Who cares, and why?
One of the questions that seems to keep coming up is, "How can someone tell whether a rug's dyes are natural or synthetic?" We might first ask why it makes any difference. Although most rug collectors agree that natural dyes are more attractive than synthetics, knowing whether a particular color came from a plant extract or from an organic chemical laboratory shouldn't change the aesthetics one whit. The real reason for caring is, or ought to be, that old rugs are worth a lot more than new ones are (all other things being equal), and knowing whether the dyes are natural or synthetic helps us estimate a rug's age and market value.

General methods for determining whether a dye is natural or synthetic
There are two broad classes of methods by which we can tell whether a color is of natural or synthetic origin. One is chemical analysis, the other is the so-called "experienced eye test" (a term coined, I believe, by George O'Bannon). Each has certain advantages and disadvantages.

Chemical analyses of dyes generally make use of some form of chromatography (a class of methods in which the chemicals in a mixture are separated from each other, their behavior during the separation providing information about their structure), sometimes combined with spectroscopy (basically, a group of physical methods for quantifying what our eyes identify as colors). Even the worst of these methods is more accurate than the "experienced eye" in identifying dyes, but even the best of them is fallible.

Offsetting the obvious advantage of being more reliable, chemical methods have a number of drawbacks. For one thing, they are expensive. The least expensive, thin-layer chromatography, could probably be set up in somebody's home for an investment of a few thousand dollars. The better methods, more sophisticated forms of chromatography, have startup costs in the tens of thousands. If spectroscopy is to be included, the cost of the laboratory will almost certainly amount to $100,000 or more.

In addition, being able to make use of these facilities requires skill and training. For most folks, being able to send samples to some centralized laboratory for analysis would be much more practical than setting up the equipment at home.

There are other disadvantages to chemical testing. Samples must be taken or sent to the testing facility and it will be some time (hours, at least, and probably days or even weeks) before results are available, a serious problem for a potential bidder at an auction or for what many dealers refer to as "opportunistic collectors". Since most rugs have 5 to 15 colors, a number of samples would have to be taken from each rug for analysis. And since each sample requires removing a little of the rug, we can't expect sellers to be too enthusiastic about the whole thing.

It isn't surprising that chemical analysis is rarely used. The "experienced eye" test is not nearly as trustworthy as chemical analysis, and is especially prone to the error of identifying some synthetic dyes as being of natural origin. Notwithstanding this problem, it has a number of advantages in practical situations. Nearly anyone can learn enough to have his own "experienced eye", at which point he has a testing system that is completely portable, very rapid, absolutely cost-free, involves no damage to the rug of interest, and is fairly reliable.

How can someone learn to identify natural and synthetic dyes?
There are a few elements of dye identification by eye that can be learned from books, but not from photos in books because color reproduction is so imprecise. The only way to learn is to be taught by someone who already has an experienced eye (that is, who has been taught by someone else with an experienced eye, and so forth back in time).

It's obvious that there is a danger that errors propagated from person to person can become accepted as truths simply by repetition. It's also a fact that there's so wide a variety of colors in both groups of dyes that educating our eye will require seeing thousands of examples under tutelage.

Where can we find people with appropriate levels of experience? And when we discover such people, how can we get them into a place with large enough numbers of rugs to provide specimens with which to educate us?

Knowledgable dealers can be very helpful, using their inventories as examples. However, it would be far better to have many teachers and many more specimens than any small number of dealers is likely to be able to show us. The two best places to go for education are rug conventions that include Dealer Fairs, and previews of sales at major auction houses like Sotheby's, Christie's or Skinner's. These provide the opportunity to see and handle several hundred to several thousand rugs in a single place, with the opportunity to discuss them and ask questions of large numbers of people who have some expertise. Collectors, dealers, and auction house staff are all potential teachers, and are usually generous with their time and knowledge.

Some rules of thumb for beginners
There's no way to develop an educated eye without looking at lots of real rugs and getting taught by people who already have some experience. However, there are some simple beginnings that can be made in the comfort of your favorite reading nook.

The issue of whether a blue color is natural or synthetic is moot. The natural dye used for producing blues is exactly the same chemical as the synthetic one, indigo. Indigo is even the dye of choice for blues in most contemporary fabrics, including blue jeans. One of the interesting things about indigo is that it doesn't dye the entire thickness of most yarns, but only coats the surface. Thus, when the surface abrades, the dyed part of the yarn is gone and its natural color peeks through. This is why blue jeans "fade". In fact, they don't. Denim has high and low areas; when the high areas abrade off, they appear white and the jeans look faded.

If you've struggled with grass stains on kids' clothes, you might be surprised to learn that natural green dyes are fairly rare. Some very early Turkoman pieces have a natural dark green in their palette, but most of the greens that are seen on rugs made prior to the availability of synthetic dyes are produced by consecutively dying the yarn with indigo and a natural yellow. These are usually easy to identify. Some natural yellows are very sensitive to light. In pieces using these dyes the color on the surface exposed to light will be blue while the back of the piece will show that it was once green. Another tipoff that a green isn't synthetic is that the surface of the yarn often abrades in some places, exposing yellow splotches amid the green (where the indigo layer has been worn off).

Blacks and dark browns were sometimes derived from sheep with wool in those colors prior to the availability of synthetic dyes, but most often they were derived from dyes with corrosive effects on wool. The dark brown or black areas will be more worn than the others, giving a relief effect. In many old rugs the blacks or dark browns are worn to the point that the pile is completely gone from those areas. One cautionary note: many modern rugs, particularly Turkish products in Caucasian designs, have blacks that have been intentionally cut to shorter pile height than other colors. One way to distinguish these from natural corroded blacks is that the wear in the areas dyed with corrosive blacks is variable from one part of the rug to another. The rugs with blacks that have been cut to lower pile heights are cut to a uniformly lower height throughout the rug.

Natural purple dyes are relatively uncommon, but easily distinguished from early synthetics, which were very sensitive to light. These typically have faded tips on areas that are vivid purple when the pile is pushed back to expose the lower levels or when the rug is turned over so the back can be seen. There are also some weaving groups that used a corrosive natural purple.

The most difficult group of colors from the standpoint of visual discrimination between natural and synthetic dyes are the reds. There is a very wide range of hues possible in this group with both natural and synthetic dyes, and no amount of verbiage can substitute for seeing large number of examples. Early synthetic reds (and, for that matter, contemporary red dyes) are very prone to color runs, but the existence of red color runs doesn't preclude the possibility that a dye is of natural origin. Indeed, some synthetic red dyes are so similar in appearance to some natural hues that even the most educated of eyes can be fooled by them. On the other hand, those with truly well educated eyes also have educated brains that are aware of this problem.

Oranges and yellows present similar problems to those posed by reds, but usually occur in far smaller areas of rugs than do the reds. Except for an early synthetic orange of remarkable exuberance, these are seldom the only synthetic dyes in a rug's palette.

Finally, I ought to mention the rugs with palettes having a number of very vivid colors that have faded drastically at the tips, which becomes very evident when the pile is spread so that the bases of the knots can be seen. These include large numbers of synthetic dyes, and are usually Persian or Caucasian weavings made between World Wars I and II (roughly, the second quarter of the twentieth century).

Some concluding remarks
There is no doubt that dye identification by chemical analysis is very much more reliable than that done by the "experienced eye" method. On the other hand, chemical analysis is often simply not practical for a number of reasons, and is too expensive for widespread adoption in the near future to be likely. The rugs for which the expenditure is most likely to only be a small fraction of the total cost are those that are most expensive, usually of great age. But these also tend to be the ones with which the "experienced eye" is least likely to err.

There are few practitioners of testing rug dyes, the best known being Paul Mushak (of Durham, North Carolina). I would guess that fewer than 500 rugs have had even a single color subjected to dye analysis, and the number that have had all dyes tested is probably in the range of 10 to 20, at most. When we consider that the major auction houses on the east coast of the USA offer about 10,000 rugs for sale each decade, just about every one of which has an estimated age published in the catalogs partly based on visual inspection of the dyes, we can appreciate how firmly entrenched the "experienced eye test" is in Rugdom.

I believe that every serious collector owes it to himself to educate his own eyes, and that there is no way to do so except by seeing very large numbers of rugs and letting other, more experienced folks, be our teachers.