THE CHEMICAL Growing Synthetic Advances in ultraviolet and infrared spectroscopy call for more and larger optical crystals than are found in nature.
The
glasses, which are plastic materials, transmit rays only slightly beyond the visible range,
and
for
penetration
beyond
this range crystalline optics only suffice.
will
Spectroscopic methods of analy-
sis are used by many oil companies as well as allied industries, for the rapid detection
of
impurities to
the
near-
est 0 . 1 % , a high degree of accuracy.
The first step in the new method for growing sodium nitrate crystals is to fold aluminum foil around glass form into shape of the dish. The .method was first described in the Journal of the Optical Society of America by Cutter D. West and his associate, Frederick J. Binde, both of the Polaroid Corp.
Below. Diameter of mica is 1/4-inch less than that of dish, permitting easy floating and removal after crystal cools. Mica isfloatedat one end first, then lowered gradually to surface of melt, which technique removes air bubbles
Aluminum foil dish is placed inside asbestos insulating mold and filled with sodium nitrate crystals. M o l d is placed in muffle oven preheated to 320° C. (Can be grown in any oven)
Right· As soon as it has cooled, crystal is removed from oven. Aluminum foil is then peeled from hardened crystal. Mica is removed with needle or sharp pointed knife. If care is taken the same piece of mica can be used many times. A careful technique developed for its removal is to let a stream of water flow between mica and crystal after needle has laid back a small Hap of mica 812
C H E M I C A L
A N D
E N G I N E E R I N G
NEWS
NEWS PARADE Optical Crystals These instruments have been made possible by large synthetic prisms which allow more light to pass through and give better definition of bands.
A new and
simple method is described by the Polaroid Corp., Cambridge, Mass., for growing crystal sections from sodium nitrate, starting on a mica cleavage surface. The mica acts as an insoluble infusible seed for the growth of the alien crystal. The stripped sodium nitrate surface reproduces exactly the mirror qualities of the mica.
Freezing occurs first at the top of the melt, resulting in unoriented sections at the back of the large hardened crystal. These unoriented sections may be readily stripped off with a sharp knife. It is of particular interest that sodium nitratecanbe grown from afloatingpiece of mica downwards
Below. Crystal is polished on wet blotter for smooth surface. These crystals have a pronounced thermoplasticity and a notable resistance to thermal shock, the latter of the H U M order or better than glass under same conditions
The procedure of growth from the bottom up from a disk of mica sunk in the melt is preferred, as it allows for the escape of gas bubbles. Such as occur can be removed with sandpaper
Left. This is how a large sodium nitrate crystal looks between two crossed polarizers. A fine geometrical pattern is always produced. Since 1884 there have been repeated attempts to grow single crystals of sodium nitrate from which polarizing prisms could be prepared. The above method is said to be the simplest to far described for growing single crystals of considerable size V O L U M E
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