2 DISCUSSION

In 1934, W.T. Bone published a short communication in which he observed that when an end of a strip of pure bleached cotton fabric was dipped into pure water, a brown line appeared, and that methylene blue, cuprammonium fluidity, and Harrison's solution indicated that the cotton in the browned area at the wet-dry cellulose interface suffered oxidative degradation.4 Only the water-air-cellulose interface suffered damage: the completely wet and the completely dry portions showed no change. The importance of the phenomenon to the dye industry was emphasized in the subsequent discussion, when several speakers commented that bleached cotton or rayon goods left damp would dye unevenly.

A more detailed study was published in 1950 by Bone and Turner5 who suggested that the brown line effect might have five possible causes:

(1) a restricted dispersion of free or slightly aggregated cellulose molecules by water, and their transfer by convection to the boundary region; (2) modification of cellulose over the whole area of the test strip, and the collection of the modification product occurring in the wet part by convection at the boundary line; (3) modification by water or by water and air, in the wet region only, and transport of the modified product to the boundary line; (4) initial presence, or formation during the experiment, in the wet area, of a substance capable of reacting with the cellulose, and the concentration of much of this substance at the boundary line before it has been able to react with the cellulose in the wet region generally; (5) a specific change in the cellulose which can take place in the conditions which exist in the boundary region, and nowhere else. [p. 326].

During an extended series of experiments it was shown that although the brown lines are water- and ethanol-soluble, a fluorescing modification remained at the water line whenever the rinsing was not effected promptly. The authors concluded that, “The presence of a non-dispersible and non-transportable modification at every position in the cloth where a boundary between wet and dry has existed for any length of time reduces the probability of alternatives (1), (2), and (3).” The fourth solution resembles that advanced by conservators hypothesizing motion towards and concentration at the stain edge of existing soil and degradation products.

In the published experiments, the fabric used was a new bleached, scoured, and washed muslin. The authors were careful to avoid contamination of solvent, fabric, glassware, and atmosphere, but found that the brown line still occurred. Repeated use of the same sample caused repeated formation of the line, although a slight diminution of the browning on the second trial was noted. Subsequent lines on the sample did not vary in intensity if similar conditions were maintained. Eliminating impurity as a primary cause of the phenomenon, experiments suggested that the interaction of air, water, and cellulose is responsible for the formation of the brown line. Furthermore, it was concluded that the energy of the water molecules at the interface, while sufficient to permit evaporation, was probably not enough to cause degradation. The authors also demonstrated that the reaction occurs in the absence of light, heat, atmospheric oxygen, iron, bacteria, and waxy materials.

One of the numerous studies of weathering—the effect of light, moisture, and atmosphere—prompted further examination of the results described above. Testing the effects on cotton fabric of repeated wetting and drying, Bogaty et al2 found that the brown of weathered cotton fabric was comparable to the fluorescing material generated in the brown line experiments. Elaborating some of Bone's studies, they were able to confirm several of his findings and to furnish new data.

Like their predecessors, they concluded that the brown line effect did not result from iron or prior existence of the brown substance, and noted that the rate of production was nearly constant in repeated trials over eight months. The degraded, acidic nature of the boundary area was confirmed, and the product subjected to further analysis. The soluble brown was determined to be of low molecular weight, and the breaking strength of the fabric (previously aged at elevated temperatures) was shown to have diminished. When Aspergillus niger was introduced on the fabric sample, initial growth occurred rapidly and exclusively along the brown line, spreading only later to other areas.

Both groups considered their conclusions speculative, and refrained from postulating a reaction mechanism. Such caution is understandable, given the difficulty of identifying “pure” cellulose, excluding all oxygen, controlling the breadth of the line formed, and producing sufficient brown material for experimentation. Schaffer et al.16 were among those assessing the relative importance of atmosphere, substrate, and solvent in the reaction. They concluded that continuous evaporation at a wet-dry interface resulted in a fluorescing brown line (sic), accompanied by modification of the substrate or the solvent. Related studies were performed by Madaras and Turner,11 who examined the evaporation of water from cotton fabric in nitrogen and in vacuo. The latter procedure hindered brown line formation without preventing modification of the substrate at the interface, leading the authors to suggest the formation of – COOH groups despite the absence of atmospheric oxygen. They summarized their agreement with Bone and Turner and with Bogaty and coworkers as follows:

1. Evaporation of distilled water from a fabric, under the conditions defined, resulted in a sharp boundary between wet and dry regions.
2. Modification of the cellulose took place at this boundary with the production of a brown, water-transportable, fluorescent reaction product.
3. After extraction of this brown material, there was evidence of modification of the residual cellulose at the place where it had been formed. The methylene blue absorbtion was markedly greater, and there was evidence of a slight local increase in cuprammonium fluidity as compared with that of the wet or dry regions of the cloth.
4. Periodic lowering of the evaporation level in the same cloth during the same experiment gave rise to a series of fresh brown lines of undiminished intensity, although, at each lowering, the area of wet cloth from which water could evaporate was correspondingly reduced.

Observation (4) seems to be conclusive evidence that the brown line does not represent merely a concentration at the boundary of non-cellulosic material present originally in the cloth sample or formed during the experiment in the whole of the wet region. [My emphasis]

The authors also note that if oxygen is required for brown line formation, the amounts are “minute” (p. 374).