Preparation of Woad Cakes.

Chapter XX. On the Cultivation of Woad, and the Extraction of Indigo from it.
Chymistry applied to Agriculture.
By John Antony Chaptal, court of Chanteloup, Peer of France, Member of the Institute, &c.
Translated from the Second French Edition.
Boston: Hilliard, Gray, and co.
1836

ARTICLE II.
Preparation of Woad Cakes.

The manufacturer of woad cakes should avoid cutting the leaves of the plant, till the period when they are richest in indigo; this substance is, to be sure, contained in the leaves of the isatis, during all the periods of its vegetation; but the coloring principle does not present itself at all times in the same quantity or of the same quality. In the young leaves the coloring principle is of a delicate blue, in those of a middle age the color is deeper, and in the ripe leaves it approaches to black. It has likewise been proved by observation, that the coloring principle is obtained from the young leaves with more difficulty than it is from those advanced towards maturity.

It appears, then, that the most advantageous time for gathering the leaves of woad, is when they have acquired their full growth. But by what marks is this to be determined?

The manufacturers of woad cakes govern themselves upon this subject according to their own observations, and their modes of procedure vary more or less in different countries.

In England and Germany, the leaves are cut as soon as they begin to droop, and their bluish color to degenerate into a pale green.

In Thuringia, the leaves ate gathered when they begin to droop, and to give out a strong, penetrating odor.

In Tuscany, the time for cutting the leaves is judged of by the color which a leaf affords when pressed between two linen cloths.

In the Roman states, the leaves are considered to be matured when they lose the intensity of their color, and begin to fade.

In Piedmont, the leaves are gathered when they begin to fall.

In the south, the leaves are considered as being mature, when they exhibit a violet shade upon their borders.

We are indebted to M. Giobert, of Turin, for an excellent treatise upon woad, in which he states that, according to his observations, the quantity of indigo contained in the leaves of the plant in the most favorable seasons, increases progressively from the eleventh to the sixteenth day of their vegetation, after which time it remains stationary during four or five days, and then begins to decrease. The observations of M. Giobert have been confirmed in the south of France, at Bedford, and in nearly all Italy; and from them may therefore be deduced a general rule, by which the cutting of the leaves of woad may be governed, whenever the vegetation of the plant has been favored by the combined action of a good soil, a warm atmosphere, and a suitable degree of moisture; for without this the leaves will not have reached maturity in twelve or sixteen days, and they should not be gathered before approaching that state.

The extraction of the indigo is uniformly performed with more ease at an earlier period of vegetation, than when the 1eaves are perfectly mature; the quantity of coloring matter obtained is equally great, and the hue of it is handsomer.

The leaves of the isatis are gathered by plucking them off with the hand, or by cutting the stalks with a knife or pair of scissors; but whichever way is practised, care must be taken not to injure the stalks or tops of the plants; the cuttings may be repeated once in six or eight days, so as not to allow time for the quality of the leaves to degenerate. A mixture of the leaves of strange plants, and of the bastard woad, with those of the isatis tinctoria, must be carefully avoided.

The leaves, when gathered, are put into baskets and conveyed to the work-shop in which the manufacture of woad cakes is carried on; when they have begun to wither, they are ground between two millstones equally channelled; the bruised substance being frequently stirred with a shovel, and the grinding continued till the nerves of the leaves can no longer be perceived by the eye. All the juice which flows out during grinding, is carefully preserved to moisten the paste with when it is fermenting.

The paste is carried under a shed, the ground of which is a little sloping, and paved with cemented stones, in which are little channels for conveying into a reservoir the juice which flows out. Under the highest part of the shed is formed a bed of the paste three or four feet in length; to render this bed as compact as possible, it is beaten down with heavy pieces of wood. Fermentation commences in a short time, the mass swells and cracks, and there flows out from it a black liquor, which is conducted into the reservoir by the channels in the pavement. In some manufactories, this liquor is allowed to run off upon the ground without the shed; but the odor which it diffuses in this case is very offensive.

Whilst fermentation is going on, attention is paid to reuniting the mass when it cracks, and to moistening it either with urine, or with the juice which flowed from it when between the millstones.

After the paste has fermented well for three or four days, the mass is again beaten down, and this operation is renewed several times during the twenty- or thirty- days that the fermentation lasts; the paste being in the intervals moistened with the juice, and the surface of it united.

In a cold season, or when the leaves are poor and dry, fermentation will not be completed in a month; in Italy they often allow four months for it, and sometimes the bed is not removed till the following spring.

There is a kind of worm which often takes possession of these beds, and sometimes in such numbers as to devour all the indigo contained in them; in this case the beds must be turned over, and, if this be not sufficient, the whole must be again ground in the mill.

After fermentation, the paste seldom appears of a uniform texture, and there will be found in it some remainsof nerves which are visible to the eye - for this reason it is subjected to a second grinding, after which it is ready to be made into cakes; this is done by filling round wooden moulds with it, or by forming loaves four or five inches in diameter, and eight or ten in height, and usually weighing about three pounds and a quarter. In the south of France the moulds are usually much smaller, and the loaves of woad, known by the name of shells, weigh but little movie than one pound. These cakes should, when broken, appear of a violet color, and exhale a good odor.

The cakes are placed upon hurdles, and carried to a dry and airy place to harden.

In most countries the cakes are sold in this state to the dyers, who make use of them either to heighten their woad dyes, or for dying by themselves a soft blue; but in general they are made to undergo. another process, by which they are improved; this is called refining. This last operation is, however, seldom performed by the manufacturers, but by the dealers to whom they are sold in large quantities; the reason of this is, that the process of refining can be performed advantageously only on large masses, and the proprietor of the fields for cultivating woad has only the product of his harvest, and the conveniences n_ecessary for making it into cakes.

For refining the woad cakes, it is necessary that they should either be ground in a mill or broken in pieces with an axe; the fragments are made into beds about four feet high, and sprinkled either with water, or, what is preferable, with the juice of the leaves; heat is developed in a short time, and a violent fermentation takes place. At the end of six days, the bed is turned, so as to bring the interior or under portion upon the top; this is watered in the same manner, and, five or six days after, the bed is again made over with the same care. These operations are renewed at short intervals, till the mass, having ceased to ferment, becomes cold; in this state all the animal and vegetable portions, with the exception of the indigo, are decomposed, and it is now sold to the dyers to the greatest advantage.

The mode of making woad cakes here described, is undoubtedly the most perfect one, but it is not everywhere practised. At Genoa they do not refine them; in the department of Calvados, and upon the Rhine, they pile up the leaves without grinding them; and they mould the cakes as soon as the division of the mass will allow of this operation.

It is necessary to observe, that an immense variety inthe quality of the cakes is produced, not only by the nature of the soil and climate, but also by the difference of seasons, and by the care bestowed upon the cultivation of the plant and the gathering of the leaves; and from these circumstances arises the different estimation in which they are held in commerce, and consequently the various prices at which they are sold. The leaves of woad yield about 1/3 their weight of good cakes; these, when used with indigo to form dyes for producing a permanent blue color, serve not only to facilitate fermentation, but add the indigo which they contain, to that which is brought from India, and thus render the dye less expensive.

The cakes, especially those that have been. refined, contain alone a sufficient quantity of indigo to give to cloth all the shades of blue, which can be procured from the imported material. M. Giobert states, that M. Alexander Mazéra, in the presence of several skilful dyers and manufacturers, and of the commissaries of the Academy of Turin, colored with the cakes four pieces of fine cloth of four different shades, and they were judged to be at least equal in brilliancy and durability to those obtained from the best Bengal indigo.

M. de Puymaurin has published an account of a process by which the inhabitants of the island of Corfu color, with the leaves of the isatis, the woollen stuffs of which they make their clothing. The practice with them is to cut the leaves when the plant is in flower, and, after carefully drawing out all the nerves, to reduce them to paste in a mortar; this paste is dried in the sun, and when it is to be used for coloring, is placed in a bucket and moistened with water; the mixture gradually heats, and at length ferments strongly; water and a little weak ley of ashes is added, and the paste undergoes the putrid fermentation. Into this composition the cloth which is to be colored is plunged, and allowed to remain eight days, turning it from time to time; in this way it acquires a deep and lasting blue. The ease with which this process is executed, would render it very useful in farmers' families.