Februar 



V I, I 



900] 



NATURE 



11^ 



lie changed from 96o"78 to 96i"-so, this aheration being based 

 on the discussion of 35,842 meridian observations made at the 

 principal observatories of the world. 



A new departure in the administration of the Observatory was 

 the appointment, in June 1899, of a Board of Visitors by the 

 Secretary of the Navy, whose duty will be to examine and 

 report upon the condition and requirements of the institution. 



ON THE BLUE COLOUR IN WO AD. 

 'PHEold East Anglian proverb, "As blue as wad," occurs to 

 one visiting the Woad Mill described by Mr. Darwin in 

 Nature, in 1896 (vol. Iv. p. 36), as evidence that woad once 

 yielded a blue dye. As a natural sequence one wonders what 

 sort of blue it was and how it was obtained. .\ some- 

 what extended series of inquiries amongst those engaged in 

 the woad industry, amongst those who have written on woad, 

 and amongst botanical, archaeological and chemical friends, 

 failed for a long time to elicit the desired information. Curious 

 as it may appear, an appeal to botanical and chemical works, to 

 dictionaries and encyclopxdias was equally unsuccessful. The 

 last-named were pretty uniform in their statements about woad, 

 in that it " was formerly used for dyeing blue, but is now super- 

 seded by indigo." Many of the books give an account of th« 

 woad-vat in which the manufactured woad is used with bran 

 and lime as a ferment to change the insoluble indigo-blue into 

 the soluble indigo-white ; but they give no clue as to how woad 

 may be used as a blue dye alone. It has been said that the 

 l)Iueness of woad was more or less a myth, and even if it ever 

 possessed this quality it has long since been lost by continued 

 cultivation. 



As some of the facts elucidated in the attempt to find the 

 bhie colour may be of interest to others, they are herewith 

 iletailed :^ 



.\t the present time woad is grown and is manufactured in 

 four places in the Fen country, viz. at Algarkirk, Wyberton, 

 Skirkbeck, and Parson Drove ; its use being as above stated, as 

 a ferment in the indigo-vat to dissolve the indigo-blue. This 

 (irocess of dyeing by woad is difficult, cumbersome and expen- 

 sive, but it yields the most permanent results. A genuine woad- 

 dyed cloth resists sunshine, rain, and sea-air better than any 

 other, but it is so expensive that only the very best articles are 

 dyed in this way. The fastness of woad-dyed cloth is so 

 proverbial that Prof. Hummel, of the Yorkshire College, Leeds, 

 tells me the adjective " woaded " is now applied in the trade 

 to any fast or permanent indigo dye : a woaded black meaning a 

 black that has an indigo ground colour. The wool is dyed 

 before it is woven, and the cloth may be distinguished by having 

 pale blue or yellow threads in the selvedge. 



But to return to the plant, many methods have been suggested 

 by which the indigo in it may be extracted. In the earlier 

 years of the present century, when we were at war with France, 

 so great was the difificulty experienced by that nation in obtain- 

 ing indigo that the Government offered a substantial prize for 

 an efficient substitute. Attention was consequently re-directed 

 to woad, and more than one method was suggested for the 

 separation of indigo from it. However effective these may have 

 proved in France and Italy, with me they failed, and failed so 

 uniformly as to render the very numerous experiments extending 

 over a period of five months unworthy of further notice. Suffice 

 it to say that the experimental material was obtained from 

 Parson Drove, Boston, Cambridge Botanic Gardens, and wild 

 plants from Gloucestershire. Eventually the presence of indigo 

 was demonstrated in these plants by the simple method of Dr. 

 Hans Molisch,' who kindly further advised me, in a letter, to 

 examine particularly the younger and expanding leaves. The 

 method consistsin keeping the fresh leaves for twenty-four hours 

 in an atmosphere of ammonia, and then for a similar period in 

 absolute alcohol. The ammonia precipitates the indigo in the 

 leaves, while the alcohol dissolves out the chlorophyll : so that by 

 cutting sections one can see the exact tissues in which the indigo 

 ccurs. These are those containing chlorophyll. The fibro- 

 vascular tissue, the hairs, the epidermis, excepting the guard 

 cells of the stomata, are free from it. 



The quantity of indigo varies very much in different leaves ; 



some turn a beautiful blue, while others come out of the absolute 



1 " Ueber d.-« Vorkommen von Indican if Chlorophyllkorn der Indican- 



planzen." (" Berichten der Deutsch Botan. Gesell.," 1899. Bd. xvii. Hf. 6, 



p. 228, t. xviii.) 



NO. 1579. VOL. 61] 



alcohol showing only a faint trace towards the base of the leaves. 

 As a rule, the younger the leaf the more indigo it contains ; 

 some young leaves, however, hardly contain any. Old leaves 

 have practically none in them, and/ become yellowish-green o? 

 greenish -white brittle objects after the above treatment. 



The process of separating the indigo is more delicate— 

 perhaps it would be more correct to say it is a process simple 

 enough in itself, but one in which certain precautions must be 

 observed. Prof. Beijerinck, whose paper " On the formation of 

 indigo from woad"' is summarised in Nature, November 16, 

 1899 (p. 71 ), gives the following method : The woad leaves are put 

 into a stoppered bottle, which is then filled with hot water, in 

 such a way that all the air is expelled and the stopper put in so 

 that no air bubble is allowed to remain between the top of the 

 water and the lower part of the stopper. The water assumes a 

 pale yellowish tint — the colour of sherry — with a green fluor- 

 escence. On the addition of a caustic alkali it darkens and 

 becomes greenish. If a dilute acid be now added the indigo 

 falls as a blue precipitate. The first time this method was tried 

 with Parson Drove woad its success was complete ; the long 

 sought-for blue colour fell in abundance. Prof. Beijerinck tells 

 me that in the month of September hp obtained '09 per cent, of 

 pure indigo-blue from plants grown in Holland. 



A considerable number of experiments have been made with 

 the Parson Drove woad, the outcome of which may thus be 

 summarised. The elaborate precautions for excluding the air 

 are not absolutely necessary— simply pouring boiling or nearly 

 boiling water on fresh woad leaves, so that they are completely 

 covered, answers well enough. If to the infusion thus obtained 

 caustic potash, caustic soda, strong ammonia or lime-water ite 

 added, the colour changes from yellowish to greenish. Any 

 woollen fabric now dipped into this alkalised infusion will, on 

 exposure to the air, pass from greenish to blue — not the dark 

 blue one had expected, but a beautiful pale azure blue. This 

 change takes place at once if the fabric be immersed in any 

 of the dilute mineral acids. The blue colour thus obtained 

 cannot be called fast, as it will not withstand the action of 

 alkalis or even of soap. It is very subject to variation, being 

 often greenish-blue, grey, or even dove colour. This depends 

 on the age and quality of the woad leaves, as well as on the 

 details of manipulation. In brief, the process consists in simply 

 making an infusion and treating it first with a caustic alkali, 

 then with an acid. The following points have, however, to be 

 attended to : the leaves must be young, they must be fresh, the 

 water must be boiling or nearly so, the infusion must not be left 

 too long before the alkali is added, nor must the addition of the 

 acid be too long delayed. The infusion must be cold before it 

 is treated. If these precautions be not observed, instead of the 

 indigo-blue, that peculiar brownish-black compound is formed 

 which is the bete noir of the woad experimenter. 



In order to determine the quantity of indigo in Parson Drove 

 woad in plants of various ages : half a kilogramme of leaves, 

 28, 30, 34 and 66 days old, was found to yield respectively I "5, 

 24, 2'i, and o*6 grammes of impure indigo. 



The indigo obtained from different experiments varied much 

 in colour ; one specimen was an exceedingly beautiful light 

 blue ; mostly, however, it was dark blue, which became when 

 dry more or less green. When this, however, was powdered 

 and dissolved by the aid of slaked lime and ferrous sulphate, 

 it dyed cotton articles bright indigo- blue. With regard to 

 the time the leaves should be allowed to infuse, a series of 

 experiments in which 30 c.c. of an infusion were examined at 

 the end of 30 minutes, li, 2, 3, 6, 9, 12, 24 and 48 hours, 

 showed that the first 30 c.c, i e. at the end of 30 minutes 

 infusion, contained as much indigo as any of the others ; 

 while after the 6th hour the indigo-blue was replaced by the 

 black-brown precipitate. It is probable that the agitation of 

 the vessel in pouring off" the successive quantities was the cause 

 of this, for I have obtained indigo-blue from infusions at the end 

 of 10 or 12 hours when they have not been disturbed. 



May I ask if any of your readers can help me by suggesting 

 the process by which the mediceval dyers got a blue dye from 

 the prepared woad ? Indigo was not introduced into Europe 

 as a commercial article till the middle of the sixteenth century, 

 and even then its employment was for some considerable time 

 more or less prohibited by legal enactments. That woad was 

 used in this country long prior to this is shown by the indenture 

 which still exists between the woad merchants of Amiens and 



1899. 



Koninklijke Akademie van Wetenshappen te .\msterdam," October 25, 



