Popular Science Monthly/Volume 8/January 1876/Prince Rupert's Drops

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WHEN from fluidity glass is cooled to a solid structure in the ordinary temperature of the atmosphere, it is found to be very brittle or liable to fracture.

If the glass is so shaped as to be of unequal thickness in its different parts, it can seldom be cooled without fracture, and, if unbroken when cool, is liable to fracture with any subsequent change of temperature or by a sudden jar. Often this fracture takes place, in articles of considerable thickness, with an explosive force, perhaps breaking the glass into a thousand pieces. When glass breaks in this manner, it is said to "fly."

In order to prevent such liability to "fly," glass-ware is annealed.

The process of annealing glass consists in reducing its temperature more slowly than would occur in the air at ordinary temperatures.

An oven is so constructed that the heat of the glass is maintained by a current of heated air in which articles to be annealed are placed, and mechanism so contrived as very slowly to draw away the ware into currents of lower temperature. Or the ware is annealed in kilns, which are closed and scaled at a temperature a little less than that at which glass becomes plastic, and heated air being thus confined the kilns are many hours, often many days, in cooling. The more carefully and slowly glass is annealed, the less liable it is to "fly."

By cooling glass more rapidly than could occur in ordinary atmospheric temperatures, that is, by a process the reverse of annealing, Prince Rupert's drops are made.

The ordinary way to make these scientific curiosities is to drop a small quantity, usually less than half an ounce, of perfectly fluid glass into water. In falling, the glass will assume the form of a tear, with an elongated end extending into a thread.

Rupert drops are clear, bright, and hard, and may be struck with much violence upon the larger end without fracture, but if the thin, though tough and very elastic thread of the other extremity be broken off, the whole drop will explode into numberless fragments, much finer than the sand of which the glass was originally composed.

Why does this happen? and why must glass-ware be annealed in older to be serviceable? There is evidently such similarity of phenomena occurring in the drops and in unannealed glass that a satisfactory theory for the one ought to lead to the explanation of the other.

In an article on "Tempered Glass" contributed by Perry F. Nursey, C. E., to the Popular Science Review, and published in the September number of The Popular Science Monthly, the following theory of the Prince Rupert's drops is given: "Glass and water, and—as far as present knowledge goes—no other substances besides, expand while passing from the fluid into the solid condition. The theory of the Rupert drops is, that the glass being cooled suddenly, by being dropped into cold water, expansion is checked by reason of a hard skin being formed on the outer surface. This exterior coating prevents the interior atoms from expanding and arranging themselves in such a way as to give the glass a fibrous nature, as they would if the glass were allowed to cool very gradually. An examination of the Rupert's drop shows the inner substance to be fissured and divided into a number of small particles. They exist in fact in a state of compression, with but little mutual cohesion, and are only held together by the external skin. So long as the skin remains intact, the tendency of the inner particles to expand and fill their proper space is checked and resisted by the superior compressive strain of the skin. Nor is the balance of the opposing forces disturbed by blows on the thick end of the drop, which vibrates as a whole, the vibrations not being transmitted from the exterior to the interior. But, by breaking off the tail of the drop, a vibratory movement is communicated along the crystalline surface, admitting of internal expansion, by which the cohesion of the particles composing the external skin is overcome, and the glass is at once reduced to fragments."

In the "American Cyclopædia" (revised edition), under the word "annealing," are found the following enplanations: "When this" (glass) "is melted and shaped into articles which are allowed to cool in the air, the glass becomes too brittle for any use. The exterior cools first and forms a contracted crust, which shelters the interior particles; so that these continue longer in a semi-fluid state, and are prevented from expanding, as glass does in cooling, and uniting with the rest to form an homogeneous mass. The inner parts are thus constantly tending to expand. If, on the contrary, the glass is placed in a hot oven, and this is allowed to cool very slowly, the particles of glass appear to assume a condition of perfect equilibrium of cohesive force without tension, so that the mass becomes tough and elastic." And, again, in the same article: "Dr. Ure explains this phenomenon" (the explosive breaking of Prince Rupert's drops) "by referring it to the tendency of a crack once formed in the glass to extend its ramifications in different directions throughout the whole mass."

In the "Encyclopædia Britannica" (ninth edition), under the word "annealing," is found as follows concerning the phenomena of unannealed glass and Prince Rupert's drops: "The particles of the glass have a cohesive polarity which dictates a certain regularity in their arrangement, but which requires some time for its development. When the vessels are suddenly cooled, the surface-molecules only can have had time to dispose themselves duly, while those within are kept by this properly-formed skin in a highly-constrained situation; and it is only so long as the surface-film keeps sound that this constraint can be resisted. In the Rupert's drops it is plainly visible that the interior substance is cracked in every direction, and ready to fly to pieces."

The practical glass-maker, desirous of thoroughly understanding the true theory of annealing glass, that from such a comprehension he may endeavor to accomplish more perfection in his process, refers to the authorities quoted above, and finds himself bewildered by the theories and explanations here given. He notices that the foundation of the theory of the Rupert drop, and of the process of annealing, in the article of The Popular Science Monthly, and in the "American Cyclopædia," is based upon the assertion that in passing from a fluid to a solid condition glass expands. Although well aware that certain substances, as water,[1] bismuth,[2] gray cast-iron,[3] and antimony,[4] expand while solidifying, yet he is constantly reminded, by phenomena occurring in the glass-house every moment under his eye, that the reverse of this takes place in the substance of glass.

Upon the supposition that glass contracts in cooling, he bases the construction and working of his moulds, in which glass-ware is pressed, and the success of their operation assures him that he is working upon a safe conclusion.

For further assurance, he replaces an article of glass-ware, when cold, in the mould in which it was originally pressed, and finds that it easily returns to its place, and fails to fill the mould. With his calipers he measures carefully the glass and the mould, and finds the shrinkage has been about one-fiftieth of the original bulk.

He remembers that he has on his book-shelf a work[5] by Apsley Pellatt, in which that careful and accurate observer states as follows: "A piece of unannealed barometer-tube, forty inches long, measured when just drawn, will become about one-fourth of an inch shorter if annealed; whereas, if quickly cooled without annealing, it will only contract about one-eighth of an inch." It must be borne in mind that the barometer-tube, when just drawn, at the time when it is first measured, has already considerably cooled from a fluid state of the glass, and has effected a part of its shrinkage, although not yet solid or rigid in its structure.

As the gray cast-iron before mentioned is said to expand at the moment of solidifying, but afterward to contract with farther cooling, he experiments with the view to ascertain if an analogous action takes place in glass. He tests the cooling of a crucible full of this molten material, to note if at any time in the cooling process an expansion of its substance takes place. Even from the first moment, when the crucible is taken from the extreme heat of the furnace, he finds that the surface of the vitreous mass takes a concave form, this concavity becoming more considerable as the cooling process goes on.

If there were expansion at the moment of solidifying, the mass would then bulge upward, that is, the concave line of the surface would be disturbed. But, as the concavity of this surface constantly and uninterruptedly increases until the mass becomes cold, he finds renewed proof of the shrinkage of solidifying glass.

His ordinary observation thus confirmed by careful tests and by other authority, he feels that there is no possibility for him to be in error in regard to this contraction of glass, which he sees constantly going on.

When he reads, in the article of The Popular Science Monthly, that the exterior coating produced by the immediate chill of the surface of the glass "prevents the interior atoms from expanding and arranging themselves in such a way as to give the glass a fibrous nature, as they would if the glass were allowed to cool very gradually," he tries to remember an instance, where, in some very perfectly-annealed glass, there has been an indication of such fibrous nature, but finds himself unable, in his own experience, or in that of his workmen, to recall such structure in any case. He finds the substance of glass always presenting the same vitreous, amorphous appearance, except in cases of devitrification, and, in the absence of any proof of such condition, cannot bring himself to believe in glass of a fibrous structure.

He finds in "a cohesive polarity, which dictates to the particles of glass a certain regularity in their arrangement, but which requires some time for its development," as laid down in the "Brittanica," a theory which is far from satisfying or giving him any useful aid, and he requires some proof (which he cannot find) of such "polarity" before absolutely adopting this theory.

He looks in vain for the fissured character of the interior substance of the Rupert drop, mentioned in the article of The Popular Science Monthly, and in the "Encyclopædia Britannica," but finding, even under the microscope, that the substance of the interior, as well as the exterior, of the drop is apparently solid and undisturbed, gives up his attempt to understand the authorities, and even Dr. Ure's explanation in the "American Cyclopedia," of the Rupert-drop phenomena, fails to satisfy him.

He now feels that, to pursue this subject further, he must put together the facts in his possession, and ascertain if their combination will not suggest a more satisfactory theory than those laid down in the books.

He begins, of course, upon the foundation which his twenty years' experience in the glass-house has strongly impressed on him, viz., the fact that in passing from a fluid to a solid state glass shrinks.

His next fact is that glass is a poor conductor of heat, as he has often noticed in the manipulation of heated glass, during its process of manufacture, that in the same piece of glass, and close together, are portions, the one solid and the other fluid.

To these facts he puts the third fact, that the surface of fluid or semi-fluid glass chills very quickly upon exposure to the air, and very quickly becomes solid.

Here are all the facts necessary by which to construct a theory for the explanation of the phenomena of fracture in unannealed glass and in the Rupert drops.

Watching a thick mass of glass cool, he notes the color: by an oblique look, he perceives that the surface has a green tint; while through this transparent tinted medium a direct look shows the centre still of a glowing red color. He knows by experience that the green tint in cooling crystal glass indicates solidification, while the red glow tells that such glass is yet soft. But, not depending upon his experience of color, he tests the surface with an iron tool and finds it absolutely rigid; then with a hammer breaks this rigid surface, and finds, as its color indicates, the centre still semi-fluid.

Here is proved, the condition of an outer skin or shell of rigid glass, and an interior substance, still soft, plastic, and constantly strained by a tendency to contract, to occupy smaller boundaries; but those boundaries cannot be moved without breaking. It is a struggle of forces. If the thickness of the glass be considerable, the constantly-increasing strain of contraction pulls so hard upon the shell, that the force of cohesion is unable to withstand it, and the shell, yielding with a shock, shivers the whole substance into fragments.

In the process of annealing, the heat of the oven keeps the surfaces of the glass articles from absolutely becoming rigid, so that they yield sufficiently to the strain of the contracting interior portions; and if the whole substance of each article cools exactly together, the exterior and interior all the time at the same temperature, there is no strain and the ware is perfectly annealed.

As it is practically impossible to accomplish a perfect equality of temperature, a perfect equilibrium of the molecules cannot be obtained; but so near an approach to it is accomplished in a well-constructed annealing oven, that the cohesion of the glass is easily able to withstand the trifling strain.

In this view the action of cooling glass is simple and easily understood, surely more simple than to imagine a tendency toward a fibrous constitution of substance, or the imperious "cohesive polarity" of the "Britannica" article.

Test this theory upon the Rupert-drop phenomenon, and its explanation will answer as well.

A small amount of fluid glass, when dropped into water, will immediately, by the action of its heat, envelop itself in a garment of steam, which protects its surface from contact with the water, until that surface is so cooled that such contact fails to crack it. To test this assumption, try the experiment with partially cooled or soft glass, and the result will be that all the drops will break in the water, on account of cracked surfaces. With fluid glass, many drops will be lost, not from the same cause, if the drops be not too large, but from excessive contraction; perhaps, out of a dozen only one or two will be saved.

The steam chills the surface of the glass much more rapidly than the air does, consequently the inner and fluid glass in the Rupert drop is inclosed in larger boundaries than if the drop had cooled in the air. Hence contractive force is very strongly exerted to draw in such excessive boundaries, but the curved form of the drop presents arches of strength to aid the power of cohesion and resist destruction.

One drop bursts in the water, another does the same, but perhaps the third is drawn forth entire, though curled and twisted, as if in the agony of its strain. Two of Nature's forces struggle fiercely for the mastery in this little drop, that gives no indication of the contest as it lies quietly before us. But break off the thread, and down goes the first of the little arches, that are holding up the surface against contraction. One arch, falling, brings down another, and, once started, they go in such rapid succession that the ear detects but one sound, one explosive burst, in which the imp of contraction exults in the ruin he has wrought.

The peculiarities of the Rupert drops are toughness, elasticity, and the property of breaking into small fragments when any fracture, however slight, is made; their strength to resist such fracture is, however, greater than that of annealed or unannealed glass.

When we consider that these same peculiarities are the characteristics of the so-called "toughened glass" of M. de la Bastie, and that the method of treating his "toughened" glass, in the cooling process, is at least analogous to that of the Rupert drops, we are forced to believe in a certain relationship between them.

The Rupert drop falls into a water-bath; M. de la Bastie's glass into an oleaginous bath, the exact composition of which has not been made public.

M. de la Bastie's glass is not malleable, is not unbreakable, but simply tougher, harder to break than the ordinary annealed glass; so also is the Rupert drop.

As the characteristic distinction between annealed glass and the Rupert drop is the excessive strain upon the molecules of the latter—contraction versus cohesion—it is fair to infer that the superior strength, toughness, and elasticity, of the drop are due to such strain. As it is harder to displace the key-stone of a loaded arch than of an unloaded one, the simile may hold good in this case, and the strain of contraction upon the molecules of the glass of a Rupert's drop may help resist any outside force tending to disturb cohesion. If an outside force could be so exerted as to act exactly in the same direction as the power of contraction acts, undoubtedly such force would be aided by contraction to destroy cohesion; but, acting in any other direction, contraction would aid cohesion to resist it. As the molecules of glass are exceedingly small, and as, in the cooling process, they one after another individually become rigid, the lines of their contractive strain become so complicated that it is very unlikely any outside force can be exerted in such direction as to unite its impulse with theirs against cohesion.

As the toughened glass of M. de la Bastie flies into many pieces when fracture is effected, in a manner analogous to the breaking of the Rupert drop, it is probable, at least, considering the process of the oil-bath, that such flying into fragments is due to a strain of contraction exerted by the molecules of its substance. And if such a strain exists, as the flying seems to prove, it is also reasonable to suppose that, exactly as in the case of the Rupert drop, this strain of contraction among the molecules of its mass produces the superior toughness, strength, and elasticity, which are claimed for this newly-invented glass.

  1. Ganot's "Physics," edition of 1873, p. 261.
  2. Miller's "Chemistry," vol. ii., p. 604.
  3. Bauerman's "Metallurgy of Iron," p. 233.
  4. Miller's "Chemistry," vol. ii., p. 595.
  5. "Curiosities of Glass-making," by Apsley Pellatt, London, 1849, p. 63.