THE PREPARATION AND PRESERVATION OF LARGE CRYSTALS OF CHROME ALUM
The method used to grow crystals of chrome alum weighing over twelve pounds i s described. Various attempts to preserve these crystals are also mentioned. The use of crystal growing as a project for high-school chemistry pupils i s suggested.
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The great interest shown by teachers and pupils who have seen the crystals described herewith has ied the author to record the method used in growing them, for the benefit of others who are interested. The crystals were first shown a t a meeting of the Chemistry Teachers' Club of New York City. Later they were exhibited at the annual dinner of the combined science clubs of New York City. The i n t e r e s t of the author in crystal growing was aroused by an article in ~ ~ ~ J O U R N A L O F C H E M I CAL EDUCATION'that described c r y s t a l s of chrome alum which had grown to a size of 700 grams in sixteen months. It appeared that it might be interesting to permit crystals of this substance to grow over an extended ~ e r i o dof time. The lirt step in the procedure was to obtain seed crystals. This was done by making a saturated solution of chrome alum, using cold water. Heat cannot be applied as it usually is in making saturated solutions, because, upon boiling, the violet solution turns green due to the different degree of hydration in each case.% The green solution does not crystallize readily and is therefore unsuited for the experiment. The cold, saturated solution was filtered and set aside to crystallize. After several days a cluster of small crystals was found on the bottom of the beaker. From this deposit, fragments of single, octahedral crystals were separated and were placed in several small beakers, each containing some of the saturated solution. These beakers were then set aside in aroom having as uniform a tem-
' RORRMAN AND TAYLOR. J. CHEM.EDUC., 6, 473-8
(Mar., 1929). Smrrn, "College Chemistry," Rev. Ed., The Century Co.. New York City, 1929, p p 699. 700. a
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perature as it was possible to find. This happened to be the photographic dark room. The crystals were removed from the solutions about twice each week. At this time small bud crystals forming on the main crystals were scraped off and the beakers were refilled with freshly filtered solution. The crystals were then replaced. At the beginning of the experiment the growth was rather slow, but as the crystals became larger the rate of growth increased until they grew as much as 100 grams in a week. At all times during the growth of the crystals they were allowed to rest on the bottom of the beakers. In order to aid symmetrical growth they were rested on a diierent face after each removal from solution for scraping. Of the six original crystals started in this experiment four were removed a t various stages of their growth. The larger of the remaining two (shown in the photograph) now weighs 5500 grams, or slightly over 12 pounds. Each of the triangular faces forming the eight sides of the octahedral form measures 20 centimeters on edge. When the crystals were permanently removed from solution, difficulty was encountered in preventing them from disintegrating. Chrome alum efflorescesrapidly and the violet crystals soon crumple to a light-colored powder. Various protective coatings were applied to the crystals to prevent the evaporation of the water of crystallization. Among the substances tried were sprayed colorless lacquer, Valspar varnish, melted paraffin, and paraffin dissolved in petroleum ether. None of these substances gave the desired degree of protectio~. In a second series of beakers the author grew several crystals of ordinary alum, potassium aluminum sulfate. When these crystals were removed from their saturated solution they retained their water of crystallization and also their form. It was then decided to try to protect the crystals of chrome alum by coating them with the potassium aluminum alum. This was accomplished by immersing them in a saturated solution. until a layer about one-eighth inch in thickness had been deposited. Crystals thus treated have remained exposed for more than a year and are still intact. These rather detailed directions are given for the benefit of teachers and pupils who may be interested in growing crystals as a hobby. They may also prove useful to teachers who may wish to use crystal-growingas a pupil or chemistry-club project. For pupil projects copper sulfate is also a good substance to use. The crystals grow rapidly, the material is cheap, and they have the interesting asymmetric form. The author has in his possession crystals of copper sulfate weighing over 1300 grams that grew to that size in two semesters. The interest of the pupils in our chemistry classes in this particular project makes it seem worth recommending to other teachers of chemistry.
