This article was adapted from the first segment of the author’s complete ANTEC® Orlando 2015 paper, which covers multiple mechanisms of color transfer. Call SPE customer service at U.S. 203-775-0471 to request the full paper.

There is quite a laundry list of words which in some way suggest the movement of a desired color from an object intended to be that color to a second object not intended to be that color (like crocking, rub-off, bleeding, blooming, blushing, transfer, migration, extraction, sublimation, exudation, plateout, diffusion). 

Many of the terms above are used interchangeably, or may be peculiar to only a single industry. Take, for example, the term crocking. The Merriam-Webster Dictionary (online) defines crock as an intransitive verb: “to transfer color (as when rubbed or washed). ”To take the same approach to the definition of migration brings the result that migrate is also an intransitive verb and can be taken to mean “to change position within an organism or substance,” or “to move from one place to another … as of an atom or group: to shift position within a molecule.”

Clearly migration could be a synonym for crocking—but could crocking really be a synonym for migration? Our purpose in this discussion is to attempt to place clear-cut industry-related definitions in terms that try to create a more clear understanding within and across different industries that all deal with color transfer.

Before we go further, for the purposes of the following discussion, it is important to establish at least one ground rule. For this discussion it is presumed that the color species, pigment or dye, is sufficiently distributed and adequately dispersed in the polymer system of choice.If not, any number of secondary issues could easily contribute to the transfer of color and hide what may actually be occurring.

Crocking Tests

Fortunately for us, crocking may be the best or most consistently defined phenomenon in question. It is, to this author’s knowledge and experience, used only in reference to the transfer of color from fabric to another fabric. This does seem to be inclusive of all fabric, whether made of leather or fiber, synthetic or natural.

The phenomenon is quantified by a procedure from the American Association of Textile Chemists and Colorists (AATCC) Test Method 8-1996 (or later). This test uses a piece of standardized white cotton cloth on a device (“finger”) that is moved in a back-and-forth motion over the fabric to be tested. The cloth is moved in reciprocal fashion ten times forward and ten times back at the rate of one cycle per second for ten complete cycles, or twenty passes. The amount of transfer is quantified by trained observers using the gray scale. This test is performed in both dry and wet conditions; only the rubbing cloth is wetted in the wet testing. 

If the test is evaluated with an objective eye, the first conclusion is that yes of course color may have transferred to the white rubbing cloth. The question becomes why and how. The “why” seems obvious: there was color available on the surface of the fiber or fabric to transfer. But was there?

Consider the surface of the rubbing cloth and compare this to the surface of a piece of very-fine-grit sandpaper or perhaps a jeweler’s rouge. It seems more than possible and even likely that in some instances the color transfer was one of simple abrasion. While it is certainly possible that there was color at the surface of the fabric that simply transferred to the white cloth, let’s consider what was required to get the color into the fabric to begin with. 

The color had to be dispersed or dissolved into the polymer that is used to make the fabric. This requires that the polymer was heated, that the color was added, and the color then needed to be mixed and distributed, and then the color had to be dispersed. It is certainly possible that there was color at the surface of the fabric; however, given the nature of the individual fibers making up the fabric, it is likely that this color was intrinsically trapped within the fiber. Other than friction during the crocking test, there was no heat to soften the fiber. While the crock meter device could certainly be thought of as offering a certain mixing or agitation, it is unlikely that you could consider it to be dispersive or distributive. 

It is therefore proposed that the likely mechanism in this instance is one of simple abrasion, the act of the crock meter rubbing against the fabric and breaking or otherwise damaging fibers that then either transfer microscopic particulates or fracture and liberate particulate color. It must be considered that pigments are particulate inclusions in the fine denier of the fabric. For this reason, the pigments can be thought of as contaminants that offer a point of weakness within a fiber that fractures when abrasion takes place. 

Dyes and Crocking

What might be the circumstance when considering the use of a dye? In this case we are considering the use of a colorant whose function is based on its presence at a molecular level. Unless the dye is overloaded within a mixture, it is very unlikely that the dye will be present as a particulate. It cannot be ruled out that color transfer of a dye would occur as a result of soluble transfer from one surface to another—this process requires time and heat in significant quantities, neither of which are likely to occur in the space of the ten seconds required to accomplish the crock meter test. Consider also that the cotton test cloth is a natural fiber, and while yes it can be colored, it is not particularly known to be a solvent for dyes.

So how did this occur? It seems again likely that the cause of this transfer would be abrasion. The dye being present on the molecular level doesn’t mean that there may not be a weak point, or simply that the fiber is susceptible to the abrasive forces at work. 

Finally, it cannot be discounted that the colorant, pigment or dye, is somehow mobile in the polymer phase of the fiber. This mobility in the case of the dye can be rationalized when considering that the polymer is acting as a high-viscosity solvent for the dye. While the polymer is frozen in the fiber, this doesn’t mean that the molecular motion has stopped. Add to this the potential activity of any additives that may act as plasticizers for the polymer or as additional solvents for the dyes. It is important to consider in this possibility that the motion of the colorant molecule is subject to the influences of temperature, concentration, and even molecular weight.

Pigment Mobility

The mobility of pigment is a bit more clouded. It is important to recall the old axiom that everything dissolves in something. At very low concentrations of pigment, it is entirely possible for the pigment to be slightly soluble and therefore mobile like a dye.

Consider that once this phenomenon occurs, that solubility is generally considered to be an equilibrium condition. This means that as pigment dissolves and moves to the surface, it necessarily removes itself from the equilibrium by recrystallizing on the surface. This will leave the pigment within the polymer in an out-of-balance equilibrium which may indeed result in the solubilization and mobilization of the pigment.

Hypothesis

The following hypothesis is therefore put forward. The word crocking is most commonly associated with the transfer of color from a fabric surface to a surface. This transfer is the result typically of abrasion of the colored fiber by the fiber incumbent on the crock meter.

It is proposed to test this theory by performing crock meter tests and evaluating the results in the normal gray scale comparisons, but also through optical microscopy, where the goal is to evaluate the colored fibers for fractured fibers and the target cotton patch on the crock meter for the presence of broken colored fibers. It is proposed that the presence of broken fibers in either location would confirm the transfer as a result of abrasion.