Feature

Carpet Circles

A strange phenomenon that has plagued the carpet trade for many years has been clarified by wool researchers.

By Geoff Crawshaw

If you had bought a carpet, say a very expensive wool carpet, and a couple of months after installation it developed a stained appearance, as if the building had been flooded, you would feel like complaining to the manufacturer.

The manufacturer would groan inwardly -- another case of carpet shading. He would know that the change in appearance was due to an optical effect resulting from the carpet pile laying in different directions, but would have no idea why the pile reversal had occurred. As far as he was concerned, there was no manufacturing fault. An unsatisfactory dispute would inevitably result.

Hope may be at hand as a result of a project carried out by the Wool Research Organisation of New Zealand.

Research Difficult

Research on the subject has been difficult to carry out, as laboratory simulation of the problem has been virtually impossible. Studies over the years have generated conflicting ideas as to what initiates shading -- dimensional changes in the backing, uneven moisture content, air currents, variations in floor levelness, underfloor heating, hot water services, even the "earth rays" allegedly detected by water diviners. A theory that seemed to explain one case failed in the next. The only certain knowledge was that all types of pile materials are susceptible to shading and that pile reversal is always present.

Shading problems were seen as particularly serious in wool carpets because of the high investment costs in the flooring. This prompted Dr Garth Carnaby, head of WRONZ Physical Sciences, to organise a study project in association with the University of Manchester Institute of Science and Technology. The research team tried a variety of approaches, including generation of shading by floor trials -- walking backwards and forwards over the carpet and walking in figure-of-eight patterns. They also sectioned the pile, photographed it, measured how it twisted and analysed its collapse mathematically.

Pile Problems

Shading was found to be an optical effect, caused by apparent differences in colour, depending on the direction from which a tuft in the pile is viewed. Shaded carpets have areas of pile that lie in various directions. Usually, the side of a tuft in cut-pile carpet reflects more light than the end, so an area of carpet with the pile laying away from an observer appears lighter than an area with the pile laying the other way.

The visual impact of pile reversal depends on the construction and composition of the carpet and the lighting. In some cases, such as loop-pile carpets, or if a bold pattern overwhelms the pattern of reflected light resulting from pile reversal, shading may not be visible even though pile reversal has occurred.

In use, the pile of virtually any carpet collapses to some extent in a variety of directions, and the direction of collapse is governed by the horizontal shear forces applied to the pile surface. WRONZ scientists found that the reversal came from the top of the carpet, suggesting that carpet backing did not play a role.

In many instances, the direction of pile collapse is predictable from the pattern of foot traffic. Pile is pushed radially away from turning traffic, and towards the sides in a straight corridor. In such cases, shading lines may cross seams, even when the manufacturing direction of the adjacent piece of carpet differs.

Permanent Effect

When the forces generated by the traffic patterns are not predictable, the pattern of shading is determined by the pile lay generated the first time the carpet is crushed. This may occur during manufacture, handling or laying. Subsequent wear consolidates this pattern. If pile crushing has been avoided up to this point, shading may be triggered by the very first sequence of footsteps.

Perhaps the most serious aspect of shading is that it is a permanent effect. The pile cannot be brushed back to its original configuration, except by drastic measures that seriously change the texture of the carpet.

The WRONZ scientists have been able to explain why this happens. Following a footstep or other crushing force, a carpet tuft never returns fully to its original position, due to frictional effects. After a subsequent footstep, the pile recovers to a new position, also further away from the original position, and so on.

The Next Step

The WRONZ explanation of shading has been received by the carpet industry with some relief. When confronted with a case of shading, manufacturers can now at least confidently explain the phenomenon to the customer.

Manufacturers can now visually distinguish between shading caused by a highly defined pattern of foot traffic and that caused by incorrect handling of the carpet. Moreover, they can anticipate shading in commercial and public buildings, and recommend carpets with favourable optical properties for such locations.

Two important questions remain -- how to prevent pile reversal, and how to repair a shaded carpet. WRONZ researchers hope to have the answers to these in the not-too-distant future.

Geoff Crawshaw is the manager of Information Services at WRONZ.