Proper Care Will Prolong the Life of
CHEMICAL GLASSWARE By E V A N J. LEWIS, Corning Glass Works SINCE
the publication of "Extending the
Life of Chemical Glassware"1 by J. T. Littleton and G. A. Dasney, much additional work has been done in the „.ibora«.orv of Corning Glass Works on this same problem, and the results obtained support the conclusions of the previous work. Therefore it seems timely to reiterate those conclusions and to add other information which may prove useful in ensuring maximum service from chemical ware. However, maximum life and most economical service are not necessarily the same, for there may be oases where it is more economical to push the apparatus to the limit and thereby accelerate the work even though more breakage results. Chemists interested only in the maximum number of analyses in the shortest possible time will not be too critical of the length of service of the glassware, for there are limits to the thermal and mechanical shocks that even the be grades of laboratory glassware can withstand. Present-Da y Glassware Practically all chemists now actively engaged in chemical analysis and research have been using throughout their lives apparatus of highly stable, heat-resistant glasses. The present-day chemical glassware has, to a degree not before known, the ability to withstand thermal and mechanical shocks. Therefore, quite unintentionally, the user often demands a service too severe even for the best glassware obtainable (Figure 1). Breakage results and the improved quality fails to be recognized. This breakage is usually the result of either mechanical (Figures 2, 3, and 4) or thermal shocks (Figures 4, 5, 6, and 8). The glass manufacturer sets wall thickness standards which he believes will render the best service to the average user. The ability t o withstand thermal shock increases with a decrease in wall thickness, but on the other hand the ability to withstand mechanical shock increases with an increase in wall thickness (Figure 6) Accordingly, articles intended for use in heating processes are made thinner than ware intended for operations where mechanical strength is the only requisite, as in filter flasks. Other Uses than Intended Glassware, designed for a specific service, is sometimes used for a radically different purpose than that for which it was intended and failure in service follows. 1 I N D . E n g . CHEM.. 1 9 , 1271 (1927).
552
1.
Deformation and wire screen imprint resulting from overheating
Had the manufacturer received with, the order details of the intended use, an article more nearly meeting the requirements might have been furnished, thus eliminating dissatisfaction and delay. Chemical glass is usually more severelyattacked by concentrated solutions of alkali than by acids, and this attack is accelerated b y heat and by scratches. The latter have sharp points upon -which bubbles collect and then rise to the surface, leaving a localized highly concentrated solution between the bubble and glass. Convection currents thus set up bring fresh liquid to the point, thus increasing the attack as new bubbles are formed over and over at the same point, repeating the CHEMICAL
process at a rapid rate. Determinations, such as the Kjeldahl method for nitrogen, requiring the heating of alkali solutions should be watched closely and stopped as soon as the necessary liquid has been removed, thus avoiding unnecessarily high temperatures and concentrations. Strain in Service Although chemical glassware has been well annealed and is practically strain-free when shipped from the factory, strain may be introduced in service. Glasses of relatively high expansion break when rapidly cooled after overheating, while those having a relatively low expansion may merely take on strain. The strain thus introAND ENGINEERING
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«iucecl is localized and is particularly dangerous because it is in that part of the article subjected to the most severe shocks. Permanent strain of this nature i s introduced in the better grades of chemical glassware only when the glass Has been heated beyond its strain point, and then allowed to cool without annealing. (Pyrex Brand chemical glass No. 774 lias a strain point of 510° C. Vycor Brand 96 per cent silica glass No. 790 Has a strain point of 900° C.) Permanent strain is most often introduced in distillations which have been allowed to continue too long and in evaporations to dryness. TJnder such use, poor glassware would undoubtedly go to pieces the first time this occurred, but high-grade ware frequently -withstands the service causing the strain and may break later under normal service or even when standing unused on a shelf (Figure 4). Evaporating to dryness over a flame should therefore be carried out with extreme care and the vessel should be removed from the flame before the glass has reached a temperature equivalent to its strain, point. Sharp-pointed, intense flames should always be avoided. Large soft flames which heat slowly and uniformly will not introduce strain as long as they do not strike the glass above the liquid level. Unfortunately, certain standard methods of test and analysis introduce strain because they do not take into consideration the limitations pointed out b y the glass manufacturer. Scratches and Abrasions Scratches and abrasions on either inner
2.
Outside impact. Characteristic "Star** break
or outer surface often result in breakage, as will b e easily seen if the common method of breaking glass tubing by means of a file scratch is recalled. . "When a beaker or flask is scratched by a. stirring rod unprotected by a piece of rubber tubing (rubber policeman) or in cleaning, its mechanical strength is greatly reduced. Scratches on the inner surface are also particularly dangerous from a thermal point of view.
3.
Breakage from cantilever action in nèck of vial.
When a beaker containing a liquid is heated, the hotter outer surface tends to expand, but is held back by the cooler inner surface, which therefore is placed under tension. If there are scratches on this inner surface, the tensions are localized and fracture is more likely to occur (Figure 5). The user of chemical glassware should avoid scratching or abrading the surface of the ware in cleaning, scrap-
f& '
5.
4. Thermal break from service strain caused by boiling dry
VOLUME
21, NO.
8
»
» » APRIL
25,
1943
Thermal break from inside scratch
553
6.
Thermal break οί thick-walled drying tray resulting from heating directly over hot plate
ing, and stirring. Unscratched glass can withstand only a certain definite tempera ture gradient. Rapid heating or rapid cooling, such as placing a hot vessel on a wet or cold metallic surface or suddenly cooling it with water, may cause the giass t o crack. Sometimes chemists are at a loss to understand breakage of an article which gives way for no apparent cause after long service in special work. They say, "The
7.
554
method has not been changed and the only way I can explain the failure is that the article was defective or deteriorates with repeated heating and cooling." The body of the glass does not change, but the sur face becomes scratched and bruised and is therefore no longer able to withstand as severe shocks. Longer service will be obtained from chemical glassware if the following rules are observed:
Inside impact. Characteristic "Star" break
8. C H E M I C A L
1. Use the article for the service for which it was designed. 2. Do not heat the glass beyond its strain point or maximum temperature limit as defined by the manufacturer. 3. Never use an unprotected stirring rod. 4. Use a large soft flame and not a sharp hot one in heating operations. Avoid severe local heating. 5. Avoid abrupt temperature changes, particularly unnecessarily rapid cooling.
Thermal break from inside bruise A N D
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