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Deck-Sheathing Compositions A Technical Discussion of the Various Materials Used as Flooring for Ships HE steadily increasing cost of : wood sheathing for ships' decks has led to the invention and application of numerous substitutes of a more or less' successful nature. These substitutes may be roughly di- vided into three classes, as follows: 1.--Those made of sawdust, bound together by a stone-like cement, the one usually employed being oxychlor- ide of magnesium. The great propor- tion of the cements in actual use falls in this class. 2.--Those made of sawdust, cork dust, sand or other 'earthy materials, bound together with bitumen or a bituminous mixture. These substances are somewhat akin to the asphalt used commonly for road paving, and are used in small quantities on' exposed decks by some shipping companies. 3.--Artificial wood itself, made by the strong compression of sawdust with a suitable binder. This substance is almost as expensive as ordinary hardwood, however, and requires as much labor in laying down. It is, therefore, rarely used in Great Britain. The compositions belonging to the first class are, as already stated, far the most largely used. The saving in cost over wood: is twofold, in that the cost of the materials themselves is less than that of wood, and the cost of laying them down is still lower, as they are put down, like cement, with a trowel while in a soft, pasty condition. All depend upon the same chemical reaction for their hardening properties. When magnesium oxide (commonly known as calcined magnesia) is mixed with a suitable proportion of a solu- tion of magnesium chloride in water, the paste so formed is converted in a few hours to a hard, solid mass, which is only broken with great difficulty, and is capable of taking a high polish. This change is brought about by the chemical union of magnesium oxide with the magnesium chloride, whereby a new substance, known as hydrated magnesium oxychloride, is formed. Theoretically, this oxychloride ce- ment contains magnesia, magnesium chloride and water, in the following proportions: Magnesium oxide, 33 per cent; mag- nesium chloride, 16 per cent; water, 51 per cent. In practice, however, this proportion is altered by the fact that there is an excess of magnesia present and also a number of foreign substances which existed as impurities in the original magnesia. The large proportion of water in the cement, even when out- wardly perfectly dry, is noteworthy, and it is this absorption of water which causes the wet cement mixture to. set hard and dry right through in a few hours. If this cement be subjected to the prolonged action of water, the mag- nesium chloride is slowly removed and dissolves in the water, leaving a hydrated magnesium oxide which con- tains 60 per cent magnesium oxide and 40 per cent water. The cement has very strong binding powers; so much so that it will tenaciously hold to- gether more than 20 times its weight of sand. The sheathing compositions under consideration are made by mixing magnesium oxide and sawdust in the required proportions, together with a suitable coloring matter, such as In- dian red or sienna, and then making into a paste with a solution of mag- nesium chloride. In this condition they are laid on the deck and smooth- ed over with a. plasterer's trowel till a good surface is obtained. The floor so produced has a number of advantages besides low cost . of. manufacture. It will hold nails and screws as well as wood, thus permit- ting large areas to be done in passen- ger vessels at one time, before the cabin bulkheads and partitions are built, these being fastened down to the floor with ordinary screws. It is quite fireproof and non-inflammable, comparatively noiseless to walk upon, and, being a poor conductor of heat, is warm to the foot. When laid in saloons, it can be polished with bees- wax to as smooth a surface as wood, and, if properly colored, has a very pleasing appearance. Must Be Shkillfully Laid The necessity for preparing and lay- ing such a floor with skill and due precaution, however, cannot be too strongly emphasized, as neglect of these essentials may cause rapid cor- rosion of the steel deck under the floor. Cases are by no means un- By John Hamilton Paterson known where thousands of pounds have had to be spent on repairs to steel decks so covered on compar- atively new ships. The reason for this trouble is not far to seek. If a piece of one of the compounds be examined under the microscope, it will be found to be full of quite large holes, mostly communi- cating with one another. It follows, therefore, that these substances are porous, and water. can _ percolate through them without much difficulty. If the compound should, through bad mixing, contain an excess of mag- nesium chloride, it follows that the water which reaches the steel deck will contain this magnesium chloride, and, as is well known to engineers, such a_ solution is powerfully cor- rosive. If the composition, with the defect noted, is laid in an exposed position, such as a weather deck, or ii it is washed down daily with water, as in an alleyway, it will not be long before the air dissolved in the water, aided by the magnesium chloride, will have caused serious corrosion of. the steel deck, . ; It is obvious, therefore, that great care should be taken that the com- pound.does not.contain an excess of. magnesium chloride, but rather an ex- _ cess of magnesia, which has no action cn steel. Even this precaution, how- ever, while lessening the danger of corrosion, does not: obviate it; as the magnesium oxychloride, of which the: cement is formed, splits up slowly in the presence of water, as previously shown, producing free magnesium chloride, and hence corrosion. Corrosion of the Deck As corrosion of the deck underlying the composition is not caused by the composition itself, but by the water and air which penetrate it, it is evident that a sheathing material, which is waterproof and strongly adherent to the deck under it, would be quite safe under any conditions. Up to the pres- ent, however, so far as the writer knows, no completely satisfactory solu- tion of this problem has been found. Much can be done by taking care to use only fine sawdust and to mix it very thoroughly with the magnesia, as examination shows that a great proportion of the holes in the com-