JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS thou. and that each resurfacing involves the removal of 1 thou. of metal (2 thou. increase in diameter) it is theoretically possible to achieve a total of 5 runs over the 4 to 6 thou. wall thickness range. In practice, this is seldom achieved, since an unusually deep score may be made by a foreign particle on the surface of the dump, necessitating the removal of more than 1 thou. of die surface to clear it, or an accidental smash may prematurely terminate the life of the tools. I have gone into this detail because of the repeated 4 to 6 thou. wall thickness cycle and its effect upon the so-called hardness of the tube. Hardness in the true sense is independent of wall thickness, but a customer uses the term loosely to describe a combination of true hardness and the effect of thickness, a quality which I prefer to call "rigidity." This characteristic is very dependent upon thickness being proportional to the 3rd power. While the wall thickness has increased by 50 per cent it has gone up by 238 per cent. Thus the expert responsible for the working of a filling machine is quite right when he says that the tubes he is using to-day are stiffer or more rigid than those going through earlier in the week. He will only be justified in blaming inadequate annealing if there is also evidence of residual spring in the metal. So far so good you have liked the cost and the appearance and have resigned yourself to the necessity of accepting a tube of a slightly stiffer character. There are no difficulties so far as decoration is concerned and you are left with the question of possible corrosion. With aluminium it is particu- larly necessary to proceed with caution, as quite a large number of compounds are reactive towards it. Since I have been asked to speak particularly on corrosion and seepage I shall deal with these general subjects after completing the review of the metals. TIN It is no exaggeration to say that tin is the ideal metal for making a collapsible tube. It has an excellent surface appearance which does not tarnish with the passage of time. It is not subject to work-hardening and has perfect manipulation characteristics while the product is being e:kpelled from the tube. Only a very limited range of products are unsuitable from the corrosion angle, and toxic effects are absent. Cost alone has allowed alumin- ium to supplant it on such a wide scale, but there are still certain types, such as the familiar eye ointment tube, for which it is essential, since elongated nozzles are not practicable in the harder metal. LEAD This metal in its natural state is rather too soft for making satisfactory tubes and is often hardened by small additions of other metals. It is still by a narrow margin the cheapest tube available, but its appearance leaves 11o

THE COLLAPSIBLE TUBE much to be desired. This, and the toxic dangers associated with it, restrict its use to such things as printing inks, enamel toners, adhesives, rat poisons and the like. It is difficult to get an enamel for it which does not show serious loss of flexibility and adhesion over a period of several months owing to the development of a surface oxide film which is partially absorbed by the layer of enamel and acts as a catalytic "drier." Storage under certain types of atmospheric conditions--proximity to some types of wood will do it--can give rise to a heavy surface deposit which disfigures the shoulder. TIN/LEAD ALLOYS Tubes based upon actual alloys of these two metals as distinct from a plating of tin on a lead core have not achieved any prominence in this country. For a year or two during recent times, a well-known toothpaste was packed in thbes which were made from a 5 per cent tin, 95 per cent lead alloy, but this is the only prominent one which I can recall. On the Continent, alloys such as 48 per cent tin, 3 per cent antimony and 49 per cent lead, and 12 per cent tin, 1 per cent copper, 3 per cent antimony, 84 per cent lead were successfully used for toothpastes. These have a high degree of corrosion resistance and a reasonable appearance. It is my opinion that such alloys would have achieved considerable importance had the competition from the cheap aluminium tube--plain or internally lacquered--been less severe. TIN-PLATED LEAD This is an old favourite, and continues to find a wide variety of uses. It is free from any unpleasant rigidity effects, relatively cheap, and its appearance enables it to masquerade as its tin counterpart until one examines it closely. Normally it consists of a core which contains from 2 to 3 per cent of tin--resulting from the use of plated waste in the melt--with 2 per cent by weight of tin on each side. As manufacturers of tubes it is our job to give the customer what he asks for without asking too many questions. As a chemist, and it is in that role that I appear before you to-night, I suggest that we should look at this system more closely. Even allowing for the difference in specific gravity of the two metals, a tube wall 5 thousands of an inch thick will have a surface layer of tin 0.15 thou. thick. The intending user should bear this fact in mind when assessing its value as protective coating for preventing corrosion. As a barrier between product and metallic lead it is quite effective providing active corrosive tendencies are abse-nt. I prefer to stress its decorative character and leave such matters as the inevitable areas of exposed lead at the end of the nozzle to look after them- selves. 111