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Tricks Of The Trade

by Rachel Petkewich
January 16, 2006 | A version of this story appeared in Volume 84, Issue 3


Tricks Of The Trade


Glass is a generic term for a fused-silica material with many different compositions. Beverage bottles and some art glass are made from a soft glass known as soda lime. For most scientific glassware, low-expansion borosilicate glass is used, because it is inert to most chemicals and can withstand dramatic changes in temperature. Various manufacturers sell this kind of glass under different brand names including Kimble's Kimax and Schott's Duran.

Corning has been making Pyrex glass since 1915 by heating sand and borax to 1,600 °C. Directly from the furnace, honeylike molten glass is shaped into finished pieces such as beakers or flasks or into intermediate forms such as tubing.

Scientific glassblowers, also called lampworkers or glassworkers, manipulate the tubing to make catalog or specialty apparatus.

Tubing. Glass tubing is prepared from molten glass that continuously flows from a furnace orifice onto a steel rod called a mandrel. The glass is pulled along a 100-foot horizontal rolling conveyor before it is cooled and cut into 48-inch sections. The tube’s dimensions are controlled by pull rate, temperature, and air pressure.

Beakers and Erlenmeyer flasks. In the U.S., these pieces are made by a high-speed, automatic blowing machine. A ball of molten glass is delivered to the continuously operating machine, which forms the required article at a rate of 20–30 pieces a minute, according to Jeffrey C. Andes, manager of operations strategies at Corning.

Flamework. Also known as lampwork, flamework involves manipulating intermediate forms of glass with a torch and other special equipment. For example, to add a ground-glass joint to a machine-blown, round-bottom flask “blank” (an intermediate form that is essentially the round part of the flask), a glassworker must skillfully attach the joint with an intense flame. Just as welders do, glassblowers use various types of torches. At the bench, glassworkers use standing burners and hand torches to add multiple necks to smaller round-bottom flasks or to make specialized apparatus. Rotating stands called lathes hold large pieces of glass.

Annealing. After extensive flamework, glass should spend a few minutes in a “warm-out” fire to prevent cracking. To remove the remaining strain of heating, completed pieces must be uniformly heated at approximately 560 ºC for about an hour in an oven known as a lehr, which is similar to a kiln for firing clay or ceramics.

Small glassware. Pieces smaller than 125 mL emerge from tubing reworked with machine blowing or hand tooling. Take a separatory funnel as an example. In the machine process, the bulb is formed by blow-molding hot tubing into the funnel shape and sealing it on a ground-glass neck and stopcock assembly with automated equipment. This kind of manipulation is called tooling, and it’s a labor-intensive method whereby tubing is shaped with handheld cast-iron molds. A separatory funnel requires two different molds: one for the bulbous section and a second for the neck. The glassblower squeezes the bulb tool around the red-hot glass, reheats the glass, and applies the neck tool. The stopcock is attached with a flame.

Large glassware. Larger sizes of glassware, between 2 and 22 L, are “done in the old traditional manner of the 1800s,” Andes explains. A glassworker will reach into the furnace forebay with a blowpipe; gather a ball of glass on the end of that blowpipe; and, working with a shop team, blow that glass glob into a cast-iron mold. This raw shape is then modified with flamework to fit the desired specifications, such as joints and flanges.

Ground-glass joints. Tubing is cut to length with a diamond saw and then the joint is formed by flamework and tooling. Although a grinding tool aids in initially grinding inner joints, up to four additional grinds—most applied by hand with different kinds of wet grit—are needed to make each joint a precise and uniform size. Outer joints are first tooled from tubing in a manner similar to that used to make small glassware, and then the inside is ground down by hand with various grits.

Calibration. The utility of certain glassware, such as volumetric flasks, graduated cylinders, or burettes, depends on precise volume control and may be certified to nationally recognized standards. Although the pieces are made to a standard size and shape, they may have small but important inconsistencies. Each piece is individually measured and marked in a climate-controlled environment. That explains why two 100-mL volumetric flasks, for example, could have meniscus marks at slightly different points.

Fritted disks. These disks are often found at the bottom of a column or filter funnel. Glass particles, which can vary in size from as fine as talcum powder to as coarse as granular sugar, are formed into white disks with different porosities. Corning crushes glass to bits. Ace Glass pulls the glass into threads and then crushes them to the appropriate size. This method, Ace says, ensures a more uniform-porosity and abrasion-resistant surface. Both companies sort the particles for uniform particle size, pack the particles into a mold, and fire them in an oven just hot enough to sinter the disks.


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