Definition of mordant. The principal mordants. The mordanting of silk and wool. Of linen and cotton. Astringents for cotton. Alum. Various examples of using alum for wool, silk, cotton and linen. Iron. Examples of iron mordants. Tin. Examples of tin mordants. Chrome. Examples of chrome mordants. Copper. Examples of copper mordants. General observations. Tannin and the galling of cotton and linen. Examples of various galling processes.

Mordants.—Dyes are divided into two classes. First, the substantive dyes, which give their colour directly to the material with which they are boiled: and second, the adjective dyes, as they are sometimes called. These latter include the greater number of dyes and require the use of a mordant to bring out their colour.

There are thus two processes concerned with the dyeing of most colours; the first is mordanting and the second is the colouring or actual dyeing. The mordanting prepares the stuff to receive the dye—(mordere, to bite.) The early French dyers thought that a mordant had the effect of opening the pores of the fibres, so that the dye could more easily enter; but according to Hummel and later dyers the action of the mordant is purely chemical; and he gives a definition of a mordant as "that body, whatever it may be, which is fixed on the fibre in combination with any given colouring matter." The mordant is first precipitated on to the fibre and combines with the colouring matter in the subsequent dye bath. But, whether the action is chemical or merely physical, the fact remains that all adjective dyes need this preparation of the fibre before they will fix themselves on it. The use of a mordant, though not a necessity, is sometimes an advantage when using substantive dyes.

In early days the leaves and roots of certain plants were used. This is the case even now in India and other parts where primitive dyeing methods are still carried on. Alum has been known for centuries in Europe. Iron and tin filings have also been used. Alum and copperas have been known in the Highlands for long ages. Stale urine is also much used in Scotland and Ireland, but perhaps more as a clearing agent than as an actual mordant.

Silk and wool require very much the same preparation except that in the case of silk high temperatures should be avoided. Wool is generally boiled in a weak solution of whatever mordant is used. With silk, as a rule, it is better to use a cold solution, or a solution at a temperature below boiling point. Cotton and linen are more difficult to dye than wool or silk. Their fibre is not so porous and will not hold the dye stuff without a more complicated preparation. The usual method of preparing linen or cotton is to boil it first with some astringent. The use of astringents in dyeing depends upon the tannic acid they contain. In combination with ordinary mordants, tannic acid aids the attraction of the colouring matter to the fibre and adds brilliancy to the colours. The astringents mostly used are tannic acid, gall nuts, sumach and myrobalams. Cotton has a natural attraction for tannic acid, so that when once steeped in its solution it is not easily removed by washing.

ALUM. (Aluminium sulphate.)—This is the most generally used of all the mordants, and has been known as such from early times in many parts of the world. For most colours a certain proportion of cream of tartar should be added to the alum bath as it helps to brighten the ultimate colour. The usual amount of alum used is a quarter of a pound to every pound of wool. As a rule, less mordant is needed for light colours than for dark. An excess of alum is apt to make the wool sticky.

"For dyeing worsted and stuffs yellow, you make use of the usual preparation, viz., of tartar and alum. You allow four ounces of alum to every pound of wool, or twenty-five pounds to every hundred. With regard to the tartar, one ounce to every pound is sufficient for yellow, though it requires two for red."—Hellot.

The usual length of time for boiling with alum is from ½ an hour to 1 hour; but some dyers give as much as 2½ hours.

Various examples of mordanting with alum.—

For silk. Wet out the silk thoroughly with water and wring out. Then work it about a little in a strong solution of alum, previously dissolved in hot water, and steep for several hours (or over night). Then wash well. It should not be allowed to dry before dyeing. "Silks are always alumed in the cold, because when they are alumed in a hot bath, they are apt to lose a portion of their lustre." Berthollet.

For wool. ¼ lb. of Alum and 1 oz. Cream of tartar for every pound of wool. This is dissolved and when the water is warm the wool is entered. Raise to boiling point and boil for one hour. The bath is then taken off the fire and allowed to cool over night. The wool is then wrung out (not washed) and put away in a linen bag in cool place for four or five days, when it is ready for dyeing.

For cotton and linen. After boiling in water (some use a sour water, some an alkaline ley) the cotton is put into the alum bath, ¼ lb. of Alum to 1 lb. of cotton. The alum is dissolved in hot water with soda in the proportion of 1 part soda to 16 of alum. (Some add a small quantity of tartar and arsenic). The cotton is well worked in this solution and left 24 hours. It is then washed, and afterwards galled.

For linen. ¼ lb. alum for every pound of linen. Boil for 2½ hours and immediately put into the dye bath.

For wool. 6 to 8 per cent. of alum and 5 to 7 per cent. of tartar of the weight of wool.

IRON. (Ferrous Sulphate, copperas, green vitriol)

Iron is one of the oldest mordants known and is largely used in wool and cotton dyeing. It is almost as important as alum. With wool it should be used in combination with cream of tartar. The temperature of the mordanting bath must be raised very gradually to boiling point or the wool will dye unevenly. A general method of dealing with copperas is to boil the wool first in a decoction of the colouring matter and then add the mordant to the same bath in a proportion of 5 to 8 per cent. of the weight of wool: and continue boiling for half an hour or so longer. With some dyes a separate bath is needed, such as with Camwood or Catechu. If used for cotton, the cotton is first dyed in a boiling decoction of the dye stuff and then passed through a cold solution of ferrous sulphate. Probably the commonest way of applying copperas in cotton dyeing is to prepare the cotton with tannin, pass through clear lime water and then through a copperas solution. Great care is needed in the using of copperas, as, unless it is thoroughly dissolved and mixed with the water before the wool is entered, it is apt to stain the wool. It also hardens wool if used in excess, or if boiled too long.

Copperas is mostly used for the fixing of wool colours (Fustic etc.) to produce brown shades by the "stuffing and saddening" method (see page 14), the wool being boiled first in a decoction of the dye for about an hour, and then for ½ an hour with the addition of 5 to 8 per cent. of copperas. If used for darkening colours, copperas is added to the bath, after the dyeing, and the boiling continued for 15 to 20 minutes.

Examples of various proportions for Mordanting.—

8 per cent. of copperas and 20 per cent. of cream of tartar is a mordant used for some colours.

4 per cent. copperas, 10 per cent. cream of tartar gives good olive colours with weld.

8 per cent. copperas without tartar with single bath method, for dark olive brown with old fustic.

2 oz. copperas and 2 oz. cream of tartar to 2½ lbs. wool.

2 oz. copperas, 1½ oz. oxalic acid to 2½ lbs. wool.

TIN.—(Stannous chloride, tin crystals, tin salts, muriate of tin.)

Tin is not so useful as a mordant in itself, but as a modifying agent with other mordants. It must be always used with great care, as it tends to harden the wool, making it harsh and brittle. Its general effect is to give brighter, clearer and faster colours than the other mordants. When used as a mordant before dyeing, the wool is entered into the cold mordanting bath, containing 4 per cent. of stannous chloride and 2 per cent. oxalic acid: the temperature is gradually raised to boiling, and kept at this temperature for 1 hour. It is sometimes added to the dye bath towards the end of dyeing, to intensify and brighten the colour. It is also used with cochineal for scarlet on wool, in the proportion of 6 per cent. of stannous chloride and 4 per cent. of cream of tartar. Boil for 1 to 1½ hours. Then wash well. The washing after mordanting is not always essential. Also 6 to 8 per cent. of oxalic acid and 6 per cent. of stannous chloride, for cochineal on wool. This mordant produces bright fast yellows from old fustic, by boiling the wool from 1 to 1¼ hours, with 8 per cent. of stannous chloride and 8 per cent. of cream of tartar. One recipe gives 2 oz. tin and 4¼ oz. cream of tartar to 2½ lbs. wool in 10 gallons of water. It is not a suitable mordant alone for cotton, but can be used to brighten the colour in combination with other mordants. "The nitro-muriate of tin (dyer's spirit) although it produces good yellows with quercitron bark, produces them in a much weaker degree than the murio-sulphate of that metal, which is really the cheapest and most efficacious of all the solutions or preparations of tin for dyeing quercitron as well as the cochineal colours." —Bancroft.

CHROME. (Potassium dichromate, Bichromate of Potash.)

Chrome is a modern mordant, unknown to the dyer of 50 years ago. It is excellent for wool and is easy to use and very effective in its action. Its great advantage is that it leaves the wool soft to the touch, whereas the other mordants are apt to harden the wool. In commercial dyeing it is now almost exclusively used, as it has proved itself the most generally convenient. By some it is said not to be so fast to light as the other mordants, but it produces brighter colours. The wool should be boiled for one to one & a half hours with bichromate of potash in the proportion of 2 to 4 per cent. of the wool. It is then washed well and immediately dyed. Wool mordanted with chrome should not be exposed to light, but should be kept well covered with the liquid while being mordanted, else it is liable to dye unevenly. An excess of chrome impairs the colour. 3 per cent. of chrome is a safe quantity to use for ordinary dyeing. One recipe gives 1½ oz. of chrome to 2½ lbs. of wool. It should be dissolved in the bath while the water is heating. The wool is entered and the bath gradually raised to the boiling point, and boiled for three quarters of an hour.

In the dyeing of cotton, it is used for catechu browns and other colours. The cotton is soaked in a decoction of catechu, and afterwards passed through a boiling solution of chrome, or it is worked for half an hour in a bath of chrome at 60°C., and then washed. It is usual to wash wool or cotton after mordanting with chrome, but some dyers do not think it necessary.

COPPER. (Copper Sulphate, Verdigris, blue vitriol, blue-stone.)

Copper is rarely used as a mordant. It is usually applied as a saddening agent, that is, the wool is dyed first, and the mordant applied afterwards to fix the colour. With cream of tartar it is used sometimes as an ordinary mordant before dyeing, but the colours so produced have no advantage over colours mordanted by easier methods.

Examples.—6 per cent. of copper is used as a mordant for weld to produce an olive yellow. 4 to 5 per cent. is used with old fustic for yellow. 10 per cent. of copper gives to wool a reddish purple with cochineal.

Mordants should not affect the physical characteristics of the fibres. Sufficient time must be allowed for the mordant to penetrate the fibre thoroughly. If the mordant is only superficial, the dye will be uneven: it will fade and will not be as brilliant as it should be. The brilliancy and fastness of Eastern dyes are probably due to a great extent to the length of time taken over the various processes of dyeing. The longer time that can be given to each process, the more satisfactory will be the result.

Different mordants give different colours with the same dye stuff. For example:—Cochineal, if mordanted with alum, will give a crimson colour; with iron, purple; with tin, scarlet; and with chrome or copper, purple. Logwood, also, if mordanted with alum, gives a mauve colour; if mordanted with chrome, it gives a blue. Fustic, weld, and most of the yellow dyes, give a greeny yellow with alum, but an old gold colour with chrome; and fawns of various shades with other mordants.

TANNIN.—(Tannic Acid.)—Tannins are used in the dyeing of cotton and linen. Cotton and linen possess the remarkable power of attracting tannins from their aqueous solution, and when these substances are prepared with tannins, they are able to retain dyes permanently. Cotton saturated with tannin, attracts the dye stuff more rapidly, and holds it. Tannic acid is the best tannin for mordanting cotton and linen, as it is the purest and is free from any other colouring matter. It is, therefore, used for pale and bright shades. But for dark shades, substances containing tannic acid are used, such as sumach, myrobalans, valonia, divi-divi, oak galls, chestnut (8 to 10 per cent. of tannin), catechu.

Cotton and linen are prepared with tannin after they have been through the required cleansing, and if necessary, bleaching operations. A bath is prepared with 2 to 5 per cent. of tannic acid of the weight of the cotton, and a sufficient quantity of water. For dark shades, 5 to 10 per cent. should be used. The bath is used either hot or cold. It should not be above 60°C. The cotton is worked in this for some time, and then left to soak for 3 to 12 hours, while the bath cools. It is then wrung out and slightly washed.

The following gives the relative proportions of the various substances containing tannin:—1 lb. tannin equals 4 lbs. sumach, 18 lbs. myrobalans, 14 lbs. divi-divi, 11 lbs. oak galls.

A few examples taken from various recipes of cotton dyeing:—

For 10 lbs. cotton use 12 oz. tannin.
For 50 lbs. cotton use 10 lbs. sumach.
For 40 lbs. cotton use 10 lbs. sumach.
For 20 lbs. cotton use 2 lbs. yellow catechu or black catechu.
For 20 lbs. cotton spend 3 lbs. of catechu with 3 oz. of blue vitriol.

Some recipes soak the cotton for 24 hours, others for 48 hours.