Indigo - dyeing - woad vat

In this vat, indigo in couched woad is reduced by bacterial fermentation to the water soluble leucoindigo. The essential parameters, temperature and pH, have been determined. After the 16th century, woad was increasingly replaced with Indian indigo until by the end of the 19th century only a small amount was used to initiate the fermentation.
Padden, A.N., John, P., Collins, M.D., Hutson, R. and Hall, A.R., Indigo-reducing Clostridium isatidis isolated from a variety of sources, including a tenth century Viking woad vat. Journal of Archaeological Science, 2000, 27, 953-956
John Edmonds, Medieval textile dyeing, Historic Dyes Series No 3, (2003) ISBN 0 9534133 2 2, a woad vat recipe from Florence (1418), pp. 66-73
John Edmonds, The history of woad and the medieval woad vat, Historic Dyes Series No 1, (1998) ISBN 0 9534133 0 6
Cardon D, Natural Dyes : Sources, Tradition, Technology and Science, Archetype Publications, 2007, 367-377

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The woad vat (1)
WOAD VAT.-These vats are extensively employed at Louviers, and in the manufactories of the north of France. The bath is prepared in the same manner as in the foregoing case; the finely-cut root is introduced into the copper along with 2 lbs. of pounded indigo, 9 lbs. of madder, and 15 lbs. of slaked lime. The liquor is, after the necessary ebullition, poured upon the woad. This substance contains but a very small quantity of coloring principle; we must, therefore, add some indigo when preparing the vat, so as to indicate the precise instant when the mixture arrives at the point of fermentation so necessary for imparting hydrogen to the coloring principle, and for rendering it soluble. We must also use a larger quantity of lime, since the woad contains no ammonia resulting from previous decomposition, such as we find to be the case with the pastel of the south. When the vat is in a suitable state of fermentation, a rusty color becomes manifest, in addition to the signs already described in speaking of the pastel vat; besides the ammoniacal odor, the bath always retains the peculiar smell of the woad. The pounded indigo is now added, and we proceed, in the manner already detailed, to reduce it to a state of solution fit for dyeing.
The vats prepared by means of pastel have greater durability than those made with the woad; but it is thought that the colors given by the latter are more brilliant than those obtained from the former dye.
Napier J (1853) Chemistry Applied to Dyeing. Henry Carey Baird, Philadelphia, p. 337.

The woad vat (2)
On blue dying.
I shall begin with the woad vat.
An English vat, of the size described, seven feet six inches in depth, the same in diameter across the bottom, and six feet across the top, is set with five hundred and sixty pounds of the best woad, five pounds of umbro madder, one peck of bran, four pounds of copperas, and a quarter of a peck of dry-slacked lime. The woad will have to be chopped into small lumps with a spade, and thrown into the vat before the liquor is put in ; let the madder be broken into the vat in small pieces, and the bran and lime thrown in upon them. When the materials are in the vat, it should be filled up with water that has been boiled and cooled down to about 195 Fahrenheit, from the furnace, and the contents kept stirred all the time it is filling. When the vat is full, within four or five inches of the top, give it a good stirring for half an hour, and then cover down close. A dye-house bucket should hold four gallons, and whilst the vat is stirring after it has been filled, put in one bucket of well-ground indigo, containing fifteen pounds of the dry article. The vat should be set about four or five o'clock in the afternoon, and be attended and stirred about nine o'clock the same evening ; by this time, if every thing goes on regular, the fermentation will so far have progressed, that, when a small portion of the liquor is let run from either a scoop, or any tin vessel, between the person viewing it and the light, it will appear of a dark bottle-green. When well stirred, let it be covered down, and if the weather should be cold, throw some mats or wool-bags over the covers to keep in the heat, which will prevent its cooling too low before the liquor comes to work. The person who manages the vat, must attend at five o'clock the following morning ; let him take off both covers, and plunge the rake into the vat, so as to bring up some of the air to the surface that will be carried down by the rake, when a part of the sediment of the vat will rise with the bubbles. If the fermentation has progressed, as it should do, the air-bubbles will appear of a fine blue, and a number of copper-coloured scales will float on the surface of the liquor. Should these appearances take place, and the liquor, when viewed by transmitted light, be of a dark olive-green, put into it another bucket of ground indigo, and a quarter of a peck of the slacked lime ; stir the liquor for twenty minutes, and cover down close. The heat of the vat should now be at about 140 Fahrenheit, and if it has lowered down below 135, and it be a fire vat, a fire must be applied to raise and keep it at the latter heat. Two hours after this stirring it must be stirred again, when, if the fermentation is found to have gone on in regular progression, the liquor will be of a brighter olive than in the morning, the bubbles will be of a richer purple, and the surface more generally covered with copper-coloured scales ; should these symptoms make their appearance, add another quarter of a peck of lime, stir for ten minutes, and cover down close as before. The liquor must now be stirred every two hours, and if the appearance continue to improve, a quarter of a peck of lime will have to be added at each stirring, until there have been given from eight to ten quarters, including the one that was put in when the vat was first set. By the time eight have been added, the liquor will look very rich in the bead, the bubbles will rise of all sizes, from the bulk of an hen's egg to that of a small hazel-nut, and none of them will break so as to disappear ; but many of them will collapse, and as they fall together, will appear of a rich smalt colour, coated with a fat-looking skin. A large quantity of bubbles will have risen by this time, which, laying on the surface in a compact mass, will look rich, and the greater part will have passed from a blue to a copper colour. The indigo, when raked up, will show in the liquor in clouds ; its appearance will be a rich yellow-olive, clouded with indigo. When the vat assumes all these appearances, it is said to be in fine condition, and every thing will have gone on in regular order ...
Partridge W (1834) A Practical Treatise on Dying, Woolen, Cotton and Silk, &c. Partridge, New York. p.68

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