sometimes they are more or less dried upon the face. The old working sand is considerably less adhesive than the new, and of a dark brown colour; this arises from the brick dust, flour and charcoal dust, used in moulding, becoming mixed with the general stock, which therefore requires occasional additions of new sand or loam, so that when slightly moist and pressed firmly in the hand, it may form a moderately hard compact lump.
Red brick-dust is generally used to make the partings of the mould, or to prevent the damp sand in the separate parts of the flask from adhering together.
The face of the mould which receives the metal, is generally dusted with meal-dust, or waste-flour; but in large works, powdered chalk, and also wood or tan ashes, are used, from being cheaper. The moulds for the finest brass castings are faced either with charcoal, loamstone, rottenstone, or a mixture of the same; the moulds are frequently inverted and dried over a dull fire of cork shavings, or when dried they are smoked over pitch or black resin lighted in an iron ladle. *
The cores or loose internal parts of the moulds for forming holes and recesses, are made of various proportions of new sand, loam and horse-dung, as will be explained in the section on cored works. They all require to be thoroughly dried, and those containing horse-dung must be well burned at a red heat; this consumes the straw and makes them porous and of a brick-red.
In making the various moulds, it becomes necessary to pursue a medium course between the conditions best suited to the formation of the mould, and those best suited to filling them with the red-hot metal, without risk of failure or accident. Thus, within certain limits, the more loam and moisture the sand contains, and the more closely it is rammed, the better will be the impression of the model; but at the same time, the moist and impervious condition of the mould would then incur the greater risk of accident, both from the moisture and from the non-escape of the air; therefore the policy, on the score of safety, is to use the sand as dry as practicable, so as to avoid the delay of after-drying, and also to keep the mould porous.
The founder, therefore, compromises the matter by using a little faring sand containing rut her more loam, for the face of the green moulds for general work; and in those cases where much loam is used, the moulds are thoroughly dried by heart, which is not generally necessary with ordinary sand moulds.
The gold and silver casters frequently use a lighted link for facing their sand-moulds, and some of the type-founders' metallic moulds are smoked over a lamp; all these modes deposit a fine layer of soot upon the moulds.
The power of conducting heat is considerably less in red-hot iron than in copper and brass, and therefore the moulds for the latter require to be in a drier condition than those which may be used for iron; but in either case the presence of superfluous moisture is always attended with some danger to the individual as well as to the work.*
Another point has also to be considered: as castings contract considerably in cooling, in moulding large and Blight works the face of the mould must not be too strongly rammed, nor too much dried, or its strength may exceed that of the red-hot metal, whilst in the act of shrinking. The result would be, that in contracting, the casting would be rent or torn asunder from the restraint of the mould; whereas it should have the preponderance of strength, so as to pull down the face of the sand instead of being itself destroyed. But the exact condition both of the mould and of the melted metal, must be determined by the nature of the object to be cast; matters which can be only referred to with the development of the practice of the foundry, and upon which we shall now commence.
The sand having been prepared, and the appropriate flask and boards selected, the moulder first examines every pattern separately, to determine the most appropriate way of inserting it in the flask, as explained by fig. 140, p. 319; also to see that patterns, such as f and h, therein shown, are smallest at the parti entering the most deeply into the sand, in order that they may deliver well. It should also be noticed whether they arc perfectly smooth, and that there is no glue hanging about them, which would cause them to adhere and to pull down the moist sand.
The bottom flask, 4, 5, p. 320, is placed on a board not less than an inch or two longer and wider than itself, with the face 4, downwards, and it is filled from the side 5. A small portion of the strong facing-sand is rubbed through a fine sieve; the remainder is thrown in from the trough with the shovel, and the moulder drives the whole moderately hard into the flask, either with a mallet, the handle of the spade, or other rammer; or else he jumps up by aid of the rope suspended from the ceiling, and treads the sand in with his feet. The surface is then struck off level with a straight metal bar or scraper, a little loose sand is sprinkled on the surface, upon which another board is placed, and rubbed down close.
* The above is the reason generally assigned for the fact, that the iron-founders may and do use their moulds with safety when sensibly more moist than is admissible for brass and copper castings. It is confirmatory of the fact that the more dense the mould, the drier it must be: as the sand used by iron-founders is also coarser and therefore more porous than that employed by brass-founders.
The two boards and the flask contained between them, are then all three turned over together; this requires them to be brought to the front of the moulding-trough, so that the individual may rest his chest against them, and his fore-arms upon the edges of the top board; he then grasps the three together at the back part with his outstretched hands, and thus retained in contact, the whole are quickly turned over upon the front edge of the moulding-trough, and then slid back upon the transverse bearers or blocks, to the usual position.