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November , 1934 AIRCRAFT ENGINEERING 305 Th e Results of Further Investigations, with the Conclusions Reached After a thorough investigation it was realised N exhaustive search of the uses to which also altered their colour on heating, but tha t the temperatures at which iodine was temperature-recording paints have not in so marked a manner as was experienced evolved, or could be made to evolve by making been made reveals the fact that com with the mixed iodides. More success was suitable additions, could not be depended upon. paratively few of these compositions have secured by following the colour changes with received continuous service. Incidentally, the range of colours and tints the assistance of ultra violet-ray glass. experienced when using mixed iodides of the One of the reasons which is principally re A considerable number of compositions were rarer metals is appreciably increased, when the sponsible for this state of affairs is that a made up, using both mixed iodide and oxide rising temperature leads to partial decom certain amount of decomposition occurs when salts and following the changes in natural position. sunlight, and under the medium of the rays. the paint is exposed to heated conditions. It should be clearly understood that paints which The starch paste cannot be made up with I t was then realised that, while a wide range display any tendency to decompose are really water in the ordinary way, as this would be of colour changes could be made use of, this of little practical value, since colour changes, evaporated with the heat. would mean suitably applying these inde although at first registering the temperature pendently in each case. with accuracy, fail at a later date. Clays and Kaolins Few subjects that have been dealt with Meanwhile, still another series of investiga Decompositio n Effects in these pages during the past five years tions was in progress, using clays and kaolins. Consistency of composition is imperative if have aroused more interest than the Osmo-kaolin is prepared electrolytically from it is desired to compete with the long-established suggested use of specially prepared china clay. The product is exceedingly light, methods using th e accurately calibrated thermo paint s for recording temperature as a result of the system of deposition. The meter and pyrometer. After a considerable changes in baths for heat treatment. density of the deposit can be altered by amoun t of research work had been carried out, Th e point was first raised in an article, manipulating the currents employed and the and samples of the paints tried in practice, it "Improve d Salt Baths for Duralumin," amount of solid matter retained in suspension. published in AIRCRAFT ENGINEER was realised that decompositions which set in ING , Vol. V, October 1933, p . 242. A selected variety of electrolytically de led to much trouble. If the colour changes posited clays opened up another avenue, since had altered in a manner similar to the initial In response to the large number of these were also found to alter their colour changes (i.e. a t partially modified tints), such inquiries that resulted from this publica systematically with a temperature change. tion, the author dealt with the subject in difficulties would have been more or less over mor e detail, giving a synopsis of the This class of colour change was found to exist come by experience. The decompositions, how histor y of the process, in a second article, only when an exceedingly faint layer of material ever, led to fresh colours being produced, and "Paint s for Recording Temperatures," was coated. A thick layer such as that applied this created confusion as to the degree of heat which appeared in Vol. VI, March 1934, by hand was of little practical value. which was supposed to be registered. The p . 76. conclusions drawn were that to make an Since then the autho r ha s continued his Intensifying by Heat absolute success of this work no form of investigations, the results of which we This method of intensifying the colour pro decomposition whatsoever must exist, even now publish. They are, as will be seen, duced by heating was tried with metallic though the paint composition has been raised to a large extent nugatory, but it by no oxides, i.e. applying an exceedingly fine coating to a temperature beyond that for which it mean s follows that other investigators by electrolytic means. But this was fraught ma y not be able to overcome the diffi was specified. with many practical difficulties, since hydrates culties. If so, we should have a cheap, are deposited instead of pure oxides. The clearly visible method of controlling Extendin g the Range temperature s that would have many deposits were made on discs of metal covered The original paints prepared by Tonner, applications . with graphite, following the methods adopted Pinnock, Jones and others depended upon the for other similar electrolytic depositions. The change in colour of mixed metallic iodides. graphite does not necessarily mask the colour I t was at first thought that such changes of the product, but leads to the possibility of were free from decomposition, but a closer I t might be argued that since the colour reduction of the oxides. A somewhat cir stud y revealed that this was not absolutely change is the result of chemical decomposition, cuitous method was then evolved for arriving accurate. As with numerous other research on cooling the mass will have so altered as a t the desired deposition. investigations, the finding of one error proved t o be valueless for further use. But this is not the solving of another problem, namely that so. Anothe r Method a still wider range of colours could be made The initial heating with a faint blue tint use of. The characteristic colour produced The predetermined mixture known to give present, but not predominating, causes a when iodine is set free in presence of starch ultimately the chameleon type of paint was range of colours to be produced. paste is known to all chemists. first deposited. In order to remove the water On cooling, the colours also return, but in This is generally represented by a deep of hydration, this deposit was gently heated. a different order, since the mass is no longer blue colour. The fine mass was then blown into a cloud in of the same composition. a small chamber and the suspended matter B y repeating the heating several times, the Use of Starch precipitated by the electrostatic method at a changes will be found to approximate more This depth of colour can be increased and comparatively high voltage, following the closely. Other organic compounds were sub reduced by the manner in which the starch methods adopted for fume recovery. The mass stituted for the starch, but on the whole it paste is made up, apart from the quantity was made to deposit in the form of a film was realised that damp compositions, irre of free iodine which is evolved. For example, which could be directly applied as a paint. spective of the fluid material employed, were if an ordinary chemical analysis of a deep red, This method, although at first sight ap generally unsatisfactory. Although abandoning yellow, or brown mixed metallic iodide is pearing somewhat complicated, far surpasses this section of the work, it was realised that carried out, and it is desired to ascertain the earlier direct electrolytic system of de could such colour changes be secured without (|ualitativcly if free iodine is present, starch position, since there is practically nothing decomposition, the products would be of paste can be employed in the usual manner. present which can decompose. commercial value. Although the blue colour of the latter is In conclusion, it should be stated that the generally sufficient to mask the original colour besetting sin which appears to be encountered Clays and Metallic Oxides of the iodides, the starch can be so manipulated with temperature-recording paints is the tha t only a change of tint is recorded. In a While the foregoing work was in progress, possibility of ultimate decomposition. At the similar manner, if such iodides are subjected other methods were attempted in order to moment practical commercial application is to a temperature whereby they become faintly arrive at the same conclusions. retarded for this reason and no other, since their decomposed, the slight evolution of iodine A search of earlier literature on the different efficiency for recording temperature changes will again alter the tint. metallic oxides proved that many of these within a small margin of degrees is proved.
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Nov 1, 1934
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