J. F. Salmon, S. J. and C. A. Polley Loyola College Baltimore, Maryland 21210
An Inexpensive Method to Produce Plastic Models of Solids
A recent AC3 Newsletter has discussed a need for formal undergraduate education in solid state chemistry: "The solid state presents some of the most exciting opportunities for illustration and application of chemical principles and for the introduction of chemical systems of great research and technological potential." 1 The discussion goes on to emphasize a need for teaching materials. The second goal could be the development of materials-again a variety of materials would be required-that would assist the teacher with some interest in and knowledge of the solid state to set up a solid state chemistry course, perhaps at the juniar-senior or early graduate level. Such a course might be one part of a physical or inorganic chemistry sequence or might be a quarteror semester-long course like those now given on kinetics, thermodynamics, spectroscopy, and so forth . . . But the route to the incorporation of such material in a regular or newly conceived chemistry course is not obvious and is obviously not easy. The use of models to introduce structure and symmetry can be invaluable. Various models of molecular structures are quite common in courses and new model types for demonstrating molecular structure and symmetry are regularly proposed in this Journal. For general chemistry and inorganic chemistry courses we have found solid models of crystalline solids also invaluable for discussing the solid state; especially for demonstrating such topics as the relation of external symmetry of crystals to symmetry of internal arrangement, crystalline habits, constancy of interfacial angles, stereographic projection, and the crystalline classes and systems. In order to make durable solid models available to all students during a demonstration or for laboratory sessions, we have sought an inexpensive source of morphological models of the various crystal classes. A search in the literature of supply houses and in this Journal did not resolve our problem. It has only been since the recent renewal of interest by chemists in the solid state itself that there has appeared in the literature much discussion of the external symmetry of crystals. We have resorted to sets of wooden and cardboard models. Good hard wooden models can be purchased but the price for sets is quite expensive.2 Our efforts with cardboard models have had some success but they are fragile and limited in their morphology.3 In our efforts to find a supply of inexpensive structural models we have developed a process to produce plastic models which are more durable than commercial wooden models and rugged enough to be handled continually by students in laboratory sessions. The method is simple enough to permit production by a teacher or students. The process consists in pouring some type of molding material into a wooden frame around a pattern of the crystal model. After the mold has hardened the pattern is removed and a liquid plastic is poured into the mold to fill the cavity. When the plastic has solidified, it can be removed from the mold. Any excess plastic produced during pouring, can be removed readily by trimming. The same mold can be reused for making more than one model of a particular crystal. Thus sets of models of all 32 symmetry classes can be produced for use by students in the laboratory. The features on these models are sharp and excellent for demonstrating crystalline morphology and external 726
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T w o halves of a moulage m o d m d another half w t h the exposed pattern. Various model castngs are also shown
symmetry, a difficult task for some students to imagine from textbook pictures. Plaster of Paris is a common molding material, but we found that, for our purposes, it has several limitations. We have found moulage to he both economical and more practical. Moulage is used in criminology laboratories to produce a faithful representation of objects of evidential value.4 This moulage is an agar mixture which melts near the boiling point of water and solidifies a t about 70°C and is sufficiently pliable after setting to permit the withdrawal of undercut surfaces. Cracks in the moulage mold are easily repairable, even after many plastic models have been produced from it. Once a particular mold has been used to make the necessary quantity of plastic reproductions, the moulage mold may be cut up and remelted to form new molds. We experimented with various moulage compositions and found an effective and inexpensive one to contain: 100 g agar, 200 ml glycerine, 40 g magnesium sulfate, 15 g methyl cellulose,and 1000 ml water. Procedure
Preparation of Moulage: A solution of 200 ml of glycerine in 800 ml of water in a metal pot or beaker is brought to about 150% and maintained at that temperature while 100 g of agar is added with stirring. Into 200 ml of hot tap water in a second beaker there are added 40 g of magnesium sulfate and 15 g of methyl cellulose. After this mixture is thoroughly stirred it is added to the first mixture and the container is heated at about 150°C for onehalf hour with intermittant stirring. Upon cooling, the moulage solidifies in the container and can be cut into small pieces with a spatula and stored in a glass jar. The moulage has a grainy appearance initially, but it becomes smooth with repeated use.
'Advisory Council on College Chemistry Newsletter, Number 16, June, 1969. See, for example the literature of Sepor Laboratory, Box 1366, Tarrence. California 90505. For example, Flexagons of Forde Corporation, 2132 Pacific Avenue, Tacoma, Washington. *Clarke, C. D., J. Crim.Lam and Crim,26,928 (1936).
Production of Models: The mold frame, lid down, is placed an a flat surface: The model is placed inside the frame and held in position, either by hand or two-way tape. Then moulage, which had been melted to a creamy consistency, in a dauhle-boiler, is poured into the frame until the model is half covered. The moulage is allowed to coal and harden for at Least two hours in a water atmosphere. A satisfactory water atmosphere can be supplied by a desiccator whose bottom section contains water. A second portion of the moulage is then poured into the mold until the frame is filled and the moulage is again permitted to harden. The lid is removed and the moulage mold is pushed out of the frame. Slight pressure is then applied with the fingers along the interface between the two halves of the mold. The halves can then he separated and the pattern removed. The figure shows the two halves of a mold and another half with the exposed pattern. The halves are rejoined and placed back in the mold frame which rests on a porcelain plate or similar smooth surface. The mold is now ready for casting. An aoorooriate volume of liouid olastic. about 110% of the volume of the model, is poured into a small b e a k e r v h e hquid plastic ia pwred lnto the mold thruugh [he upen face unrrl the mold ia completely filled. After pouring, the mold frame almg wrth the porcelain plate is placed in the water atmosphere where it is allowed to harden. In two to three hours the casting is ready to be removed from the mold. In order to extract the model, the mold is removed from the frame, and the upper half of the mold is drawn carefully away from the model. The bottom half with the cast is then inverted over a smooth surface such as the oareelain olate and the cast model is allowed to 41de out of the mold and on to the porcelain plate. 'l'hr cast i i then allowed to remain in thia position cxpored to rhe air until thc plairir is no longer tack). At thts poinr the new cast model may be removed from the plate and is ready for use. The mold is placed in a container of water for two hours in
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order to replenish water last during the casting process. The mold surface then can he blotted dry and employed to reproduce another identical cast model, stored in a water atmosphere until its next use, or cut up to he melted for making new molds. Summary
The process using moulage molds t o produce cast plast i c models of any crystalline solid affords a n inexpensive method t o produce teaching materials for solid s t a t e chemistry. These models may he used during lecture demonstrations; also sets may be generated for laboratory sessions. T h e method is simple a n d safe enough t o permit students t o make their own models. Students, who have participated in this project have found satisfaction in t h e knowledge a n d insight they have gained about t h e casting process, one of t h e most ancient, and yet modern, examples of technology. Acknowledgment We wish t o thank Professor Henry C. Freimuth and Paul Nagengast for valuable suggestions a n d t h e Division of Chemical Education-duPont Small Grants Program for support of this project. 5Any lidded container with its bottom removed is satisfactory, e.g., aplastic ice cream container with its bottom cut out. Liquid casting plastics which are obtainable from hobby shops can be used. We have found "Clear Cast," produced by American Handicrafts Company, Fort Worth, Texas to he satisfactory. This particular plastic requires stirring together in the beaker for 15 min before pouring two drops of hardening catalyst far every 10 ml of liquid plastic. The catalyst is available with the liquid.
