Robert H. Dalton Corning Glass Works Corning, N e w York
Recent Developments in Glass
This is an age of rapid scientific progress in many firlds. Glass technology is no exception. The recent developments in the field of glass have been so numerous and diverse that any attempt a t comprehensive coverage in a brief article such as this would be so superficial as to be of little value. This discussion will deal only with a few representative examples of these recent developments which are important and of special interest to chemists. Before starting on these recent developments, however, a brief look a t the past will be of interest to chemists. Glass making is one of the oldest chemical industries. Tlmre are reliably dated sp~cimensthat. go back 4000 years and no doubt the real beginnings were much earlier. One of the first products was ornamental jewelry, and this is still a flourishing business-a remarkable record for the survival of a product. As the uniformity, durahility and clarity improved, glass was used in urns, vases, drinking vessels, bottles, windows, etc. Because of the comparatively durahle nature of glass, a numher of examples of early glass have come down to us. Thc first topic deals with some recent and very interesting discoveries
regarding the chemical attack on these old glass items. A New Look at Old Gloss
Anyone who looks a t archaeological specimens of glass in a museum will soon notice that many are coated with a crust of veathering products. Figure 1 shows such a specimen, a Roman bottle of around the first century am. The old glasses were in general of the alkali-lime-silicate type wit,h a rather high content of sodium or potassinm and were rather susceptible to hydrolytic attack. Through the years of exposure the alkali and alkaline earth elements were gradually leached out of t,he glass leaving behind a residue that consists largely of hydrated silica. Figure 2 shows a piece of the neck of a hottle of early American glass
Figure 2.
Figure 1.
A Roman bottle from the first century A.0
Neck of bottle IAmerironl from t h e ~evenreenthcentury.
(seventeenth century) that was excavated in 1959 a t Kingston, Massachusetts. A thick white crust is clearly visible. If we look a t a cross section of such a crust under high magnification (Fig. R), me find that it is not just a simple uniform layer but is almost always stratified; that is, it consists of a series of very fine layers, more or less parallel to the glass surfacc. These layers are only of the order of a micron in thickness and the whole picture would represent less than a tenth of a millimeter. It was first suggested that this pattern was a type of Liesegang ring phenomenon, but Brill and Hood ( 1 ) at Corning conceived the idea that it might result from a cyclical factor in the chemical attack such as could be caused by annual variations of temperatnre Volume 40, Number 2, February 1963
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or moistnre. To check this t,heor,v they have counted the st,rata on a number of specimens of more or less well-rst,ablished history. The results are shown in Table 1 . In general, there is good agreement between the dates calculated by counting layers and those known from historical records or other evidence. We thns have a new method of dating ancient glass pieces,
Figure 3. Photomicrograph of corrosion loyer on gloss.
or a t least determining the period of exposure. The method will also be useful in establishing the age of archa~ologicalsites and may even serve as a source of information about the weather in ages past. I n some ways it is analogous to the well-known method of counting tree rings, although with glass, the time sequence is reversed, that is, the latest layer is on the inside. Obviously, not all samples of ancient glass will be amenable to this dating method. 111many cases it tnrns ont that part of the crust has been lost either by
method of high purity. As you know, the conve~~tiol~al making glass ronsists of fusing together an intimate mix of t h ~raw materials (usually sand, soda and limestone) in a refractory unit such as a pot or tank. In contrast, the starting point for these glasses is not ordinary mineral substances which cannot generally be obtained in a very pure form, but synthetic chemical products which can be refined by chemical means to a very high degree. of purity. The mannfacturing process for these glasses is also unique in that the materials are not fused in a refractory container, but are formed in the gas phase and deposited without contact with other solid materials. The method consists in feeding very pure compounds of the deaired glass forming elements through a flame which converts them to minute fused droplets of the oxides. When these impinge on a surface they build up a layer of glass much as a layer of ice builds up during a sleet storm. Some of the compounds which lend themselves readily to this process are shown in Table 2. The reaction, in all cases, is one of hydrolysis with t,he water vapor in the flame. Other volatile compounds may be used instead of the halides. I n fact this has the advantage that we then get away from the halogen acids which are always a nuisance. However, the cost factor usually favors the halogen compounds. I t should he pointed out that the last two materials shown in t,he table will not form Table 2.
Flame Reactions
Table 1. Dating of Ancient Glass
S v b m q c d 1781 Buried 1639.1670 Buried cr. 1660-1675 Submerged
1931
ca. 1600.L640
1916 1919 1919 1919
1931 I931
Buried 1676 or Inter Submerged 1692 Submerged 1621
M.n"fnctund eighth-tenth ccnluri
m. 1910
Ninth.thirteenth ccnlury Nin,h.thirfctnlh
(1. 1910
century
(1.
L s l third-ly seventh century
1910
1960
accident or by overzealous cleaning. Also in some cases the cyclical factor is not strong enough to give layers t,hat can be accurately counted. However, in spite of these difficulties, it represents a novel and valuable tool in helping us to establish our record of the past. F l a m e Formed Glasses
Since chemists are often concerned with the purity of materials, the second general topic that I have selected has to do with some special glasses made by a new process which permits the attainment of extremely 100
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glasses by themselves, but only when used in conjunction with one of the prior materials. The principal commercial interest a t present centers on the first reaction which is being used for the preparatioo of a very high purity Si02 glass. The principal properties of this material are listed in Tahle 3. The low impurity level makes it useful as a container for melting super high purity semiconductor materials. The transmission in the far ultraviolet is greatly superior to other glasses, as is the resistance to discoloration by high energy radiation. Surprisingly Table 3.
Properties of High Purity SiOr Glass (Code 7943)
SiOz R.0,
99.99+%
