(via The Journal of Gemmology, Vol 12, No.7, July 1971) Robert Webster writes:
For some time it has been the writer’s intention to consider some aspects of the nature of threads used to string pearls and beads into necklets, for the subject, which one might readily admit is not gemology proper, does have some importance in answering questions sometimes posed to a laboratory.
The reason which has prompted further investigation into this sphere devolved from some remarks made by the late Dr V B Meen, of Canada, after the writer had read a paper on damage to gemstones at the XIIIth International Gemmological Conference held at Brussels in 1970. During this talk the blackening of some cultured cultured pearls by the action of cosmetic creams was mentioned.
Dr Meen asked if any attention had been paid to the type of thread which had been used in stringing the pearls in the case mentioned, as some types of thread were more prone to attract grease than others. It was possible to show that in the records of this case there was a note to the effect that the string did not appear to be normal, but that this aspect was not pursued.
There may be other problems, too, where some information on the nature of the string used in threading beads may well be needed. This article is an attempt to give a short survey and to provide a basis for any future investigation. It will be readily seen that any full-scale investigation on the types and characters of threads would be a long-term project, but one which might well be worthwhile.
According to Walls threads, the more scientific name being fibres, can be classified into four groups as follows:
1. Animal hairs such as wool, mohair (Angora goat), camel, none of which have any place in the present study.
2. Vegetable fibres, which are divided into two groups. (a) Seed hairs, such as cotton and kapok, cotton being the only one which needs consideration here. (b) Bast and structural fibres; these are exemplified by flax (linen), jute, hemp and sisal. Only flax is of interest.
3. Fibres produced by the solidification of a liquid extruded through a fine orifice; these are again subdivided into sub-groups:-
(a) Natural, of which silk is the only important member.
(b) Artificial (man-made); there are two well defined groups:-
(i) Those made from animal or vegetable raw materials, such as regenerated cellulose (Rayon); cellulose esters, usually acetate (Tricel, Arnel, Trilon); alginates (from seaweed); regenerated proteins (casein) and others.
(ii) Purely synthetic fibres: such as polyolefines (polyethylene, etc); polyamides (Nylon, Brilon); polyesters (e.g. Terylene—Dacron in the United States of America—a polyethylene terephthalate); acrylonitrile polymers (Orlon, Acrilan, Courtelle, etc); vinyl chloride and vinylidene chloride polymers (Vinyon, Saran, and others).
4. Miscellaneous fibres; such fibres are natural mineral fibres (asbestos) and fibres of glass, metal, etc. which have no place in this study.
Of these numerous fibres, most of which are used in the textile industry, the only ones which need to be discussed are silk, cotton, linen, and some of the artificial fibres, particularly Nylon.
Silk
The most important fibre used for pearl stringing, silk is produced by the caterpillar of the moth Bombyx mori, which, when fully grown, spins a cocoon with a secretion produced by the caterpillar from a pair of tubular spinning glands. Each of these glands produces a single fibre, which is at first in a fluid condition.
These two fibres are then, by muscular action, and possibly aided by another secretion, formed into a single thread. This is silk, an albumoid, and the fibres normally receive certain cleaning treatments before being spun and woven into fabrics.
When a flame is applied to the end of a silk thread it burns, but does not readily flame, and the thread forms a shriveled blob. The flame does not tend to travel along the thread and is quickly extinguished. Under the microscope the threads are seen to be more or less structureless cylindrical rods which at places may flatten or bulge out. The fluorescence varies considerably due, as mentioned by Radley and Grant, to the fact that dressing agents, oils and dyestuffs, often completely alter the color of the fluorescence, and these writers also state that small traces of fluorescent dyestuffs added as brightening agents may produce complicating effects in ultraviolet light. Fluorescence can have scant discriminative value in the detection of the fibres themselves.
Cotton
Threads of cotton are not normally used for pearl stringing but some mention is included in this survey for there is reason to believe that they have been used for such a purpose, and, further, they are certainly used for stringing necklets of amber, coral, jet and some ornamental stones, such as malachite and rhodonite. Star sylko or Clark’s anchor stranded embroidery silks are often used for stringing such necklets.
The most important of the vegetable fibres, cotton, consists of white or yellowish-colored fibres which are obtained from the seeds of various species of the genus Gossypium of the order Malvaccae. Well bleached cotton is said to be nearly pure cellulose.
When a flame is applied to cotton threads they readily burn with the flame traveling along the thread leaving very little ash. On extinction of the flame the glowing embers emit a smell of burnt wood. When fibres of raw cotton are examined under the microscope the general appearance resembles that of a wrinkled, twisted irregular ribbon which may be likened to an exhausted rubber tubing. After treatment, such as mercerizing, which imparts a luster to the cotton so that it resembles to some extent silk, the typical appearance of the threads may not be so apparent under magnification.
Linen
The name is derived from the flax plant Linum usitatissimum, which is pulled just after flowering. The linen fibre, being the first layer under the epidermis of the stalk is separated from the rest of the stalk by retting in stagnant or running water. Following this the mucilaginous substances contained in the flax are removed by suitable treatments.
Like cotton, linen burns fairly readily with, on extinction, a smell of smoldering wood. Examined under the microscope the central canal is not so marked as in cotton and the fibre has notches at irregular intervals, and, further, may show diagonal striations. Linen lacks the convolutions shown by cotton fibres.
Artificial fibres
The beginning of the artificial fibre industry began with the work of Count Hilaire de Chardonnet of France during the 1880’s, although as early as 1665 the idea of making artificial silk was mooted by Robert Hooke. Chardonnet’s silk was nitrated cellulose, a type of celluloid, and was far too inflammable and, indeed, the sale of this silk was eventually banned in France.
The next advance in the production of artificial silks was due to the work of C F Cross and E J Bevan, who produced the rayons. These fibres were and are produced by regenerating cellulose. Plant cellulose, such as cotton linters, wood-pulp, are chemically dissolved and then regenerated and extruded through a diaphragm pierced with very fine orifices called spinnerette. Such regenerated cellulose fibres are Viscose rayon, cuprammonium rayon, and acetate rayon, but these fibres have little importance in this discussion.
The most important of the man-made fibres for this study is the super-polyamide called nylon, the production of which was the outcome of researches carried out by the American plastics chemist Carothers, aided by some of his colleagues. Made from hydrocarbons obtained from coal, and from ammonia, nylon, of which there are at least three types, is said to be twice as strong as silk and to be more elastic. It is probably due to this latter effect that nylon is not favored for pearl stringing as it does not knot well is inclined to stretch. There are two kinds of nylon cord which may be purchased for pearl and bead stringing. The first is made up of twisted fibres and resembles the true pearl silk, while the other, more used for beads than pearls, consists of a single strand, like a flexible rod. There is a variation of this latter type which is wound round with a spiral of fine metal wire and presumably used for heavy beads. If the single strand nylon cord is used for pearls or lightweight beads the necklet tends to bow and does not hand at all well.
Nylon melts before burning, producing a sticky blob which follows the flame traveling along the string. This blob of molten material, if dropped on to the skin, burns like hot fat. The flame of burning nylon is, however, readily extinguished. Nylons, like most of the other man-made fibres, are usually assumed to be structureless internally, but microscopical examination has shown that there appear to be masses of fine bubbles oriented parallel to the length of the fibre. This phenomenon seems to be more prominent in the case of nylon and this may be accounted for as nylon is extruded from a molten mass and not from a liquid which is expelled through the spinnerettes into a coagulating bath which solidifies the fibres.
In order to ascertain the probability of differential absorption of grease by various fibres a series of experiments were undertaken. A frame was constructed consisting of a length of channel aluminum, bored with eight small holes through both sides, which was screwed down to a suitable baseboard. The metal channel, intended to carry the grease, was closed at each end by aluminum angle plates. A series of different threads were then threaded through the holes in the channel and anchored so that they were fixed at the outer face of the channel. The threads then passed across the channel and across the baseboard for a distance of some 12 centimeters, where the ends were anchored to a fixed wooden strip by the aid of drawing pins. Suitable lettered labels were stuck down on the baseboard in order to identify the strands.
It was considered that suitable grease would be a cosmetic cream and Pond’s cold cream was used. To ascertain if the grease traveled along the string a small quantity of the chemical rhodamine was mixed with the cold cream, the notion being that it would not only give color to the but would show up under ultraviolet light, for rhodamine is highly fluorescent and glows with an orange or reddish brown light. During the experiment the frame was kept in a glass-topped box.
The strings used in the first experiment were as follows:
(a) Silk (dyed brown)
(b) Pearl silk
(c) Sylko mercerized cotton
(d) Linen thread
(e) Terylene thread
(f) Nylon (single thread)
(g) Spun nylon pearl silk (Nylcord)
(h) Clark’s anchor stranded embroidery cotton
The result observed after the grease had been placed in the channel was striking in that within an hour the grease had traveled down some of the threads, admittedly not very far but with significant differences. Strangely the seepage seemed to stop at these points and there was little further increase even after a week. The distances the grease, which colored the strings, had traveled along the strings were then measured, giving the following results:
(a) Dyed silk: no apparent effect
(b) Pearl silk: very weak seepage of about 1.5 mm
(c) Sylko mercerized cotton: 5mm
(d) Linen: 2mm
(e) Terylene: 4mm
(f) Nylon single cord: no effect
(g) Twisted nylon (Nylcord): 11mm
(h) Embroidery cotton: 3mm
The frame was then unstrung and the grease removed and kept, and the frame itself thoroughly cleaned and restrung with different threads as under:
(a) A fine tacking cotton
(b) Brown linen thread
(c) Terylene, same type of thread as in (e) in first run
(d) Pearl stringing silk. A different source from (b) above
(e) Pure silk (yellow dyed Regal)
(f) Polyethylene thread (blue dyed)
(g) Terylene (Coat’s white)
(h) Embroidery silk (cotton) as in (h) above
The cold cream plus rhodamine, which was removed from the channel after the first run, was then mixed with as much again of the cold cream but no more rhodamine added. Thus the concentration of rhodamine was only half that of the previous mixture.
After some hours the frame and threads were examined, but the results appeared to be disappointing, mainly because the lower concentration of rhodamine did not strikingly color the threads, and the fluorescence effects were masked by the strong whitish glow emitted by the threads themselves. However, some trace of differential seepage was apparent.
There was always the question of how much body heat would affect the mobility of grease in the case of a necklet worn for some time around the neck. To test this, the frame was removed from the glass-topped box and placed on a warm print dryer. This print dryer gave off much more heat than the heat given off by a human body, and, hence, the results obtained would be expected to be much more rapid, as, indeed they seemed to be as the following shows:
(a) Cotton thread: diffuse staining decreasing in intensity up to 5 to 6 cms
(b) Linen: a little staining up to 2 to 3 mm
(c) Terylene: slight tinting for practically the full length of the string
(d) Pearl silk: very slight staining for 3 to 4 cms
(e) Yellow dyed pure silk: very slight staining
(f) Polyethylene: color of the dyed thread precluded much in the way of observation or by fluorescence
(g) Terylene: weak staining for a considerable distance, and in fact seemed to behave rather like (c)
(h) Cotton embroidery silk: staining for about 15 mm
Note: the very slight staining of the Terylenes (c) and (g) could only be identified by comparison with thread taken from the original reels.
In conclusion, it may be said that the experiments have shown that there is some justification for the suggestion that there is differential absorption and percolation of grease along fibres of different natures. However, much more information is required and far more experimentation needs to be carried out on a greater number of kinds of fibres for a really full study of the subject. It seems apparent that twisted fibres tend to carry grease more readily than single strand material, as exemplified by the nylon samples tested. This was rather to be expected and most probably due to the greater possibility of capillary attraction between the threads.
Most of the threads carried grease for upwards of two millimeters, and as the nacreous shell of a cultured pearl is seldom more than a millimeter thick, it is clear that the grease would reach the discontinuation layer between the skin and the bead of the cultured pearl and tend to travel along it, as indeed, was found to have happened in the case mentioned at the beginning of this article.
What is further to be considered is that the experiments were carried out with static threads on a frame. There is movement of the beads on a necklet when it is worn and this would assist the grease to seep along the string. What does seem to be proved by the experiments is that silk is the best material to use in stringing natural and cultured pearls and to use other types of thread may well lead to trouble for the jeweler.
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