INFLUENCE OF CERTAIN FACTORS ON THE STRIATION OF FOURCAULT GLASS

A. T. Logvinenko

At the “Belyi Bychek” glassworks (Leningrad Region), work was carried out to study the influence of individual factors on the striation of Fourcault glass under ordinary conditions of sheet-glass production.

Over the course of two months, observations were made of the operating conditions of the machines and of the degree of striation of the glass. Samples for study were taken in the form of ribbons across the entire width of the machine.

In the present article, the results of the observations carried out are set forth in abbreviated form.

I. DEGREE OF STRIATION OF THE GLASS

During the indicated period, the plant produced glass with varying degrees of striation, ranging from coarsely striated to practically non-striated—the weak striation of which could be established only with the aid of an instrument.

The photograph presented below (made according to a method proposed by me¹) characterizes the striation that was present (Fig. 1). Sample 1 (top)—third grade; samples 2 and 3—export glass; sample 4—made to a special order.

During the work, no instance was observed in which all the machines simultaneously produced either strongly striated glass or weakly striated glass. Usually, glass with different degrees of striation was being drawn on different machines.

II. WEAR OF THE BOAT

With systematic observation of the operation of the boat, from the moment it was inserted into the machine chamber until its removal as unsuitable, the following may be noted: as a rule, during the first run after insertion of the boat, the machine produces glass with very weak striation. Sometimes, against the general background of weak striation, a sharply defined zone of striation appears, but it does not depend on the quality of the boat; the causes of its occurrence will be discussed below. Over time, the sharpness of the striation gradually increases. After the machine is renewed, it again becomes weak.

The increase in striation due to wear of the boat occurs slowly and becomes clearly noticeable only when comparing samples taken after the first run and after 10–15 days of operation of the boat.

However, if the increase in striation due to wear of the boat proceeds slowly in the middle part of the ribbon, the same cannot be said of the areas located near the edges. Here the intensification of striation occurs extremely rapidly. After 8–10 days of operation of the boat, zones 15–20 cm wide on the side of each edge (cone) acquire a constant, sharply defined striation.

Fig. 1

Fig. 2

More severe wear of the boat in the cones, as compared with the middle part, occurs, on the one hand, because the viscosity of the glass in these places is higher than in the middle, owing to which the mechanical wear of the boat by the streams of passing glass is intensified; and, on the other hand, in the cones crystals grow more rapidly and more vigorous undercutting takes place, causing destruction of the lip.

An old, worn boat, after renewal, gives glass with strong striation, increasing rapidly and sharply.

III. PREPARATION OF THE MACHINE FOR START-UP

The normal time for heating up the machine at the plant is 6 hours. During this time the machine is cleaned, heated up, put into “halm,” and then knocked.

The heating-up time often fluctuated, both upward and downward. In the course of the work it was possible to observe, on the same machines, long heat-ups (10–12 hours) and short ones (up to 4 hours). When samples were compared after equal intervals of machine operation in both cases, it was established that after short heat-ups the machines had more strongly striated glass; moreover, it was often possible to observe that during the first period of operation of the machine there was severe striation, which at first decreased to a minimum (4–5 hours after start-up) and then, in the usual manner, increased. Along with this, in the first hours of operation of such a machine, the appearance of oblique striation—“svil”—was observed. These phenomena may be explained by the fact that during heating in the under-machine chamber thermal homogeneity of the glass had not been achieved, and after start-up, along with hot glass, comparatively cold glass also entered the machine, causing intensified striation.

Heating the under-machine chamber with overturning of the boat gave no noticeable improvement with respect to striation.

IV. POSITION OF THE MACHINE ON THE FOURCAULT CANAL

There is a widespread opinion that, with the Belgian system of arranging machines, on machines located far from the middle of the Fourcault canal it is impossible to obtain glass with weak striation, since over the long and complicated path (narrowing of the canal, bridges, preheating chambers) the homogeneity of the glass is disturbed (Fig. 3).

However, the practice of the plant’s operation does not confirm this. On the Fourcault canal, machines Nos. 5 and 6 produce a wider ribbon than the others, and after them the canal narrows; this should have had a particularly unfavorable effect on the striation of the glass of the subsequent machines, i.e., Nos. 4 and 7, but this was not observed: the latter machines very often produced weakly striated glass. Likewise, a number of cases were recorded when the end machines (Nos. 1, 2, 3, 9) produced weakly striated glass, while at the same time on the middle machines (Nos. 5, 6) strongly striated glass was being drawn.

Cases were repeatedly noted in which the end machines, after operating for 50–60 hours after start-up, continued to produce weakly striated glass.

Thus, weakly striated glass can be obtained on every machine, and striation is determined not by the position of the machine on the canal, but by the quality of the boat, the preparation of the machine for start-up, and the duration of operation of the machine.

V. STABILITY OF STRIATION

Striation on a machine is not stable. If two glass samples are taken from the machine, one after the other after some interval of time, and compared, it can be seen that the striation of one sample does not correspond to the striation of the other.

If a series of such samples is made at comparatively short intervals of time, then it is possible to trace the displacement of individual striae, the disappearance of some, and the appearance of new ones. Only a small number of striae, mainly near the edge, remain stable; the greater part of them disappear and reappear. But along with the instability of striation in its position, it should be noted that it is very stable in sharpness. The pattern of striation changes, but its overall sharpness remains unchanged over short intervals of time; over longer intervals one can establish its slow but steady increase.

Fig. 3.

Trimming the machine, when striation is strong, gives positive results; a considerable improvement occurs at the edges and a smaller one in the middle part. But trimming the machine is used very rarely as a measure for combating striation. The machine is trimmed only in the case when streaking occurs on it.

VI. COOLERS, GAS TRANSFER, GLASS DISCHARGE

One-sided supply of cold water and removal of heated water in the coolers has no noticeable influence on striation. Throughout the entire period of machine operation, the coolers operate approximately with the same temperature regime. Striation after start-up and before breakage is different. Likewise, the operating regime of the coolers on machines with strong and weak striation does not differ substantially. The degree of striation of the edge located on the side of the cold-water supply is no greater than that of the edge where warm water is removed. Consequently, direct, sharp cooling of the crest of the ribbon by the coolers has no substantial influence on striation. Likewise, no influence of gas transfer on striation in the Fourcault channel was established.

Discharging glass from the cul-de-sacs causes some weakening of striation on the last machines, which is explained by an increase in the temperature of the glass in the under-machine chambers.

Repeated observations have shown that the starting and stopping of neighboring machines is reflected neither positively nor negatively in the striation of the operating machine; and newly started machines located beside an old machine producing strongly striated glass often produce glass

with very slight striation. The latter gives grounds to suppose that in the under-machine chambers there are zones of glass which take part only to a very insignificant degree in feeding the boat with glass and, being replenished with glass mass from the stream of glass mass passing through the channel, practically do not themselves participate in this stream. The temperature of the glass in these zones decreases during operation of the machine, which leads to a continuous intensification of striation on the machine; but since this glass is not drawn further, it does not affect the striation of neighboring machines.

This consideration is supported by the following: because of a shortage of fuel at the plant, all machines were temporarily stopped. During the interruption in operation, the temperature in the under-machine chambers fell so much that the glass crystallized, and after start-up, stones appeared on the machines for a long time. The stones were of two kinds: one surface type, coming from the lips of the boat, and another deep-seated type—consisting of melted crystals located inside the ribbon and coming from the Fourcault channel.

The degree of stone formation on different machines varied. Some, located closer to the middle of the machine channel, had more deep-seated stones than those located farther away.

The stones disappeared after 10–15 hours of machine operation after start-up. The end machines, as those started earlier and having worked for a longer period, were the first to clear themselves of the deep-seated stones; when only surface stones were appearing on them, on the machines located closer to the middle both kinds of stones were appearing.

Obviously, in each under-machine chamber there was a different amount of deep-seated stone that filled a zone which did not touch the general stream of glass moving along the channel. As the stones were drawn out by the ribbon, the glass on the machine improved independently of its quality on neighboring machines; thus, for example, machine No. 8 had very many stones, machine No. 9 very few, machines No. 2 and No. 4 were stopped because of a large number of stones, while machine No. 3, at the same time, was producing satisfactory glass.

VII. THERMAL FLOWS AND STRIATION

After start-up of the machine, one can observe a characteristic phenomenon, especially pronounced when working with new boats. Against the background of general weak striation across the entire width of the machine, in one place there appears a region of sharp striation, 10–20 cm wide. It appears more often in the middle of the sheet and is less often shifted toward one side or the other. It appears soon after the machine is started and persists from 20 to 100 hours. The appearance of the striation of such a region changes with time. It shifts to the right or to the left, but the sharpness of the striation remains stable and increases in accordance with the increase of striation over the whole glass ribbon.

After stopping and restarting, such a region may appear in another place or may not appear at all. Consequently, a defect in the boat is not the cause of its occurrence. Likewise, the formation of stones on the lips of the boat cannot cause the appearance of a region of sharp striation, since the period of machine operation is too short. The cause of the formation and long persistence of this striation can only be thermal flows of the glass.

The surface of Fourcault glass is not flat, but has the form of alternating cylindrical convex and concave lenses; this character of the surface also gives rise to the phenomenon called striation. Usually on one of the surfaces the irregularity is expressed more sharply, and on the other more weakly; this can be verified if these surfaces are alternately leveled. The indicated operation can be carried out with the aid of a flat glass plate.

and a liquid having the same refractive index as the glass. If, for example, Canada balsam or cedar oil is poured onto the surface of a glass sample and then covered with a flat glass plate, the surface of the glass will be optically leveled; when light is projected through such glass onto a screen, we can determine the extent to which that side affects the striation of the glass. In Fig. 2 the upper strip is a photograph of the striation of the glass sample; the next strip (in the portion bounded by the frame) shows the striation when one of the sides is “leveled,” then when the opposite side is “leveled,” and, finally, the lower strip—when both sides are “leveled” simultaneously.

It is clearly visible in the photograph that the striation is caused predominantly by one side; the other side, attenuated, repeats this pattern without detail. When both sides of the sample are “leveled,” the striation has a very weak, uniform character, which in no way reflects the character of the sample’s striation. With regard to the above-mentioned region of sharply expressed thermal striation, it should be noted that, in all observed cases, the greater surface irregularity was found on the side opposite the inflow of glass from the tank furnace.

When a good bait is present, a machine that after start-up produces weakly striated glass gradually begins to produce increasingly striated glass, and old machines, as a rule, produce glass with strong striation. After the machine has been renewed, it again produces glass with insignificant striation. This character of the fluctuations in striation is difficult to explain by chemical inhomogeneity of the glass. Likewise, from this point of view, it is impossible to explain the differing degrees of striation of two neighboring machines and the obtaining, from one canal, of glass with sharply different striation, since there are absolutely no grounds for asserting that in some chamber the glass is chemically more homogeneous than in a neighboring one.

Chemical inhomogeneity is inherent in glass melted in a tank furnace and is present to the same degree in both weakly striated and strongly striated glass.

The most probable cause of the appearance of striation (after poor bait quality and crystallization of the glass on the lips of the bait) is a drop in the temperature of the glass in the machine chamber during operation of the machine and the formation of a region of relatively cold glass, in which the viscosity of the various layers differs sharply from one another.

Such glass is drawn into the ribbon and causes increased striation. To reduce striation, the following are required: increasing the mechanical strength of the lips of the bait, periodic trimming of the machines, and ensuring a stable glass temperature in the machine chamber throughout the entire operating time of the machine. To fulfill the last condition, it is necessary to isolate the glass surface in the machine chamber from the action of the coolers and to insulate the machine chambers at the ends.

1. Logvinenko A. T. An instrument for controlling the striation of Fourcault glass. Ceramics and Glass, 1934, No. 4, p. 15. ↩