January 3rd, 2010, 08:24 PM   1
Jeff Krauss

Join Date: Oct 2008
Posts: 5
Natural vs. Synthetic Indigo

As some of you know, I collect Japanese textiles, and I have an interest in indigo dyeing. Coincidentally, today I both got an email from John Howe about this salon, and a Japanese textile dealer's blog that had a discussion of indigo dyeing.

From this salon, the major difference between natural and synthetic indigo is that natural indigo has a higher percentage of impurities such as indirubin (which itself is a separate and interesting area of study). The molecular structure of natural and synthetic indigo are the same.

But the textile dealer's blog led me to a different distinction. Several papers concluded that natural indigo is more likely to be subject to "dye aggregation." One of the articles uses the terms "dye aggregation" and "aggregated indigo clusters" without defining them or specifying what is the mechanism that would produce this aggregation in natural indigo. I assume it means that several indigo molecules are somehow physically connected, but I don't understand what physical mechanism might produce this. Anyway, the conclusion is that this "aggregation" causes natural indigo to have poorer penetration into the yarn and more dye remains on the surface, compared with synthetic indigo. But the paper does not explain why this "aggregation" occurs. See http://www.mdpi.com/1996-1944/2/2/661/pdf. And the "apparent molecular weight" of natural indigo is much higher than synthetic indigo.

I wonder if this provides a way to distinguish between yarn dyed with natural indigo and synthetic indigo, which was not produced commercially until perhaps 1897.
January 4th, 2010, 02:24 AM   2
Pierre Galafassi

Join Date: Oct 2009
Posts: 30

Hi Jeff,
As you write, the molecule of synthetic and of natural indigo are strictly identical.

None of the available indigo qualities (natural or synthetic) is pure though. Although purification would be relatively easy, it is an added cost and brings no advantage for dyeing. The main impurity Indirubin (a bluish-red vat dye with good dyeing properties, similar to those of its isomer indigo) is therefore present in all indigo qualities. In general natural indigo contains more indirubin than synthetic indigo does. But synthetic indigo is a commodity now, manufactured in factories of all sizes, equipment levels and competences, thus the level of impurities and in particular the proportion of indirubin can vary to a certain extend.

Aggregation is frequent with most dye molecules (again synthetic ones as well as natural ones). It is caused by physical attraction between dye molecules, due to various types of interaction known as hydrophobic forces, Van der Waals forces etc.
In general the average aggregate involves a few dozens or a few hundreds single dye molecules, but the so-called direct dyes for cotton (large and flat molecules) are notorious for aggregates averaging 100 000 or more single dye molecules. Indigo too will form aggregates.
Aggregates are relatively weak, reversible structures and can be reduced in size, even eliminated completely from a dye-bath or from a printing paste, for example by using hydrotropic substances like urea, thiourea etc. Increasing the dye-bath temperature, decreasing the concentration of electrolyte are other usual ways to reduce average aggregate size.
It is true that aggregates can influence the dyeing outcome, in particular they tend to reduce the speed of dyeing (it takes longer to achieve the final equilibrium, thus the final shade). They are particularly obnoxious in coloration techniques characterized by a short operation time, as in printing, continuous dyeing or in some indigo applications.
The presence of some specific chemical impurities can help keeping the size of aggregates down, by disturbing and destabilizing their geometric construction (we call such molecules "spacers" and they are added on purpose in some synthetic dye brands). I am not aware whether indirubin has such an "aggregate killing" capability in indigo brands, but it is not unlikely.
Anyway, it is not too farfetched to think of differences of dye-ability between different qualities of commercial indigo, natural ones included.

Besides, pigments (indigo is a pigment) are known to crystalize in various crystalline systems depending on production process details. Crystals are much more stable arrangements of dye molecules than aggregates. To destroy them (converting them in a so-called amorphous form) or to convert them in another crystalline form more favorable to the coloration process, may require quite energy-and time-intensive operations, like heating, milling, etc..
In the case of indigo the various qualities, starting with un-sophisticated powders and ending with pre-reduced stabilized liquid formulations indeed show very different dyeing properties, especially in terms of ease of use and shade reproducibility.
Hope this helps.
January 6th, 2010, 08:06 AM   3
Pierre Galafassi

Join Date: Oct 2009
Posts: 30

Hi Turkotekers,

Since I am affected by a strong case of «l’esprit de l’escalier», I realize only now that I have not answered convincingly enough Dinie’s and Steve’s concern:
« I have read that it (indigo) does not really penetrate the fiber, but forms a kind of coating on the outside. That was why jeans fade so well and why a green made by dyeing blue over yellow, tends to become yellowish. Indigo rubs off. Is this correct?»

Dinie’s remark in a previous post is partly correct: Even seasoned denim dyer’s do indeed believe and write, erroneously, that indigo stays all on the surface of the cotton fibre, forming layers of pigment and that it is the cause of the peculiar fading of jeans. By analogy many ruggies believe that indigo sits on their favorite rug surface too.

In fact, as this micro-photography of a cross section of a denim warp clearly shows, even in a dyeing process specifically designed (*) to keep reduced indigo («leuco») from penetrating too deeply into cotton, the external individual cotton warp fibrils are indeed penetrated by the dye, which forms a so-called «ring dyeing». The very short contact time and low dyeing temperature explain that the dye-bath only wets properly the external fibrils of the warp, but even under these, purposely unfavorable, dyeing conditions the leuco molecule quite easily penetrates any single cotton fibril which it is allowed to meet.
The picture also shows a few agglomerates of «surface dye» (for example on top of the first fibril on the left). Given the low affinity of leuco for cotton, its strong tendency for premature oxidation (back to indigo pigment) and the peculiar dyeing process, a significant amount of indigo on the surface is unavoidable. However most of it is eliminated from the warp yarn at the end of the dyeing process, during a two-steps (hot & cold) wash-off operation.
The famous «stone-washing» performed on the jeans garment is something different: It is a severe treatment of the textile surface with pumices, which rubs-off some of the surface fibers and the ring dyeing of others fibers, showing the white parts.
A mere elimination of indigo agglomerated on the fibre surface would not call for such a hard abrasion.
In dyeing of rug wool with indigo, the pre-industrial dyer did his very best to avoid deposits of pigment on the yarn surface (thoroughly pre-wetting the wool to get air out of the fiber, keeping it under the dye-bath surface to avoid contact with the foam of indigo pigment which usually floats on top, agitating only very gently the fiber, to avoid carrying air into the dye-bath, keeping the pH and the reductive power of the dye-bath high etc.). In old recipes, one often notes that deposits of surface dye eliminated during the final washing-off, leave colorless spots on the fiber. No doubt one of the causes of the typical abrash of indigo-dyed wool.
Besides, wool dyeing conditions are much more favorable to leuco penetration into the fiber than it is the case in cotton denim warp dyeing. Although each indigo wool dyer seems to have his own recipe, the dyeing temperature averages 50°C (instead of 25°C) and the dyeing time averages 30 minutes for each dip (instead of 30 seconds).

(*) the cotton warp yarn is dipped for less than 30 seconds in a leuco dye-bath, at room temperature, «aired» (exposed to air) for 60-80 seconds and this process is repeated, on average, 6 times)

Hoping that you have all recovered from season excesses,
best regards
January 6th, 2010, 08:27 AM   4
Steve Price

Join Date: May 2008
Posts: 90

Hi Pierre

I understand what you're saying, but still don't understand two common phenomena that I've been attributing to indigo simply abrading off the surfaces of wool and cotton.

1. Greens on antique rugs presumably are made by double dying with indigo and a yellow dye. It's common to see areas on surfaces that are subject to abrasion in which there are yellow splotches in the green areas.

2. Blue jeans "fade" in normal use. The loss of color is most pronounced in areas subject to abrasion and seems to occur only in the yarn that's on the surface.

What's going on?


Steve Price
January 6th, 2010, 11:02 AM   5
Pierre Galafassi

Join Date: Oct 2009
Posts: 30

Good morning Steve,

1. If yellow spots seem to be due to abrasion on some antique carpets, it is indeed possible that they occurred at a place where some indigo pigment sticked onto the surface. It also seems probable that the dyer chose the less logical and less safe way of dyeing the wool yarn first with the yellow dye and then over-dyed it with indigo. (If indigo would have been dyed first, the surface covered with indigo pigment would probably have been "reserved" and, after abrasion, would rather have been colorless than yellow).
2. Denim cotton warp fibrils are "ring dyed", meaning that only the external 1/3 or 1/4 of each fibril diameter is colored. At points of major friction these fibrils are worn and show more and more their white core.
The most exposed fibrils of each warp (which due to the specific dyeing methods are the ones which got actually dyed) are worn-off, showing white(r) fibrils beneath.
I do not know which of these two occurrences is the main cause for jeans fading.

Besides, indigo has only a limited chlorine-fastness. Each washing will therefore reduce a little the color-strength of the garment. My guess is that this slow bleaching takes place in a homogeneous way on the whole garment, but I might be wrong and it could also be that chlorine bleach is more active at places of major fiber abrasion due to the more open fiber structure.

I have a feeling that I am venturing on rather thin ice now.