which is taken as the standard, the others with the other dye but containing 2, 5 and 10 per cent. more dye-stuff, and all these are dyed together, and after drying a comparison can be made between these and the standard swatch, and a judgment formed as to the relative strength of the two dyes; a little experience will soon enable the dyer to form a correct judgment of the difference in strength between two samples of dye-stuff.

The colorimetric test is based on the principle that the colour (p. 217) of a solution of dye-stuff is proportionate to its strength. Two white gla.s.s tubes, equal in diameter, are taken; solutions of the dye-stuffs, 05 gramme in 100 c.c. of water, are prepared, care being taken that the solution is complete. 5 c.c. of one of these solutions is taken and placed in one of the gla.s.s tubes, and 5 c.c. of the other solution is placed in the other gla.s.s tube, 25 c.c. of water is now added to each tube and then the colour of the diluted liquids is compared by looking through in a good light. That sample which gives the deepest solution is the strongest in colouring power. By diluting the strongest solution with water until it is of the same depth of colour as the weakest, it may be a.s.sumed that the length of the columns of liquid in the two tubes is in proportion to the relative strength of the two samples. Thus if in one tube there are 30 centimetres of liquid and in the other 25 centimetres, then the relative strength is as 30 to 25, and if the first is taken as the standard at 100 a proportion sum may be worked out as follows:--

30: 25 :: 100 : 833;

that is, the weakest sample has only 833 per cent. of the strength of the strongest sample.