Flask star cracks

Star cracks. A little talked about phenomenon that turns an ordinary R.B. flask into a potentially explosive monster. Stress, whether prolonged heating in one spot, or indiscriminate trouncing upon hard surfaces, can cause a flask to develop a star crack (Fig. 20) on its backside. Sometimes they are hard to see, but if overlooked, the flask can split asunder at the next lab. 

Heating a flask. Since they are cold-blooded creatures, flasks show more of their unusual behavior while being heated. The behavior is usually unpleasant if certain precautions are not taken. In addition to star cracks, various states of disrepair can occur, leaving you with a benchtop to clean. Both humane and cruel heat treatment of flasks will be covered in (see Chapter 13, Sources of Heat ), which is on the SPCG (Society for the Prevention of Cruelty to Glassware) recommended readings list. 

Placing a wire gauze between the flame and the flask spreads out the heat evenly. Even so, the burner may have to be moved around. Hot spots can cause star cracks to appear in the flask (see Chapter 4, Round-Bottom... 

Round-Bottom Flasks, Pennyhead Stoppers, Plugs, and Other Loose Items. Many items are stored in drawers and (as the drawer is opened and closed) can bang around. This situation applies especially to round-bottom flasks, which are hard to prevent from rolling around. When these flasks bang around, star cracks, or flaws for future fracture, are created. For round-bottom flasks, Erlenmeyer... 

Fig.4.3 Star cracks are caused by hitting one point of a laboratory flask against a hard surface with enough force to crack the glass, but not hard enough to destroy the flask. The crack lines radiate from the point of impact. They frequently occur in glassware left to roll loose in drawers.

Safety note You rnust check all flasks and glassware for star cracks and chips. If in doubt, replace. 

Always check your flask for star -cracks. 

Caution Carry out all procedures in a well-ventilated fume-hood, wear appropriate disposable gloves, a lab-coat, and safety glasses. All vacuum-line work should be performed while standing behind a protective Perspex screen. Check all glassware for star cracks before use with any reduced pressure system. Never use flat-bottomed flasks with rotary evaporators. 

Examine your glassware for cracks and other weaknesses before assembling the distillation apparatus. Look for "star cracks in round-bottom flasks, because these can cause a flask to break upon heating. 

Check to see that your thermometer reads about 22-25°C (20°C = 68°F), normal room temperature. Examine the mercury column to see if the thread is broken—i.e., that the mercury column is continuous from the bulb up. Replace any flasks that have star-shaped cracks. Remember that apparatus with graduations and porcelain apparatus are expensive. Erlen-meyer flasks, beakers, and test tubes are, by comparison, fairly cheap. 

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