kind should get on the surface. Also, temperatures above 1200°C should be limited as much as possible.

Note that devitrification is a nucleation process. That is, devitrification does not confine itself within the area where it begins. Rather, once started, it spreads like mold on a piece of fruit. Thus, a drop of tap water, a fingerprint, or any other localized contaminant can initiate devitrification, which can then spread over the entire surface of the container.

Quartz is not the only glass that is prone to premature destruction due to devitrification. Aluminosilicate glass (see page 14) used on halogen lamp bulbs are also prone to this problem. Handling halogen lamp bulbs with bare fingers can significantly decrease their operation life by depositing fingerprint oils on the surface. To maximize the life of halogen lamp bulbs, they should only be handled with cotton cloves. Although cleaning the bulbs after handling will help, it is best not to handle them at all.

1.1.5 Different Types of Glass Used in the Lab

Different properties of glass can be exploited by combining different oxides in a glass mix. There are thousands of different commercially made glasses (although many of these glasses overlap in type and characteristics). The glass used in a laboratory can be divided into three major categories: soft glasses, hard glasses, and high-temperature and UV-transmission glasses. Table 1.2 in the next section lists selected properties of these various glasses.

Among the properties of glass that are of particular interest to glass scientists are its expansion properties (the amount of size increase in relation to temperatures increases, refractive index (how light passes through the glass), viscosity (the consistency of the glass as temperature increases), and its dielectric properties (how electrically resistant the glass is at various temperatures).

One of the problems with glass composition and its relations to its properties is that many of the properties change depending on its temperature. For example, as glass gets hotter, it becomes less electrically resistive while at the same time permeation rates increase. Thus, one may need to consider the temperature the glass will be used in while selecting a glass for specific purposes.

When creating a glass, one has to watch all the properties. This is because as you increase one material to obtain certain desirable properties, other properties are likely to change which may or may not be desirable. For example, by adding PbO to silicate glass, there is an increase in viscosity while at the same time there is a decrease in electrical resistivity.9

This section occasionally lists the contents of glass types. The glass industry typically lists the component chemicals of glass in their oxide states as percentages of total weight. Chemically speaking, this practice does not make sense because when you add CaO and Na2O to SiO2, you are actually adding Ca + 2Na + Si + 4O. Chemists do not consider this industrial approach proper because it does not look at the reaction from a proper chemical perspective. Regardless, I

spot easily and are difficult to clean may be the result of a low lime or high alkalinity concentration glass rather than bad soap or hard water.

An extreme example of a glass that can be easily weathered is sodium silicate, also known as soluble silicate or, informally, waterglass. The sodium content in sodium silicate is so high that the glass can be dissolved in water and shipped as a liquid glass. Sodium silicate is used as an adhesive, cleaner, and protective coating material.

Sodium silicate is stored and shipped as a liquid, but when exposed to air, it will turn into a hard, glass-like material. In reality, it is a glass in a totally anhydrous form; the sodium hydroxide content is 34% by weight, and the remaining 66% is silicon dioxide. These proportions are theoretical, however, as an anhydrous sodium silicate is not really possible because it would always be absorbing water from the air.

When sodium silicate is shipped, its contents are typically

These proportions are all approximate because the water percentage can vary significantly. Once the water has evaporated from the sodium silicate, there is still a high level of water left behind. The dried sodium silicate will have the following approximate contents (by weight), although the actual percentages will again vary depending on the atmospheric water:

Note that although dry soda content would normally be listed as Na2O, in sodium silicate all the soda is hydrated and, therefore, is identified in the form NaOH.

Many disposable glass laboratory items are made of soft glass. If a laboratory glass item does not specifically bear one of the words Pyrex®, Kimax®, or Duran® in printed or raised letters, it is likely to be a soda-lime type of glass.

Beginning chemistry students usually receive their first glassblowing lesson using soft glass tubes over a Bunsen burner to make bends or eye droppers, because only soft glass is malleable at Bunsen burner temperatures. Thus, if you have a glass tube soft enough to bend over a Bunsen burner, it is a soft glass. Currently, only Wheaton Glass and Schott glass make soda-lime tubing. Wheaton's glass can be purchased from either Wheaton or Friedrich & Dimmock. Schott's soft glass (AR) can be purchased from Schott or from Glass Warehouse. (See Appendix C for addresses and phone numbers.)

The other common soft glass is lead glass. Commercially, we see this glass in lead crystal glass, where 22-25% (by weight) of the component material is PbO2. Although misnamed as a crystal, lead crystal glass nevertheless has beautiful opti-

cal properties that are often associated with crystals. The high refractive index of lead glass (one of the highest refractive indexes of all glasses) is the foundation for its effects with light. The other important property of lead glass is its high electrical resistivity. A typical lead oxide glass may be 100 times more electrically resistive than a soda lime glass. A side property of lead crystal glass is its resonance: Decorative bells of lead glass have a beautiful chime, while those of borosilicate glass often have a dull, dead "clank."

One additional application in the laboratory for lead glass is as a shield to protect workers from the harmful rays produced by x-ray machines. For example, the glass window behind which an x-ray technician stands is made of lead glass. These windows can be made of up to 75% PbO2. This greater quantity of lead oxide in this glass makes the glass three times heavier (6.0 gm/m3) than standard laboratory borosilicate glass (2.1 gm/cm3).

Lead glass is very vulnerable to impact abrasion (it is almost twice as soft as soda-lime) and can be extremely sensitive to temperature changes (temperature induced failure is common in lead glass). On the other hand, as previously mentioned, lead glass is among the most electrically resistive glasses available. Because of that property, lead glass is one of the chief glasses used in electrical components. Neon signs and TV tubes are typically made of lead glass. (Borosilicate glass is being used more and more often for neon work.)

Another term for lead (or lead-potash-silica) glass is flint glass. This name can be confusing, because the term has two other meanings in the glass industry: It is used by the container industry to connote colorless glass. The term flint glass is also used by the optical glass industry to connote glass that has a high refractive index and dispersion* of optical rays. (Soda-lime [or potash-lime-silica] glass has a low refractive index and dispersion of optical rays. It is called crown glass.)

In addition to soda-lime and lead glass, there are several "speciality" soft glasses in the laboratory. One of the more notable is Exax® made by the Kimble Corporation. This glass repels a static charge, making it particularly useful for holding or weighing powders.

The biggest drawback with all soft glasses is that any apparatus made with these glass types are essentially nonrepairable. Because of their relatively high thermal coefficients of expansion, they are likely to shatter when the flame of a gas-oxy- gen torch touches them. Additionally, replacement soft glass components such as stopcocks and joints are generally not easily available. Thus, unless an item has particularly unique qualities or value, the cost of component repair is far greater than wholesale replacement. Although soft glass items may be less expensive ini-

*The dispersion of glass is related to its index of refraction and is based on an analysis of the passage through the glass of a yellow helium line (587.6 nm), blue and red hydrogen lines (486.1 and 656.3 nm, respectively), and green mercury line (546.1 nm).

tially, their long-term costs can be much greater than the more expensive (but less expensively repaired) borosilicate ware.

The Hard Glasses. Borosilicate glass,* the most common glass found in the laboratory, is one example of a hard glass. It is considered a hard glass for two reasons. First, its ability to resist impact abrasion is over three times the level of soft glass. Second, it sets at a higher temperature and thus, gets "harder" faster. (This second quality was a physical characteristic that scientific glassblowers found particularly challenging in the early days of borosilicate glass.) Finally, because the hard glasses have much lower thermal coefficients of expansion than do the soft glasses (see Table 1.2 to see the thermal coefficients of expansion for various glasses), they can withstand much greater thermal shocks than soft glasses. The hard glasses are also more chemically resistant to alkaline solutions and many other chemicals.

Commercially, borosilicate glasses are found in many consumer products. In the kitchen, oven windows, baking containers, and cooking pot lids all take advantage of the thermal strength of borosilicate glass. In addition, measuring cups are also made out of borosilicate glass, not only for their thermal abilities (like pouring boiling water into a cold measuring cup), but also for their ability to withstand abrasion and impact. Measuring cups are typically nested (smaller cups placed within the bigger cups) and banged around in cupboards and drawers. Borosilicate glass is also used for automobile headlights, as well as for floodlights used in indoor/outdoor lighting.

There is more than one brand and type of borosilicate glass. Pyrex® (by Corning), Kimax® (by Kimble), and Duran® (by Schott) are all brand names of particular borosilicate glasses of similar composition made by different companies.1 The term "Pyrex" is to borosilicate glass as "Xerox" is to photocopying equipment: It is an almost generic term. There is little difference between the products of these three major companies as far as the chemical makeup of the glass is concerned; in theory, they are all interchangeable. What is important to the user, however, is the quality of the glass itself and the quality and uniformity of its manufacturing.

Borosilicate glass is chemically more resistant than soft glasses and will therefore resist the weathering effects of standing water better than soft glasses. Although they are resistant to most chemicals, hard glasses are still susceptible to damage from hydrofluoric acid, hot phosphoric acid, and strong alkali solutions.

*Any glass containing 5% or more (by weight) of B2O3 (boron oxide) is considered a borosilicate glass.

tAlso, not all borosilicate glass is the same chemical mix as these glasses. The borosilicate glass used in pipettes is designed for greater chemical resistivity and cannot be sealed to other laboratory ware such as a beaker.

crystal can't be manipulated to other shapes without changing it into quartz glass, this advantage would be lost.

Aluminosilicate glass is very difficult to work with because it is very prone to reboil. That is, while bringing it a working temperature, the glass develops bubbles over the surface that are impossible to remove. One other challenge with aluminosilicate glass, which is different from other glasses, is that you can't clean it with hydrofluoric acid (see Sec. 4.1.10). If attempted, it will cause the surface to develop a translucent sheen that is also impossible to remove. It is safe (for the glass) to clean the surface with nitric acid.

The High-Temperature and UV-Transmission Glasses. The last types of glass found in the laboratory are the quartz and UV-transmission glasses. From the many names that are used to describe this type of material, there may be confusion as to what to call it. It is often just called quartz, but this name can be deceiving because the term quartz could equally refer to the mineral quartz* (which is a crystal) or amorphous silica (which is a glass).

Historically, fused quartz referred to transparent products produced from quartz crystal rock, and fused silica referred to opaque products produced from sand. With the advent of new manufacturing techniques, transparent products can now be produced from sand, so the old distinction is no longer applicable. Currently, the term fused quartz is used whenever the raw product is either quartz rock or sand. The term fused silica is used whenever the raw product is synthetically derived (from SiCl4). Generically, the term quartz glass or, better yet, vitreous silica can be used to cover the whole range of materials.

The average consumer is only likely to come across vitreous silica when purchasing special high-intensity, high-brightness light bulbs for specialized lamps such as stadium lights, flash lamps (for cameras), or stroboscopes. Because of its purity, vitreous silica is essential in the manufacture of many consumer products that would otherwise be impossible to manufacture. One such product is the ubiquitous silicon chip that controls everything from computers and calculators to toys and cars. To maintain the purity of chips during construction, the silicon wafers are baked in large tubes of ultrahigh-purity fused silica. ^

Whereas other glasses obtain their unique properties by the addition of various oxides, the lack of other materials provides quartz glasses with their unique properties. Most distinctive of all quartz glass characteristics are their thermal and UVtransmission abilities. Unlike other glasses that deform and/or melt at temperatures in excess of 1200°C, quartz glass maintains a rigid shape. In addition, quartz glass has an extremely low thermal coefficient of expansion (approximately 5.0 x 10"7 Acm/cm/°C), meaning that it can withstand thermal shock that would likely shatter all other glasses. Furthermore, although all transparent materials limit various frequencies of light, quartz glass has the potential to transmit the broadest

Silica, or SiO2, occurs in nature as quartz, cristobalite, or tidymite.

+Fused quartz is likely to have sufficiently high impurity levels to jeopardize the purity (and thereby the quality) of the final product.