Source: Procedures in Experimental Physics
by John Strong
With illustrations by Roger Hayward,
edited and amended by Shawn Carlson

THE fundamental operations in glass blowing for laboratory use are cutting, rotating, bending, blowing, and welding. By various combinations of these operations, apparatus is constructed from glass tubing and glass cane. This TechBanner describes how these operations are executed.

Fig. 1.

Hard glass, such as Pyrex, is used extensively for making laboratory apparatus. It is harder to manipulate than soft glass because it has a higher working temperature and thus congeals quickly when it is removed from the flame. However, hard glass as a much lower coefficient of thermal expansion and so it is much more durable in the laboratory. You can plunge red-hot Pyrex into liquid nitrogen without the piece cracking. Moreover, hard glass is easier to anneal. So this TechBanner will deal exclusively with techniques related to shaping this material.

Fig. 2.

A typical glass-blowing workbench is shown in Fig. 1. Cross-fires heat the glass to softness, a method that may be termed American, since German glass blowers ordinarily use a single-blast burner. Compared with the blast burner, cross-fires heat the glass more rapidly and uniformly. Either method may be used for most of the operations. However, some of them require a pointed flame, which is more easily obtained with a blast burner.

Fig. 3.

The hand torch, mounted as shown by the dotted lines in Fig. 1, provides a pointed flame. Natural or artificial gas is used for fuel in the burners. Compressed air is used for working soft glass; but in order to obtain the higher temperature required to work hard glass, oxygen or a mixture of oxygen and air must be used. In an ordinary blast burner, however, acetylene can be used as fuel with compressed air. Map gas, which can be purchased at any hardware store, burns much hotter than natural gas and also makes a fine gas to use went building glassware.

Fig. 4.

Accessory equipment includes a collection of corks of various sizes, some fitted with closed glass tubes to serve as handles for rotating the work, and others with open tubes for blowing. Pieces of rubber hose of various sizes fitted with closed glass tubes, to close up the ends of small tubes, are also included. A swivel L and mouthpiece device with a connecting rubber hose, shown in Fig. 1, is convenient for blowing rotated work that is large or otherwise awkward to bring to the mouth. Forceps and molding tools used for spinning glass are shown in Fig. 2. A file for cutting small tubes and a hot-wire device for cutting larger tubes are shown in Fig. 3. To sharpen the corners of the file, the narrow sides are ground on an emery wheel. When the file requires tempering, it is heated until it becomes a dull red and plunged into ice cold water.

Pyrex tubes of various sizes, capillaries, and cane are kept in stock for constructing apparatus. There should also be a supply of other glasses, such as soda glass, lead glass, and Nonex. These should be well labeled and kept apart from the main stock.

Fig. 5.

Some physical properties of glass. The thermal expansion of various glasses and metals is shown in Fig. 4 and Table I. Other characteristic temperatures of glass and quartz are given in Table II. The variation of viscosity with temperature for a typical glass is shown in Fig. 5. The viscosities corresponding to important characteristic temperatures—annealing temperature, working temperature, and melting temperature—are indicated on the curve in Fig. 5. The significance of the first two temperatures is that internal strain is relieved in about 4 hours when glass is heated to the yield point, while only about 4 minutes are required at the annealing temperature. At the yield point the viscosity is about 10¹³ poise. At the annealing temperature it is about 10¹² poise. In the working range of temperature the viscosity varies between the limits 10⁵ and 10¹⁰ poise, with the optimum working viscosity about 10^8.6 poise. Glass is considered molten when the viscosity is less than 10² poise.

Fig. 6.

Cutting tubes and bottles. To cut small glass tubes (to 1/2 inch in diameter) for the operations of glass blowing, they are first scratch-marked with the sharp edge of a file, care being taken that the scratch, a few millimeters long, is accurately perpendicular to the tube. A break is then made by a combined bending and pulling force as illustrated in Fig. 6. Tubes can be broken at the scratch-mark by means of a stroke with the file as shown in Fig. 7. This technique is suitable when the tube is hot or when it is to be cut near the end.

Fig. 7.

Tubes larger than 1/2 inch in diameter require a different technique. After being scratch-marked with the file, they may be cracked by applying the tip of a small piece of glass cane, made incandescent in the flame, to one end of the file mark. The crack thus produced may or may not completely encircle the glass. If not, it can be made to do so by leading it with repeated applications of the glowing cane tip, each application being just ahead of the end of the crack.

A tube or bottle of several inches in diameter is cut by first filing a narrow scratch-mark around its circumference. A piece of stiff paper or cardboard may be used to guide the file in making this mark. The wire of the device shown in b Fig. 3 is adjusted to fit in the mark. The ends of the wire a must not touch. An electric current is passed through the wire, heating it to a red heat for a few seconds, and water is applied to the scratch-mark and wire with a pad of wet cotton. This procedure will produce a clean crack around the circumference. Small irregularities in the crack may be removed by grinding on a brass plate with Carborundum grits, or after the glass has been softened in the flame they may be pulled off with forceps or cut off with shears.

Cleaning. Good welds cannot be made with contaminated glass. Therefore, the first operation after cutting should be cleaning. Sometimes washing with water is sufficient, but nitric acid may be substituted if necessary. In extreme cases, hot chromic acid "cleaning solution" may be required. Water used to rinse glass tubing is removed from the outside with a clean cloth and from the inside with a wad of cotton pulled through with a string or blown through with air. Or, if distilled water is used, the tube may be dried by drawing air through it with a water aspirator and by warming it gently at the same time.

Preheating. Glass tubing and especially large glass apparatus must be preheated carefully before they can be safely exposed to the local intense heat of the cross-fires or hand torch. By one procedure for preheating, the work is first exposed to the relatively cool flame of a Meker burner with the air shut off. As the glass temperature rises, more and more air is admitted to the Meker burner, giving a hotter and hotter flame, until finally, when the work is thoroughly heated, it is safe to expose it to the intense heat of the cross-fires or blast burner. By an alternative procedure the work is exposed to the heat of the cross-fires for a fraction of a second, after which it is quickly withdrawn to allow temperatures to equalize, and then after a few seconds another section of the work is exposed. This operation is repeated in such a way that the temperature of the work as a whole is uniformly elevated. The exposure to the flame is increased and the interval outside the flame decreased as the heating progresses, until the work is brought to a temperature at which it distills enough sodium vapor to make the flame yellow. This sodium test usually indicates a temperature at which it is safe to begin the operations of shrinking, blowing, molding, and so forth. Some things, such as tubes, require preheating only in the zone around the region to be worked.

Fig. 8.

The rotation of the work. Rotation of work is a fundamental operation. It should be executed uniformly and with good coordination of the two hands. Glass properly rotated in the flame becomes uniformly soft, and the effect of gravity on it is symmetrical.

The lower surfaces of hot glass cool more rapidly than the upper surfaces. For this reason it is also important to continue uniform rotation even after the work is removed from the flame.

The beginner will have difficulty manipulating the work in the flame, particularly after the glass connecting the two parts on either side of the flame becomes soft, when one may "tie up" the work. To avoid this, one should practice rotation with a model consisting of two glass tubes connected with fairly heavy cloth. One should be able to rotate these in the manner shown in Fig. 8, so that the cloth does not wrinkle or twist and is under neither compression nor tension. Then you'll be ready to begin operations with the flame.

Fig. 9.

The work is manipulated by the thumbs and forefingers so that, despite differences in diameter, the sections of the work on either side of the soft zone in the flame are rotated in synchronism, the motion consisting of a series of angular displacements of about 45 degrees. The left hand always handles the heavier section of the glass, while the right manipulates the section beyond the soft zone. The right hand has the more delicate though lighter task, since it must rotate its section in phase and without undesired stretching or compression relative to the main section of the work. The hands are held as shown in Fig. 8 to facilitate the application of the right end of the work to the lips for blowing.

Fig. 10.

Bending tubes. A tube to be bent is heated in the cross-fires with continued rotation until it is quite soft along a length equal to several diameters. It is then removed from the flame and bent to the desired angle with the apex down as shown in Fig. 9. As large tubes are difficult to heat uniformly, imperfections often occur. They are also present in small tubes, particularly in small, thin-walled tubes, that have been bent to a sharp angle. Imperfections are worked out in every case by local heating with a pointed flame. When one portion of the tubing wall is heated until it is soft, the general form of the bend is maintained by the portion on the opposite side of the axis of the tube. If the outside tends to flatten as shown in Fig. 9(b), it is corrected by' blowing while the glass is soft. If the inside surface folds' as shown at (c), it is locally heated with a sharp pointed flame and worked by alternating shrinking with blowing until it is uniform. These corrections are followed by a general heating to anneal the whole bend.

A glass coil is made on a mandrel. The mandrel is usually either a steel or brass tube covered with asbestos paper. The paper is applied wet, the ends being lapped and cemented with sodium silicate. After the paper is dry, this lap joint is sandpapered. One or more coats of stove polish or some other | form of carbon will prevent the glass from adhering to the asbestos. Notches in the end of the tube secure the coil to the mandrel. The procedure is illustrated by Fig. 10.

Fig. 11.

Shrinking. Since softened glass is a liquid, its surface tension tends to deform it in such a way that the total surface is decreased. Shrinking at elevated temperatures is restrained by the viscosity of the glass, and this restraint is greater at the lower limit of the working range. Shrinking may yield both desirable and undesirable changes in the work and it is controlled by the use of spinning tools and by blowing into the work. Fig. 11 shows the use of forceps to counteract the undesirable tendency of the end of a tube to decrease its diameter by shrinking, while the desirable effect of increased wall thickness is achieved.

Annealing. The annealing of complicated and elaborate work is one of the most difficult operations in glass blowing. It is also an important one, since, if the work is not properly annealed, it may break in cooling or, what is worse, fail after it is put into operation. The purpose of annealing is to bring the glass from the working temperature to room temperature with the introduction of a minimum amount of strain. Annealing is properly executed when all parts of the work are maintained at a uniform temperature while the glass is gradually cooled. Large, complicated work should be annealed in a suitably regulated oven. Small work in which the wall thicknesses are uniform can be successfully annealed either with a Meker burner or in the cross-fires.

When the manipulations have been completed, the work is heated until it is above the annealing temperature. The temperature is then gradually lowered by applying the procedures of preheating in reverse order. It is important that the temperatures be kept uniform during the cooling by special extra applications of heat on those parts which tend to cool more rapidly, either because they are thinner or because they are subject to greater heat losses by radiation and convection. When the temperature is judged to be well below the strain point, the work may be set aside for final cooling in a place free from drafts.

Fig. 12.

Pulling a point. "Pulling a point" is a technical term used by glass blowers indicating that a tube is heated in the flame and drawn out as illustrated in Fig. 12 to give a "point," which is usually some 6 inches in length. The point may have several functions. It may serve as a handle for rotation or, with the tip removed, as a mouthpiece through which to blow; or it may afford a means of closing the work. Also, pulling a point is a preliminary to several other operations.

We will assume, for the purpose of our discussion here, that a section of tubing is required with points on both ends as an element of some apparatus under construction, and furthermore that this is to be obtained from a longer stock tube. First, a point is pulled on the end of the stock tube. If it is long, the stock tube may be supported on the left by a V-block as shown at (a) in Fig. 12. After preheating the tube by the second procedure outlined above, it is softened a few diameters back from the end. Then the glass is gathered together at the tip with forceps; the work is removed from the flame, and with continued rotation the soft glass is drawn out as shown at (b). The capillary section is fused in the middle as shown at (c), or, if the point is to serve as a mouthpiece, it may be cut and fire-glazed by momentary exposure to a flame.

Fig. 13.

The tubing is then heated until it is soft at a suitable distance back of the first point, and the desired section is drawn off, forming at the same time the, second point.

It is important to have the walls of the point symmetrical about the axis of the tube. Errors may be corrected by heating the shoulder of the point until it is soft and manipulating the glass from the end of the capillary. It is advisable to work the glass at a low temperature when making corrections.

Closing a tube. Pulling a point is the first operation in closing a tube as shown in Fig. 12(d) to (h). The point is removed with a sharp flame as shown at (d) and (e). Excess glass at the tip is removed with forceps or with a piece of cane (f), and the end is then heated to shrink it (g); then it is blown to the final hemispherical shape (h). The hand torch is usually used for this operation.

"Cutting" a tube in the fire. The first step in "cutting" a tube in the fire is to pull a point. Again the point is removed as described above, Fig. 12(d) and (e), and excess glass removed, Fig. 13(a). The end is then heated (b) and blown with a strong puff to yield a thin kidney-shaped bulb (c), which is broken off with the file or forceps as shown at (d). The edges are now heated to shrink them and thicken them to the size of the tubing elsewhere (e). The diameter is increased by a spinning process and the use of forceps as in (f) or flanging tool as in (g). If forceps are used, they are introduced and allowed to expand slowly as the glass is rotated in the fire. The end of the tube is then squared with the carbon plate (h). If a flange is required, the end is spun out with the arrowhead spinning tool and squared with the carbon plate as shown at (h) and (I). Metal spinning tools are wet with beeswax to prevent sticking to soft glass.

Fig. 14.

Preparations for making joints. Thorough cleaning of the glass tube and careful attention to the preliminaries of cutting, flanging, or drawing and expanding it facilitate the manipulations in the flame. A common fault in the beginner is thinking that it is easily to correct deficiencies in these operations after the work is in the flame. Good glass blowers do not handicap themselves by carelessness with these preliminaries.

Fig. 15.

The elements that are to be welded to form a joint must have approximately the same diameter and wall thickness. If a large tube is to be joined to a smaller one, the large tube is first prepared as shown in Fig. 14(a) by pulling a point on it and then cutting off the point in the flame where the shoulder has the same diameter as the small tube.

A capillary or thick-walled tube is prepared as shown at (b). It is heated to softness and blown until it has the proper wall thickness and then pulled until it has the same diameter as the tube to which it is to be sealed.

A bulb or cylinder to which a small tube is to be joined is first preheated. A soft flame is then directed on the place selected for the joint until it is soft, and a slight bulge is blown as shown in Fig. 15(a). This bulge is strongly heated at its apex with the tip of a sharp flame as at (b). Then, after removing it from the flame, a small thin-walled bulb is blown as at (c). This is then broken off with the forceps or file. The edges of the hole thus made are softened with the flame, flanged with the carbon taper, and squared with the carbon plate as shown in Fig. 15(d) to (h).

Fig. 16.

A straight tube is prepared for making T's by opening the side as described above. When several T's are to be made, a holder for the straight-through tube as shown in Fig. 16 is convenient. Y's are made by first bending a tube to an acute angle. This is then opened at the apex as shown in Fig. 17.

Fig. 17.

Making a joint. The elements are heated with rotation in a flame whose diameter is approximately the same as the diameter of the tubes. They are arranged facing each other, as shown in Fig. 18, with the axis of the joint perpendicular to that of the flame. When the tubes are thoroughly soft at their ends, they are removed from the flame and touched together at right angles as shown at (b). This contact is used as a hinge to steady the hands while the tubes are brought into exact register and pushed together (c). With continuous rotation, the joint is held in the flame until it shrinks to a uniform outside diameter (d). It is then withdrawn from the flame and blown out until it has a uniform wall thickness (e), and stretched at once to a uniform outside diameter (f). Since it is necessary to blow a joint, obviously all openings except the one applied to the lips must be temporarily closed.

Fig. 18.

Large tubes that are to be joined must have flanges. When it is necessary to make a joint on apparatus which cannot be rotated, the squared ends of the elements of the joint are accurately fitted together and heated, a section of the circumference at a time. The welding of the flanges is effected with the heat of the hand torch and pressure applied with the forceps working around the circumference as shown in Fig. 19. After this the joint is locally heated, a small section of the circumference at a time, until it is soft, and the softened area is worked by alternate shrinking and blowing until the wall is smooth. Then the while circumference is uniformly heated for final blowing, alignment, and annealing.

Fig. 19.

Ring seals. When a tube is inserted in a bulb or a larger tube, a ring seal joins the tubing wall to the edge of the aperture in the bulb or large tube. First, the glass around the aperture is accurately molded with the carbon taper until it is slightly larger than the outside diameter of the small tube to be inserted. The small tube is prepared for the seal by heating a narrow zone around its circumference with a pointed flame and swelling it as shown in Fig. 20(a). This is accomplished by blowing and simultaneously applying a longitudinal compression. The small tube is then inserted and held exactly concentric with the larger tube by an improvised support, such as a roll of asbestos paper, as illustrated in Fig. 20(b). The place to be sealed is exposed to a pointed flame with continued rotation until the glass at the ring is soft. Then the weld is made by pushing the swelling of the smaller tube against the constricted opening of the larger tube. The work is removed from the flame, blown, and aligned, while at the same time the small tube is given a slight pull. Fig. 20 shows the construction of a water aspirator which requires two ring seals. A tapered wooden dowel which just slips into the first tube centers the second while it is being sealed. Ring seals require careful annealing.

Fig. 20.

Another procedure for ring seals, particularly suited for inserting a small tube through the side of a larger tube, is illustrated by Fig. 21. The inner section of the insert is flanged and molded to conform to the inside wall of the large tube and is supported in contact with it as shown at (a). The area of the outside wall of the large tube opposite the place where the section makes contact on the inside is then heated until the two tubes are sealed together. A bulge is blown and opened with a sharp flame at the center of this seal as shown at (b). The opening is molded and a small side tube is joined to the edges of it to form a continuation of the inner section as shown at (c) and (d).

Fig. 21.

Blowing bulbs. Difficulty may be experienced in making large bulbs (of 2 inches in diameter or larger), for which it is necessary to heat heavy masses of soft glass to a uniform temperature in the flame. Also, the work must be skillfully managed outside the flame to make the effects of air cooling symmetrical. Because of these difficulties it is advisable to use commercial balloon flasks for bulbs rather than to make them from tubing. Small bulbs, less than 1 inch in diameter, are not so difficult to make.

Fig. 22

The first operation in making bulbs is to heat the end of a glass tube until the glass collects as shown in Fig. 22(a). As glass collects, it is alternately blown out and shrunk to distribute it uniformly until enough has collected for the final bulb. The collected glass is then heated to a uniform temperature and removed from the flame.

After the work has been rotated a few seconds about a horizontal axis, it is expanded by blowing through an appropriate mouthpiece. The blowing is gentle at first and stronger as the glass stiffens. The work is continuously rotated. However, if one portion of the surface tends to expand more rapidly than the other portions, it is turned down and cooled to restrain its expansion, since the under side of the work cools most rapidly.

To blow a bulb in the middle of a tube, the operation of collecting glass, as described above, is carried out in zones until several adjacent ones are obtained as shown in Fig. 23(a) to (c). Then, by blowing and shrinking, these are combined in a single uniform zone (d). This is well heated, removed from the flame, allowed to cool a moment, and blown to the desired form (e).

Fig. 23.

Constrictions. Two types of constrictions may be required. One, useful for preventing kinetic overflow of mer-cury in a manometer tube when the pressure suddenly changes, has a constricted inner wall but uniform outside di-ameter. The second type, useful as a "seal--off" for a vacuum system, has a uniform wall thickness. To make either type, the glass tubing to be constricted is heated and worked until the glass walls thicken. This operation is essentially the same as the preliminary operation for blowing a bulb in the middle of a tube as shown in Fig. 23(a). After the walls have been thickened, the glass is removed from the flame, and the tube is rotated and pulled instead of blown as for a bulb. To get a constriction of the first type, the tube is pulled until the outside diameter of the tube is uniform, while to get a "seal-off" constriction the tube is pulled until the wall thickness is uniform.

Correction of errors. Owing to errors of manipulation, the walls of glass apparatus frequently are not uniform. This lack of uniformity not only detracts from the workman-like appearance of the finished apparatus but also increases the difficulty of annealing, since the thick and thin portions tend to cool at different rates, a circumstance which causes strain in the glass.

Fig.24.

Excessive glass can be drawn off from a region in the walls of an apparatus by using a piece of cane as illustrated in Fig. 13(a). After the required amount of glass is drawn off, the region is worked by blowing and shrinking until the wall thickness becomes uniform. Also, if the wall of a region is too thin, glass can be added from a piece of cane and worked out smooth by blowing and shrinking. Holes may inad-vertently appear in the work. They are closed by drawing their edges together with a piece of cane.

Platinum seals. Formerly, the only satisfactory method of making a metal-to-glass seal was by the use of platinum and soft glass. Such seals are rarely used now because of the high price of platinum. Also, hard glass, which seals directly to tungsten, is now used extensively for making laboratory apparatus. However, a plat-inum tube may be required to introduce pure hydrogen by diffusion into a glass apparatus. For this and other special pur-poses, platinum-soft glass seals are required. Fig. 24 shows a platinum electrode in a soft-glass tube. In making this platinum seal, a small bead of soft glass (either lead or soda glass) is first fused to the platinum wire. The bead and wire are heated to about 1000C. to obtain a good glass-to-metal bond. Then the bead is sealed into the wall of the tube as shown in the illustration.

Fig. 25.

Tungsten-glass seals. Tungsten wires may be sealed . through Pyrex if their diameters are less than 0.060 inch, Larger tungsten wires, to diameters of twice as much, are first sealed in a sleeve of Nonex glass, which in turn is sealed into the wall of glass apparatus. This latter operation is necessary, especially if the seal is to be exposed to the heat of a baking-out oven. Nonex glass has a lower softening temperature than Pyrex, and between the strain point and room temperature the total thermal expansion of Nonex is almost equal to the expansion of tungsten for the same tem-perature interval.

Fig. 26.

A tungsten wire is prepared for sealing through glass by heating it to a white heat in the gas flame. If this is not done, bubbles appear at the surface of the seal. The surface of the tungsten is cleaned for sealing by heating and touching it with a piece of potassium or sodium nitrite. The tungsten is then washed, and a short sleeve of Pyrex (or Nonex, depending on the size of the wire) tubing is fused to it as shown in Fig 25(a). The intense heating required to shrink the glass should be started at one end of the sleeve, so that the shrinking progresses from that end. This avoids trapping air bubbles between the metal and the glass. The interface between glass and tungsten is red, because oxide on the surface of the tungsten dissolves in the glass and dyes it. After the sealing operation between glass and metal is finished, the sleeve is welded into the apparatus as shown at (b). In making metal-to-glass seals, it is important to cool the glass slowly to avoid excessive strain.

Fig. 27.

Tungsten wire is frequently fibrous, having longitudinal channels which may leak if it is sealed into a vacuum apparatus. To avoid such a possibility, the tip of the tung-sten should always be closed by fusing nickel or advance wire over it. The nickel or advance tip also affords a place for attaching copper wires. Copper can be fused to these tips, whereas it cannot be easily welded to tungsten directly.

Discharge tube electrodes are made from coiled aluminum wire of about 3/32 inch in diameter and a tungsten-Pyrex seal as shown in Fig. 26. The aluminum-wire projection of the coil is fused to make the connection to the tungsten wire. The tungsten wire with a nickel enlargement to secure it in position is pushed into the fused aluminum. The projection is wrapped with copper foil to preserve its form. After the aluminum has solidified, the copper foil is removed. A glass sleeve, shaped as illustrated, is then sealed to the tungsten. This sleeve fits the aluminum projection and affords additional support for it.

Fig. 28.

Copper-to-glass seals. It is possible to seal copper to Pyrex or soft glass by the technique developed by W. G. House-keeper.¹ The copper has a much larger coefficient of thermal expansion than either type of glass╤it is the arrangement of the seal which prevents the glass from breaking. When the copper is thin, it is deformed to absorb differences between its expansion and that of the glass, a circumstance made possible by its high ductility and low yield point. The construction details of various , seals developed by Housekeeper are shown in Figs. 27 and 28. For the constructions shown in Fig. 27 it is important to prevent the glass from passing over the rim of the copper. The seal shown in Fig. 28 is made with a copper wire which is hammered out to have thin sharp edges. Care is necessary in heating the Pyrex to avoid melting the copper.

Fig. 29.

Kovar and Fernico.² The rate of expansion of glasses increases near their softening temperatures, as Fig. 4 shows. On the other hand, the thermal expansion for most metals is nearly linear. However, the expansions of two new alloys, Kovar and Fernico, closely duplicate the expansion of some of the commercial glasses.³ These alloys yield metal-to-glass seals which are unstrained under all annealing conditions, and they may be sealed to appropriate glasses without any of the special procedure required for Housekeeper seals. Large seals of 4 inches in diameter and 1/8 inch in wall thickness have been made between Kovar and 705 AJ glass. Such seals as the ones shown in Fig. 29 have made modern all-metal radio tubes possible.

Fig. 30

Porcelain-Pyrex seals. Porcelain, particularly the grade known as Insulite,⁴ may be sealed directly to Pyrex in small diameters (less than 1/2 inch),.'or it may be sealed to Pyrex in large diameters with an intermediary glass ring of Nonex as shown in Fig. 30.

¹ Housekeeper, W.G., Electrical Engineering, 42, 954 (1923).

² The fundamental study of expansion properties of Fe-Ni-Co alloys, on which this kind of metal-to-glass seal is based, was made in the Westinghouse Research Laboratories by Howard Scott, Technical Publication 318, American Institute of Mining and Metallurgical Engineers (1930). These alloys are manufactured under U. S. Patent 1,942,260, held by the Westinghouse Electric and Manufacturing Company. Further information is contained in Scott, Howard, Frank. Inst., J., 220, 733 (1935); Burger, E. E., Gen. El Rev.; 37, 93 (1934); and Hull, A. W., and Burger, E. E., Physics, 5, 384 (1934). The Westinghouse product, called Kovar, is obtainable from the Stupakoff Laboratories, 6627 Hamilton Avenue, Pittsburgh, Pennsylvania. Fernico is obtainable from the General Electric Company, Schenectady, New York.

³ According to A. W. Hull, "Fernico is capable of existing at room tempera-ture in either the gamma face-centered phase, or in the alpha, body-centered phase. When annealed irom 900 or more, it has the face-centered structure and the characteristic low expansion, and is stable in this condition at any temperature above╤40C. Exposure to liquid air temperature or me-chanical strain will transform it into the alpha phase, which has a different expansion and is to be avoided." According to Mr. Scott, "To obtain the desired low and reversible expansion characteristic of Kovar and Fernico, their composition is so adjusted that transformation from the gamma to alpha phase occurs between╤80 and ╤180C. Seals, however, cannot be cogled safely below╤40C. because of the progressively increasing expansion between metal and glass on cooling below room temperature. Special compositions can be made which make possible cooling to somewhat lower temperatures."

⁴ Insulite is obtainable from Stupakoff Laboratories, 6627 Hamilton Avenue, Pittsburgh, Pennsylvania.