THE PRINCIPLES AND PRACTICE OF COTTON DYEING.
Students of cotton dyeing should have a good knowledge of the principles that underlie the processes of dyeing cotton fabrics. It is only by recognising these principles and then endeavouring to apply them to each individual case of dyeing, that the dyer or student will obtain a thorough grasp of his subject. It is the aim of the author to lay down these principles in a clear and intelligible form. Cotton is dyed in its loose raw condition, as yarn in the form of hanks, yarn in the form of cops, and in the woven pieces of every kind. Formerly the idea was prevalent among cotton dyers that the process which succeeds with piece goods would not answer with yarns. It is now recognised however that this is not so, that a process which will dye cotton yarn will also dye cotton piece goods or loose cotton. The differences which do exist in the practical working of the processes entirely arise from the difference in the form in which the cotton is presented to the dyer, for it must be obvious to any one that the mode of handling a piece of cotton cloth during the time it is in the dye-bath must be different from that of a hank of yarn, a parcel of loose cotton or a number of cops. The various machines used for dyeing all these forms and the manner of working them have been already described.
The dyes, whether natural--derived from the various dye-woods, etc.--or artificial--prepared from coal tar--may according to their varied chemical composition and constitution be divided into seventeen or eighteen distinct groups, but it is not intended here to give any account of them; the reader is referred to other books such as _The Dictionary of Coal Tar Colours_, by George H. Hurst; _The Chemistry of Coal Tar Colours_, by Benedikt and Knecht; or _The Chemistry of Organic Colouring Matters_, by Nietzki, where the composition and properties of the dyes are fully described.
From the manner in which the various dyes are applied to cotton, linen, wool and other fibres we can divide them into five groups, thus:--
Group 1. Direct dyes.
" 2. Basic dyes.
" 3. Acid dyes.
" 4. Mordant dyes.
" 5. Miscellaneous dyes.
First group, direct dyes, are now very numerous; they dye cotton, linen and other vegetable fibres from a plain bath, and do not require those fibres to be prepared in any way. Hence the reason of their being named direct, or by some the substantive colours. They will also dye wool and silk.
The second group, basic dyes, comprise some of the oldest of the coal-tar dyes; they dye wool and silk direct from plain baths, but require cotton, linen and other vegetable fibres to be previously prepared in baths of tannic acid, sumach or other tanning material.
The third group, acid dyes, are very numerous, and from both their chemical composition and mode of dyeing can be divided into several sub-groups. Their principal feature is that they dye wool and silk from baths containing Glauber's salt and some acid, hence their name of "acid dyes". They do not dye cotton or linen well, some not at all, others are absorbed to a slight extent by the cotton, but only pale tints are produced, while others may be used along with metallic mordants to dye bright but pale and fugitive shades. The acid dyes comprise such as Acid green, Formyl violet, Acid magenta, Azo scarlet, Orange. Thiocarmine R., Patent blues, Wool greens, indigo extract, Eosines, etc.
The fourth group, mordant dyes, includes the alizarine series of coal-tar colours, logwood, Brazil wood and most natural colours, and some others. The principal feature of these dyes is that they require the cotton to be prepared with some metallic oxide, like those of chrome, alumina and iron, before dyeing, and the colour which is got depends partly upon the particular dye-stuff used and partly upon the oxide with which the cotton has been prepared.
The fifth group includes a few dyes like indigo, which are dyed on to cotton by various and special processes.
The processes of cotton dyeing employed to-day may be comprised under eight heads, namely:--
(1) Direct dyeing.
(2) Direct dyeing followed by fixation with metallic salts.
(3) Direct dyeing followed by fixation with developers.
(4) Direct dyeing followed by fixation with couplers.
(5) Dyeing on tannic mordant.
(6) Dyeing on metallic mordant.
(7) Developing the colour direct on fibre.
(8) Dyeing by impregnating the cotton with the dye-stuff, followed by oxidation or steaming.
It is of course not easy to elaborate a simple scheme of grouping the processes that shall definitely include all processes, but the above grouping will be found as successful as any, and each will be considered as fully as is deemed necessary.
(1) DIRECT DYEING.
Nothing in the history of cotton dyeing caused such a revolution in the methods of working as did the introduction some fifteen years ago of the forerunners (Congo red, Benzo purpurine, Chrysamine, Azo blue) of the now numerous group of direct dyes, followed as they were by the Benzo, Congo, Diamine, Titan, Chicago and Zambesi members of the group. Prior to their introduction cotton dyeing was always more or less complicated and mordanting methods had to be employed.
With the introduction of the direct dyes cotton dyeing has become even more simple than wool or silk dyeing, and now all that is necessary is to prepare a dye liquor containing the necessary amount of dye-stuff and Glauber's salt, or common salt or soda, or some similar body, or a combination thereof. The method of working is to place the cotton in a lukewarm or even in a hot bath, raise to the boil, allow the goods to remain in the boiling bath for half an hour to an hour, then take them out, wring, wash and dry. This method is simple and will answer for all the dyes of this group. There are some that do not require the working to be done boiling; it is simply needful to enter the cotton into a boiling bath and work without steam until the bath has cooled down.
Furious boiling is not needed--a gentle simmer gives the best results.
Uneven dyeing seems to be an impossibility in this group of dyes, unless the cotton is dirty; no matter how the operations are carried on, level dyeing is the rule not the exception. An enormous variety of shades and tints can be obtained from these dyes, and they can be combined together in every conceivable manner and proportions.
No satisfactory explanation has yet been given as to what feature in the chemical composition of these dye-stuffs give them such an affinity for the cotton fibre as to enable them to dye in so simple a manner such fast shades as they do; it is a fact there is such an affinity and there the matter must rest.
It has been found in practice that the efficiency of the dyeing operation depends, primarily of course, on the particular dye-stuff used, but also upon other factors, that a certain assistant be used.
Some dyes work on the cotton better from a bath containing Glauber's salt, while with others common salt works best, while a little soda along with the salt facilitates the dyeing in some cases. It is practically impossible to specify here the best assistant for all the direct colours, on account of the great number of such dyes which are known, but in the practical recipes given below much useful advice will be found. Then the quantity of such assistants used is of much importance; there is one proportion at which the best results are obtained for each dye. The dyer should find out for himself by experiment and the use of the dyes he employs in his work what assistant and how much is best, and make his baths up to that strength. With some dyes 10 per cent. of the assistant will be found sufficient, while with others, 25 to 30 per cent. will not be too much. The percentage refers to the weight of the cotton that is taken.
One function of the assistants must be pointed out here: it is that in some cases they--especially the alkaline ones, soda, potash, borax, phosphate of soda--help the dyeing by promoting the solubility of the dye-stuff in the bath, thereby retarding the exhaustion of the bath and ensuring the production of level shades.
The following formulas show the application of the foregoing principles to the dyeing of numerous shades on to cotton and also the dyes which are applicable, some of the combinations which are possible with these direct dyes, and give some idea of the tints and shades of the colours that can be got by their means. The best assistants to use are also indicated in the formulae.
All the formulae here given and all that will be given in future chapters are intended for 100 lb. weight of cotton fabrics in any condition, whether of loose cotton, yarn in cops, hanks or wraps and woven fabrics of every kind.
_Bright Red._--Dye with 3 lb. Benzo purpurine 4 B, 3 lb. soda and 15 lb.
Glauber's salt. This dye may also be used with 3 lb. soap and 10 lb.
soda in the bath with equally good results.
_Pale Salmon._--Prepare a dye-bath with 3 lb. salt, 5 lb. phosphate of soda, 1 lb. soap, 1/2 oz. Benzo orange R. For a pale shade like this it is not necessary to heat to the boil, a temperature of 170 to 180 F.
is sufficient.
_Dark Plum._--Prepare a dye-bath with 20 lb. of Glauber's salt, 2-1/2 lb. soap, 1-1/2 lb. Diamine black R O. and 2 lb. Diamine red N. Enter at 180 F., work for a few minutes, then raise to boil and dye to shade; lift, wash and dry.
_Turkey Red._--Prepare a dye-bath with 1-1/2 lb. Benzo purpurine 4 B, 1 lb. Brilliant purpurine, 2 lb. soap, 10 lb. borax. Enter the cotton at the boil and work for one hour; lift, wash and dry.
_Lilac Red._--Prepare the dye-bath with 2 lb. soap, 5 lb. soda, 3 lb.
Rose azurine G. Work at the boil for one hour.
_Pink._--Prepare a bath containing 10 lb. soda, 1 oz. Rose azurine B.
Enter at a boil and work for one hour, boiling to shade; lift, wash and dry.
_Bordeaux._--Prepare a dye-bath with 15 lb. Glauber's salt, 5 lb. soda crystals, 3 lb. Diamine fast red F, 1 lb. Diamine violet N, 1 lb. Rose azurine G. Enter cold, then raise to the boil, and work for one and a half hours; lift, wash and dry.
_Rose Pink._--The dye-bath is made with 2 lb. Erika B, 20 lb. Glauber's salt and 3 lb. soap, working at near the boil to shade.
_Brilliant Red._--Make the dye-bath with 24 lb. Brilliant purpurine R and 25 lb. Glauber's salt, working at the boil for one hour.
_Deep Pink._--Make the dye-bath with 1/2 lb. Diamine rose B D, 1/2 lb.
soda, 1 lb. soap and 5 lb. Glauber's salt, working at 150 F. for half an hour.
_Dark Red._--Use in the dye-bath 3 lb. Diamine red 5 B, 2 lb. soda and 20 lb. Glauber's salt, working at the boil for one hour.
_Pink._--Prepare the dye-bath with 4 oz. Diamine rose B D, 1 lb.
Turkey-red oil, 40 lb. Glauber's salt. Dye at the boil for one hour.
_Scarlet._--Prepare the dye-bath with 4 lb. Diamine scarlet 3 B, 1 lb.
Turkey-red oil, 20 lb. Glauber's salt. Dye at the boil for one hour.
_Scarlet._--Prepare the dye-bath with 3 lb. Titan scarlet C, 1/2 lb.
Titan orange, 50 lb. salt. Work at the boil for thirty minutes, then lift, wash and dry. The dye-bath is not exhausted and may be used for further lots.
_Crimson Red._--Prepare the dye-bath with 5 lb. Titan scarlet D and 50 lb. salt. Work at the boil for fifty minutes, then lift, wash and dry.
The bath is not exhausted, the cotton taking up only about 3 lb. of the dye-stuff; it may therefore be kept for further use, when for each succeeding lot 3 to 3-1/2 lb. of colour and 25 lb. of salt only need be added.
_Scarlet._--Prepare the dye-bath with 5 lb. Rosophenine 5 B, dissolved in 50 gallons hot water, 2 gallons caustic soda lye (60 Tw.). When thoroughly dissolved add 150 lb. salt. Make up the bath to 100 gallons.