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What Are Shading and Pile Reversal?

To varying degrees, most cut-pile carpets exhibit a characteristic known as shading—apparent shade variations caused by relatively slight changes in pile lay from traffic, vacuuming and general use. Since the sides of fibers reflect more light than their tips, pile laying away from the observer appears lighter, while pile laying toward the observer appears darker. Areas that appear dark when viewed from one direction appear light when viewed from the opposite direction, and vice versa. These changes in pile lay are temporary and usually can be removed easily by vacuuming or brushing the pile.

In time, the forces of traffic may strongly orient the pile in a particular direction. This condition is commonly referred to as pile reversal, which, in contrast with simple shading, occurs in fairly predictable patterns and cannot easily be removed by vacuuming or brushing the pile.

The simplest form of traffic-induced pile reversal occurs where changes in the direction of traffic flow create shear forces that strongly orient the pile; i.e., the force of traffic turning to the left causes the pile to lay to the right, and vice-versa. (See figure 2.) This form of pile reversal occurs in fairly predictable patterns and is more or less permanent.

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The most controversial form of pile reversal-commonly called pooling or watermarking-is characterized by sharp changes in pile direction at apparently random, wavering lines called interfaces. (See figures 1, 3a and 3b.) The pile at the interface lies in roughly opposite directions, usually away from the interface, without regard for the pile's inherent lay created during manufacturing. (See figures 4 and 5.)

The interfaces typically are located in or immediately adjacent to trafficked areas, but they often appear not to correspond to the flow of traffic. In many cases the patterns of pile reversal continue across seams, even onto different carpet, while in other cases it stops or even reverses at seams. (See figure 8.)

While the pile on either side of the interface usually exhibits very little distortion, the pile within the interface often is severely distorted. This distortion appears to be attributable to the combination of traffic and loss of density created by the pile laying away from the interface.

The terms pooling and watermarking are synonymous and describe the typical appearance of an affected carpet, in which areas of the pile may appear wet. (See Figure 1.) Puddling is a term sometimes used to describe areas of pile reversal that are completely surrounded by interfaces. (See Figure 6.) Despite the terms used to describe it, this form of pile reversal is not related to exposure to moisture.

The textbook pattern of pile reversal, illustrated in figures 3a and 3b, occurs far more frequently than is apparently realized. This condition actually occurs in most installations with moderately to very dense cut-pile carpet and concentrated, directional traffic. Though the condition exists, factors such as poor lighting conditions often prevent it from being noticed.

Like the simple traffic-induced pile reversal described above, pooling is relatively permanent.

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Theories About Pooling

If there is anything humorous about pooling, it has to be its history of fueling imaginative and far-fetched theories to explain its occurrence. Following are some of the more popular ones:

  • Static Electricity: This theory actually is based partly on a real observation: sharp differences in static charge that sometimes can be measured on either side of interfaces. Problem: No evidence yet indicates that the static is the cause of the problem rather than a secondary elect of some other cause, such as traffic.
  • Electromagnetic Fields: A cousin of the static electricity theory, this hypothesis suggests that the changes in pile direction are caused by electromagnetic fields. Problem: No supporting evidence, and pooling occurs in the apparent absence of sources of electromagnetic fields.
  • Air Flow: The winding, swirling lines of pooling's interfaces, combined with reports of its appearance near HVAC intakes and/or floor registers, apparently led to the perpetuation of this idea. Problem: No supporting evidence, and pooling has been documented in plenty of cases in the absence of such airflow.
  • The "Domino Effect": This is the theory that the initial orientation of the pile is "set" the first few times the carpet is trafficked and that subsequent traffic exaggerates this orientation. As tufts in one area begin to lay over, they in turn begin pushing surrounding tufts over, and the pile reversal is thus spread throughout trafficked areas. Problem: None really. This is one theory that is fairly consistent with the observed characteristics of pooling, though it does not explain all of its behavior.
  • Subfloor Irregularities: This theory suggests that slight irregularities in the subfloor telegraph through the carpet and result in pile reversal when the carpet is trafficked. This idea is not totally without merit, as a number of documented cases show a degree of correlation between the pattern of the pooling and features of the subfloor. It appears that while subfloor irregularities do not cause pooling, they may influence the pattern in which it occurs. (See Figure 7.) Problem: Pooling often occurs over relatively smooth surfaces, such as hardwood floors and stone.



What Causes Pooling?

Though any or all of the theories described above may have some influence, the exact cause or causes of pooling have not been conclusively determined. However, there are two common factors present in virtually every documented case of which we are aware:

1. Traffic: The forms of pile reversal discussed in this bulletin, including pooling, occur almost exclusively in and immediately adjacent to trafficked areas, though the volume of traffic required is minimal. Though stories of pooling occurring on untrafficked carpet still circulate, additional investigation almost invariably reveals that the affected areas received at least some traffic.

Stories of pooling being discovered on new, uninstalled rolls of carpet also circulate. Though some relatively rare forms of roll crush can be somewhat similar in appearance to pooling, the condition described in this bulletin occurs only with traffic.

2. Carpet Style: Pooling occurs primarily in relatively dense, untextured, cut-pile styles. Thickness does not appear to be a significant variable, as it occurs with comparable severity in short commercial styles as well as thicker residential products. Low-density constructions (e.g., shag) and loop-pile styles appear to be virtually immune to visible pile reversal.

Because it occurs in carpet and rugs of all fiber types (wool, nylon, polyester, acrylic, polypropylene, coir, silk, etc.), all construction types (woven, tufted, fusion bonded, modular, etc.), in all methods of installation, under virtually every imaginable combination of circumstances, the conclusion that pile reversal and pooling are not related to any of these factors is well-supported.

Documented cases support the conclusion that normal traffic is the primary cause; i.e., pile reversal and pooling are normal reactions to traffic of certain styles of carpet. Therefore, pooling, like random shading, is accurately considered a performance characteristic rather than an abnormality. While some may find this explanation too simple to account for the wild, swirling patterns in which pooling appears, it is the most consistent with the hundreds of cases on which reliable documentation exists.

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Can Pooling be Corrected?

The short answer is, No. In nylon and wool carpet, pooling sometimes can be reduced or eliminated by aggressive steaming (actual steam, not "steam" cleaning) and pile lifting, but it invariably reappears within a few days or weeks of correction.

It has been reported that diligent maintenance based on frequent pile lifting has minimized the degree to which pooling develops on nylon carpet in some commercial installations.

In one case of moderately affected nylon carpet, pooling was removed and did not recur. However, the furniture in the room was rearranged in such a way that the affected areas remained untrafficked.

Because of their poorer resilience and minimal response to steam, polyester and polypropylene usually do not yield even temporary correction.

Unusual Patterns of Pooling:

Figure 6, 'puddling- areas of reversed pile completely enclosed by interfaces;

Figure 7, patterns of pooling influenced by high areas in subfloor (indicated by arrows);

Figure 8, pooling crossing and reversing at a seam where two different axminster carpets are joined.


Industry References

Shading and pooling are recognized throughout the carpet industry as inherent characteristics of most cut pile products and are not considered a valid basis for claims.

In the Carpet and Rug Institute's Carpet Claims Manual, shading is defined as an "apparent color difference between areas of the same carpet caused by ... random difference[s] in pile lay direction. It is a characteristic of all cut-pile carpet and ... is not a manufacturing defect. The sides of fibers reflect more light and appear brighter and lighter than the ends, which absorb more light and appear to be duller and darker in color."

The Carpet Claims Manual also states, "Pile crushing, pile shading, watermarking, and soiling are not manufacturing defects and will not be considered as a basis for claims."

The Carpet Manufacturers Association of the West's Statement of Obligations and Responsibilities of Carpet Manufacturers and Purchasers of Carpet and Carpet Performance and Claim Guidelines states, "Highlighting and shading are differences in light reflection between surface areas and are not defects." This document also states, "Watermarking or pooling is a color change effect which arises from reversal or bending of the carpet pile fibers so that light is either absorbed or reflected from the pile. This is a common condition and is not related to carpet construction or fiber type and is not the basis for a claim."

The Carpet and Rug Institute's bulletin Pile Reversal ("Shading"; "Water Marking") states that "a thorough search of available literature has not revealed any reason as to why carpet, after it has been installed, may or may not develop pile reversal. The many theories have been studied, tested, and evaluated by one or more sources. It has been concluded that the theories were not valid.... At the present time, the only conclusion which can be drawn is that pile reversal may develop on the surface of some carpet after it is installed, and that pile reversal is not due to the materials which are used to produce the carpet, the manufacturing process, or any combination of these factors."

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That bulletin also makes the following observations:

  • "it [pooling] occurs on the very expensive handmade Oriental rugs as well as machine-made carpet."
  • "The area may start to appear with several days after installation or may not become apparent for several months."
  • "A higher, cut pile, denser carpet which will not develop these areas is more than the consumer should expect," and, "Those who object to this type of change should consider purchasing another style of carpet, as it is not possible to assure the purchaser that pile reversal will not develop [in dense, cut-pile carpet]."

The Carpet Institute of Australia's Technical Information Bulletin Number 3, entitled Permanent Pile Reversal Shading, states, "Studies conducted by both independent researchers and major carpet manufacturers in Australia and overseas into the causes of Permanent Pile Reversal Shading have been largely inconclusive. A number of theories have been advanced over the years but most have been discarded or at best remain unproven. However, the consensus of expert opinion about Permanent Pile Reversal Shading is that:

  • "it can occur in any cut pile carpet (or rug) including hand knotted, tufted, woven, bonded, knitted or hand-made carpets and rugs;"
  • "its occurrence has not been linked with the various different carpet manufacturing processes or the component products used to make carpets;"
  • "it can occur in carpets made from all common carpet fibers and blends of different fibers (e.g. nylon, wool, acrylic, polyester, polypropylene and their blends);"
  • "its occurrence will not lead to premature wear of the carpet and it will have no effect on the durability of the carpet;"
  • "it has not been linked to methods of installation."

Preventing Pooling

The following guidelines have proven helpful in minimizing the visibility of the various types of shading, including pile reversal and pooling:

  • Pile ripe: Because it occurs almost invariably in cut-pile styles, loop-pile styles are a virtual guarantee against pooling.
  • Pattern: The more busily and boldly patterned a carpet is, the better it will disguise shading and pile reversal.
  • Shade: Lighter carpets sometimes tend to show less contrast between darker and lighter shaded areas, whereas darker colors often tend to exaggerate these differences.
  • Luster: Fibers with duller lusters soften some of the "sheen" that is characteristic of bright fibers, thereby reducing the contrast between dark and light areas.

—REM, 12/9O, revised 5/96

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@ 1996 by Floorcovering Forensics Company, 909-597-8501. Please do not duplicate.