Materials in the Lab

1.1 Glass

1.1.1 Introduction

When one talks of chemistry (in general) or working in a laboratory (in specific), the average person usually envisions an array of glassware—test tubes, beakers, coiled tubes, and the like. This list obviously is not the sum and substance of laboratory equipment (or chemistry for that matter), but items made of glass will always play a predominant role in most laboratories.

Glass is used in the laboratory because it has three important properties. First, it is transparent and thus allows the user to observe a reaction taking place within a container. Second, it is very stable and is nonreactive to many materials used in the laboratory. Third, it is (relatively) easily malleable, allowing new designs and shapes of apparatus to be created, produced, and, if broken, repaired.

The nature of glass, its structure, and its chemistry, as well as the shapes and designs made from it, have come from years of development. The standard shapes of the beaker, Erlenmeyer flask, and round-bottom flask were each developed to meet specific needs and functions. Variations in wall thicknesses, angles, and height-to-width ratios are all critical for specific functions of each container.

Before discussing any container, however, it may be best to talk of the containment material: what it is, what makes it unique, and what are its strengths and weaknesses. We can then relate these properties to specific types of containers and know why they are made as they are. By understanding the shape, design, and function of laboratory glassware, one can achieve greater efficiency and proficiency in the laboratory.

1.1.2 Structural Properties of Glass

The standard definition of glass is a "noncrystalline solid." In 1985, the ASTM (American Society for Testing and Materials) referred to glass as "... an inorganic product of fusion that has cooled to a rigid state without crystallizing." However, since 1985, glasses of organic and metallic materials have been produced. There-

Materials in the Lab

Silicon

Fig. 1.1 The molecular structure of a quartz crystal and quartz glass.

fore, a more accurate definition of a glass would be "any material that is cooled fast enough to prevent the development of crystals." Any further reference to the term "glass" in this book refers specifically to the inorganic glasses that we normally associate as glass. As we shall see, this specific type of glass has properties that facilitate the creation of the "glass state."

The key structural concept of glass, is that it is not a crystal. This concept holds true for high-temperature quartz glass and for ornate lead crystal glass. A crystal, by definition, is a collection of atoms in a repeating sequence and form that develops or has symmetry in its structure. By comparison, the structure of glass at the atomic level is a three-dimensional web of interconnecting oxides of materials, frozen in place by rapid cooling. Thus, glass made of inorganic materials may be defined as a super-cooled liquid composed of a mixture of oxides in a solution.

To better understand what glass is, and why different glasses have different properties, one should first compare crystals and glass at the molecular level. This comparison begins with the molecular structure of a quartz crystal. The quartz crystal is composed of (essentially) pure SiO2, the same molecular composition of quartz glass, and the chief component material of most glass. The quartz crystal is a silicon tetrahedra composed of one atom of silicon surrounded by four oxygen atoms, in a tight three-dimensional network of high-energy bonds. The extra two oxygen atoms, bridging oxygens, interconnect the silicon atoms into a threedimensional network. The left side of Fig. 1.1 illustrates a two-dimensional representation of one plane of such a network.

If you take a quartz crystal, heat it until it melts, and allow it to rapidly cool, the even network of rings is broken into nonuniform irregular chains (see right side of Fig. 1.1). These glass chains have no discrete crystalline form as they interconnect throughout the entire glass item. The cooled SiO2 is now called either "fused quartz," "fused silica", or "quartz glass." The bonds between the Si and O atoms are still high-energy bonds, and tremendous amounts of thermal energy are still required to disturb and break those bonds. One can also see from Fig. 1.1 that the interstice regions in the glass form of SiO2 is much larger than the crystal form.

glass scientists define important transitional changes based on specific viscosity ranges of the glass in question. There are four significant viscosities of glass (in comparison to the viscosity [poises] numbers presented below, the viscosity of glass at room temperature is 1022+ poises):

1.Strain Point (~10145 poises) Anything above this temperature may cause strain in glass. Internal stresses may be relieved if the glass is baked in an oven for several hours at this temperature

2.Annealing Point (=1013 poises) If an entire item were uniformly baked at this temperature, the item would be relieved of strain in about 15 minutes. The annealing range is considered to be the range of temperatures between the strain point and the annealing point.

3.Softening Point (=107 6 poises) Glass will sag under its own weight at this temperature. The surface of the glass is tacky enough to stick (but not fuse*) to other glass. The specific viscosity for the softening point depends on both the density and surface tension of the glass.

4.Working Point (=104 poises) Glass is a very thick liquid at this point (like honey) and can be worked by most conventional glassblowing techniques. At the upper end of the working range, glass can be readily fused together or worked (if the glass is heated too high, it will boil and develop characteristics that are different from those of the original glass). At the lower end of the working range, glass begins to hold its formed shape.

The fact that glass is a solid at room temperatures should not be underestimated, and the statement "solids cannot flow" cannot be over emphasized. There is a romantic notion that windows in old churches in Europe (or old colonial homes in the United States) have sagged over time. The common belief that the glass is thicker on the bottom than on the top because of such sagging is incorrect. Studies by F.M. Emsberger, an authority on viscoelastic behavior of glass, showed that glass will not permanently sag under its own weight at room temperature.

Ernsberger took several V4" glass rods and bent them 1.7 cm off center over a 20-cm span. The amount of bending stress was calculated to be about 150,000 lb/ in.V significantly greater than any stress received from simple sagging. After 26

*Metals can be welded or stuck together so that they cannot be separated. Glass, on the other hand, must be fused by heating two separate pieces of glass to the point at which they flow together and become one piece. If glass pieces are simply "stuck" together, it is relatively simple to break them apart.

+This amount of stress without fracture was accomplished by covering just newly made glass with lacquer, which prevented surface flaws and surface hydration. Normally, glass cannot receive this amount of stress without breaking.