414 



NA TURE 



[August 22, 1901 



cuticular cells (excepting the guard cells of the stomata) and 

 fibro-vascular bundles are free from it. When the chlorophyll 

 has been thus extracted, the leaves have a blue colour of 

 greater or less depth according to the amount of indigo they 

 contain. 



Although the extraction of indigo is so difficult and unsatis- 

 factory a process, yet woad has been used as a blue dye from 

 remote antiquity. Pliny refers to it as having been used tn stain 

 chalk blue for the adulteration of indigo, which was then a 

 pigment of great rarity, as it had to be imported by the " over- 

 land route " from India. 



The first printed reference to woad as a blue dye occurs in 

 Ruellius (" De Natura Stirpium," 1536), who remarks in words 

 of which the following is a translation : — " They crush the 

 green plant in mills, so as to expel the vegetable juices, then 

 when the moisture has been removed they make the woad up 

 into large balls, and these they allow to lie on the floor and 

 decay till they fall into ashes (dust). In many places they call 

 woad ' pastel,' from the loaf like shape into which the woad 

 balls are made up. They heat (the dust of) these balls in vats, 

 in dyers' shops and dip woollen cloths and skins therein, that they 

 may absorb the blue colour. The blue scum floating on the 

 surface, which the vats throw up when warfning on the fire, our 

 dyers call indigo ; this they dry for the use of painters." 



In 1555 Crolach published his small book on woad and its 

 culture and preparation, from which it appears that Thuringia, 

 one of the great woad producing districts of Europe, was already 

 beginning to feel the effect of the introduction of indigo into 

 Western Europe by the Cape route. A century later this was 

 more pronounced, judging from what Wedelius says. His 

 account of the woad industry is very good ; so much so that 

 Ray, the first professor of botany at Cambridge, copies it almost 

 verbatim with due acknowledgment, which in its turn was copied 

 and translated by the author of the English edition of Tourne- 

 fort's "Herbal." The latter tells us. "the ground, which is 

 plow'd in Autumn, must be left all ]Vi)iter to be soak'd by the 

 rain, till the Fitrification of Our Lady. After Lady Day, when 

 the air is somewhat softer and milder, it is proper to sow it, 

 and your end will be better answer'd if you sprinkle a little 

 snow over it, and take care that you do not sow it too thick 

 .... and after Whitsuiitidi you must weed all other herbs 

 from it. After St. /'ohn's Day in the Beginning of Harvest it is 

 ripe." It is interesting to add that the wadmen of the present 

 day say that, " no wad should be gathered after Martimas Day " 

 (November 15). Wedelius was essentially a chemist, and the 

 main object of his book was to show that ammonia was produced 

 from plants. He showed that ammonia was given off in large 

 quantities during the couching of woad, and he also argued on 

 theoretical grounds that woad contained sulphur, in both of 

 which assertions he was correct. He tells us, as early as 1577 

 a decree was made at Frankfort to prevent the fraudulent and 

 injurious substitution of indigo for woad, and on April 21, 1654, 

 at Ratisbon, an edict was promulgated inflicting the penalty of 

 confi.scation against the further importation of indigo. The 

 days of woad as a dye were, however, rapidly drawing to an 

 end, and yet, paradoxical as it may seem, the dyers of the 

 " greater dye" could not do without it. At this time no other 

 equally good method was known by which indigo could be dis- 

 solved and used for dyeing. We find woad culture an important 

 industry during the seventeenth and eighteenth centuries ; 

 accounts are to be found in the contemporary agricultural 

 writers — Ellis, Trowell, Miller and Young. It was mostly 

 carried on by itinerant "wadmen," who, with their families, 

 travelled from place to place, growing the woad on newly 

 broken up pasture land for which very high rents were paid. 

 These gangs built their huts and wad mills with the sods from 

 off the land, and were brought up to the industry from their 

 childhood. They seldom stayed more than two or three 

 seasons in the same spot, moving to a fresh location as soon as 

 the soil became exhausted. Abroad Schreber's monograph, 

 published in 1752, gives a very complete account, not only of 

 the culture, but of the history of th^ subject, as well as copious 

 extracts from the more important writers on the subject, with 

 copies of the various proclamations, edicts, &c. In the ap- 

 pendix to this volume a German translation is given from 

 Hellot's chapter on dyeing wool with indigo and woad. This 

 book (Hellot's) was subsequently translated into English, 

 anonymously. Under the " greater dye " or dyeing " colours 

 in grain," it gives the modus operandi of working a woad or 



pastel vat, which was the best then known way of dyeing with 

 indigo. The directions are sufficiently quaint ; for instance, 

 the writfr begins l)y saying, "Your copper cauldron should be 

 placed as near as possible to the vat and then filled with pond 

 water : if the water be not sufficiently putrid you put in a hand- 

 ful of hay. When the copper is full the fire should be lighted 

 under it at three o'clock in the morning." Then again, for 

 every ball of pastel you throw in a full measure of ware (slaked 

 lime), and sundry mysterious stirrings and coverings are 

 enjoined, until the vat has "come to." When the indigo is 

 put into it, there follow more stirrings and additions of ware, 

 until the vat is ready for the " overture," or first piece of stuff 

 to be dyed. " Towards the latter end of the week you dye the 

 light blues, and on Saturday night, in order to preserve it 'till 

 Monday, you garnish with a little more ware than on the day 

 preceding." On Monday morning the vat was reheated, fresh 

 indigo added to replace that which had been taken out by 

 articles dyed during the preceding week, while bran and lime 

 were added in the proper proportions. In point of fact a woad 

 vat, once started, was kept going for many weeks or months, 

 adding the indigo from time to time as required, as well as the 

 requisite proportion of bran (sharps) and slaked lime (ware). 

 The whole process was an exceedingly delicate one ; if the lime 

 was deficient the vat became putrid, if used too freely the vat 

 " got the kick " and did not work at all ; this was also the case 

 if the proper temperature was not maintained. 



What really takes place in a woad vat is concisely this : — 

 Indigo blue is a very insoluble substance ; it will not dissolve in 

 any of the ordinary solvents, such as hot or cold w"ater, dilute 

 acids, alkalies, alcohol, ether, chloroform, &c. Hence it is a 

 very fast dye if it can only be made to attach itself to a fabric. In 

 order that this may be done, it is necessary to dissolve it ; but, 

 as we have seen, none of the ordinary solvents will do this. In 

 the woad vat the chemical composition of the insoluble indigo- 

 blue is altered ; it is, as chemists say, reduced to indigo-white ; 

 now indigo-white is soluble in weak alkaline solution, hence 

 the use of the slaked lime. If a skein of wool be dipped into a 

 vat containing indigo-white in this state, the solution soaks into 

 the tissues of the wool fibres ; when the wool is taken out and 

 exposed to the air, the oxygen unites with the reduced indigo 

 and the skein passes from a greenish-yellow to a deep blue, the 

 insoluble indigo-blue being thus formed and the fabric dyed in 

 such a way that no mordant is required. The chemical changes 

 which take place in the woad vat when once started are, that 

 the starch of the bran is converted into grape sugar, which be- 

 comes lactic acid. The lactic acid becomes butyric acid, and 

 in so doing nascent hydrogen is liberated, which reduces the 

 indigo to indigo-white. Indigo is soluble in strong sulphuric 

 acid, and there are other processes by which it can be reduced ; 

 but the above is the rationale of the woad vat, which has held 

 its own from the time when the medi;^;val dyers added a little 

 indigo to the vat to improve the colour of the blue down to this 

 present time. It is an expensive, awkward and difficult pro- 

 cess, but it has this one advantage — the colour produced is 

 extremely durable. In actual practice a little madder is added ; 

 this is done, the dyers say, " to kill the green " in the indigo. 



Woad was used long before indigo came into Europe, not as 

 a solvent, but as a dye per se. Woad contains no indigo ready 

 formed ; not the slightest trace of any blue colour can be de- 

 tected in it. With water it forms a dark brown mixture, which 

 colours woollen fabrics olive-green. In order to dye with woad, 

 all that is necessary is to pour boiling water on the woad and 

 keep it in a well-covered vessel for fifteen or twenty hours at a 

 temperature of about 110° to 140' F., not going above 150° or 

 letting it fall below 100". In about thirteen to fourteen hours 

 bubbles of gas begin to rise ; a very small quantity of slaked 

 lime should now be added, and in a few hours woollen articles 

 allowed to remain in it for an hour or two change from yellow 

 to blue as they are taken out and exposed to the air. When 

 the vat is in full working order the liquid is of an olive-brown 

 colour, on the surface of which darker veins appear which change 

 their position, slowly moving, appearing and disappearing 

 spontaneously. The froth which at this time gathers on the 

 surface of the vat is blue from the indigo precipitated by contact 

 with the atmosphere. This constitutes the caeruleum sputnam 

 Ruellius speaks of as being dried and sold to the painters. It 

 was also the " flowers of the woad " which the dyers of Coventry 

 were accused of skimming off the woad vats in which they dyed 

 their customers' goods and added to those vats in which they 



NO. 1660, VOL. 64] 



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