Robert 1. Anselmi New
Mexico
Highlands University Las Vegas, 87701
hexpensive Safety Shields
Safety shields in the undergraduate laboratory, although useful in teaching the student the safety habit, are rare because of their relatively high cost. We have been able to fabricate safety shields from discarded automotive windshields obtained a t no cost from the local glazier. These were cut to the desired size and shape, eliminating the defects if possible, and slipped into metal stands. The glass was cut by scratching both surfaces and flaming the scratch with a small hot flame on a Hoke oxy-gas torch (obtainable from William Dixon, Inc., Newark, New Jersey). The glass in these shields was sufficiently heavy to make extra weight in the stands unnecessary. The stands were constructed by welding together angle-type braces which happened to be in plentiful supply. An immediate objection to the use of these shields was raised. An explosion behind the shield, it was argued, might loosen glass from the pera at or'^ side of Safety sield support (1 in. = 1 2 the shield and send it fly- in).
ing toward him. These spall particles could he stopped by a plastic sheet laminated or clipped to the shield but the original economy would not be realized and considerable time might be lost in the fabrication. A thin coating of plastic applied by allowing a solution of the plastic in benzene to evaporate on the surface proved to be ineffective in appreciably reducing either the number or size of spall particles. Experiments conducted to determine the seriousness of spalling indicated that little or no danger exists unless glassware of a thickness greater than 1 mm is involved in the explosion. Explosions were produced by adding ethyl ether to chlorine in a dry ice bath and allowing it to %arm. After repeated tests, the surface near the sealed tubes was badly pitted but no spalling could be detected on the remote surface. To mimic more serious explosions, a sealed tube 25 mm o.d., 4-mm thick wall and 20 cm long containing 5 ml ether and 5 ml liquid chlorine was allowed to warm 14 cm from the shield. No spall particles were found above 20 cm from the hood surface, calculations indicated that all cavities were a result of contact with pieces of the sealed tube that traveled the shortest distance to the shield. These shields are not recommended for research use unless a plastic sheet ('/gl/in.) , is included to stop spa11 particles. Of course they should never be used without safety glasses.
Volume 45, Number 4, April 1968
/
259
