Principles of dyeing

Preface

This book is the result of two years' instruction in dyeing, in the class room and in the laboratory. It aims to be a systematic presentation of the principles underlying the art of dyeing, illustrated and emphasized by laboratory exercises. It attempts to apply to the teaching of dyeing the same methods of class-room work, coordinated with experiments in the laboratory, which have proved so successful in the teaching of inorganic chemistry and other branches of science.

The reader will not find in this book, therefore, a collection of recipes for the production of particular colors, or detailed descriptions of more than a few of the 'more important processes of dyeing, or even a hst of the multitude of dyes which are at the service of the dyer. For information on these points, special works, or manuals of dyeing, must be consulted. A clear survey of the field of dyeing does not require these things ; rather, a multitude of details would obscure the general view which it is desired to present.

While intended primarily for the student, it is believed that this work will prove of benefit to the practical dyer who desires a fuller knowledge of the principles underlying his art. The experiments hardly call for more apparatus and reagents than a dyer should possess for the purpose of making necessary tests of his dyes and chemicals.

17174 vi PREFACE

The experiments, so far as is practicable, are the processes used in dye-house work ; necessarily modified for use as laboratory experiments. It has seemed desirable to reduce the time of the operations, so as to admit of the performance of a larger number of experiments by the student in the time at his disposal. Samples of all dyed material should be pasted in a suitable scrap-book, and marked with the name of the dye and the process used for dyeing. The dyed samples used for fastness tests, and all other samples possible, should be entered in the scrap-book and suitably marked, so that the pages of the scrap-book will exhibit a complete record of the student's work. In addition, a note-book should be kept, with descriptions of the experiments, observations, results, and answers to questions, after the manner of note-books used for experimental inorganic chemistry.

Before entering on this course, all students should be tested for color-blindness.

In the preparation of this book, a number of books andl original articles have been consulted. It is impossible tO) make special acknowledgment to all of these, but the following deserve particular mention : Knecht, Rawson, andl Lowenthal's " Manual of Dyeing," Hummel's " Dyeing off Textile Fabrics," Paterson's "Science of Color Mixing,"' Gardner's " Mercerization der Baumwolle," Beech's "Dyeing of Cotton Fabrics," and the publications of the Casselai Color Company, the Farbewerke Hoechst (Victor Koechl,, United States agent), the Berlin Aniline Works, and the Farbenfabriken von Elberfeld. Acknowledgment is due also to Messrs. P. R. French and E. B. Owen for reading^ manuscript and proofs, and Professor W. A. Withers andl Professor D. H. Hill for suggestions.

Raleigh, North Carolina, September 1902.

CHAPTER I Introduction

Object of the Dyer — Laws of Dyeing — Textile Fibers — Animal Fibers — Vegetable Fibers — Artificial Fibers — Coloring Matters

— Plan of Study.

Chapter II

Congo Red — Properties — Action of Acids — Reduction of Congo — Behavior to Fibers — Function of the Salts in Dyeing — Action of Hard Water — Hard Water — Stripping of Congo — Behavior to Washing, Light, etc. — Fastness — Direct Cotton Colors

— Primuline — Properties — Behavior toward f ibers — Diazotizing and Developing — Precautions in Diazotizing — Developers — Fastness of the Colors — Summary.

PAGE

I

Chapter III

Composition — Properties — Leuco Compounds — Rosaniline Tannate — Other Salts — Action toward Fibers — Dyeing Cotton with Fuchsine — Mordants — Fixing Tannic Acid — Action of Hard Water — Printing Fuchsine — Basic Colors.

CHAPTER IV B lEBRicH Scarlet — Alkali Blue 29

Properties — Action toward Fibers — Action of the Acid — Assistants — Stripping Biebrich Scarlet — Dyeing Cotton — Hard Water — Alkali Blues — Composition — Properties — Action toward Cotton and Wool — Acid Colors.

vii viii CONTENTS

Chapter V Page

Preparation — Composition — Properties — Salts — Action toward Fibers — Beiiavior of Fibers to Mordants — Polygenetic Colors

— Assistants for Metallic Mordants — Methods for Dyeing with Logwood — Saddening — Soaping — Application of Logwood — Mordant Colors.

Chapter VI

Indigo — Preparation — Composition — Artificial Indigo — Properties — Reduced Indigo — Dyeing with Indigo — Fastness of Indigo — Indigo Extract — Chrome-yellow — Production — Properties — Retrospect — Theory of Dyeing — Classes of Fibers.

Chapter VII

Vegetable Fibers — Cotton — Motes — Grades of Cotton — Microscopic Appearance • — ■ Dead Cotton — Other Defects — Composition — Cellulose — Preparation and Properties — Solvents for Cellulose — Cellulose Nitrates — Oxycellulose — Action of Acids — Carbonization — Action of Alkalies — Mercerization — Nature of the Change — Properties of Mercerized Cotton — Tendering of Cotton — Cotton Manufacture.

CHAPTER VIII Linen — Other Vegetable Fibers 73

Linen — Microscopic Appearance — Composition — Properties

— Detection of Cotton in Linen — Jute — Composition and Properties — Hemp — China Grass — Other Fibers — Other Vegetable Materials.

Chapter IX

Animal Fibers — Wool — Grades — Microscopic Appearance — Kemps — Physical Properties — Composition of Raw Wool — Composition of the Fiber — Solution of Wool — Chlorinated ix

PAGE

Wool — Oxidation of Wool — Tendering of Wool — Absorptive Power — Behavior to Dyes — Detection of Wool — Carbonization — Shoddy and Mungo — Rare Wools — Wool Manufacture.

CHAPTER X Silk 9°

Silk — Plow the Worm Spins — Preparation of Silk — Grades of Silk — Physical Properties — Composition — Solution of Silk — Absorptive Power — Metallic Salts — Coloring Matters — Tendering of Silk — Oxidation of Silk — Estimation of Silk, Wool, and Cotton — Wild Silks — Composition and Properties of Tussur Silk — Detection of Tussur Silk.

CHAPTER XI Operations Preliminary to Dyeing — Bleaching Cotton and

Linen 98

Objects — Wetting out — Bleaching — Chemistry of Bleaching

— Saponification — Methods — Decolorization — Decolorizing Agents — Oxycellulose — Bleaching Loose Cotton — Yarn Bleaching — Cop Bleaching — Preparing the Chemic — Control of the Chemic — Titration of the Chemic — Control of the Sour — Cloth Bleaching — Printer's Bleach — Market Bleach — Turkey-red Bleach — Defects in Bleached Goods — Other Bleaching Processes

— Linen Yarn — Linen Cloth — Jute Bleaching.

Chapter XII

Washing Loose Wool — Methods and Machinery — Soaps — By-products — Other Methods of Wool Washing — Yarn Scouring

— Methods and Machinery — By-products — Cloth Scouring — Crabbing — Wool Bleaching — Sulphur Bleach — Hydrogen Peroxide Bleach — Silk — Methods of Boiling-off — Bleaching — Souple Silk — Other Operations — Weighting of Silk.

CHAPTER XIII Raw Stock — Dyeing — Washing — Drying — Sliver or Slubbing

— Skeins — Hand Dyeing — Machine Dyeing — Washing — Ex-

X tracting Water — Drying — Warps — Dyeing Machines — Sizing — Drying — Dyeing on the Cop — Cloth — Drying — Printing of Slabbing and Warps.

Chapter XIV

Dissolving the Dye — Hard Water — Water Purification — Production of Level Dyeings — Feeding of Colors — Standing Baths — Mixing of Dyes — Dyeing to Shade — Fastness of Dyes— Requirements for Fastness — Defects in Dyeing — Soaping — Silk Dyeing.

Chapter XV

Properties — Application to Cotton — Methods of Dyeing — Assistants — Production of Level Colors — Mixing Direct Cotton Colors — Exhaustion of the Bath — Standing Baths — After-treatment of Direct Cotton Colors — Diazotizing and Developing — Control of the Diazotizing Bath — Developing — Application — Shading Developed Dyeings — Coupling — After-treatment with Metallic Salts — Application — Defects in Dyeing — Sulphur Colors

— Properties — Methods of Dyeing — After-treatment — Soaping — Defects in Dyeing — Topping Direct Cotton Colors with Basic Colors

— Fastness of Direct Cotton Colors —Application to Linen— Application to Wool — Application to Silk — After-treatment,

Chapter XVI

Composition — Properties — Application to Cotton — Tannic Acid — Salts— Affinity for Cotton — Antimony Salts — Mordanting with Tannic Acid— Fixing Tannic Acid — Dyeing — Janus Colors

— After-treatment with Tannic Acid — Mordanting with Turkey-red Oil and Soap — Alum Mordant— Dyes as Mordants — Fastness of the Colors — Defects in Dyeing — Basic Colors on Wool — Dyeing— Sulphur Mordant — Fastness of the Colors — Defects

— Basic Colors on Silk — After-treatment.

xi

Chapter XVII Page

Properties — Application to Cotton — Methods of Dyeing — Application to Jute — Application to Wool — Methods of Dyeing Alkali Colors — Level Dyeing — Mixing — Exhaustion of Bath

— Defects in Dyeing — Fastness — Acid Dyes on Silk — Methods

— AlkaU Colors.

Chapter XVIII

Methods of Application — Natural Mordant Colors — Artificial Mordant Dyestuffs — Mordant Colors on Cotton — Alizarin — Methods of Application — Turkey-red Processes — New Turkey-red Process Alizarin-red Processes — Alizarin on Chromium Mordants—Alizarin on Iron Mordants — Logwood — Defects — Cutch Other Mordant Colors — Mordant Dyestuffs on Linen — Mordant Colors on Wool — Ordinary Mordant Colors— Mordanting with Aluminium — Mordanting with Chromium — Dyeing — Saddening — Level Dyeing — Defects in Dyeing — Single-bath Method Mixing — Logwood — Acid-mordant Colors — Mixing and Shading — Mordant Colors on Silk — Mordanting — Dyeing.

Chapter XIX

Oxidation Colors — Aniline Black — Properties — Application — Production on Loose Cotton and Yarn — Production on Cloth — Topping Aniline Black — Soaping — Defects in Dyeing — Indigo — Application to Cotton — Dyeing — Topping Indigo —Application to Wool — Insoluble Azo Colors — Paranitraniline Red — Soaping — Defects — Other Azo Colors — Mineral Colors — Manganese Brown — Iron-buff and Nankin Yellow — Prussian Blue.

Chapter XX

Mercerization — Production of Embossed Effects — Production of Lustered Cotton — Process of Mercerization — Machines for Mercerization — Properties of Lustered Cotton — Dyeing Mercerized

Xll

PAGE

Cotton — Direct Cotton Colors — Basic Colors — Scroop Feel — Level Dyeing — Artificial Silk — Cellulose Silk — Properties — Dyeing — Detection — Gelatin Silk.

Chapter XXI

Dyeing Cotton-wool Goods — Dyeing with Direct Colors — Application — Acid Dyes — Basic Colors — Shoddy Dyeing — Cotton and Silk — Wool and Silk — Methods of Dyeing.

CHAPTER XXn

The Solar Spectrum — Absorption-spectrum — Dichroism — Primary, Secondary, and Complementary Colors — Hue, Tint, Shade, Purity — Absorption-spectrum of Mixtures — Primary and Secondary Mixing-colors — Mixing of Dyes — Effect of Light on Colors — Color-blindness — Dye Testing — Comparative Dye-trials — Testing for Mixtures — Detection of Dyes.

LIST OF ILLUSTRATIONS

FIIGURE PAGE

1. Cotton 57

2. Linen 74

3. Wool 79

4. Low-pressure Kier 100

5. Bleaching under Sieve 108

16. Cloth- WASHING Machine 113

7. Square Beater Washing Machine 115

8. Cloth-scouring Machine . . . . . . . .124

9. Klauder-Weldon Raw Stock Dyeing Machine . . . 134

lio. Raw Stock Drying Machine 135

11. Klauder-Weldon Skein-dyeing Machine 138

i;2. Skein-washing Machine 139

i;3. Hydro-extractor 141

1:4. Warp-dyeing Machine , 143

115. Drying Cans , 144

1(6. Dye-beck and Wince 145

1(7. Jiggers 146

1(9. Single-color Printing Machine 148

220. Aniline Black Ageing Chamber 226

221. Solar Spectrum 251

222. Absorption Si'ectrum of a Yellow Dye (A), A Blue Dye (B),

and the Mixture of the Two (C) 254

xiii

Introduction

The term " dyeing," in its broad sense, covers the coloring of leather, paper, feathers, and other articles, besides the coloring of textile fabrics.

This book will be confined to a discussion of the dyeing of textile fibers, both the raw material and the finished fabric.

Object of the Dyer. — The object of the dyer is to produce any desired color on the material given him, an object usually accomphshed by means of dyes, but sometimes by the use of bleaching agents, such as in the case of the production of a white piece of goods. The dyer is usually furnished with a sample of goods of the exact shade desired ; but besides matching the sample, the color he produces must come up to certain requirements as regards fastness, cost, etc. In some cases it does not matter if the color washes out with water, provided it resists the action of light; while in other cases the color must be as fast to soap and hot water as possible. These requirements are met by the selection of different dyes and methods of dyeing, according to the demands of the work.

The color produced by the dyer must also be such that it will not undergo undesirable changes during the further

B I manufacture of the goods, or in any of the finishing processes to which they may be subjected. For example, the color must not rub off from dyed threads to uncolored threads, giving the goods a dirty appearance, or change while the goods are being dried or ironed.

Laws of Dyeing. — The laws of dyeing rest, on the one hand, on the chemistry and physics of the dyes and textile fibers, and, on the other hand, on the principles involved in the application of the dye to the fiber. The different textile fibers have their individual peculiarities, and differ in their behavior toward the same dye; processes which are beneficial to one fiber may be positively injurious to another. Dyes also differ greatly in their behavior to fibers, and require different methods of application according to their nature.

Textile Fibers. — Cotton, wool, silk, linen, etc., are knowin as textile fibers. The only common property of all th e textile fibers is their fibrous nature. They are divided int o three groups : —

I. Mineral fibers ; examples, asbestos, glass wool, amd slag-wool.

II. Animal fibers; examples, silk and wool. III. Vegetable fibers; examples, cotton, linen, and jutes.

Mineral Fibers are incombustible; they are never dyecd or bleached, and are only mentioned for the sake of comipleteness. Asbestos is a natural silicate of calcium ancd magnesium, and, on account of its being a non-conducto^r of heat, is used principally as a covering for steam-pipess. Glass wool is spun glass. Slag-wool, obtained by blowin^g

INTRODUCTION a strong current of air through a stream of molten slag, is used for covering steam-pipes.

Animal Fibers. — Silk and wool are the two principal members of the group ; wool is the outer covering of the sheep, and silk is the product of a caterpillar. The animal fibers contain carbon, hydrogen, nitrogen, and oxygen ; wool contains sulphur in addition. In their action toward dyes and in other of their properties, animal fibers are quite different from vegetable fibers, as will be brought out later.

Leather, feathers, bone, horn, and ivory resemble the animal fibers in many of their properties, and behave in a similar way to dyes.

Vegetable Fibers. — Most of the vegetable fibers are obtained from the stem or leaves of plants, but the most important one — cotton — consists of the hairs which surround the seeds of a species of plant (Gossypium). The vegetable fiber next in importance to cotton is linen, then come hemp, jute, and China grass. The vegetable fibers contain carbon, hydrogen, and oxygen, but no nitrogen or sulphur. They do not have as much chemical activity as the animal fibers.

Straw (for hats), vegetable ivory (for buttons), wood, and paper resemble the vegetable fibers in their properties, and may be placed in the same class.

Artificial Fibers. — Imitation silks, spun artificially, have attained some commercial importance. They will be discussed later.

Coloring Matters. — According to their origin, coloring matters may be divided into two classes — natural and artificial.

The natural colors are of vegetable or of animal nature, though the only animal dye of any importance is cochineal, which is found in a bug.

Until about fifty years ago, the dyer was obliged to rely entirely upon natural colors, and some few mineral colors, to produce the effect desired. In 1856 the first artificial color (mauveine) was manufactured by W. H. Perkins, and captivated the world by its purity and beauty. Soon others were prepared, and a very large number of highly colored organic compounds are now known, though they do not all find practical apphcation as dyes. The number of artificial dyes is increasing so rapidly every year that it is difificult to keep up with the new dyes which are constantly being put on the market.

Another advance made in recent years has been the discovery of methods for the preparation of natural coloring matters in the factory, the first one so prepared being alizarin. Alizarin is the coloring principle of the madder plant, which had been used for a long period to produce a fiery red color upon cotton goods. At present the artificial color has replaced the natural one almost completely. Indigo, a very important blue dye, is another natural dye which is being prepared by chemical methods, and just now there is a decided competition between the natural indigo and the artificial or synthetic indigo.

Plan of Study. — At present an enormous number of coloring matters are known, and the number is increasing rapidly. The matter is still further complicated by the fact that the same dye is often sold under several different names, and in different degrees of purity. The large number of

INTRODUCTION dyes known need not, however, be a source of uneasiness to the student. The dyer does not need to be acquainted with every individual color, but should have a knowledge of the most important colors, and know how to test new dyes and work out new processes.

Without regard to their origin or chemical constitution, coloring matters can be separated into a number of groups for convenience in study, just as we can separate the elements into groups; as, for example, the chlorine group, including chlorine, bromine, and iodine ; the oxygen group, in which are included sulphur, selenium, and tellerium. The members of these groups have certain resemblances to each other, and also their pecuharities, which distinguish them from the other members.

It is the object of this work to present a clear view of the subject of bleaching and dyeing of textile fibers. It is proposed to consider only a comparatively small number of dyes, either important in themselves, or those which can be used to emphasize important principles. In general, detailed information regarding particular dyes will not be given. It would hardly be profitable for the beginner to burden his mind with too many details at first, — he may thereby lose the general view, which it is important that he should gain quickly. Nevertheless, seemingly unimportant details, neglect of which may make or mar the work of the dyer, have been emphasized. It is the attention to details, which must be learned, and learned well, which makes the good dyer.

The plan of study which will be followed is briefly this : — Of the first group of dyes, two will be studied with special attention to their action toward the different fibers. Members of the second group of dyes will then be taken up, and so on, until all the groups have been considered,

A clear idea of the relations between the groups of dyes, their general properties, and their behavior toward different fibers having been gained in this way, the next step will be the study of the groups of textile fibers.

Following this will come the consideration of the machinery for dyeing and bleaching, and the methods of preparing the fibers or fabrics for the dyeing proper. Finally the different groups of dyes will be studied, with the principles involved in their application to the different classes of material.

Chapter II

Congo red, or congo, was discovered in 1884, and was the first artificial dye known to have the property of dyeing cotton directly without the aid of any other substance. For this reason other dyes of the same kind, which were afterward discovered, are sometimes called congo colors.

Properties. — Congo red is a red-brown powder, readily soluble in water. Its solubility is decreased by the presence of salts, — such as common salt (sodium chloride) or Glauber's salt (sodium sulphate). Salt in sufficient quantity will precipitate the dye from solution.

Experiment i. — Dissolve a little congo red in 200 cc. water. Add common salt to a portion of the solution, stirring from time to time, until no more dissolves. Let the solution stand for half an hour, stirring occasionally, and filter. The first few drops may be colored, but the remainder of the filtrate will be nearly colorless. The dye has been " salted out " from solution.

Practical appHcation is made of the power of salts to precipitate dyes and other compounds from solution in the manufacture of dyestuffs and other substances. In dyeing with congo and similar colors, salts are usually added to the dye-bath to decrease the solubility of the dye, and cause more of it to go upon the fiber.

Action of Acids. — Congo is the sodium salt of a dibasic acid. If we add hydrochloric acid to a solution of coigo, the free color acid (combined with hydrochloric acid) separates as a dark-blue, flocculent precipitate. The reaction is as follows : —

CgaHa^NgS-PgNaa + 2 HCl = C32H22N6S2O6H2 + 2 NaCl.

Congo red. Color acid.

Or, if we let Ac Nag represent congo red, the reaction may be written in a condensed form : —

AcNa2 + 2 HCl = AcHj -f 2 Na CI.

Ac merely stands for a complex group of atoms of acid nature.

If the blue precipitate is treated with sodium hydroxide, it dissolves, congo red being formed again.

Experiment 2. — Add hydrochloric acid to a solution of congo red. Explain what happens, and write condensed reaction.

The acids always present in the air, especially in the air of cities, cause the bright color of goods dyed with congo to become dull gradually by liberating the free color acid ; an effect which can be counteracted, to a certain extent, by saturating the goods, before drying, with a solution of sodium carbonate, which neutralizes the acid.

Reduction of Congo. — Reducing agents, such as zinc dust and acetic acid, transform congo red into colorless bodies.

Experiment 3. — To about 20 cc.of a solution of congo red add 20 CO. acetic acid (25 per cent) (what happens?), and then about 5 g. zinc dust. Mix well, heat to boiling, and boil a few minutes. Filter. Explain results.

The reaction between acetic acid and zinc dust is as follows : —

Acetic acid. Zinc acetate.

The nascent hydrogen is taken up by congo red, and splits it into two new colorless compounds. All organic coloring matters may be reduced to colorless compounds by suitable choice of reducing agents. They vary greatly in the ease with which the reduction may be effected.

Behavior to Fibers. — If we boil cotton or wool with a solution of Congo red, the fiber will withdraw a portion of the dye from solution, and become colored. Further, the color cannot be washed out with water. The dye has an affinity for the substance of the fiber, and dyes it ; such dyes are called direct colors or substantive colors.

Experiment 4. — Boil five lo-gram skeins of cotton yarn with 500 CO, water and i g. sodium carbonate until they are thoroughly wet. Wash them and place them in clean water until needed. Cotton yarn should always be "boiled off" or "wetted out " before dyeing.

Prepare a dye-bath with 2 per cent (of the weight of the yarn) Congo red,^ 2 per cent sodium carbonate, and 20 per cent Glauber's salt (sodium sulphate), and make the volume up to 200 cc. Hang a lo-gram skein of cotton yarn in the bath on a V-shaped glass rod, and boil gently half an hour, working the yarn from time to time by lifting it out of the cup, and turning it with the aid of a glass rod. Remove, rinse, and dry. Save the dye-bath.

1 To save time, dyes and salts are furnished in solution, so that the student has only to measure out a certain volume of liquid. For example, 25 cc. congo solution contains o.i g. ; 10 cc. sodium carbonate solution contains O.I g. ; sodium sulphate (anhydrous) 5 cc. = o.i g. The dyeings are usually made in agate-ware cups, j-pint capacity, heated directly by the burner.

Dye a 5-gram skein of wool in the same way, without the sodium carbonate. Which skein is the darker? Compare the dye-baths. Which fiber takes up the more dye ?

If we examine the two solutions after dyeing cotton and wool with Congo, we find : —

1. The wool has exhausted the dye-bath ; that is, it has withdrawn almost all dyestuff from solution.

2. The cotton has not exhausted the bath, but has left a considerable portion of the dye unabsorbed.

The experiment illustrates the characteristic differences between animal and vegetable fibers in their behavior toward direct dyes. Animal fibers, represented by wool, have a great afifinity for direct colors, and withdraw them readily and completely from solution. Vegetable fibers have no great affinity for them, and rarely exhaust a dye-bath.

It is evident that if we throw away the dye-bath after dyeing cotton with congo red or similar colors, a part of the dye is lost. In dyeing with such colors, the dye-bath is used continuously whenever possible, its strength being restored by fresh additions of dyestuff and salts after each dyeing.

Function of the Salts. — In dyeing with congo red and similar colors, salts are always added to the dye-bath.

Glauber's salt or common salt is added to decrease the solubility of the dye, and thereby cause more of it to go on the fiber. Sodium carbonate is added to make the bath alkaline and to counteract to some extent the action of acids of the air, as the sodium carbonate is not entirely washed out.

Action of Hard Water. — The calcium and magnesium salts of the color acid of congo red are not soluble in water, and are precipitated when a calcium or magnesium salt and congo red are brought together in solution.

Water containing calcium salts (hard water), when used for the preparation of a dye-bath, precipitates congo red with loss to the dyer.

Experiment 5. — Dissolve o.i g. congo in 75 cc. water, add 25 cc. lime-water, and boil about 10 minutes. What is the precipitate?

Prepare two dye-baths, each with exactly 2 per cent congo (measure the solution with a pipette), 2 per cent sodium carbonate, and 20 per cent sodium sulphate. Add 50 cc. hme-water to one solution and make the volume of each 200 cc. Dye two skeins of cotton yarn (boiled out) as described in Experiment 4. What is the effect of hard water?

Hard Water. — The purest water found in nature is rain-water. As soon as rain-water comes in contact with the earth, and begins its course toward the ocean, it begins to take up various substances, according to the character of the soil with which it comes in contact. Natural waters which come in contact with Hmestone (calcium carbonate) gradually take up more or less of the carbonate with the aid of the carbonic acid of the air, forming calcium acid carbonate, Ca(H €03)3, and becoming hard. On heating the water, this salt decomposes, carbon dioxide is given off, and calcium carbonate is precipitated. The hardness removed by boiling is called the temporary hardness.

Water, when it comes in contact with calcium sulphate, dissolves it, and a hard water is produced, the hardness of

PRINCIPLES or DYEING which is not removed by boiling, and is therefore called permanent hardjiess. Magnesium sulphate acts in the same way to produce permanent hardness.

By boiling hard water with sodium carbonate or soap, insoluble calcium compounds are precipitated and soft water can be drawn off. These, and other methods of purifying water, will be discussed later.

Hard water is often injurious in dyeing.

Stripping of Congo. — When cotton yarn dyed with congo is boiled in water, part of the color will be removed or stripped from the fiber.

Experiment 6. — Plait part of the cotton yarn dyed with congo with undyed cotton yarn, place it in 50 cc. water, and boil 15 minutes. Proceed with dyed wool in the same way, plaiting it with wool yarn. Which fiber loses its color more easily?

The experiment shows that the color is more easily stripped from cotton than from wool. It also shows, in the case of the cotton yarn, that part of the color stripped off is taken up by undyed fibers ; the dyed and undyed yarn will become the same color if boiled long enough with water.

Behavior to Washing, Light, etc. — If we subject the dyed yarn to the action of warm water and soap, — that is, imitate the action of washing, — the solution becomes colored, but the color of the yarn does not fade appreciably. If white material is present, it becomes colored ; the color is said to bleed.

Experiment 7. — The test for fastness to washing is conducted as follows : —

Plait part of the dyed cotton with undyed yarn. Make up a solution^ of 10 g. soap in a Uter of water, heat to 60° C, and pour about 25 cc. on the yarn. The plait should be worked and squeezed in the liquid, and allowed to remain in it 10 minutes. It is then rinsed in cold water, allowed to lie in water 15 minutes, then wrung out, washed, and dried. Note any change in the color of the yarn, how much color is taken up by the soap, and whether the color bleeds. Test the dyeing on wool in the same way.